cranial nerves part 1

CRANIAL NERVESCRANIAL NERVESPART IPART I

Appearance of Normal Cranial Nerves(REVIEW)

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Abbreviations

• Olfactory nerve: CN1• Optic nerve: CN2• Oculomotor nerve:

CN3• Trochlear nerve: CN4• Trigeminal nerve:

CN5• Abducens nerve: CN6• Facial nerve: CN7

• Vestibulocochlear nerve: CN8

• Glossopharyngeal nerve: CN9

• Vagus nerve: CN1O• Accessory nerve:

CN11• Hypoglossal nerve:

CN12

Overview

• Cranial nerve groupings based on area of brainstem origin - Diencephalon: CN2 - Mesencephalon (midbrain): CN3 and CN4 - Pons: CN5, CN6, CN7, and CN8 - Medulla: CN9, CN1O, CN11 and CN12

Imaging Approaches

• CN1, 2, 3, 4 and 6: Include focused orbital sequences• CN5: Include entire face to inferior mandible if V3

affected• CN7: Include CPA, temporal bone and parotid space• CN8: Include CPA-lAC and inner ear• CN9-12: Include basal cistern, skull base,

nasopharyngeal carotid space• CN10: Follow carotid space to aortopulmonic window on

left, cervicothoracic junction on right• CN12: Remember to reach hyoid bone to include distal

loop as it rises into sublingual space

CN1CN1Olfactory Nerve

Olfactory nerve: CN1

• First cranial nerve• Special visceral afferent cranial nerve for

olfaction (sense of smell)• Olfactory nerve segments

- End receptor in olfactory epithelium in nasal vault - Transethmoidal segment through cribriform plate - Intracranial olfactory bulb, tract and cortex

CN1: Nasal Epithelium

• Approximately 2 cm² nasal epithelium in roof of each nasal cavity - Extends onto nasal septum and lateral wall of nasal cavity including superior turbinates

• Bipolar olfactory receptor cells (neurosensory cells) located in nasal pseudostratified columnar epithelium - Peripheral processes of receptor cells in olfactory epithelium act as sensory receptors for smell

• Olfactory glands (of Bowman) secrete mucous which solubilizes inhaled scents (aromatic molecules)

CN1: Transethmoidal Segment

• Central processes of bipolar receptor cells traverse cribriform plate to synapse with olfactory bulb

• Hundreds of central processes traverse cribriform plate as unmyelinated fascicles (fila olfactoria) - Fila olfactoria are actual olfactory nerves - Each side of nasal cavity has - 20 fila olfactoria

CN1: Intracranial, Olfactory Bulb and Tract

• Olfactory bulb and tracts are extensions of the brain, not nerves - Historically bulb and tract are called "olfactory nerve"

• Olfactory bulb closely apposed to cribriform plate at ventral surface of medial frontal lobe - Rostral enlargement of olfactory tract - Bipolar cells synapse in olfactory bulb with secondary neuronal cells (mitral and tufted cells) - Mitral cell axons project posteriorly in olfactory tract - Granule cells modulate mitral cells

CN1: Intracranial, Olfactory Bulb and Tract

• Olfactory tract divides into medial, intermediate and lateral stria at anterior perforated substance - This trifurcation creates olfactory trigone - Anterior perforated substance is perforated by multiple small vascular structures - Olfactory tract is made up of secondary sensory axons, not primary sensory axons - Majority of fibers project through lateral olfactory stria and intermediate stria

CN1: Intracranial, Central Pathways

• Complex pattern of central connections• Lateral olfactory striae

- Formed by majority of fibers of olfactory tracts - Course over insula to prepiriform area (anterior to uncus) and amygdala - On way to prepiriform area collaterals are given to subfrontal or frontal olfactory cortex - Fibers to subthalamic nuclei with collaterals / terminal fibers to thalamus and stria medullaris


• Medial olfactory striae - Majority terminate in parolfactory area of Broca (medial surface in front of the subcallosal gyrus) - Some fibers end in subcallosal gyrus and in anterior perforated substance - Few fibers cross in anterior commissure to opposite olfactory tract

• Intermediate olfactory striae - Intermediate olfactory stria terminate in anterior perforated substance - Intermediate olfactory area contains anterior olfactory nucleus and nucleus of diagonal band


• Medial forebrain bundle - Formed by fibers from basal olfactory region, periamygdaloid area and septal nuclei - Some fibers terminate in hypothalamic nuclei - Majority of fibers extend to brainstem to autonomic areas in reticular formation, saliva tory nuclei and dorsal vagus nucleus

CN1: Imaging Recommendations

• Coronal sinus CT is best study for isolated anosmia - Identifies nasal vault and cribriform plate lesions

• MR of brain, anterior cranial fossa and sinonasal region used in complex anosmia cases - Identifies intracranial dural and parenchymal lesions

CN1: Imaging "Sweet Spots"

• Intracranial: Include anterior cranial fossa floor and medial temporal lobes

• Extracranial: Include nasal vault and cribriform plate

• Imaging Pitfalls - Coronal sinus CT insensitive to intracranial pathology

CN1: Clinical Importance

• CN1 dysfunction produces unilateral anosmia - Each side of nose must be tested individually

• Esthesioneuroblastoma arises from olfactory epithelium in nasal vault

• Head trauma may cause anosmia: Cribriform plate fracture or shear forces; anterior temporal lobe injury

• Seizure activity in lateral olfactory area may produce "uncinate fits", imaginary odor, oroglossal automat isms and impaired awareness

Cranial Nerve I:The Olfactory Nerve

• Unlike most cranial nerves, the olfactory nerve consists of white-matter tracts and is not surrounded by Schwann cells

• The neurosensory cells for smell reside in the olfactory epithelium along the roof of the nasal cavity

• The axons of these cells extend through the cribriform plate of the ethmoid bone into the olfactory bulb at the anterior end of the olfactory nerve

• The nerve then courses posteriorly through the anterior cranial fossa in the olfactory groove

• Posterior to the olfactory groove, the cisternal segment of the nerve runs below and between the gyrus rectus and the medial orbital gyrus

RadioGraphics 2009; 29:1045–1055

Cranial Nerve I:The Olfactory Nerve

• These secondary axons in the olfactory nerve eventually terminate in the inferomedial temporal lobe, uncus and entorhinal cortex

• To avoid confusing the olfactory nerve with the gyrus rectus on axial images, it is important to remember that the olfactory nerve is situated deep in the olfactory groove, inferior to the gyrus rectus

• Coronal images are easiest to interpret because the nerves are seen in cross section


Olfactory nerve. Axial and coronal 0.8-mm-thick SSFP MR images show the olfactory nerve (white arrow) within the CSF-filled olfactory groove and the optic nerve (black arrow) ringed by highsignal-intensity CSF within the dural sheath. Coronal 1.0-mm-thick SSFP MR image shows the cisternal segment of the olfactory nerve (arrow), which is located inferior to and between the gyrus rectus (r) and the medialorbital gyrus (o).


CN2CN2Optic Nerve

CN2

• Second cranial nerve

• Nerve of sight

• Visual pathway consists of optic nerve, optic chiasm and retrochiasmal structures

Overview

• Optic nerve not true cranial nerve but rather extension of the brain - Represents collection of retinal ganglion cell axons - Myelinated by oligodendrocytes not by Schwann cells as with true cranial nerves - Enclosed by meninges - Throughout its course to visual cortex nerve fibers are arranged in retinotopic order

Overview

• Optic nerve has four segments - Intraocular, intraorbital, intracanalicular and intracranial

• Partial decussation CN2 fibers within optic chiasm - Axons from medial portion of each retina cross to join those from lateral portion of opposite retina

• Retrochiasmal structures: Optic tract, lateral geniculate body, optic radiation and visual cortex

Intraocular segment

• 1 mm length

• Region of sclera termed lamina cribrosa where ganglion cell axons exit globe

Intraorbital segment• 20-30 mm in length• Extends posteromedially from back of globe to orbital apex within

intraconal space of orbit• CN2 longer than actual distance from optic chiasm to globe allowing

for movements of eye• Covered by same 3 meningeal layers as brain

- Outer dura, middle arachnoid and inner pia - Subarachnoid space (SAS) between arachnoid and pia contains cerebrospinal fluid (CSF); continuous with SAS of suprasellar cistern - Fluctuations in intracranial pressure transmitted via SAS of optic nerve-sheath complex

• Central retinal artery - 1st branch of ophthalmic artery - Enters optic nerve about 1 em posterior to globe with accompanying vein to run to retina

Intracanalicular segment

• 4-9 mm segment within bony optic canal

• Ophthalmic artery lies inferior to CN2

• Dura of CN2 fuses with orbit periosteum (periorbita)

Intracranial segment

• About 10 mm length from optic canal to chiasm

• Covered by pia and surrounded by CSF within suprasellar cistern

• Ophthalmic artery runs inferolateral to nerve

Optic chiasm

• Horizontally oriented; "X-shaped" structure within suprasellar cistern

• Forms part of floor of 3rd ventricle between optic recess anteriorly and infundibular recess posteriorly

• Immediately anterior to infundibulum (pituitary stalk), superior to diaphragma sellae

• Anteriorly chiasm divides into optic nerves• In chiasm nerve fibers from the medial half of retina

cross to opposite side• Posteriorly chiasm divides into optic tracts• Medial fibers of optic tracts cross in chiasm to connect

lateral geniculate bodies of both sides (commissure of Gudden)

Optic tracts

• Posterior extension of optic chiasm• Fibers pass posterolaterally curving

around cerebral peduncle and divide into medial and lateral bands - Lateral band (majority of fibers) ends in lateral geniculate body of the thalamus - Medial band goes by medial geniculate body to pretectal nuclei deep to superior colliculi

Optic radiation and visual cortex

• Efferent axons from lateral geniculate body form optic radiations (geniculocalcarine tracts)

• Fan out from lateral geniculate body and run as broad fiber tract to calcarine fissure - Initially pass laterally behind posterior limb internal capsule and basal ganglia - Extend posteriorly around lateral ventricle passing through posterior temporal and parietal lobes - Terminate in calcarine cortex (primary visual cortex) on medial surface of occipital lobes

Imaging Recommendations

• CT best for skull base and optic canal bony anatomy

• MR for CN2, optic chiasm and retrochiasmal structures - Axial and coronal thin-section T2, T1 and T1 C+

• Imaging Pitfalls: orbital CT may see subtle calcified optic sheath meningioma when MR may not

Clinical Importance

• Lesion location - Optic nerve pathology: Monocular visual loss - Optic chiasm pathology: Bitemporal heteronymous hemianopsia (loss of bilateral temporal visual fields) - Retrochiasmal pathology: Homonymous hemianopsia (vision loss in contralateral eye)

• Increased intracranial pressure transmitted along (SAS) of optic nerve-sheath complex - Manifests clinically as papilledema - Imaging shows flattening of posterior sclera, tortuosity and elongation of intraorbital optic nerves and dilatation of perioptic SAS (subarachnoid space)

Cranial Nerve II:The Optic Nerve

• Like the olfactory nerve, the optic nerve is a white-matter tract without surrounding Schwann cells.

• It includes four anatomic segments: retinal, orbital, canalicular, and cisternal.

• The retinal segment leaves the ocular globe through the lamina cribrosa sclerae (the optic foramen of the sclera).

• The orbital segment, which is surrounded by a dural sheath containing CSF, travels through the center of the fat-filled orbit.

• The canalicular segment is the portion that lies in the optic canal, below the ophthalmic artery. This segment of the nerve is frequently overlooked on radiologic images, so it should be specifically sought when imaging for vision loss.

• Finally, the cisternal segment of the nerve can be visualized in the suprasellar cistern, where the nerve leads to the optic chiasm. The anterior cerebral artery passes over the superolateral aspect of the cisternal segment of the nerve.


Cranial Nerve II:The Optic Nerve

• Key anatomic landmarks in the suprasellar cistern include the infundibulum (stalk) of the pituitary gland, the anterior cerebral artery, and, posterior to the chiasm, the mamillary bodies.

• The optic nerve terminates at the optic chiasm, where the two nerves meet, decussate, and form the optic tracts.

• The optic tracts travel around the cerebral peduncles, after which most axons enter the lateral geniculate body of the thalamus, loop around the inferior horns of the lateral ventricles (Meyer loop), and enter the visual cortex in the occipital lobe.


Optic nerve. Axial oblique 0.8-mm-thick SSFP MR image shows three of four segments of the optic nerve: the retinal (black arrow), orbital (black arrowheads), and canalicular (white arrowhead) segments. The infundibulum of the pituitary gland (white arrow) also is seen. The fourth (cisternal) segment of the optic nerve would be visible on more superior images. The cisternal segment of the optic nerve (white arrow) leads to the chiasm, which resembles the Greek letter χ in this plane. The optic tract (white arrowheads) leads backward from the chiasm to the thalamus. Important anatomic landmarks include the mamillary bodies (black arrowhead) and the anterior cerebral artery (black arrow).


Residual tumor near the optic chiasm in an 18-year-old woman after resection of a pituitary adenoma. Axial oblique 0.8-mmthick SSFP MR image shows a thin layer ofcerebrospinal fluid (arrow) between the residual tumor (t) and the left optic nerve and chiasm, a finding suggestive of resectability.The residual tumor was removed successfully with an expanded endonasal approach.


CN3CN3Oculomotor nerve

Overview

• Third cranial nerve• Motor nerve to extraocular muscles except

lateral rectus (CN6) and superior oblique muscles (CN4); parasympathetic to pupillary sphincter and ciliary muscle

• Mixed cranial nerve (motor and parasympathetic)

• Four anatomic segments: Intra-axial, cisternal, cavernous and extracranial

Intra-Axial Segment

Oculomotor nuclear complex• Paired paramedian nuclear complex

- Located in midbrain anterior (ventral) to cerebral aqueduct at level of superior colliculus - Partially embedded in periaqueductal gray matter - Bounded laterally and inferiorly by medial longitudinal fasciculus

• Consists of five individual motor subnuclei that supply individual extraocular muscles

Intra-Axial Segment

Edinger-Westphal parasympathetic nuclei• Located dorsal to oculomotor nuclear complex in

poorly myelinated periaqueductal gray matter• Preganglionic parasympathetic fibers exit

nucleus,course ventrally with motor CN3 fibers• Innervation of internal eye muscles (sphincter

pupillae and ciliary muscles)

Intra-Axial Segment

• Oculomotor fascicles course anteriorly through medial longitudinal fasciculus, red nucleus, substantia nigra and medial cerebral peduncle - Exit midbrain into interpeduncular cistern

• Parasympathetic Perlia nuclei - Located between the Edinger-Westphal nuclei - Thought to be involved in ocular convergence

Cisternal Segment

• Courses anterolaterally through interpeduncular and prepontine cisterns

• Passes between posterior cerebral (PCA) and superior cerebellar arteries (SCA)

• Courses inferior to posterior communicating artery and medial to free edge of tentorium cerebelli

• Crosses the petroclinoid ligament and penetrates dura to enter roof of cavernous sinus

Cavernous Segment

• Enters roof of cavernous sinus surrounded by narrow oculomotor CSF cistern

• Courses anteriorly through lateral dural wall of cavernous sinus

• CN3 remains most cephalad of all cranial nerves within cavernous sinus

• CN3 superolateral to cavernous internal carotid artery

Extracranial Segment

• CN3 enters orbit through superior orbital fissure and passes through annulus tendineus (annulus of Zinn)

• Divides into superior and inferior branches• - Superior branch supplies levator palpebrae superioris

and superior rectus muscles - Inferior branch supplies inferior rectus, medial rectus and inferior oblique muscles

• Preganglionic parasympathetic fibers follow inferior branch to ciliary ganglion of orbit - Postganglionic parasympathetic fibers continue as short ciliary nerves to enter globe with optic nerve - In globe short ciliary nerves to ciliary body and iris - Control papillary sphincter function and accommodation via ciliary muscle


• Bone CT best for skull base, bony foramina• MR for intra-axial, cisternal, cavernous segments

- Thin-section high-resolution T2 MR sequences in axial and coronal planes . Depicts cisternal CN3 surrounded by CSF with high contrast and high spatial resolution

• Imaging "Sweet Spots“: CN3 nuclear complex and intra-axial segment not directly visualized - Find periaqueductal gray matter to localize

• Imaging Pitfalls: Negative MR and MRA does not completely exclude posterior communicating artery aneurysm - Cerebral angiography still represents gold standard to exclude this diagnosis

Clinical Importance and Findings

• Uncal herniation pushes CN3 on petroclinoid ligament• During trauma downward shift of brainstem upon impact can

stretch CN3 over petroclinoid ligament• CN3 susceptible to compression by PCA aneurysms• CN3 neuropathy divided into simple if isolated and complex if with

other CN involvement (CN4 & CN6) - Simple CN3 with pupillary involvement

• Must exclude PCA aneurysm as cause• Explanation: Parasympathetic fibers are peripherally distributed

- Simple CN3 with pupillary sparing .Presumed microvascular infarction involves vessels supplying core of nerve with relative sparing of peripheral pupillary fibers

• Oculomotor ophthalmoplegia: Strabismus, ptosis, pupillary dilatation, downward abducted globe and paralysis of accommodation

Cranial Nerve III:The Oculomotor Nerve

• The oculomotor nerve originates from nuclei deep to the superior colliculus, ventral to the cerebral aqueduct, and inferior to the pineal gland.

• The nerve then travels across the midbrain from posterior to anterior.

• The oculomotor nerve root emerges into the interpeduncular cistern, and this root entry zone in the cistern is a good way to identify the oculomotor nerve on axial SSFP MR images.

• In the prepontine cistern, the nerve travels between the superior cerebellar and posterior cerebral arteries, which makes it easy to identify on coronal SSFP images.


Cranial Nerve III:The Oculomotor Nerve

• The cavernous segment of the oculomotor nerve runs along the lateral wall of the cavernous sinus and is the most superior of the nerves in this sinus.

• The oculomotor nerve then enters the orbit through the superior orbital fissure, before splitting into superior and inferior divisions lateral to the optic nerve.

• Knowledge of this anatomy may be helpful for identifying the precise location of a nerve abnormality.


Oculomotor nerve.Axial oblique 0.8-mm-thick SSFP MR image shows the nerve (small arrows) where it emerges from the interpeduncular cistern (large arrow), which lies medial to the cerebellar peduncle (p).Coronal 0.8-mm-thick SSFP MR image shows the oculomotor nerve (white arrow) in cross section between the posterior cerebral artery (white arrowhead) and the superior cerebellar artery (black arrowhead), which are distal branches of the basilar artery (black arrow).


Oculomotor nerve compression in na 82-year-old woman with ptosis of the right eye. Axial 0.8-mm-thick SSFP MR image shows displacementand compression of the right oculomotor nerve in the root entry zone (long arrow) by the distal basilar artery (short arrow). The left oculomotor nerve (arrowhead), in comparison, appears normal.


CN4CN4Trochlear nerve

Overview

• Fourth cranial nerve

• Motor nerve to superior oblique muscle

• CN4 is a pure motor nerve

• Four segments: Intra-axial, cisternal, cavernous and extracranial

Intra-Axial Segment

Trochlear nuclei

• Paired nuclei located in paramedian midbrain - Anterior to cerebral aqueduct - Dorsal to medial longitudinal fasciculus - Caudal to oculomotor nuclear complex at level of inferior colliculus

Intra-Axial Segment

• Trochlear nerve fascicles course posteriorly and inferiorly around cerebral aqueduct - Fibers then decussate within superior medullary velum - Key concept: each superior oblique muscle is innervated by contralateral trochlear nucleus

• CN4 exists dorsal midbrain just inferior to inferior colliculus - Key concept: CN4 is the only cranial nerve to exit dorsal brainstem

Cisternal Segment

• CN4 courses anterolaterally in ambient cistern - Runs underneath the margin of the tentorium - Passes between free edge of tentorium cerebelli and midbrain just superolateral to pons

• Passes between posterior cerebral artery above and superior cerebellar artery below - Oculomotor nerve travels this gap as well - CN4 is just inferolateral to oculomotor nerve

• Penetrates dura to enter lateral wall of cavernous sinus just inferior to oculomotor nerve

Cavernous Segment

• Courses anteriorly through lateral dural wall of cavernous sinus

• Intracavernous relationships of CN4 - Remains inferior to CN3 - Superior to ophthalmic division of trigeminal nerve (CNV1) - Lateral to cavernous internal carotid artery

Extracranial Segment

• CN4 enters orbit through superior orbital fissure together with CN3 and CN6

• Crosses over CN3 and courses medially

• Passes above annulus of Zinn (CN3 and CN6 go through annulus)

• Supplies motor innervation to superior oblique muscle


• CT best for skull base, bony foramina

• MR best for brainstem, cisternal, cavernous and intra-orbital imaging

• Intra-orbital segment not visualized by any imaging modality or sequence

Imaging "Sweet Spots"

• CN4 nucleus and intra-axial segment not directly visualized - Nuclei position inferred by identifying periaqueductal gray matter and cerebral aqueduct at level of inferior colliculi on high-resolution MR

• MR for intra-axial, cisternal and cavernous segments - Thin-section high-resolution T2 and T1 C+ MR in axial and coronal planes

• Coronal imaging margins: Fourth ventricle to anterior globe

• Axial imaging margins: Orbital roof-diencephalon to maxillary sinus roof-medulla

Imaging Pitfalls

• Difficult to visualize CN4 despite best MR imaging efforts

• During image interrogation by radiologist, view known landmarks along its course - Midbrain → tentorial margin → cavernous sinus → superior orbital fissure → extraconal orbit

Normal Measurements

• CN4 is smallest cranial nerve

• CN4 has longest intracranial course (- 7,5 cm)

Clinical Importance

CN4 neuropathy divided into simple and complex• Simple CN4 neuropathy (isolated)

- Most common form; usually secondary to trauma - Cisternal segment injury by free edge of tentorium cerebelli or from posterior cerebral or superior cerebellar artery aneurysm - Contusion of superior medullary velum

• Complex CN4 neuropathy (associated with other CN injury, CN3 ± CN6) - Brainstem stoke or tumor - Cavernous sinus thrombosis, tumor - Orbital tumor

Clinical Findings

• Paralysis of superior oblique muscle results in extorsion (outward rotation) of affected eye

• Extorsion is secondary to unopposed action of inferioroblique muscle

• Patient complaints: Diplopia, weakness of downward gaze, neck pain from head tilting

• Physical exam: Compensatory head tilt usually away from affected side

Cranial Nerve IV:The Trochlear Nerve

• The trochlear nerve is the only nerve with a root entry zone arising from the dorsal (posterior) brainstem.

• After exiting the pons, the trochlear nerve curves forward over the superior cerebellar peduncle, then runs alongside the oculomotor nerve between the posterior cerebral and superior cerebellar arteries.

• The trochlear nerve then pierces the dura to enter the cisterna basalis between the free and attached borders of the cerebellar tentorium.



• After completing its cisternal course, the trochlear nerve runs through the lateral cavernous sinus just below the oculomotor nerve and enters the orbit through the superior orbital fissure to innervate the superior oblique muscle.

• The nerve is named for the trochlea, the fibrous pulley through which the tendon of the superior oblique muscle passes.



• The cisternal segment of this tiny nerve is most easily identifiable posterolateral to the brainstem.

• Along part of its intracranial course, the trochlear nerve lies between dural layers, where it is difficult to visualize on radiologic images.

• Particular attention should be given to the anterior aspect of the tentorium in patients in whom the presence of isolated trochlear nerve palsy is suspected.


Trochlear nerve. Axial 0.8-mm-thick SSFP MR image shows both trochlear nerves (arrows) where they emerge from the dorsal midbrain to crossthe ambient cisterns. The characteristic course of the trochlear nerves allows their differentiation from the nearby superior cerebellar artery (arrowheads).


CN5CN5Trigeminal Nerve

Overview

• Trigeminal nerve: CN5, CNV• Ophthalmic division, trigeminal nerve: CNV1• Maxillary division, trigeminal nerve: CNV2• Mandibular division, trigeminal nerve: CNV3• Fifth cranial nerve, nervus trigeminus• Great sensory cranial nerve of head and face;

motor nerve for muscles of mastication• Mixed nerve (both sensory, motor components)• Four segments: Intra-axial, cisternal,

interdural and extracranial

Intra-Axial Segment

Four nuclei (3 sensory, 1 motor)Located in brainstem, upper cervical cord

Mesencephalic nucleus CNS• Slender column of cells projecting cephalad from pons to

level of inferior colliculus• Found anterior to upper fourth ventricle/aqueduct near

lateral margin of central gray• Afferent fibers for facial proprioception (teeth, hard

palate and temporomandibular joint)• Sickle-shaped mesencephalic tract descends to motor

nucleus, conveys impulses that control mastication and bite force

Intra-Axial Segment

• Main sensory nucleus CNS - Nucleus lies lateral to entering trigeminal root - Provides facial tactile sensation

• Motor nucleus CNS - Ovoid column of cells anteromedial to principal sensory nucleus - Supplies muscles of mastication (medial and lateral pterygoids, masseter, temporalis), tensor palatine/tensor tympani, mylohyoid and anterior belly of digastric

• Spinal nucleus CNS - Extends from principal sensory root in pons into upper cervical cord (between C2 to C4 level) - Conveys facial pain, temperature

Cisternal (Preganglionic) Segment

• Two roots: Smaller motor, larger sensory• Emerges from lateral pons at root entry zone

(REZ)• Courses anterosuperiorly through prepontine

cistern• Enters middle cranial fossa by passing beneath

tentorium at apex of petrous temporal bone• Passes through an opening in dura matter called

porus trigeminus to enter Meckel cave

Interdural Segment

• Meckel cave formed by meningeal layer of dura lined by arachnoid - Cave is filled with cerebrospinal fluid (CSF) (90%) and continuous with prepontine subarachnoid space

• Pia covers CNS in trigeminal cave• Preganglionic CNS ends at trigeminal ganglion

(TG) - TG located in inferior aspect of Meckel cave - TG synonyms: Gasserian or semilunar ganglion

Divisions (Post-Ganglionic) of CNS

Ophthalmic nerve (CNV1)• Courses in lateral cavernous sinus wall

below CN4 - Exits skull through superior orbital fissure - Enters orbit, divides into lacrimal, frontal and nasociliary nerves

• Sensory innervation scalp, forehead, nose, globe


Maxillary nerve (CNV2)• Courses in cavernous sinus lateral wall

below CNV 1• Exits skull through foramen rotundum• Traverses roof of pterygopalatine fossa• Continues as infraorbital nerve in floor of

orbit• Exits orbit through infraorbital foramen

- Sensory to cheek and upper teeth


Mandibular nerve (CNV3)• Does not pass through cavernous sinus• Exits directly from Meckel cave, passing inferiorly

through foramen ovaIe into masticator space• Carries both motor and sensory fibers

- Motor root bypasses TG, joins V3 as it exits through foramen ovale - Divides into masticator (muscles of mastication) and mylohyoid nerves (mylohyoid and anterior belly of digastric muscles) - Masticator nerve take off just below skull base - Mylohyoid nerve take off at mandibular foramen

• Main sensory branches include inferior alveolar, lingual and auriculotemporal nerves


• CT best for skull base and bony foramina• MR for intra-axial, cisternal and intradural

segments - Thin-section T2 in axial and coronal planes

• T1 C+ fat-saturated MR of entire extracranial course

• Imaging Pitfalls: Trigeminal ganglion is small crescent of tissue found in the anteroinferior Meckel cave - Trigeminal ganglion lacks blood-nerve barrier therefore normally enhances with contrast

Clinical Importance

• Sensory complaints: Pain, burning, numbness in face

• Motor (V3 only): Weakness in chewing - Proximal V3 injury causes motor atrophy of masticator muscles within 6 weeks to 3 months - Distal V3 injury (above mylohyoid nerve takeoff) affects only anterior belly of digastric & mylohyoid

• Tic douloureux (trigeminal neuralgia) - Sharp, excruciating pain in V2-3 distributions - Look for vascular compression at REZ (on MR)

Cranial Nerve V:The Trigeminal Nerve

• The trigeminal nerve is the largest cranial nerve.• It is composed of a large sensory root that

runs medial to a smaller motor root. • The roots emerge from the lateral midpons and

travel anteriorly through the prepontine cistern and the porus trigeminus to the Meckel (trigeminal) cave, a CSF-containing pouch in the middle cranial fossa.

• Because the trigeminal nerve is large and its course proceeds straight forward from the lateral pons, it is easy to recognize on most MR images.


Cranial Nerve V:The Trigeminal Nerve

• In the Meckel cave, the nerve forms a meshlike web that can be visualized only with high-resolution imaging.

• Along the anterior aspect of the cavity, the trigeminal nerve forms the trigeminal (gasserian) ganglion before splitting into three subdivisions.

• The ophthalmic (V1) and maxillary (V2) divisions of the nerve move medially into the cavernous sinus and exit the skull through the superior orbital fissure and foramen rotundum, respectively.

• The mandibular division (V3), which includes the motor branches, exits the skull inferiorly through the foramen ovale.


Trigeminal nerve. Axial 0.8-mm-thick SSFP MR image shows the sensory (arrowhead) and motor (large arrow) roots of the trigeminal nervewhere they cross the prepontine cistern and enter the Meckel cave (small arrows).


Trigeminal nerve. Coronal 0.8-mm-thick SSFP MR image at the level of the Meckel cave shows the complex web of trigeminal nerve branches (arrows), which coalesce anteriorly to form the gasserian ganglion. The temporal horn of the lateral ventricle (arrowhead) is also shown.


CN6CN6Abducens nerve

Overview

• Sixth cranial nerve

• Motor nerve to lateral rectus muscle only

• CN6 is a pure motor nerve

• Five segments can be defined: Intra-axial, cisternal, interdural, cavernous and extracranial (intra-orbital)

Intra-Axial Segment

Abducens nucleus• Paired CN6 nuclei located in pontine tegmentum near

midline - Found just anterior to fourth ventricle

• Facial colliculus: Axons of facial nerve (CN7) loop around abducens nucleus creating this bulge in floor of fourth ventricle

Abducens nerve axons course anteroinferiorly through pontine tegmentum

Emerges from anterior brainstem near midline through groove between pons and pyramid of medulla oblongata (bulbopontine sulcus)

Cisternal Segment

• CN6 ascends anterosuperiorly in prepontine cistern toward site where it penetrates dura

• May be posterior or anterior to anterior inferior cerebellar artery

• Penetrates dura of basisphenoid to enter Dorello canal

Interdural Segment• Dorello canal represents channel within basilar venous plexus

(petroclival venous confluence) - Channel is located between two layers of dura - Basilar venous plexus is continuous with cavernous sinus anteriorly - Basilar venous plexus drains into inferior petrosal sinus

• Extends from point where CN6 pierces inner (cerebral) layer dura mater to its entrance into cavernous sinus

• Within Dorello canal, abducens nerve is surrounded by layer of arachnoid mater & occasionally dura mater

• After penetrating dura, CN6 passes superiorly through basilar venous plexus - It then arches over petrous apex below petrosphenoidalligament into upper posterior region of cavernous sinus - Bony sulcus of CN6 as it passes over top of petrous apex usually present

Cavernous Segment

• CN6 courses anteriorly within cavernous sinus - Abducens nerve is only cranial nerve to lie within cavernous sinus - Cranial nerves 3, 4, VI and V2 are all embedded within lateral wall of cavernous sinus

• Within cavernous sinus CN6 runs along inferolateral aspect of cavernous internal carotid artery

Extracranial (Intra-Orbital) Segment

• CN6 enters orbit through superior orbital fissure together with CN3 and CN4

• Passes through annulus of Zinn

• Supplies motor innervation to lateral rectus muscle


• MR for intra-axial, cisternal, interdural & cavernous segments - Thin-section high-resolution T2 and contrast-enhanced Tl in axial and coronal planes

• Depicts small structures including cranial nerves surrounded by CSF with high contrast & high spatial resolution

• Bone CT best for skull base and its bony foramina

• Dorello canal, cavernous sinus and orbital CN6 not visualized on routine MR imaging

Imaging "Sweet Spots"

• Axial and coronal MR sequences should include brainstem, fourth ventricle, cavernous sinus and orbit

• CN6 nucleus and intra-axial segment not directly visualized - Position of CN6 inferred by identifying facial colliculus in floor of fourth ventricle on high-resolution thin-section T2 MR

• Cisternal segment routinely visualized on high-resolution T2

• CN6 entrance into Dorello canal may be visualized due to invagination of cerebrospinal fluid into proximal canal

Imaging Pitfalls

• Use of fat-saturation on post-contrast T1 MR sequences can amplify blooming (susceptibility) artifact around a well aerated sphenoid sinus - Cavernous sinus & orbital apex subtle lesions may be obscured by this artifact - Remove fat-saturation and repeat Tl post-contrast MR if this artifact obscures key areas of interest

Clinical Importance

• In abducens neuropathy, affected eye will not abduct (rotate laterally)

• CN6 neuropathy divided into simple if isolated & complex if associated with other CN involvement (CN3, 4 and 7) - Simple CN6 neuropathy most common ocular motor nerve palsy - Usually presents as complex cranial neuropathy: Pontine lesions affect CN6 with CN7 Cavernous sinus, superior orbital fissure lesions affect CN6 with CN3, 4 and CNV1

Cranial Nerve VI:The Abducens Nerve

• The abducens nerve emerges from nuclei anterior to the fourth ventricle, then courses anteriorly through the pons to the pontomedullary junction and into the prepontine cistern.

• After crossing the prepontine cistern in a posterior-to-anterior direction, the abducens nerve runs vertically along the posterior aspect of the clivus, within a fibrous sheath called the Dorello canal.

• The nerve then continues over the medial petrous apex and through the medial cavernous sinus, entering the orbit through the superior orbital fissure to innervate the lateral rectus muscle.


Cranial Nerve VI:The Abducens Nerve

• It is important to note that the abducens nerve runs almost the entire length of the clivus.

• Radiologists should be vigilant for clivus and petrous apex abnormalities in the setting of abducens nerve palsy.

• Although the abducens nerve lies near the anterior inferior cerebellar artery and has a similar caliber, the two structures course in orthogonal directions and are thus easily distinguished.


Abducens nerve. Axial 0.8-mm-thick SSFP MR image at the level of the pontomedullary junction shows both abducens nerves (arrows) where they traverse the prepontine cistern. The bottom of the pons (p) and the top of the medulla (m) are visible in this section, and the cerebellopontine angle (CPA) and basilar artery (arrowhead) are important anatomic landmarks.


Abducens nerve. Axial 0.8-mm-thick SSFP MR image shows the abducens nerve where it enters the Dorello canal (arrow) along the posterior aspect of the clivus. Vascular landmarks include the basilar artery (black arrowhead) and the anterior inferior cerebellar artery (white arrowhead).


CONTINUE...CONTINUE...

References

• Diagnostic and Surgical Imaging Anatomy. Brain, Head & Neck, Spine / H. Ric Harnsberger. [et al.] ; managing editor, Andre Macdonald. n 1st ed. I:174-I:259.

• RadioGraphics 2009; 29:1045–1055 • Sujay Sheth, BA • Barton F. Branstetter IV, MD • Edward J. Escott, MD. Appearance of Normal Cranial Nerves on Steady-State Free Precession MR Images.

cranial nerves part 1

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