the posterior fossa veins

CHAPTER 3

The Posterior Fossa Veins

Albert L. Rhoton, Jr., M.D.Department of Neurological Surgery, University of Florida, Gainesville, Florida

Key words: Anatomic study, Brainstem, Cerebellum, Posterior fossa, Veins

The veins of the posterior fossa are divided into fourgroups: superficial, deep, brainstem, and bridgingveins. The superficial veins are divided on the basis of

which of the three cortical surfaces they drain; the tentorialsurface is drained by the superior hemispheric and superiorvermian veins, the suboccipital surface is drained by theinferior hemispheric and inferior vermian veins; and thepetrosal surface is drained by the anterior hemispheric veins(15, 16). The deep veins course in the three fissures betweenthe cerebellum and the brainstem and on the three cerebellarpeduncles. The major deep veins in the fissures between thecerebellum and brainstem are the veins of the cerebellomes-encephalic, cerebellomedullary, and cerebellopontine fissures,and those on the cerebellar peduncles are the veins of thesuperior, middle, and inferior cerebellar peduncles. The veinsof the brainstem are named on the basis of whether they drainthe midbrain, pons, or medulla and course transversely orhorizontally. The veins of the posterior fossa terminate asbridging veins, which collect into three groups: a galenicgroup that drains into the vein of Galen; a petrosal group thatdrains into the petrosal sinuses; and a tentorial groupthat drains into the tentorial sinuses, which empty into the trans-verse, straight, or superior petrosal sinus (Figs. 3.1 and 3.2).

THE POSTERIOR FOSSA VEINS

Superficial veins

The superficial veins drain the cortical surfaces of the cer-ebellum. They are divided on the basis of whether they drainthe tentorial, petrosal, or suboccipital surface and whetherthey drain the hemisphere or vermis. The tentorial surface isdrained by the superior hemispheric and the superior ver-mian veins; the suboccipital surface is drained by the inferiorhemispheric and the inferior vermian veins; and the petrosalsurface is drained by the anterior hemispheric veins. In addi-tion, selected cortical veins may be named on the basis of thevermian or hemispheric lobule that they drain, or on the basisof the fissure in which they course. The superficial tonsillarveins are also included in this group.

Deep veins

The deep veins course in the three deep fissures betweenthe cerebellum and brainstem near the roof and walls of thefourth ventricle and on the three cerebellar peduncles thatcourse within these fissures. The vein of the cerebellomesen-cephalic fissure arises in the cerebellomesencephalic fissureand is intimately related to the superior half of the roof; thevein of the cerebellomedullary fissure courses in the cerebel-lomedullary fissure, and is intimately related to the inferiorhalf of the roof; and the vein of the cerebellopontine fissurecourses in the cerebellopontine fissure is intimately related tothe lateral recess and lateral walls of the fourth ventricle.

The major veins on the surface of the three cerebellar pe-duncles also course within these fissures. The vein of thesuperior cerebellar peduncle courses on the posterior surfaceof the superior cerebellar peduncle in the cerebellomesence-phalic fissure; the vein of the inferior cerebellar peduncleascends on the posterior surface of the inferior cerebellarpeduncle in the cerebellomedullary fissure; and the vein ofthe middle cerebellar peduncle ascends on the lateral surfaceof the middle cerebellar peduncle in the anterior part of thecerebellopontine fissure. The deep tonsillar veins are alsoincluded in this group.

Veins of the brainstem

The veins of the brainstem are named on the basis of threecharacteristics: the subdivision of the brainstem drained (mes-encephalon, pons, or medulla); the surface of the brainstemdrained (median anterior, lateral anterior, etc.); and the direc-tion in which they course (transverse or longitudinal).

The longitudinally oriented veins are the median anteriorpontomesencephalic and the median anterior medullary veins,which course in the midline; the lateral anterior pontomesence-phalic and the lateral anterior medullary veins, which course onthe anterolateral surface of the brainstem; and the lateral med-ullary and the lateral mesencephalic veins, which course on thelateral surface of the brainstem. The transversely oriented veinsrunning in the sulci at the junctions of the pons and mesenceph-alon and the pons and medulla are the veins of the pontomes-encephalic and the pontomedullary sulci. The transverse pontineand transverse medullary veins course across the anterior and

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lateral surfaces of the pons and medulla, and the peduncularveins pass around the cerebral peduncles.

Bridging veins and major draining groups

The terminal ends of veins draining the brainstem andcerebellum form bridging veins that cross the subarachnoidand subdural spaces to reach the venous sinuses in the dura(3, 6, 20, 21, 25). These bridging veins collect into three groups:a superior or galenic group that drains into the vein of Galen;an anterior or petrosal group that drains into the petrosal si-nuses; and a posterior or tentorial group that drains into thesinuses converging on the torcula.

An outline of the veins is as follows (Fig. 3.3):

I. Superficial VeinsA. Tentorial surface

1. Superior vermian veins

2. Superior hemispheric veinsB. Suboccipital surface

1. Inferior vermian veins2. Inferior hemispheric veins3. Retrotonsillar veins4. Medial and lateral tonsillar veins

C. Petrosal surface1. Anterior hemispheric veins

II. Deep VeinsA. Cerebellomesencephalic fissure

1. Vein of superior cerebellar peduncle2. Vein of cerebellomesencephalic fissure3. Pontotrigeminal vein4. Tectal veins

B. Cerebellomedullary fissure1. Vein of cerebellomedullary fissure2. Vein of inferior cerebellar peduncle3. Supratonsillar veins4. Choroidal veins

C. Cerebellopontine fissure1. Vein of cerebellopontine fissure2. Vein of middle cerebellar peduncle

III. Veins of the BrainstemA. Longitudinal veins

1. Midlinea. Median anterior pontomesencephalic veinb. Median anterior medullary vein

2. Anterolaterala. Lateral anterior pontomesencephalic veinb. Lateral anterior medullary vein

FIGURE 3.1. Drainage patterns of the cerebellar surfaces. A,tentorial surface. The tentorial surface is drained by the supe-rior hemispheric and superior vermian veins, which are dividedinto an anterior and a posterior group. The anterior group andthe veins from the cerebellomesencephalic fissure empty pre-dominantly into the vein of Galen and its tributaries. The poste-rior group drains the posterior part of the tentorial surface andempties into the tentorial sinuses, which are tributaries of thestraight, transverse, or superior petrosal sinus, or the torcula.Some of the inferior hemispheric veins from the suboccipitalsurface pass forward under the transverse sinus and cross theposterior part of the tentorial surface to empty into the tentorialsinuses. B, suboccipital surface. The suboccipital surface isdrained by the inferior hemispheric and inferior vermian veins,which ascend toward the transverse sinus, but then turn for-ward below the sinus and commonly empty into the tentorialsinuses. Some of the inferior hemispheric veins from the suboc-cipital surface empty into the inferior vermian veins, which inturn empty into the tentorial sinuses. C, petrosal surface andanterior surface of the brainstem. The anterior hemisphericveins, which drain the petrosal surface, and the veins from thebrainstem commonly unite to form the superior petrosal veinsthat empty into the superior petrosal sinus. Ant., anterior; Cer.Mes., cerebellomesencephalic; Fiss., fissure; Hem., hemispheric;Inf., inferior; Pet., petrosal; Post., posterior; Sup., superior;Trans., transverse; V., vein; Ve., vermian.

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3. Laterala. Lateral mesencephalic veinb. Lateral medullary and retro-olivary veins

B. Transverse Veins1. Peduncular vein2. Posterior communicating vein3. Vein of pontomesencephalic sulcus4. Transverse pontine veins

5. Vein of pontomedullary sulcus6. Transverse medullary vein

IV. Bridging Veins (Major Draining Groups)A. Galenic group (to vein of Galen)B. Tentorial group (to torcula and tentorial

sinuses)C. Petrosal group (to petrosal sinuses)D. Other bridging veins

FIGURE 3.2. A–D. Venous drainage of the posterior fossa. A, superior surface of the tentorium. Some of the tentorial sinuses can be seenthrough the tentorial surface. Veins from both the cerebrum and cerebellum empty into the tentorial sinuses. The veins in the quadrigem-inal cistern and the cerebellomesencephalic fissure empty into the vein of Galen and its tributaries. B, the left half of the tentorium hasbeen removed while preserving the tentorial edge. The inferior hemispheric veins from the suboccipital surface cross the posterior part ofthe tentorial surface to empty into one of the tentorial sinuses with some of the superior hemispheric veins. Two veins from the right pos-terior temporal lobe empty into the transverse sinus. C, superolateral view of the tentorium. A complex and variable group of venoussinuses course within the tentorium and empty into the straight, transverse, and superior petrosal sinuses. The veins draining the suboc-cipital surface and posterior part of the tentorial surface empty into the tentorial sinuses. The majority of veins from the upper part of thetentorial surface drain toward the cerebellomesencephalic fissure and empty into tributaries of the vein of Galen. Some veins from thelateral part of the tentorial surface may empty into the superior petrosal sinus. D, lateral cerebral and cerebellar surfaces. The sinuses inthe tentorium receive drainage from both the cerebrum and cerebellum. Veins from the lateral and inferior surfaces of the cerebral hemi-sphere pass toward, but often turn medially above the transverse sinus to join the tentorium sinuses that empty into the transverse sinus.The inferior hemispheric veins from the suboccipital surface ascend toward, but often pass below the transverse sinus to empty into thetentorial sinuses. A mastoidectomy has been completed to expose the sigmoid sinus and jugular bulb. Cer., cerebellar; Cer. Mes., cerebel-lomesencephalic; Cist., cistern; CN, cranial nerve; Fiss., fissure; Hem., hemispheric; Inf., inferior; Int., internal; Jug., jugular; Occip., occipi-tal; Ped., peduncle; Pet., petrosal; Quad., quadrigeminal; S.C.A., superior cerebellar artery; Sig., sigmoid; Str., straight; Sup., superior;Temp., temporal; Tent., tentorial; Trans., transverse; V., vein.

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SUPERFICIAL VEINS

The superficial veins drain the tentorial, suboccipital, and petro-sal surfaces. Each surface has the vermis in the midline and thehemispheres laterally, and is divided by a major fissure namedon the basis of the surface that it divides (Figs. 3.1 and 3.3).

The three surfaces are separated by borders that are parallelto the major venous sinuses surrounding the cerebellum. Thetentorial and petrosal surfaces are separated by a border thatparallels the superior petrosal sinus; the tentorial and suboc-cipital surfaces are separated by a border that parallels thetransverse sinus; and the suboccipital and petrosal surfacesare separated by a border that parallels the sigmoid sinus. Theveins from adjoining surfaces frequently join near these bor-ders to form common trunks that terminate in a dural sinus.The veins from adjoining surfaces often anastomose acrossthese borders. These anastomoses often take place in thefissures between the folia, which are continuous from onesurface to the other. The hemispheric lobules and interfolial

fissures on the tentorial surface overlap onto the superior partof the petrosal surface, and those on the suboccipital surfaceoverlap onto the inferior part of the petrosal surface.

The cortical surfaces are drained by a mixture of longitudi-nal and transverse veins. On some surfaces the predominantdrainage is transversely oriented along the interfolial fissures,and on others the major drainage is longitudinally oriented atright angles to these fissures. The veins within the interfolialfissures may not be visible on the cortical surface.

Tentorial surface

The tentorial surface drained by the superior hemisphericand superior vermian veins, conforms to the lower surface ofthe tentorium (Figs. 3.1-3.5).

Superior vermian veinsThe veins that drain the vermian part of the tentorial sur-

face are divided into an anterior group, which ascends toward

FIGURE 3.2. E–H. Venous drainage of the posterior fossa. E, the temporal lobe has been elevated to show a group of veinsthat pass from the lower surface of the cerebral hemisphere to the tentorial sinuses. Two large lateral cerebral veins emptyinto the right transverse sinus, but the more medial veins exposed by eliminating the temporal lobe empty into tentorialsinuses. F, the posterior part of the right temporal lobe has been elevated to show the complex of veins on the inferior sur-face of the hemisphere that empty into the tentorial sinuses. G, the right half of the tentorium has been opened while pre-serving a large tentorial sinus, which receives drainage from the cerebrum and cerebellum. The temporal and occipital lobeshave been preserved on the left side. H, the posterior lip of the cerebellomesencephalic fissure has been removed. The pairedveins of the superior cerebellar peduncle ascend to join and form the vein of the cerebellomesencephalic fissure, which emp-ties into the vein of Galen.

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the vein of Galen, and a posterior group, which descendstoward the torcula (Figs. 3.3-3.5). The anterior, or ascending,veins originate near the tentorial fissure and join near the apexof the cerebellum to form the superior vermian vein thatcrosses the quadrigeminal cistern to reach the vein of Galen.The major tributaries of the superior vermian veins are thevein of the cerebellomesencephalic fissure, to be describedlater; the tectal veins from the quadrigeminal plate; and thehemispheric branches from the medial part of the hemisphere.

The posterior, or descending, superior vermian veins origi-nate in or near the tentorial fissure, course posteriorly, anddrain alone or after joining the inferior vermian veins into thetorcula or a tentorial sinus.

Superior hemispheric veinsThese veins are divided into larger anterior and posterior

groups and a smaller lateral group (Figs. 3.3-3.5). The veins in

FIGURE 3.3. A and B. Veins ofthe posterior fossa. The veinsin the posterior are dividedinto three groups: a galenicgroup (green) that drains intothe vein of Galen; a petrosalgroup (blue) that drains intothe petrosal sinuses; and a ten-torial group (brown) thatdrains into the sinuses near thetorcula. A, tentorial surface,superior view. The tentoriumhas been removed except inthe area of the tentorialsinuses. B, suboccipital surface,posterior view. The right tonsiland the medial part of thebiventral lobule have beenremoved to expose the struc-tures on the ventral wall of thecerebellomedullary fissure. A.,artery; Ant., anterior; Bas.,basilar; Br., bridging; Car.,carotid; Cav., cavernous;Cer., cerebellar, cerebello,cerebral; Cer. Mes., cerebel-lomesencephalic; Ch., choroi-dal; Com., communicating;Con., condylar; Em., emissary;Fiss., fissure; He., hemispheric;Inf., inferior; Int., internal;Jug., jugular; Lat., lateral; Lig.,ligament; Marg., marginal;Med., medial, medullary; Mes.,mesencephalic; Mid., mid, mid-dle; N., nerve; Occ., occipital;Olf., olfactory; Ped., peduncle;Pon., pontine; Post., posterior;Retroton., retrotonsillar; Sag.,sagittal; Sig., sigmoid; Str.,straight; Sulc., sulcus; Sup.,superior; Supraculm., supracul-minate; Supraton., supratonsil-lar; Tent., tentorial; Ton., ton-sillar; Trans., transverse; Trig.,trigeminal; V., vein; Ve., ver-mian; Vel., velum; Vert.,vertebral.



the anterior group drain the anterior part of the hemisphericsurface and join the superior vermian vein or the veins in thecerebellomesencephalic fissure. The other veins in the anteriorgroup cross the anteromedial margin of the cerebellum anddip into and join the veins coursing in the cerebellomesence-phalic fissure. The veins in the posterior group drain theposterior part of the tentorial surface. They usually join andform a common trunk with the inferior hemispheric veinsfrom the suboccipital surface to form bridging veins that enter

the torcula or the superiorpetrosal, transverse, or tentorialsinuses. The veins in thesmaller lateral group originateon the lateral part of the tento-rial surface and drain directlyinto the superior petrosal si-nus or one of its tributaries.

Suboccipital surfaceThe suboccipital surface,

drained by the inferior hemi-spheric and inferior vermianveins and the superficial groupof tonsillar veins, conforms tothe part of the inner surface ofthe occipital bone located be-tween the sigmoid sinuses (Figs.3.3 and 3.6-3.8). The superficialgroup of tonsillar veins is com-posed of the retrotonsillar andthe lateral and medial tonsillarveins that converge on the pos-terior surface of the tonsil andjoin to form the inferior ver-mian vein. There is also a deepgroup of tonsillar veins, the su-pratonsillar veins, which coursein the cerebellomedullary fis-sure along the inferior part ofthe roof of the fourth ventricleand join the vein of the cerebel-lomedullary fissure.

Inferior vermian veins

The inferior vermian veins drain the vermis and the adja-cent portion of the hemisphere, including part of the tonsil(Figs. 3.3 and 3.6-3.8). These paired veins are usually formedby the union of the retrotonsillar veins. They ascend along thevermohemispheric fissures and terminate in the straight ortransverse sinuses or the torcula, either directly or through ashort tentorial sinus. They may course on the vermis or the

FIGURE 3.3. C and D. Veins ofthe posterior fossa. C, petrosalsurface and left side of thebrainstem, anterolateral view.D, deep cerebellum and fourthventricle, posterior view. Theright cerebellar hemisphere andthe part of the left cerebellarhemisphere posterior to thedentate nucleus and tonsil havebeen removed to show the roofof the fourth ventricle and thecerebellomesencephalicand cerebellomedullary fissures.

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adjacent part of the hemisphere before reaching the vermo-hemispheric fissure. In a few cases, one inferior vermian veinwill cross the vermis to terminate in the inferior vermian vein onthe opposite side. There is often an anastomotic vein thatcrosses obliquely or transversely from one inferior vermianvein to the other. Some interconnect the veins after they leavethe surface of the cerebellum to form bridging veins (24). Thetributaries of the inferior vermian vein, beginning caudally,include veins from the tonsil (the superior and inferior retro-tonsillar and the medial and the lateral tonsillar veins), theadjacent part of the vermis and hemisphere, and the postero-medial part of the tentorial surface.

Inferior hemispheric veinsThe inferior hemispheric veins are oriented longitudinally

or transversely on the suboccipital surface (Figs. 3.3, 3.6, and3.7). The majority of the longitudinal veins ascend and crossthe margin between the suboccipital and tentorial surfaces tojoin the posterior group of superior hemispheric veins beforeemptying into a sinus in the tentorium. Some join the lowerpart of the inferior vermian vein. The transversely orientedveins course along the fissures between the folia and predom-inantly empty into the inferior vermian vein medially, but afew join the anterior hemisphere veins laterally.

The inferior hemispheric veins are divided into fourgroups: the superomedial, inferomedial, superolateral, andinferolateral veins, based on the part of the suboccipital sur-face that they drain. The veins in the superomedial group arethe largest. The major veins in this group usually run longi-tudinally and drain into the torcular, a tentorial sinus, or theinferior vermian vein. Transversely oriented veins in thisgroup, if well developed, drain into the torcula or the inferiorvermian vein. The inferomedial group consists of small veinsthat originate and course inferiorly on the biventral lobule tojoin the inferior retrotonsillar or the inferior vermian veins.The veins in the superolateral group usually pass superolat-erally across the posterior margin of the hemisphere anddrain either directly or through a tentorial sinus into thesuperior petrosal or transverse sinuses, but some smallermembers of this group may course around the lateral marginof the hemisphere to join the anterior hemispheric veins onthe petrosal surface. The veins in the inferolateral group drainthe lateral part of the biventral lobule and pass around theinferior margin of the hemisphere to join the anterior hemi-spheric veins.

Retrotonsillar veinsThe superior and inferior retrotonsillar veins drain the supe-

rior and inferior poles and the posterior surface of the tonsils(Figs. 3.3 and 3.8). They receive tributaries from the medialand lateral tonsillar surfaces and the adjacent part of thevermis and hemisphere. The superior retrotonsillar vein arisesnear the superior pole and courses posteriorly to join theinferior retrotonsillar vein to form the inferior vermian vein.The inferior retrotonsillar vein arises near the caudal pole ofthe tonsil and courses superiorly to join the superior retroton-

sillar vein and other tributaries from lateral and medial ton-sillar surfaces.

Medial and lateral tonsillar veinsThe medial tonsillar veins originate on the tonsillar surface

facing the other tonsil, and the lateral tonsillar veins arise onthe lateral side of the tonsil fissure between the tonsil andbiventral lobule (Fig. 3.3). These veins usually course posteri-orly and drain into the superior or inferior retrotonsillar or theinferior vermian veins.

Petrosal surface

This surface, drained by the anterior hemispheric veins,faces the posterior surface of the petrous bone (Figs. 3.3 and3.9).

Anterior hemispheric veinsThese veins arise near the border that separates the petrosal

surface from the suboccipital and tentorial surfaces, and passanteriorly to converge on the cerebellopontine fissure and themiddle cerebellar peduncle. They are divided into supe-rior, middle, and inferior groups. The veins in the inferiorgroup arise on the inferior part of the petrosal surface andconverge on the caudal part of the cerebellopontine fissure toform a common trunk. The vein of the cerebellomedullaryfissure, if it passes dorsal to the flocculus, joins the commontrunk of the inferior group. The veins in the middle groupdrain the middle portion of the petrosal surface and convergeon the apex of the cerebellopontine fissure. The commontrunk of the inferior group joins the common trunk of themiddle group near the flocculus to form the vein of thecerebellopontine fissure, which passes to the superior petrosalsinus. In a few cases, the common trunk of the middle groupdoes not join the common trunk of the inferior group, butascends to drain directly into the superior petrosal sinus. Thesuperior group, the smallest of the three groups, drains therostral edge of the petrosal surface. These veins course ante-riorly or posteriorly to join either the vein of the cerebellopon-tine fissure, the anterolateral marginal vein that courses alongthe junction of the tentorial and petrosal surfaces, or one of thesuperior hemispheric veins.

DEEP VEINS

The deep veins course in the fissures between the brain-stem and the cerebellum near the roof and lateral walls of thefourth ventricle (Fig. 3.3). The veins most intimately related tothe superior part of the roof are those that course in thecerebellomesencephalic fissure; the veins most intimately re-lated to the inferior part of the roof are those that course in thecerebellomedullary fissure; and those most intimately relatedto the lateral wall and cerebellopontine angle are those thatcourse in the cerebellopontine fissure. The structures ventralto the floor of the fourth ventricle are drained by the veins ofthe brainstem, which are considered in the section on theveins of the brainstem in this chapter.



FIGURE 3.3. E and F. Veins ofthe posterior fossa. E,midsagittal section ofcerebellum and fourthventricle. Left lateral view.The left half of the cerebellumhas been removed to exposethe fourth ventricle. F,brainstem. Anterior view. Thepart of the tentorium betweenthe temporal lobe and thecerebellum has beenpreserved. A–F, the inferiorsagittal sinus joins the straightsinus at the apex of thetentorium. The superiorsagittal sinus joins the straightsinus at the torcula. Thesuperior petrosal sinus passesalong the petrous ridge andjoins the junction of the lateral(referred to here as thetransverse sinus) and sigmoidsinuses posteriorly and thecavernous sinus anteriorly.The veins converging on thetentorium join to formtentorial sinuses that draininto the straight, lateral, andsuperior petrosal sinuses andthe torcula. The marginal sinuscourses in the dura at the levelof the foramen magnum abovethe rostral attachment of thedentate ligament. The emissaryvein passing through thecondylar foramen joins thesigmoid sinus. The vertebralvenous plexus anastomoseswith the internal jugular vein.Bridging veins pass from thesurface of the cerebellum andbrainstem to the dural sinuses.The superior hemisphericveins are divided into threegroups: an anterior group thatdrains toward the vein ofGalen; a posterior group thatdrains into the veins

converging on the straight sinus, torcula, and medial part of the lateral sinus; and a lateral group that drains into the superiorpetrosal sinus, the anterolateral marginal vein, and the lateral part of the lateral sinus. The superior vermian veins drain thetentorial part of the vermis. The veins on the superior part of the tentorial surface of the vermis ascend toward the superiorvermian vein and those on the inferior part of the tentorial surface of the vermis descend toward the torcula. The tributary ofthe superior vermian vein draining the tentorial surface of the culmen has been called the supraculminate vein. The declivalvein drains the declive and joins the inferior vermian vein or the torcula. The vein of the postclival fissure courses in thepostclival fissure. The superior petrosal veins are divided into medial, intermediate, and lateral groups, depending on whetherthey enter the middle, intermediate, or lateral third of the superior petrosal sinus. The inferior hemispheric veins drain thehemispheric part of the suboccipital surface, and the inferior vermian veins drain the vermian part of the suboccipital

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Cerebellomesencephalic fissure

The major veins in the cerebellomesencephalic fissure arethe veins of the cerebellomesencephalic fissure and the supe-rior cerebellar peduncle, and the pontotrigeminal and lateralmesencephalic veins (Figs. 3.2-3.5 and 3.10).

Vein of the superior cerebellar peduncleThe paired veins of the superior cerebellar peduncle orig-

inate deep in the cerebellomesencephalic fissure near the cau-dolateral margin of the superior cerebellar peduncles fromtributaries draining the dentate nuclei, superior cerebellarpeduncles, and the walls of the cerebellomesencephalic fis-sure (Figs. 3.3 and 3.4). They first course medially across the

peduncles and then upward on the peduncles, just lateral tothe lingula. They join near the rostral tip of the lingula to forma single trunk, the vein of the cerebellomesencephalic fissure.In a few cases, the paired veins do not join but form twoseparate veins of the cerebellomesencephalic fissure. Eachnerve of the superior cerebellar peduncle anastomoses withthe pontotrigeminal and lateral mesencephalic vein.

Vein of the cerebellomesencephalic fissureThis vein, also referred to as the precentral cerebellar vein,

arises deep in the cerebellomesencephalic fissure from theunion of the veins of the superior cerebellar peduncle. Itcrosses the quadrigeminal cistern anterior to the central lobule

Šsurface. The inferior vermian veins drain toward the tentorium and enter the torcula or a tentorial sinus. The inferior hemi-spheric veins cross the posterior margin of the cerebellum to reach the tentorial surface, where they often join the superiorhemispheric veins before terminating in the tentorial, lateral, or superior petrosal sinuses or the torcula. The inferior vermianvein is formed on the posterior surface of the tonsil by the union of the superior and inferior retrotonsillar veins. The medialand lateral tonsillar veins pass to the retrotonsillar or the inferior vermian veins. The vein of the petrosal fissure passes alongthe petrosal fissure. The anterior hemispheric veins that drain the petrosal surface of the cerebellum and are divided intosuperior, middle, and inferior groups, depending on whether they drain the superior, inferior, or middle third of the petrosalsurface. The anterior hemispheric veins converge on the lateral cerebellar incisura and join to form the vein of the cerebel-lopontine fissure that ascends to enter the superior petrosal sinus. The major veins related to the superior half of the roof ofthe fourth ventricle are the veins of the cerebellomesencephalic fissure and the superior cerebellar peduncle; the major veinsrelated to the inferior part of the roof are the veins of the cerebellomedullary fissure and the inferior cerebellar peduncle;and the major veins in the region of the lateral recesses and lateral walls are the veins of the cerebellopontine fissure and themiddle cerebellar peduncle. In the cerebellomesencephalic fissure, the paired veins of the superior cerebellar peduncleascend lateral to the lingula and the superior medullary velum and join to form the vein of the cerebellomedullary fissure,which ascends to join the superior vermian vein. The internal cerebral, basal, and superior vermian veins enter the vein ofGalen. The lateral mesencephalic and the pontotrigeminal veins course in the cerebellomesencephalic fissure. The lateralmesencephalic vein courses in the lateral mesencephalic sulcus. The pontotrigeminal vein arises on the superior and middlecerebellar peduncles and passes rostral to the trigeminal nerve. The tectal veins arise in the region of the colliculi. The veinof the cerebellomedullary fissure arises on the lateral side of the uvula and nodule and passes laterally through the cerebel-lomedullary fissure, either dorsal or ventral to the flocculus, to join one of the veins in the cerebellopontine angle. The veinof the cerebellomedullary fissure receives the medial and lateral supratonsillar veins, which pass along the medial and lateraledge of the inferior medullary velum above the superior pole of the tonsil. The dentate nucleus is drained by the supratonsil-lar veins and the vein of the cerebellomedullary fissure. The median posterior medullary vein ascends on the posteriormedulla and divides just below the obex into the paired veins of the inferior cerebellar peduncle. The veins of the inferiorcerebellar peduncle ascend on the inferior cerebellar peduncles and join the lateral medullary veins. The choroidal veinsdraining the tela choroidea and choroid plexus are tributaries of the veins of the inferior cerebellar peduncle and the cerebel-lomedullary fissure. The peduncular veins arise in the interpeduncular fossa and pass laterally to join the basal veins. The pos-terior communicating vein interconnects the medial ends of the peduncular veins. The longitudinally oriented veins in themidline on the anterior surface of the brainstem are the median anterior medullary vein, which ascends on the medulla, andthe median anterior pontomesencephalic vein that ascends in the midline on the pons and midbrain. The median anteriorpontomesencephalic vein does not usually extend the full length of the pons. The ends adjoining the absent segment oftenjoin the transverse pontine veins. The transversely oriented veins coursing in the sulci between the subdivisions of the brain-stem are the veins of the pontomesencephalic and the pontomedullary sulci. Each vein of the middle cerebellar pedunclearises in the region of the foramen of Luschka near the flocculus and ascends on the middle cerebellar peduncle to join thevein of the cerebellopontine fissure or one of the superior petrosal veins. The lateral anterior medullary vein courses alongthe preolivary sulcus near the hypoglossal nerve. The lateral anterior pontomesencephalic vein passes along the anterolateralmargin of the pons and medulla. The transverse medullary veins pass transversely across the medulla. The retro-olivary veincourses along the posterior margin of the olive, and the lateral medullary vein courses slightly dorsal to the olive, along theorigin of the rootlets arising from the dorsolateral surface of the medulla. There are diffuse anastomoses between the veinsventral to the diencephalon and third ventricle and those draining the midbrain. The deep middle cerebral and the anteriorcerebral veins join the basal vein in the region of the anterior perforated substance. (From, Matsushima T, Rhoton AL Jr, deOliveira E, Peace D: Microsurgical anatomy of the veins of the posterior fossa. J Neurosurg 59:63–105, 1983 [15].)



FIGURE 3.4. Tentorial surface and cerebellomesencephalic fissure. A, the left half of the tentorium has been removed whilepreserving a laterally placed tentorial sinus. A large sinus is seen through the right tentorial surface. B, the right half of thetentorium has been removed to expose a large inferior hemispheric vein from the suboccipital surface and a smaller superiorhemispheric vein from the tentorial surface emptying into the large tentorial sinus. The superior hemispheric veins, whichdrain the tentorial surface, are divided into an anterior group, which empties into the Galenic system, and a posterior group,like the vein shown, which empties into the tentorial sinuses. Smaller veins from both the left tentorial and suboccipital sur-faces join the laterally placed tentorial sinus near the junction of the left transverse and superior petrosal sinuses. C, thestraight and tentorial sinuses have been removed. The anterior group drains toward the cerebellomesencephalic fissure andthe vein of Galen, and the posterior group passes backward to empty into the tentorial sinuses. D, the posterior lip of thecerebellomesencephalic fissure has been removed to expose the veins of the superior cerebellar peduncle, which ascend tounite and form the vein of the cerebellomesencephalic fissure that empties into the vein of Galen. A transverse pontine veinand the vein of the cerebellopontine fissure join to form a superior petrosal vein that empties into the superior petrosal sinus.

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FIGURE 3.5. Tentorial surface and cerebellomesencephalic fissure. A, the left half of the tentorium has been removedwhile preserving the tentorial sinuses. The anterior group of superior vermian and superior hemispheric veins arise onthe upper part of the tentorial surface and ascend to reach the veins exiting the cerebellomesencephalic fissure, whichempty into the vein of Galen. The posterior group of superior vermian and superior hemispheric veins arise on the pos-terior part of the tentorial surface and descend to empty into tentorial sinuses. The inferior hemispheric veins, whicharise on the suboccipital surface, also empty into the tentorial sinuses. B, both halves of the tentorium have beenremoved while preserving the large tentorial sinuses. The superior hemispheric veins from the posterior part of the ten-torial surface and the inferior hemispheric veins from the suboccipital surface drain into the paired large tentorial sinusthat join the torcula. The veins draining the anterior part of the tentorial surface empty into the tributaries of the veinof Galen. C, lateral view of the cerebellomesencephalic fissure. The largest vein in the fissure is the vein of the cerebel-lomesencephalic fissure. The internal cerebral veins pass above the pineal to join the vein of Galen. D, the veins drain-ing the walls of the cerebellomesencephalic fissure join the vein of Galen, as do the internal cerebral and basal veins. Apineal vein also joins the Galenic group. Ant., anterior; Cer., cerebral; Cer. Mes., cerebellomesencephalic; Fiss., fissure;Hem., hemispheric; Inf., inferior; Int., internal; Occip., occipital; Post., posterior; S.C.A., superior cerebellar artery; Str.,straight; Sup., superior; Temp., temporal; Tent., tentorial; V., vein; Ve., vermian.

ŠE–F, cerebellomesencephalic fissure from another hemisphere. E, the posterior lip of the cerebellomesencephalic fissure has beenremoved to expose the tributaries of the vein of the cerebellomesencephalic fissure and the branches of the SCA. The paired veinsof the superior cerebellar peduncle unite to form the vein of the cerebellomesencephalic fissure that empties into the vein ofGalen. F, the branches of the SCA within the cerebellomesencephalic fissure have been removed. The paired veins of the superiorcerebellar peduncle ascend on the superior cerebellar peduncles and join to form the vein of the cerebellomesencephalic fissure.The veins on the surface of the middle cerebellar peduncle course laterally to join the veins emptying into the superior petrosalsinus. A., artery; Ant., anterior; Cer., cerebral; Cer. Mes., cerebellomesencephalic; CN., cranial nerve; Fiss., fissure; Hem., hemi-spheric; Inf., inferior; Int., internal; Mid., middle; N., nerve; Ped., peduncle; Post., posterior; Str., straight; Sup., superior; Tent., ten-torial; Trans., transverse; V., vein; Ve., vermian.



FIGURE 3.6. Suboccipital surface. A, the falx cerebelli, which fits into the posterior cerebellar incisura in which the vermis is partiallyburied, has been preserved. The inferior hemispheric veins drain the hemispheric portion of the suboccipital surface. A large left inferiorhemispheric vein ascends toward a tentorial sinus. A large right inferior hemispheric vein descends medially to join an inferior vermianvein, which ascends to empty into the sinuses in the tentorium. The occipital sinus courses within the falx cerebelli and joins the torculaabove and the sigmoid sinus below. B, the falx cerebelli has been removed to expose the inferior vermian veins, which ascend and passbelow the transverse sinus to empty into the tentorial sinuses. The retrotonsillar veins and other veins around the superior pole of thetonsils ascend to join the inferior vermian veins. C and D, another cerebellum. C, the branches of the PICA supplying the left hemispherehave been removed, but those on the right have been preserved. The inferior vermian and hemispheric veins on both halves of the suboc-cipital surface ascend and pass below the transverse sinus to empty into the sinuses in the tentorium. D, enlarged view of the inferior ver-mian veins that ascend to empty into sinuses in the tentorium. E, another cerebellum. A large right inferior hemispheric vein joins an infe-rior vermian vein that crosses the upper edge of the suboccipital surface and courses for a short distance on the tentorial surface beforeemptying into a tentorial sinus. F, enlarged view of another cerebellum. The large right inferior vermian vein passes forward to join thesinuses in the tentorium. A superior hemispheric vein from the tentorial surface descends to join a tentorial sinus. In the midline, a supe-rior and inferior vermian join to empty into a tentorial sinus. A., artery; Cer., cerebellar; Hem., hemispheric; Inf., inferior; Occip., occipi-tal; P.I.C.A., posteroinferior cerebellar artery; Post., posterior; Retroton., retrotonsillar; Sig., sigmoid; Sup., superior; Tent., tentorial; Trans.,transverse; V., vein; Ve., vermian; Vert., vertebral.

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to drain either directly or through the superior vermian veininto the vein of Galen (Figs. 3.2-3.5 and 3.10). Its tributaries arefrom the posterior aspect of the midbrain and the walls of thecerebellomesencephalic fissure, and occasionally include thetectal and preculminate veins.

Pontotrigeminal veinThis vein arises on the surface of the middle cerebellar

peduncle near the interpeduncular sulcus located between thesuperior and middle peduncles, passes above the trigeminalnerve, and drains directly into the superior petrosal sinus orits tributaries (Figs. 3.10 and 3.11). Its proximal end frequentlyanastomoses with the vein of the superior cerebellar peduncleand the lateral mesencephalic vein. Some of the superior

hemispheric and transverse pontine veins may drain into thepontotrigeminal vein.

Tectal veinsThe small tectal veins originate on or near the superior and

inferior colliculi and course upward in the quadrigeminal cisternto drain into the vein of the cerebellomesencephalic fissure, thesuperior vermian or internal cerebral vein, or the vein of Galen.These veins often anastomose with the vein of the superior cerebel-lar peduncle and the pineal, lateral mesencephalic, and basal veins.

Cerebellomedullary fissure

The major veins in the cerebellomedullary fissure are theveins of the cerebellomedullary fissure and the inferior cerebellar

FIGURE 3.7. Suboccipital surface and cerebellomedullary fissure. A, the veins from the region of the tonsil empty into theinferior vermian veins that ascend toward the sinuses in the tentorium. B, gentle retraction of the cerebellar tonsils exposesthe veins of the cerebellomedullary fissure crossing the inferior medullary velum. C, the cerebellar tonsils have beenremoved. The tela on the left side has been removed. The veins of the cerebellomesencephalic fissure cross the inferior med-ullary velum to join the veins in the cerebellopontine angles, which empty into the superior petrosal veins. The medial end ofthe veins of the cerebellomedullary fissure anastomose with the veins around the tonsil. D, a portion of the left half of thecerebellum has been removed. The inferior hemispheric veins from the suboccipital surface ascend and cross the junction ofthe suboccipital and tentorial surfaces to course on the posterior part of the tentorial surface, where they often form com-mon stems with the superior hemispheric veins from the posterior part of the tentorial surface before emptying into the ten-torial sinuses. A., artery; Cer., cerebellar; Cer. Med., cerebellomedullary; Fiss., fissure; Hem., hemispheric; Inf., inferior; Med.,medullary; Ped., peduncle; P.I.C.A., posteroinferior cerebellar artery; Retrotons., retrotonsillar; Sup., superior; V., vein; Ve.,vermian; Vel., velum; Vert., vertebral.



peduncle (Figs. 3.3, 3.7-3.9) (2). Both of these veins drain into thecerebellopontine angle through the communication betweenthe cerebellomedullary and cerebellopontine fissures.

Vein of the cerebellomedullary fissureThis vein originates on the lateral edge of the nodule and

uvula, courses laterally near the junction of the inferior med-ullary velum and tela choroidea, and passes dorsal or ventralto the flocculus to reach the cerebellopontine angle (Figs.3.7-3.9). If it courses dorsal to the flocculus, it terminates in the

anterior hemispheric veins or in the vein of the cerebellopon-tine fissure. If it courses ventral to the flocculus, it passesbetween the flocculus and the foramen of Luschka and joinsthe lateral medullary vein or the vein of the inferior cerebellarpeduncle or the pontomedullary sulcus to form the vein of themiddle cerebellar peduncle. The vein of the cerebellomedul-lary fissure frequently connects with its mate on the oppositeside through a transverse vein crossing the nodule or uvulaand/or with the inferior vermian vein. The medial part of thevein of the cerebellomedullary fissure is sometimes hypoplas-

FIGURE 3.8. Suboccipital surface and the cerebellomedullary fissure. A, the retrotonsillar veins pass upward in the fissurebetween the tonsil and biventral lobule and empty into the inferior vermian veins. B, the tonsils have been removed toexpose the veins of the cerebellomedullary fissure, which pass laterally on the inferior medullary velum and across the lateralrecesses to join the veins in the cerebellopontine angles. The medial end of the veins of the cerebellomedullary fissure anasto-mose with the veins around the tonsil. C, another specimen. The tonsils and part of the biventral lobules have been removedto expose the paired veins of the cerebellomedullary fissure, which cross the inferior medullary velum to empty into the veinsin the cerebellopontine angles. D, the cerebellar hemispheres, except for the right tonsil, have been removed. The right retro-tonsillar vein courses along the posterior surface of the tonsil and empties into an inferior vermian vein. The left vein of thecerebellomedullary fissure passes through the lateral recess to join the vein of the middle cerebellar peduncle, which ascendsto empty into a superior petrosal vein. The paired veins of the superior cerebellar peduncle ascend on the peduncle and jointo form the vein of the cerebellomesencephalic fissure. An interpeduncular vein courses between the superior and middlecerebellar peduncles. A., artery; Bivent., biventral; Cer., cerebellar; Cer. Med., cerebellomedullary; CN, cranial nerve; Fiss.,fissure; Inf., inferior; Interped., interpeduncular; Lat., lateral; Med., medullary; Mid., middle; Ped., peduncle; Pet., petrosal;P.I.C.A., posteroinferior cerebellar artery; Retrotons., retrotonsillar; Sup., superior; Tons., tonsillar; V., vein; Ve., vermian;Vent., ventricle; Vert., vertebral.

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tic or absent. Its tributaries drain the inferior medullary ve-lum, tela choroidea and attached choroid plexus, periven-tricular white matter, dentate nuclei, anteroinferior surface ofthe biventral lobule, and the inferior vermis.

Vein of the inferior cerebellar peduncleThis vein courses on the peduncle parallel and several

millimeters caudal to the curved inferolateral margin of thefourth ventricle (Figs. 3.3, 3.7, and 3.9). Its caudal part is visibleon the posterior surface of the medulla lateral to Magendie’sforamen, but its superior portion is hidden in the cerebellomed-ullary fissure. Inferiorly, the veins from each side join below theobex to form a single channel, the median posterior medullaryvein. Superiorly, it passes below the lateral recesses and joinsthe vein of the pontomedullary sulcus, either directly or byfirst connecting with the lateral medullary vein. It often re-ceives the vein of the cerebellomedullary fissure near thelateral end of the pontomedullary sulcus. These veins, con-verging on the lateral end of the pontomedullary sulcus, jointo form the vein of the middle cerebellar peduncle. The vein ofthe inferior cerebellar peduncle drains the posterior and lat-eral aspects of the medulla, the tela choroidea, choroid plexus,the inferior part of the floor of the fourth ventricle, the lateralrecess, and the glossopharyngeal and vagus nerves. This ros-tral part of the vein of the inferior cerebellar peduncle oftenanastomoses with the sinuses converging on the jugular fora-men through a bridging vein that passes along the nerves thatpass through the jugular foramen.

Supratonsillar veinsThe supratonsillar veins course in the cerebellomedullary

fissure near the superior pole of the tonsil (Fig. 3.3) (9). Thename “supratonsillar” suggests that these veins drain thetonsil; however, they course on and drain the opposite side ofthe cerebellomedullary fissure from the tonsil. They originatein the deep nuclei and white matter of the cerebellum anddrain the inferior half of the roof of the fourth ventricle ratherthan the tonsil. They course along the inferior medullaryvelum and drain into the vein of the cerebellomedullary fis-sure or the inferior vermian vein.

Choroidal veinsThe choroidal veins drain the tela choroidea and the at-

tached choroid plexus, and are tributaries of the veins of thecerebellomedullary fissure and the inferior cerebellar pedun-cle. The medial half of the vein of the cerebellomedullaryfissure drains the rostral part of the medial segment and themedial part of the lateral segment of the choroid plexus. Thelateral half of the vein of the cerebellomedullary fissure, andthe rostral part of the vein of the inferior cerebellar peduncledrain the lateral part of the lateral segment. The caudal part ofthe vein of the inferior cerebellar peduncle receives the drain-age of the caudal part of the medial segment (Fig. 3.3).

Cerebellopontine fissure

The major veins arising in this region are the veins of thecerebellopontine fissure and the middle cerebellar peduncle.

Vein of the cerebellopontine fissureThis is the largest vein draining the petrosal surface. It is

formed just rostral or caudal to the flocculus by the union ofthe stems of the anterior hemispheric veins (Figs. 3.9-3.11). Itcourses in or near the superior limb of the cerebellopontinefissure, or on the superior part of the petrosal surface near theanterolateral margin. It crosses the subarachnoid space rostralto the facial, vestibulocochlear, and trigeminal nerves, anddrains into the superior petrosal sinus either directly or afterforming a common stem with other veins draining into thesuperior petrosal sinus. The vein of the middle cerebellarpeduncle and the pontotrigeminal vein often join the vein ofthe cerebellopontine fissure to form one of the trunks thatdrain into the superior petrosal sinus near the trigeminalnerve. The vein of the cerebellomedullary fissure, if it passesdorsal to the flocculus, may drain into the vein of the cerebel-lopontine fissure.

Vein of the middle cerebellar peduncleThis vein originates in the fossette above the inferior olive

by the union of the vein of the pontomedullary sulcus withthe lateral medullary vein or the vein of the inferior cerebellarpeduncle (Figs. 3.8-3.11). It ascends on the lateral surface ofthe middle cerebellar peduncle near the base of the cerebel-lopontine fissure to reach the area posterior to the origin of thetrigeminal nerve. It drains directly into the superior petrosalsinus or joins other veins to form one of the common trunksthat drain into the superior petrosal sinus. Its initial partpasses either between the flocculus and the origin of thevestibulocochlear nerve or between the origins of the vestibu-locochlear and the facial nerves.

The vein of the middle cerebellar peduncle receives thedrainage of the rostral half of the medulla, the inferior half ofthe fourth ventricle, and the lateral surface of the pons. It oftenreceives the drainage of the veins of the cerebellomedullaryfissure and inferior cerebellar peduncle, some of the trans-verse pontine veins, and the veins draining the origins of thefacial and the vestibulocochlear nerves. It is large if the vein ofthe cerebellomedullary fissure courses ventral to the flocculusto join it rather than passing dorsal to the flocculus to join theanterior hemispheric veins or the vein of the cerebellopontinefissure.

VEINS OF THE BRAINSTEM

The veins of the brainstem are divided into two groupsbased on whether they course longitudinally or transversely(Figs. 3.3 and 3.9-3.11). The longitudinal veins are the mediananterior pontomesencephalic, median anterior medullary, lat-eral anterior pontomesencephalic, lateral anterior medullary(preolivary), lateral mesencephalic, lateral medullary, andretro-olivary veins. The transverse veins are the veins of thepontomesencephalic and the pontomedullary sulci, and thetransverse pontine, transverse medullary, peduncular, andposterior communicating veins.



FIGURE 3.9. Brainstem and petrosal surface. A, the vertebral and basilar arteries and their branches course superficial to theveins. The veins on the anterior surface of the pons and medulla and the petrosal surface drain predominantly into the superiorpetrosal veins, which empty into the superior petrosal sinuses. B, the arteries have been removed. The median anterior pontomes-encephalic and median anterior medullary veins ascend on the front of the brainstem. The transverse pontine and transverse med-ullary veins run transversely across the pons and medulla surfaces. The anterior hemispheric veins drain the petrosal surface andcommonly empty into the vein of the cerebellopontine fissure, which ascends to join the superior petrosal veins. The vein of thepontomedullary sulcus passes across the pontomedullary junction. C, enlarged view of the right petrosal surface. The anteriorhemispheric veins drain the petrosal surface and pass forward to empty into the vein of the cerebellopontine fissure or a superiorpetrosal vein. The vein of the cerebellopontine fissure arises at the lateral apex of the cerebellopontine fissure and crosses the mid-dle cerebellar peduncle, where it is joined by a large transverse pontine vein. D, enlarged view of the left petrosal surface. The veinof the cerebellopontine fissure arises from the union of the anterior hemispheric veins at the apex of the cerebellopontine fissure

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Longitudinal veins

Median anterior pontomesencephalic veinThis vein runs in or near the midline on the anterior surface

of the mesencephalon and the pons (Figs. 3.9-3.11). It has amesencephalic segment that courses in the interpeduncularfossa, and a pontine segment that runs in or adjacent to thebasilar sulcus. The mesencephalic segment of this vein isusually composed of the two veins, which are frequentlyasymmetrical in size and course near the oculomotor nerveson the lateral walls of the interpeduncular fossa. They usuallyanastomose rostrally with the medial end of the peduncularveins and the lateral ends of the posterior communicatingvein. The small veins exiting the posterior perforated sub-stance often join this confluence. A bridging vein may arise inthe interpeduncular fossa and pass to the tentorial edge. Thepaired mesencephalic segments join several millimeters be-low the pontomesencephalic sulcus on the upper surface ofthe pons to form the pontine segment. If the superior part ofthe pontine segment is absent, the mesencephalic segmentdivides to connect inferiorly with the lateral anterior pon-tomesencephalic vein or the vein of the pontomesencephalicsulcus.

The pontine segment, which connects caudally with themedian anterior medullary vein and the vein of the pon-tomedullary sulcus, is subdivided into superior, middle, andinferior parts. One of the three parts is usually absent. If thesuperior portion is absent, the middle portion anastomosessuperiorly with a transverse pontine vein, and the caudal partis continuous inferiorly with the median anterior medullaryvein. If the middle part is absent, the caudal end of thesuperior part and the cranial end of the inferior part anasto-mose with the transverse pontine or the lateral anterior pon-tomesencephalic veins. The pontine segment may deviate toone side away from the basilar sulcus, especially if the trans-verse pontine vein gives rise to a large bridging vein to apetrosal sinus.

Median anterior medullary veinThis vein courses in the median anterior medullary fissure

between the medullary pyramids (Figs. 3.9-3.11). It connectssuperiorly with the median anterior pontomesencephalic vein

and the vein of the pontomedullary sulcus at the pontomed-ullary junction and inferiorly with the anterior spinal vein. Itmay join the lateral anterior pontomesencephalic vein ros-trally if the inferior part of the median anterior pontomesen-cephalic vein is absent. A bridging vein may connect themedian anterior medullary vein with the sinuses around thejugular foramen.

Lateral anterior pontomesencephalic veinThis vein on the anterolateral aspect of the brainstem is

rarely continuous from the midbrain to the lower pons (Fig.3.3). At the mesencephalic level it may anastomose with thebasal and peduncular veins and the vein of the pontomesen-cephalic sulcus, and at the pontine level it anastomoses withthe transverse pontine veins. Caudally, it joins the vein of thepontomedullary sulcus near the abducens nerve. It deviatesmedially to connect with the median anterior pontomesence-phalic or the median anterior medullary veins, if the lowerpart of the median anterior pontomesencephalic vein is ab-sent. It may give rise to a bridging vein to the inferior petrosalsinus.

Lateral anterior medullary vein (preolivary vein)This vein courses in the anterolateral sulcus between the

pyramid and the olive, and is partly hidden by the roots of thehypoglossal nerve (Fig. 3.3). A segment along the lateral bor-der of the pyramid may be absent. It connects superiorly withthe vein of the pontomedullary sulcus and inferiorly with thelateral medullary or transverse medullary vein. The medianand lateral anterior medullary veins are linked together by thetransverse medullary veins that cross the pyramids at variouslevels.

Lateral mesencephalic veinThis vein runs in or adjacent to the lateral mesencephalic

sulcus and usually drains into the basal vein near the medialgeniculate body (Fig. 3.3). It drains the posterolateral aspect ofthe midbrain and sometimes receives a branch from the quad-rigeminal plate. Its inferior end anastomoses with the ponto-trigeminal vein and the vein of the pontomesencephalic sul-cus. It sometimes receives a superior hemispheric or tectalvein (1, 9, 26).

Šand ascends to be joined by a superior hemispheric vein from the lateral part of the tentorial surface before emptying intothe superior petrosal sinus. E, the cerebellum has been removed to expose the veins of the superior, inferior, and middle cer-ebellar peduncles. The vein of the superior cerebellar peduncle ascends to join the vein of the cerebellomesencephalic fis-sure. The vein of the inferior cerebellar peduncle crosses the peduncle at the inferolateral margin of the fourth ventricle andpasses around the lateral recess to join the veins in the cerebellopontine angle. The veins of the cerebellopontine fissure andmiddle cerebellar peduncle and a transverse pontine vein join to form a superior petrosal vein. The vein of the cerebellomed-ullary fissure empties into the vein of the middle cerebellar peduncle. F, posterior view of the right cerebellopontine angle.The vein of the cerebellomedullary fissure passes laterally across the lateral recess and empties into the vein of the middlecerebellar peduncle. The latter vein and the vein of the cerebellopontine fissure join to form a large superior petrosal vein. Alarge anterior hemispheric vein ascends along the petrosal surface. A., artery; A.I.C.A., anteroinferior cerebellar artery; Ant.,anterior; Cer., cerebellar, cerebral; Cer. Med., cerebellomedullary; Cer. Pon., cerebellopontine; CN, cranial nerve; Fiss., fis-sure; Hem., hemispheric; Inf., inferior; Med., median, medullary; Mid., middle; Ped., peduncle; Pet., petrosal; P.I.C.A., pos-teroinferior cerebellar artery; Pon., pontine; Pon. Med., pontomedullary; Pon. Mes., pontomesencephalic; Pon. Trig., pontotri-geminal; S.C.A., superior cerebellar artery; Sup., superior; Trans., transverse; Trig., trigeminal; V., vein; Vert., vertebral.



FIGURE 3.10. Upper brainstem. A, the veins on the anterior surface of the pons and medulla and the veins of the cerebellopontinefissure and their tributaries empty into the superior petrosal veins. The median anterior medullary vein and median anterior pon-tomesencephalic veins course in the midline, but often do not extend along the full length of the pons and medulla. The vein of thepontomesencephalic sulcus and the transverse pontine veins are transversely oriented. The veins of the cerebellomedullary fissurejoin the veins of the middle cerebellar peduncle, which ascends to join the veins of the cerebellopontine fissure. B, the veins in thecrural and ambient cistern join the basal vein, which empties into the vein of Galen in the quadrigeminal cistern. The basal veinalso drains the walls of the temporal horn, which has been opened on the right. An internal occipital vein passes from the calcar-ine sulcus and occipital lobe to the vein of Galen. C, enlarged view of the basal cisterns. The inferior ventricular vein from thetemporal horn and the lateral atrial vein join the basal vein, which also drains the walls of the crural and ambient cisterns. The cer-ebellomesencephalic fissure, an inferior extension of the quadrigeminal cistern, is drained by tributaries of the vein of Galen. D,lateral view of the cerebellomesencephalic fissure. The veins in the medial portion of the cerebellomesencephalic fissure emptyinto the vein of Galen and those from the lateral part may join the superior petrosal veins. In this case, the vein of the cerebel-lomesencephalic fissure is small, resulting in most of the fissure’s drainage being directed laterally through a pontotrigeminal vein,which passes above the trigeminal nerve to empty into a superior petrosal vein formed by a superior hemispheric and transverse pontinevein and the vein of the cerebellopontine fissure. Ant., anterior; Atr., atrial; Cer., cerebellar; Cer. Med., cerebellomedullary; Cer. Mes.,cerebellomesencephalic; Cist., cistern; CN, cranial nerve; Fiss., fissure; Hem., hemispheric; Int., internal; Lat., lateral; Med., median, med-ullary; Mes., mesencephalic; Mid., middle; Occip., occipital; Ped., peduncle; Pet., petrosal; Pon., pontine, ponto; Quad., quadrigeminal;Str., straight; Sulc., sulcus; Sup., superior; Temp., temporal; Trans., transverse; Trig., trigeminal; V., vein; Vent., ventricle.

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Lateral medullary and retro-olivary veinsThere are usually two longitudinal veins between the lat-

eral border of the olive and the foramen of Luschka (Fig. 3.3):a smaller ventral vein (the retro-olivary vein), and a largerdorsal vein (the lateral medullary vein). The lateral medullaryvein courses slightly dorsal to the retro-olivary sulcus alongthe rootlets of the accessory, vagus, and glossopharyngealnerves. It receives the retro-olivary vein from its ventral sideand the vein of the inferior cerebellar peduncle from its dorsalside, and joins the vein of the pontomedullary sulcus to formthe vein of the middle cerebellar peduncle. This vein and thevein of the inferior cerebellar peduncle often give rise to aninferior petrosal bridging vein near the foramen of Luschka,which courses along the rootlets of the nerves entering thejugular foramen to join the venous sinuses near the jugularbulb.

The retro-olivary vein usually courses along the rostraltwo-thirds of the retro-olivary sulcus slightly ventral to thelateral medullary vein. Although small, it may rarely replacethe lateral medullary vein. It often anastomoses near thelower edge of the olive with the caudal part of the lateralmedullary vein and above the olive with either the vein of thepontomedullary sulcus or the rostral end of the lateral med-ullary vein.

Transversely oriented veins

Peduncular veinThis vein arises in the interpeduncular fossa and courses

laterally around the cerebral peduncle below the optic tracttoward the basal vein (Figs. 3.3, 3.9, and 3.10). It anastomosesmedially with the posterior communicating vein, which linksthe medial ends of the peduncular veins, and with the upperend of the median anterior pontomesencephalic vein. Its me-dial end is located on the superomedial surface of the origin ofthe oculomotor nerve. The lateral end of the vein drains intothe basal vein or one of its tributaries. In a few cases it drainsthrough a bridging vein into a sinus in the edge of thetentorium.

Posterior communicating veinThis vein courses transversely across the interpeduncular

fossa on the superomedial surface of the oculomotor nerves,interconnecting the medial ends of the peduncular veins andthe rostral ends of the median anterior pontomesencephalicveins (Fig. 3.3). It usually courses in the interpeduncular cis-tern, bridging over rather than coursing on the floor of theinterpeduncular fossa. Small veins exiting the interpeduncu-lar fossa frequently join the posterior communicating vein.

Vein of the pontomesencephalic sulcusThis vein is usually small, and does not extend the entire

length of the pontomesencephalic sulcus from the midline tothe lateral mesencephalic sulcus (Fig. 3.3 and 3.10). It passesbelow the oculomotor nerves and anastomoses with the me-dian and lateral anterior pontomesencephalic veins in mostcases. The lateral mesencephalic and pontotrigeminal veins

may anastomose with the lateral end of this vein near theconfluence of the pontomesencephalic, lateral mesencephalic,and interpeduncular sulci.

Transverse pontine veinsThis is a group of veins that cross the anterior surface of the

pons at various levels (Figs. 3.9-3.11). They interconnect themedian anterior pontomesencephalic vein and the veins onthe lateral surface of the pons. The most prominent transversepontine veins are located at the midpons. Those on the upperand lower thirds of the pons are usually small and onlyinfrequently transverse the full width of the pons. Those onthe midpons are usually present bilaterally and anastomosemedially with the median anterior pontomesencephalic vein.They course laterally above or below the trigeminal nerve todrain into the superior petrosal sinus, the pontotrigeminalvein, or the vein of the cerebellopontine fissure or the middlecerebellar peduncle. They sometimes give rise to a bridgingvein to the inferior petrosal sinus. If the middle third of themedian anterior pontomesencephalic vein is absent, the endsadjoining the absent segments drain into the transverse pon-tine veins.

Vein of the pontomedullary sulcusThis vein courses in or near the pontomedullary sulcus and

connects with the longitudinally oriented veins on the ante-rior aspect of the pons and medulla (Figs. 3.3 and 3.9). It joinsthe lateral medullary or retro-olivary veins or the vein of theinferior cerebellar peduncle above the olive to form the vein ofthe middle cerebellar peduncle. It may give rise to a bridgingvein to the sinuses around the jugular foramen.

Transverse medullary veinsThese veins cross the anterior and lateral surfaces of the

medulla at the level of the medullary pyramid or below (Fig.3.9). They interconnect the median anterior medullary veinwith the veins on the lateral surface of the medulla. Theyrarely cross the full distance from the median anterior med-ullary vein to the lateral medullary vein, but usually consist ofone or two shorter veins passing transversely across the med-ullary pyramid or the olive. The largest transverse medullaryveins are usually situated at the level of the middle thirdof the pyramid. They sometimes give rise to a bridging vein tothe sigmoid or marginal sinuses.

MAJOR DRAINING GROUPS

The terminal end of the veins draining the brainstem andcerebellum form bridging veins that collect into three groups:1) a galenic group that drains into the vein of Galen; 2) apetrosal group that drains into the petrosal sinuses; and 3)a tentorial group that drains into the sinuses converging onthe torcula (Fig. 3.3). The galenic group drains the tentorialsurface, the cerebellomesencephalic fissure, and the superiorhalf of the roof of the fourth ventricle; the petrosal groupdrains the petrosal surface, the cerebellomedullary and cer-ebellopontine fissures, the inferior half of the roof of thefourth ventricle and the lateral recesses, and the anterior and



FIGURE 3.11. Superior petrosal veins. A, the superior petrosal veins drain the anterior and lateral surfaces of the brainstem, thepetrosal surface, and some of the lateral part of the tentorial and suboccipital surfaces. The veins of the middle cerebellar peduncleascend on the middle cerebellar peduncles and join the veins of the cerebellopontine fissure and the transverse pontine veins toform superior petrosal veins that empty into the superior petrosal sinuses. B, lateral view of a large superior petrosal vein formedby the union of the transverse pontine, pontotrigeminal, and anterior hemispheric veins and the vein of the cerebellopontine fis-sure. A large branch of the superior cerebellar artery and the trigeminal nerve are enmeshed in the tributaries of this superior

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lateral sides of the pons and medulla; and the tentorial groupdrains the suboccipital surface. Other less frequent bridgingveins pass to the cavernous, marginal, basilar, and sigmoidsinuses and the jugular bulb.

Galenic draining group

This group, formed by the veins converging on the vein ofGalen, includes the superficial veins that drain the tentorialsurface, the deep veins that drain the superior part of the roofof the fourth ventricle and the cerebellomesencephalic fissure,and the brainstem veins that drain the midbrain. Most of theseveins drain through the superior vermian and basal veins toreach the vein of Galen (Figs. 3.2-3.5). The superficial groupincludes the superior vermian vein and the anterior group ofthe superior hemispheric veins; the deep group includes thevein of the cerebellomesencephalic fissure and the pairedveins of the superior cerebellar peduncle; and the brainstemgroup includes the peduncular, posterior communicating,and tectal veins and the rostral portions of the medial andlateral anterior pontomesencephalic and the lateral mesence-phalic veins. All of these brainstem veins, except for the tectalvein, join the basal vein that drains into the vein of Galen. Thetectal veins join the superior vermian vein or the vein of thecerebellomesencephalic fissure.

Tentorial draining group

The tentorial draining group includes the veins that draininto the straight and lateral sinuses and the torcula, eitherdirectly or through a tentorial sinus (Figs. 3.2-3.6). It iscomposed of the superficial veins draining the suboccipitalsurface and the posterior part of the tentorial surface. Theveins from the suboccipital surface include the inferiorvermian veins and inferior hemispheric veins. The veinsfrom the tentorial surface include the posterior groups ofsuperior hemispheric and superior vermian veins. The in-ferior hemispheric veins and the posterior group of thesuperior hemispheric veins often join before entering thetentorial sinuses, which drain into the torcula or into thestraight or lateral sinuses near the torcula.

The tentorial sinuses also receive the inferior cerebral veins,the vein of Labbe, and the bridging veins to the tentorial edge.

The tentorial sinuses can course directly medially to drain intothe midportion of the straight sinus, posteromedially to draininto the torcula or the straight or lateral sinus near the torcula,immediately posteriorly to drain into the middle third of thelateral sinus, or posterolaterally to drain into the lateral andsuperior petrosal sinuses at or near the confluence of the twosinuses. Some sinuses are formed by the union of veins drain-ing the tentorium itself.

Petrosal draining group

The petrosal draining group includes the veins draininginto the petrosal sinuses (Figs. 3.1, 3.11, and 3.12) (9, 26). Thisdraining group includes the superficial veins that drain thelateral part of the cerebellar hemisphere; a deep group thatdrains the cerebellopontine and cerebellomedullary and thelateral part of the cerebellomesencephalic fissures, and theinferior part of the roof and the lateral wall of the fourthventricle; and a brainstem group that drains much of thebrainstem.

The petrosal veins are divided into superior and inferiorpetrosal veins based on whether they enter the superior orinferior petrosal sinus. The superior petrosal veins are amongthe largest and most frequent veins in the posterior fossa. Theinferior petrosal veins are represented by a few small bridgingveins. The superior petrosal veins may be formed by theterminal segment of a single vein or by the common stemformed by the union of several veins. The most commontributaries of the superior petrosal veins are the transversepontine and pontotrigeminal veins, the common stem of thelateral group of the superior hemispheric veins, and the veinsof the cerebellopontine fissure and the middle cerebellar pe-duncle. The superior petrosal veins are subdivided into alateral, intermediate, and medial group based on the relation-ship of their site of entry into the superior petrosal sinus to theinternal acoustic meatus. The intermediate group drains intothe sinus above the internal acoustic meatus, the medialgroup drains into the sinus medial to the meatus, and thelateral group drains into the sinus lateral to the meatus. Of 20superior petrosal sinuses examined in our previous study, 8received one superior petrosal vein, 10 received two, and2 received three (15). Of the 34 superior petrosal veins, 22(64.7%) were of the medial type, 3 (8.8%) were of the inter-

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petrosal vein. Care is required to avoid occluding the superior cerebellar artery when occluding a multipronged petrosal vein.C, retrosigmoid view. The right cerebellopontine angle is drained by a superior petrosal vein formed by the pontotrigeminaland transverse pontine veins and the vein of the cerebellopontine fissure. D, the tributaries of this superior petrosal veininclude the transverse pontine, pontotrigeminal, and anterior hemispheric veins and the vein of the cerebellopontine fissure.E, superior petrosal vein with multiple tributaries. The vestibulocochlear nerve has been depressed to expose the facial nerve.F, the segment of the superior petrosal sinus, which crosses above the trigeminal nerve and receives the superior petrosalveins, has been removed. The posterior trigeminal nerve passes forward below the tentorial edge and the superior petro-sal sinus to enter Meckel’s cave. The superior petrosal sinus extends medially through the upper edge of the porus of Meck-el’s cave and above the trigeminal nerve to join the cavernous sinus. Some superior petrosal veins may join the sinus on themedial side of the trigeminal nerve. A.I.C.A., anteroinferior cerebellar artery; Ant., anterior; Cer., cerebellar; Cer. Pon., cer-ebellopontine; CN, cranial nerve; Fiss., fissure; Hem., hemispheric; Med., median, medullary; Mid., middle; P.C.A., posteriorcerebral artery; Ped., peduncle; Pet., petrosal; Pon., pontine; Pon. Mes., pontomesencephalic; Pon. Trig., pontotrigeminal;S.C.A., superior cerebellar artery; Sup., superior; Tent., tentorial; Trans., transverse; V., vein.



mediate type, and 9 (26.5%) were of the lateral type. Nineteenof 20 (95%) sinuses examined had veins of the medial type, 3(15%) had veins of the intermediate type, and 9 (45%) hadveins of the lateral type. The medial group of superior petro-sal veins is usually a common trunk formed by the union oftwo or three of the following veins: transverse pontine veins,pontotrigeminal veins, and the veins of the cerebellopontinefissure and the middle cerebellar peduncle. The latter veinsmay also enter the sinus without joining another vein. Two ofthe three intermediate superior petrosal veins were formed bya single vein, the vein of the cerebellopontine fissure. Themost common veins in the lateral group are the common stemformed by the union of superior and inferior hemisphericveins and the vein of the cerebellopontine fissure.

Other bridging veins

The major bridging veins have been discussed above. Otherless frequent bridging veins run from the basal vein to a sinuscoursing in the tentorial edge; from the peduncular vein to asinus in the tentorial edge or the cavernous sinus; from thelateral or medial anterior pontomesencephalic or a transversepontine vein to the posterior portion of the cavernous or theadjoining part of the inferior petrosal sinuses just belowMeckel’s cave; from the veins of the pontomedullary sulcusand the inferior cerebellar peduncle or the lateral medullaryvein to the sigmoid and inferior petrosal sinuses near thejugular foramen or jugular bulb; from the vein of the pon-tomedullary sulcus, and the lateral anterior, lateral, and trans-

FIGURE 3.12. Inferior petrosal sinus and veins. A, posterior view of the anterior portion of the posterior fossa with thebrainstem and cerebellum removed. The inferior petrosal and sigmoid sinuses can be seen through the dura. B, the dural roofof the basilar, inferior petrosal, and sigmoid sinuses have been removed. The inferior petrosal sinuses extend from the basilarsinus above to the jugular bulbs below. The inferior petrosal veins arise on the brainstem and empty into the lower part ofthe inferior petrosal sinus, jugular bulb, or distal sigmoid sinus. C–E, posterior views into cerebellopontine angle. C, an infe-rior petrosal vein passes from the medulla between the glossopharyngeal and vagus nerves to the jugular bulb. It receives thedrainage of the vein of the inferior cerebellar peduncle, which crosses the peduncle just below the lateral recess. D, an infe-rior petrosal nerve passes behind the glossopharyngeal and vagus nerves to empty into the terminal part of the sigmoid sinus.E, an inferior petrosal vein crosses behind the nerves entering the jugular foramen to reach the sigmoid sinus. A., artery;A.I.C.A., anteroinferior cerebellar artery; Bas., basilar; Cer., cerebellar; Cer. Med., cerebellomedullary; CN, cranial nerve;Fiss., fissure; Inf., inferior; Jug., jugular; Lat., lateral; Med., median, medullary; Ped., peduncle; Pet., petrosal; P.I.C.A., pos-teroinferior cerebellar artery; Post., posterior; Sig., sigmoid; V., vein; Vert., vertebral.

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verse medullary veins to a marginal sinus at the level of theforamen magnum or to the veins in the hypoglossal canal, whichcommunicates with the marginal sinuses (Fig. 3.12) (13, 15).

DISCUSSION

The infrequent reports of adverse sequelae after the intra-operative occlusion of veins in the posterior fossa is caused bythe diffuse anastomosis between the veins. It is not surprisingthat more severe sequelae have occurred after occlusion ofbridging veins than after occlusion of veins on the surface ofthe cerebellum, since the bridging veins are formed by theterminal end of numerous surface veins. The veins crossingthe cerebellopontine angle to reach the petrosal sinuses are theones most frequently occluded in the course of operations inthe posterior fossa. Bridging veins are more frequently ex-posed and sacrificed in the rostral part of the cerebellopontineangle during operations near the trigeminal nerve than dur-ing operations in the central or caudal part near the nervesentering the internal acoustic meatus and the jugular foramen.Exposure of the trigeminal nerve through a suboccipital crani-ectomy commonly requires the sacrifice of one or more bridg-ing veins, while exposure of the nerves entering the internalacoustic meatus infrequently requires sacrifice of even a sin-gle bridging vein.

In 1929, Dandy pointed out that the petrosal vein shouldreceive special attention during posterior fossa operations on thetrigeminal nerve (4). His illustration showed a vein that coursedin the cerebellopontine angle near the rostral aspect of the tri-geminal nerve to drain into the superior petrosal sinus. Later,this common stem came to be known either as the superiorpetrosal vein or simply as the petrosal vein (4, 5, 23). No con-sideration has been given in the surgical literature to the identi-fication of the trunks that unite to form the petrosal veins, and tothe size of the area drained by their tributaries. The veins con-verging on the trigeminal nerve to form the superior petrosalveins are the transverse pontine and the pontotrigeminal veins,and the veins of the cerebellopontine fissure and the middlecerebellar peduncle. The largest vein contributing to the forma-tion of the petrosal vein near the trigeminal nerve is the vein ofthe cerebellopontine fissure, which drains most of the petrosalsurface of the cerebellum and much of the lower brainstemand the cerebellopontine and cerebellomedullary fissures. Al-though superior petrosal veins can be located at any point alongthe superior petrosal sinus, most are located just lateral to thetrigeminal nerve. Adverse sequelae only infrequently followocclusion of this medial group of superior petrosal veins; how-ever, we have seen two patients with a transient cerebellardisturbance caused by a venous infarction with hemorrhagicedema after the intraoperative occlusion of these veins lateral tothe trigeminal nerve.

The exposure of lesions such as acoustic neuromas in thecentral part of the cerebellopontine angle near the lateralrecess, by retracting the petrosal surface of the hemisphereaway from the sigmoid sinus, can usually be completed with-out sacrificing a single bridging vein. If a vein is obliteratedduring acoustic tumor removal, it is usually one of the supe-rior petrosal veins that is sacrificed near the superior pole of

the tumor during the later stages of the removal of a largetumor. Smaller tumors can often be removed without sacri-ficing a petrosal vein. The large vein encountered around thesuperior pole of an acoustic neuroma is the vein of the cer-ebellopontine fissure, which passes from the petrosal surfaceand cerebellopontine fissure above the facial and vestibuloco-chlear nerves to the area above the trigeminal nerve. This veinhas been occluded during acoustic neuroma removal withoutcausing a deficit (14).

Compression of the trigeminal nerve by the surrounding veinsis postulated to be a cause of trigeminal neuralgia (8, 11). In 411operations for trigeminal neuralgia, Jannetta found veins com-pressing the nerve in 153; however, none of these veins involvedin this compression was listed by name (11). Compression of thefacial and glossopharyngeal nerves by veins has also been pos-tulated to be a cause of hemifacial spasm and glossopharyngealneuralgia (12). The venous relationships of the trigeminal nervewhere numerous bridging veins converge on and cross the sub-arachnoid space near the posterior root is distinctly differentfrom those in the region of the facial and vestibulocochlearnerves, where the predominant veins are on the side of thebrainstem and in contact with the nerves at their junction withthe brainstem. The veins coursing on or near the junction of thefacial and vestibulocochlear nerves with the brainstem arethe veins of the middle cerebellar peduncle, the cerebellomed-ullary fissure, and the pontomedullary sulcus. There are no largeveins intermingling with the nerves at or within the acousticmeatus, as occurs with the arteries. The major veins near theglossopharyngeal and vagus nerves also course near the originof the nerves on the surface of the brainstem, although there aresmall bridging veins that course along these nerves to the ve-nous sinuses near the jugular bulb. The lateral medullary, retro-olivary, and transverse medullary veins and the vein of theinferior cerebellar peduncle course near the origin of the rootletsof the glossopharyngeal and vagus nerves.

Bridging veins are more frequently encountered in expos-ing the tentorial surface of the cerebellum than in exposing thesuboccipital or petrosal surfaces of the cerebellum. The bridg-ing veins from the suboccipital surface are often encounteredon the posterior part of the tentorial surface because thehemispheric veins from the suboccipital surface uniformlyascend to the tentorial surface before forming bridging veinsthat pass to the venous sinuses in the tentorium. Most of theveins from the petrosal surface pass to the vein of the cerebel-lopontine fissure and not directly to a venous sinus. The veinsfrom the tentorial and suboccipital surface that enter thesinuses in the tentorium are obstacles in the supracerebellarapproaches. In the infratentorial supracerebellar approach tothe pineal region, it may be necessary to divide numerousbridging veins entering the torcula and the tentorial sinuses,including some of the superior and inferior hemispheric andvermian veins, and the vein of the cerebellomesencephalicfissure. These veins have commonly been sacrificed withoutadverse effect to open the quadrigeminal region and the in-cisura (18, 22, 27). However cerebellar swelling followed tran-section of one of the bridging veins by Page (17).

Bridging veins infrequently cross from the suboccipital sur-face, tonsils, and medulla to the venous sinuses in the dura



overlying the suboccipital surface. In a few cases, an inferiorvermian or hemispheric vein will give rise to a bridging veinthat drains into the occipital sinuses below the torcula, andthe veins on the posterior and lateral surfaces of the medullawill give rise to bridging veins to the marginal or the occipitalsinuses (7, 19). In approaching the fourth ventricle, the veinsaround the tonsils, on the lower vermis, and near the inferiorpart of the roof may be sacrificed. These veins, including thevein of the cerebellomedullary fissure, have been occludedrepeatedly without sequelae.

It may be necessary to divide part of the tentorium in eitherthe occipital transtentorial or the infratentorial supracerebellarapproaches (10). In the occipital transtentorial approach, theoccipital lobe can often be retracted away from the falx andtentorium adjoining the straight sinus without sacrificing anyveins, because only infrequently are there bridging veins fromthe occipital lobe near the straight sinus to the torcula, lateral,straight, and superior sagittal sinuses. The posterior 5 cm of thesuperior sagittal sinus is frequently devoid of bridging veins.The vein of Labbe, which drains into the lateral portion of thelateral sinus, is usually lateral to this exposure, and the internaloccipital vein, which must be divided to reach the pineal region,drains not into a dural sinus, but into the internal cerebral orgreat vein in the quadrigeminal cistern. In the transtentorialapproach the tentorium is divided adjacent to and parallel to thestraight sinus beginning at the free edge and extending posteri-orly (20). The tentorial sinuses in the anterior part of the tento-rium are smaller and less frequent than those in the posteriorpart of the tentorium. Most of the tentorial sinuses found in theposterior third of the tentorium are formed by the cerebellarveins. The veins draining the lower portion of the temporal andoccipital lobes empty into the more anteriorly situated tentorialsinuses that drain into the superior petrosal or adjacent part ofthe transverse sinus. The tentorial sinuses formed by the veinsdraining the cerebrum commonly course posteromedially, pos-terolaterally, or straight posteriorly from their origin. The sinusin the anterior part of the tentorium usually receives only smallbridging veins from the midbrain, but in rare cases, the basalvein may terminate as a large bridging vein that enters theanterior part of the tentorium. The anteromedial edge of thetentorium posterior to the superior petrosal sinus may be sec-tioned through a subtemporal craniectomy to expose the trigem-inal nerve and the surrounding superior petrosal venous com-plex from their superolateral side. This provides excellentexposure of the pontotrigeminal and transverse pontine veinspassing above the trigeminal nerve, but some of the transversepontine and bridging veins may be hidden below or medial tothe nerve in this exposure.

Reprint requests: Albert L. Rhoton, Jr., M.D., Department of Neuro-logical Surgery, University of Florida Brain Institute, P.O. Box 100265,100 South Newell Drive, Building 59, L2-100, Gainesville, FL 32610-0265.

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4. Dandy WE: An operation for the cure of tic douloureux: Partialsection of the sensory root at the pons. Arch Surg 18:687–734, 1929.

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trigeminal nerve: An anatomical study with clinical correlation.J Neurosurg 52:381–386, 1980.

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13. Katsuta T, Rhoton AL Jr, Matsushima T: The jugular foramen:Microsurgical anatomy and operative approaches. Neurosurgery41:149–202, 1997.

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16. Matsushima T, Rhoton AL Jr, Lenkey C: Microsurgery of thefourth ventricle: Part 1—Microsurgical anatomy. Neurosurgery11:631–667, 1982.

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18. Pendl G: Infratentorial approach to mesencephalic tumors, inKoos WT, Bock FW, Spetzler RF (eds): Clinical Microneurosurgery.Stuttgart, Georg Thieme, 1976, pp 143–150.

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23. Takahashi M, Wilson G, Hanafee W: The significance of thepetrosal vein in the diagnosis of cerebellopontine angle tumors.Radiology 89:834–840, 1967.

24. Wackenheim A, Braun JP: The Veins of the Posterior Fossa: Normaland Pathologic Findings. New York, Springer-Verlag, 1978, pp 1–23.

25. Waltner JG: Anatomic variations of the lateral and sigmoid si-nuses. Arch Otolaryngol 39:307–312, 1944.

26. Wolf BS, Huang YP, Newman CM: The lateral anastomotic mes-encephalic vein and other variations in drainage of the basalcerebral brain. AJR Am J Roentgenol 89:411–422, 1963.

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the posterior fossa veins

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