heteronymous hemianopia

Journal ofNeurology, Neurosurgery, and Psychiatry, 1973, 36, 710-723

The anatomy of the optic chiasma andheteronymous hemianopia

J. E. A. O'CONNELL

From the Department of Neurological Surgery, St. Bartholomew's Hospital, London

SUMMARY The gross anatomy of the optic nerves and chiasma has been studied, and differences inthe tension in the crossed and uncrossed fibres after chiasmal displacement have been investigated.The anterior and posterior attachments of the medial and lateral fibres of the nerves have beenstudied. The chiasma has been dissected under low power microscopy and a three dimensionalpicture of it developed. Bitemporal hemianopia, as well as associated or independent hemianopicscotomata, results from stretching of the crossing fibres in the chiasma. Binasal hemianopia resultsfrom compression of the uncrossed fibres in the optic nerve or chiasma by the anterior cerebral or

internal carotid arteries. The compression is effective because it is sharply localized and, probably as

a result of pulsation, deeply grooves the nerve with a resulting acute distortion of fibres; it is likelythat the lax lateral fibres would be less affected by a more widely spread compression. When thisdefect develops on top of an existing bitemporal hemianopia, it is believed that its usual cause

remains the same. The crossed and uncrossed fibres of the optic chiasma differ not only anatomicallyin the areas of retina in which they arise but also physically. Tension is the force which occasionsbitemporal hemianopia and pressure that which produces nasal field defects.

With the development of neurological surgeryduring the last half century there has been muchcareful work on the visual field defects producedby lesions in the sellar neighbourhood. The com-bined studies of ophthalmologists, neurologists,and neurosurgeons have added to knowledge andrefined techniques have led to extremely accuratecharting of abnormalities in the fields. However,as will be described, certain features of the fielddisturbances associated with tumours related tothe optic chiasma remained unexplained. It isbelieved that a reexamination of the anatomy ofthe optic chiasma may throw some light on theseproblems and the results of such an examinationand its implications form the basis of this paper.The term hemianopia indicates a defect in half

of the visual field in one eye, usually a verticalhalf, and it may be incomplete. Hemianopia maybe homonymous right or left when correspond-ing half fields in each eye are affected. Thisresults from a lesion of the optic tract, geniculo-calcarine pathway or striate cortex. Such a defectis a single one of the binocular field. On theother hand, hemianopia may be heteronymous

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when both outer or both inner half fields areinvolved. Such defects will usually be secondaryto a lesion of the optic chiasma and will bebitemporal if the crossing fibres are involved, orbinasal if the uncrossed are affected. Here thedefects involve both sides of the binocular field.It is believed that the mechanisms responsiblefor binasal and bitemporal hemianopia are dis-tinct and related to chiasmal anatomy and theywill be discussed after the anatomy of this areahas been reviewed.

METHODS

In 25 postmortem subjects following removal of thecalvarium the frontal dura mater was incised trans-versely, the falx cerebri sectioned, and the frontallobes elevated to display the optic nerves, opticchiasma, and anterior parts of the optic tracts as wellas related arteries. The arachnoid mater was separ-ated from the chiasmal region and, with a nerve hook,the resistance to deformation and displacement ofthe nervous structures was estimated. The tension inthe medial and lateral portions of the optic nerveswas determined by noting the gap which developed

The anatomy of the optic chiasma and heteronymous hemianopia

in these after appropriate section. The effect on thesegaps of subsequent chiasmal and tract section wasalso determined. Photographs were made to demon-strate the results.

In 20 cases after its removal from the skull andfixation in 500 formalin solution the chiasma wasmacerated in 3000 alcohol for periods of from oneto three weeks. The specimens were then dissectedunder the microscope (magnification x 10) and theappearances sketched and photographed. Themicroscopic anatomy of the optic nerves, chiasma,and tracts was reviewed in sections, stained withhaematoxylin and eosin, haematoxylin and VanGieson, and Loyez's stains.

ANATOMY OF OPTIC CHIASMA

Not only embryologically but topographicallythe optic chiasma is an integral part of the brain(diencephalon). While it projects inferiorly andanteriorly into the cisterna basalis, it forms partof the third ventricular wall between the laminaterminalis and the tuber cinereum (Fig. 1). More-over, its posterolateral angles are continued intothe optic tracts, which are themselves attachedto the cerebrum and cerebral peduncles, and

these structures are clothed in a well-marked piallayer which passes anteriorly on to the opticnerves where these join its anterolateral angles.The dimensions of this junctional zone betweenthe optic nerves and optic tracts vary. Whitnall(1932) gives as mean values 13-28 mm trans-versely, 8 mm anteroposteriorly, and 3 to 5 mmin thickness. Its anteroposterior relationship tothe sella turcica varies. According to Schaeffer(1924) in 79%0 the chiasma lies almost whollyover the diaphragma sellae overlapping the dor-sum to a slight extent, and in 12% it lies whollyover the diaphragma sellae; in 5X0 it lies in partin the sulcus chiasmaticus of the skull base andin part on the diaphragma sellae, and in 400 itlies on and behind the dorsum sellae. Clearly themore posterior the position of the chiasma, thelonger will be the intracranial portions of theoptic nerves-their length varying from 6 to21 mm (Zander, 1896). Of the other relations ofthe chiasma, that to the neighbouring arteries-internal carotid, anterior cerebral, and anteriorcommunicating-is of particular importance. Aspointed out by Bull (1956), the vertical height of

-.*.. ..

FIG. 1. The attachments ofthe optic nerve at optic canialand chiasma.

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J. E. A. O'Connell

FIG. 2. Photograph of dissectioni to show differences in resistance to deformation and displacement in lateraland medial fibres of optic nerves.

FIG. 3. Photograph showing effects ofsagittal sectionof chiasma.

FIG. 4. A. Anterior nerve sections showing widergaping of medial incision. B and C. Intermediate sec-tions showing wider gaping of medial incision and itsincrease with chiasmal retraction. D, E, and F.Posterior section with wider gaping ofmedial incision,after chiasmal retraction, the situation being reversedafter chiasmal section. G and H. Posterior sectionwith wider gaping of medial incision increased bychiasmal retraction.

712

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the chiasma above the skull base varies, just asdoes its distance posterior to the tuberculumsellae, according to the length of the optic nerves.It is difficult to estimate the former distance, butthe latter he states to be 10 to 23 mm.

DIFFERENCES IN RESISTANCE TO DEFORMATION AND

DISPLACEMENT IN OPTIC NERVES AND TRACTSAfter exposure of the area in the manner de-scribed, pressure against the lateral margin ofthe optic nerve and chiasma reveals, at first, alow resistance to deformation and to the medialdisplacement of the nerves, the chiasma becom-ing relaxed (Fig. 2, A and C). Pressure againstthe medial margin of the nerve reveals a higherresistance to deformation and lateral displace-ment, the chiasma being stretched and the oppo-site optic nerve drawn medially (Fig. 2, B andD). These differences are particularly markedimmediately anterior to the chiasma andincreased with posterior retraction of thatstructure. Posterior displacement of the chiasmais itself strongly resisted. There is relatively littleresistance to displacement of the optic tractswhich retract like brain tissue itself. There islittle resistance to downward displacement ofeither medial or lateral fibres of the optic nerves,the degree of such displacement being, however,restricted by the underlying cranial floor. How-ever, when upward displacement is attempted,this is resisted more strongly by the medial thanby the lateral fibres.

DIFFERENCES IN TENSION IN OPTIC NERVE FIBRES ASINDICATED BY RESULTS OF SECTION AT VARIOUSSITES (1) Sagittal section of optic chiasma (Fig.3) After sagittal section of the chiasma separa-tion of the cut surfaces occurs and the opticnerves move laterally, the nerves and tractsmoving closer towards anteroposterior align-ment. The gap between the cut surfaces measured4 to 5 mm, yet the separation of the lateralchiasmal margins amounted to only 2 to 3 mm.This implies that chiasmal fibres are in partretracted into the optic nerve and tract bytension within these.

2. Partial section of optic nerves (Fig. 4, A, B,C, G, H; Fig. 5, A, B, C, D) The medial halfof one optic nerve and the lateral half of theother were sectioned. This was carried out at

anterior, intermediate, and posterior levels in thenerves of different specimens. In almost all cases,and at all levels, but more markedly at posteriorones, the incision on the medial side of the nervegaped more widely than that on the lateral side.The most posterior incisions were twice as widemedially as laterally, and it was noted that theytended to open up in a rectangular manner onthe medial side instead of remaining wedge-shaped as in the case of lateral incisions. Gentleposterior retraction on the chiasma made thedifference considerably more obvious. The gapas measured at the medial margin of the nervevaried from 1 5 to 4 mm, and at the lateral mar-gin of the nerve 0 5 to 2-5 mm. After posteriorretraction of the chiasma, the medial measure-ment was 3 to 5 mm, and the lateral one 1 to

._

I

FIG. 5. A, B, C, and D. Posterior section of medialand lateral fibres and effects of chiasmal retractionupon them. E and F. Bilateral section of medial fibresshowing decrease in gap in right nerve end and increasein that in the left nerve after left tract section.

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J. E. A. O'Connell

3 mm. These findings indicate that when thechiasma is exposed in the manner described,tension develops in the optic nerves, and is moremarked in their medial than in their lateralfibres. The degree of tension will vary from caseto case, but is invariably increased by chiasmalretraction.

3. Partial section of optic nerves combined withchiasmal and tract sections (Fig. 4, D, E, and F;Fig. 5, E and F) If sagittal section of thechiasma is carried out on a specimen in whichthe medial half of one optic nerve and the lateralhalf of the other has been sectioned, it is foundthat the gap on the lateral side widens, and thaton the medial side diminishes. In such a case,much of the tension is transferred to the lateralfibres, and the usual state of affairs is reversed.If such chiasmal section is carried out on a speci-men in which the medial halves of both opticnerves have been sectioned, the gap on bothsides diminishes in extent. Again if after sectionof the medial halves of both optic nerves, theoptic tract is divided on one side, the gap in theopposite optic nerve diminishes in width, tensionhaving been reduced in its crossing fibres.

It is believed that this series of observations,like the preceding ones, indicates that when thefrontal lobes are elevated or the chiasma re-tracted posteriorly, an increase in tension de-velops in the optic nerve fibres, and that thistension is considerably greater in the medialcrossing fibres than in the lateral uncrossed ones.It should be pointed out that, while some of themeasurements described were made on theexposed specimens, the majority were made forgreater accuracy from photographs. The magni-fication of these varied to some extent and theactual measurements are not therefore reliable;however, the plates do reveal the differences inthe gaps which develop in the medial and lateralgroups of fibres in the nerves after fractionalsection and the effects of chiasmal and tractsections upon these.

INTERNAL ANATOMY OF OPTIC CHIASMA Informa-tion on this subject is less detailed than might beexpected. It is now believed that the ratio ofcrossed to uncrossed fibres in the chiasma is53:47 and that the number of fibres of diametergreater than 0O5 pu in each optic nerve is in the

order of 11 million (Kupfer et al., 1967). More-over, it is accepted that the nasal fibres in theoptic nerves cross anteriorly in the chiasma andthe macular ones posteriorly. Further, it is statedthat the more anteriorly placed nasal fibres loopforwards into the termination of the contra-lateral optic nerve before turning posteriorly,while the more posteriorly placed and lateral ofthose fibres are described as looping backwardsinto the origin of the ipsilateral optic tract beforecrossing in the chiasma to the contralateral one.Anatomical illustrations to support these state-ments are lacking in a wide selection of ana-tomical, neurological, and ophthalmologicalworks. Reference is, however, frequently madeto the writings of Henschen (1890 and 1892) andWilbrand and Sanger (1904). In these beautifullyillustrated works, information concerning thepreparation of the specimens is scanty, thoughWilbrand and Sanger (1904) describe theirplates as 'being true to nature'. The platesappear to be drawings rather than photographsand designed to show, in one figure, the findingsfrom the examination of a number of horizontalsections of the chiasma. Polyak (1957) provides amore complete account of chiasmal anatomywith a description of the methods by which hisspecimens were prepared, though again theillustrations appear to be drawings rather thanphotographs. While it is clear that the finerdetails of fibre arrangement in the chiasma canonly be determined by histological methods, itseemed possible that dissection of this structureunder magnification could provide a better three-dimensional view of the general internal arrange-ment of the fibre bundles and the resultingphysical properties of this decussation uniquebecause of its anatomical site outside, though apart of, the central nervous system.

Chiasmal dissections The pial sheath of thechiasma was noted to be extremely well developedparticularly on its superior aspect. As had beennoted during dissections of the optic nerve inconnection with an investigation of intraneuralplexus formation (O'Connell, 1936) the fibres ofthe optic nerve could be separated into parallelrounded bundles without intercommunication.The same was true of the anterior part of theoptic tracts, the larger bundles here being flat-tened with concave medial and convex lateral

714


LO0N R.O0N

L.O.T ROT

"M b

FIG. 6. (a) Photograph of superficial dissection of superior aspect of chiasma showing crossed and uncrossedfibres. At the origin of the optic tract the latter separate into medial and lateral bundles with the crossed fibresfrom the opposite nerve between them. (b) Drawing of above dissection. L.O.N. and R.O.N.-optic nerves.L.O.T. and R.O.T.-optic tracts. U.C.F.-uncrossed fibres. L.C.F. and R.C.F.-crossed fibres with side oforigin.

surfaces. The lateral fibres in the nerve could betraced through the lateral chiasma into thelateral tract. Here a division occurred so that thesuperior fibres passed to the superomedialaspect of the emerging crossed fibres from theopposite side (Fig. 6).

Dissection of the superior aspect of thechiasma revealed that the superior crossingfibres on each side spread out in a triangularsheet of bundles with its apex at the terminationof the optic nerve and its base at the midline(Figs 6 and 7). Anteriorly were the relativelyshort medial fibres crossing in the anterior por-tion of the chiasma and posteriorly and laterallythe lateral crossing fibres passing to the posteriormargin of the structure (Fig. 8). Fibre bundles of1-5 to 2 mm in diameter could be readily tracedmedially under the dissecting microscope. How-ever, as the central area of the chiasma wasapproached, these split into smaller bundlesuntil diameters of 0-2 mm or less were reached,and further dissection became impossible as itled to their being torn. Beneath the most super-

ficial layer of crossing fibres, further similarlayers, four in number, can be made out.

Dissection of the inferior surface of thechiasma displays the crossed fibres emergingfrom it in the midline throughout its antero-posterior extent and passing into the optic tracton each side (Fig. 9). Some 15 bundles of adiameter greater than 0-25 mm could be madeout and, as they are traced into the tract, someeight flattened bundles are formed. As in the caseof the crossed fibres entering the chiasma fromthe optic nerve, so in the case of those leaving itto enter the tract, superimposed layers ofbundles could be separated (Fig. 10).

In carrying out the dissections of the inferiorsurface of the chiasma attempts were made toseparate the non-visual fibres of the supraopticdecussations-including Gudden's commisure.Separation of a considerable bundle of fibreswhich, under the dissecting microscope, appearedquite distinct from the visual ones was possible.However, this method of examination is a

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J. E. A. O'Connell

inferiorly. (b) Drawing ofabove dissection. L.O.N. and L.O.T.-eft optic nerve and tract. A.-point ofsectionof superficial group of crossing fibres. B.-distal retracted end of this group. R.C.F.-crossed fibres from rightnerve.

relatively crude one and permits statements con-cerning gross anatomy only.

MICROSCOPIC ANATOMY OF OPTIC CHIASMA Alimited examination alone was carried out. Inthe longitudinal sections of the chiasma andattached nerves and tracts, longitudinally run-ning bundles could be made out in both nerveand tract and the presence of fibrous tissueamong the bundles of the nerve and its absenceamong those of the chiasma and tract was con-

firmed. Again transversely directed fibres wereseen in the chiasma, the arrangement in bundlesbeing no longer obvious. In the median sagittalplane the decussation of fibres could be clearlyseen (Fig. 11). In the lateral chiasma between thetermination of the optic nerve and the origin ofthe optic tract it became impossible to tracefibres in continuity-doubtless because thoseseen were entering or leaving the plane of thesection. Here some fibres were cut transverselyand others longitudinally for short lengths andcurved groups of fibres could be seen. No

716


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FIG. 8. (a) Photograph ofdeeper dissection ofsuperior surface of the chiasma showing interweaving ofbundlesofcrossingfibres, those most laterally placed in the nerve crossing posteriorly. (b). Drawing of(a). L.O.N. andR.O.N.-optic nerves. U.C.F.-uncrossed fibres. R.C.F. and L.C.F.-crossed fibres with side of origin.

attempt to carry out a detailed microscopicstudy was made.

DISCUSSION

A structure such as a nerve coursing betweentwo points of attachment will be capable ofmaximum displacement transversely midwaybetween these points and minimal displacementat the attachments. Should a mass arise alongthe course of the nerve, it is perhaps better toregard the latter as being stretched as a result ofdisplacement rather than as being compressed-unless, of course, it is trapped between the mass

and some related structure.Anteriorly, the intracanalicular portion of the

optic nerve in its 4 to 9 mm course is firmlyattached around its circumference in the opticcanal by adhesion between its sheaths of arach-noid and dura mater (Fig. 1), the nerve and itssheaths filling the canal. The attachment liesfurther anteriorly than it at first appears becauseof the dural fold which passes from the anteriorclinoid process to the tuberculum sellae, whichmay be 5 mm or more in width (Fig. 12). The

internal carotid artery lies beneath the nerve atthis point and medial and, to a lesser extent,lateral mobility of the nerve at the level where a

sellar mass would impinge upon it is greater thanis the case anteriorly at the optic canal.

Posteriorly, the nerve is attached to theanterolateral angle of the chiasma and here theuncrossed fibres take an uninterrupted course

through it into the optic tract. The crossedfibres, however, spread out and intimately inter-weave in the midline of the chiasma with those ofthe opposite optic nerve before being gatheredtogether to enter the contralateral optic tract.This interweaving produces a degree of resistanceto displacement in the medial fibres of the opticnerve and in the chiasma which, as has beenshown, is greater than in their lateral fibres.Moreover, the tension which develops in themedial fibres of nerves and chiasma anchors thelatter and any displacement of the chiasmaapplies a stretching force to the bundles of de-cussating fibres within it. Thus the medial andlateral fibres of the optic nerves and chiasmadiffer not only in the areas of retina from whichthey arise but also in their physical properties

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717

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because of their differing posterior attachments.The possible relationship of these observationsto heteronymous hemianopia will now be con-sidered.

BINASAL HEMIANOPIA Binasal hemianopia hasbeen discussed in a recent paper (O'Connell anddu Boulay, 1973). The literature was reviewedand the commonly held view that the develop-ment of a binasal hemianopia necessitated bi-lateral and symmetrical lesions on the lateralaspects of the optic nerves described. It is note-worthy that, before the development of neuro-

logical surgery, numerous descriptions of thepostmortem finding of grooving of the opticnerves, tracts, and chiasma by the Willisianvessels in the presence of tumours in the sellarregion had appeared; however, these changes hadnot for the most part been related to visual fieldabnormalities. The rarity of the surgical con-firmation of such findings is thought to be due tothe relatively limited operative as distinct fromnecropsy exposures. However, utilization ofmodern methods of investigation can demon-strate the detailed relationships of a tumour tothe Willisian vessels and basal visual pathways.

718


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against the optic nerve or chiasma, or, the fibres of nerve and chiasma are most intimatelyreverse, a displacement of the nervous structures related to the vessels mentioned. These will pro-

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J. E. A. O'Connell

FIG. 12. Photograph of dural fold covering the opticnerve after its escape from optic canal.

tect the medial fibres and be cut into by thestretched pulsating vessels as though by a bluntsaw.

BITEMPORAL HEMIANOPIA It has been widelyaccepted that bitemporal hemianopia in thepresence of a tumour related to the sella turcicais due to compression of the crossing fibres in theoptic chiasma. However a number of authorshave questioned this view. Fisher (1913) re-garded it as glib and asked 'How can a tumouroriginating in the interpeduncular fossa producethe definite and precise effects of a sagittal sectionof the chiasma? It would take almost a knifeedge to do this'. He believed that the visualphenomena in many cases were explained bytraction effects on the visual pathways as thetumour extended upwards behind the chiasma

and between the optic tracts. A tumour herewould, he thought, stretch the decussating fibressituated in the midline and occasion a bitemporalhemianopia without in any way dragging on theuncrossed fibres. Subsequent field disturbancewould result from such dragging on the opticnerve or tract, depending upon the position ofthe major mass of tumour. Fleischer (1914) sup-ported the traction hypothesis. Traquair (1917)discussed the matter in detail and also ques-tioned how the rounded dome of a tumourcould produce a limited anteroposterior knife-edged pressure such as would be required to pro-duce the clear-cut field defects often found. Hedecided that the evidence showed that such pre-cise pressure is not required to produce thesefield defects. He was particularly perplexed bythe cases of bitemporal hemianopia not due tospace occupying lesions which he had studiedand stressed that these must be considered in anyattempt to explain the mechanism of this type offield defect. He thought that the traction explana-tion was open to the same objections as the pres-sure one. The third possibility considered was alocal intoxication arising from a chiasmalneuritis, local inflammatory conditions, or theproducts of tumour degeneration or disturbancesof endocrine secretion. He admitted, however,that in tumour cases mechanical factors could beimportant. Walker and Cushing (1918) in dis-cussing bitemporal hemianopia pointed out thatthe absence of an upper nasal defect when anupper temporal one has developed means thatpressure is not the sole factor at work; tensionand strangulation in the crossing fibres areadditional factors. Tension augments pressureand both lead to strangulation, the latter prob-ably occasioning the physiological block. Theysuggested that the delay in development of anasal defect is occasioned by the additionalpressure and distortion necessary before the un-crossed fibres are affected in the same way. How-ever, in the last half century it appears thatchiasmal compression has become accepted asthe cause of bitemporal hemianopia in tumourcases. Forty years after his earlier communica-tion Traquair in the seventh edition of hisClinical Perimetry (Scott, 1957) appears to haveaccepted pressure as the most attractive hypo-thesis. He did this by taking the view that pres-sure acted not on nerve fibres directly but on

720


their vascular supply, thus occasioning ischaemia.Yet the view that compression is the factor

responsible for bitemporal hemianopia is nomore readily acceptable now than it was half acentury ago and a number of problems arise inconnection with the hypothesis. Although a partof the central nervous system, the optic nervesand chiasma lie outside it, and might be expectedto respond to compression like other cranialnerves. However, the hypersensitivity of itscrossing fibres and the relative insensitivity ofthe uncrossed ones are in contrast with theinsensitivity to pressure of all the fibres of, say,the facial nerve in the presence of large acousticneurinomata. This may be so stretched and com-pressed as to be unrecognizable and yet no facialparesis may be present. Moreover, when in thelatter cases a facial paresis does appear, it affectsall parts of the face equally. Regarding thenerves and chiasma as part of the central nervoussystem, the reaction of their fibres to compres-sion is still in contrast with those of the brain andspinal cord. The clinical localization of masslesions in the cranium and spinal canal dependsupon the functional disturbance occasioned bycompression of the areas most closely related tothe mass. There is here no evidence of differingsensitivities of neurones and the clinical picturedepends upon the intimacy of the relationshipbetween eloquent areas and the compressingagent. The dimensions of the chiasma are smalland yet repeatedly the rounded dome of a sellartumour, although varying in its exact position,will produce a bitemporal hemianopia, with orwithout temporal scotomata and only second-arily and at a late stage will a nasal field lossdevelop. It might be expected that such a massbetween the optic nerves compressing theirinferior and medial fibres would give rise to adefect in the superolateral visual fields whichwould progress inferiorly and medially like atotal solar eclipse until the field loss was com-plete. However, in spite of the smallness of thetarget and the inevitable variation of the site ofthe application of the compressing force, theresult is repeatedly the same, commencing withthe picture of a progressive sagittal section of thechiasma in the midline. After a bitemporalhemianopia is established there is frequently aconsiderable time interval before vision beginsto fail in the inferior nasal quadrants and a fur-

ther delay before the upper nasal quadrantsbecome involved. Moreover as has been said,binasal hemianopia is by itself extremely un-common.

THE TENSION HYPOTHESIS As has been seen theintracranial portion of the optic nerve has twoattachments. Anteriorly, the whole circum-ference of the nerve is attached in the optic canalby adhesion between its arachnoid and duralsheaths and the bone. Posteriorly, the lateralfibres of the nerve are continued into the optictract and the attachment is a relatively lax one.The medial fibres crossing in the chiasma spreadout through it and interweave with the medialfibres of the opposite nerve before being gatheredtogether into the contralateral optic tract. Thisfirm posterior attachment of the medial fibreslies in the median sagittal plane of the chiasmawhere the interweaving of nerve bundles is atmicroscopic (nerve fibre) level. Over half amillion nerve fibres from each optic nerve crossone another in small groups, if not as individualfibres, at angles of up to 90° in the chiasma.Tension developed in these crossing nerve fibresas a result of displacement of optic nerves orchiasma would lead to mutual compression andpossibly even section. There is a further struc-tural reason for chiasmal sensitivity to tension.It has been shown that the tensile strength ofperipheral nerves is particularly dependent on theendoneurium not the epi- or perineurium, northe axons nor their myelin sheaths (Sunderlandand Bradley, 1961a). Spinal nerve roots, in whichthere is a scarcity of endoneurial collagen(Gamble, 1964), resist longitudinal tension lesswell than peripheral nerves because of thisstructural difference. In the case of the opticchiasma the protection from tensile forces is evenless good. While in the optic nerve there arefibrous tissue septa between the groups offuniculi, in the chiasma and tract collagen isabsent and, as in the rest of the central nervoussystem, the supporting tissue is purely glial-especially astrocytic. Thus the posterior attach-ment of the medial fibres of the optic nerve is notonly a firm physical one; because of the angledcrossing of large numbers of closely packed fibresin the median plane of the optic chiasma and thelack of collagenous support for them, it is to be

721

J. E. A. O'Connell

expected that the fibres in the area will be highlysensitive to injury by tension.

Irrespective of the exact site of impingement ofa tumour on the nerves or chiasma, displacementof them applies tension to the crossing fibres inthe midline and defects of function result. Dis-placement laterally of the optic nerves and dis-placement posteriorly or elevation of the chiasmaleads to an increase in tension in the crossingfibres in both nerve and chiasma (Fig. 1). Atumour between the optic nerves and anterior tothe chiasma will thus apply a stretch to theanterior transversely directed crossing fibres ofthe chiasma (especially the inferior ones) andbitemporal field defects (at first superior) willdevelop. As the tumour grows and separation ofthe nerves and posterior displacement of thechiasma proceeds, more posteriorly placed fibreswill in succession be stretched until the mostlaterally placed, which cross in the posteriorchiasma, are involved and a bitemporal hemi-anopia with splitting of the fixation pointsresults. With a more posteriorly placed tumourwhich, at an early stage, elevates the chiasma aswell as displacing it posteriorly, the moreobliquely running and lateral of the crossing fibreswill be stretched and homonymous scotomataare then added to the peripheral field defects.With tumours still more posteriorly placedrelative to the chiasma as a result of prefixationor other cause, the posteriorly placed crossingmacular fibres in the chiasma and medial optictracts will be stretched and homonymous scoto-mata will be the sole field defects. Lateral dis-placement of the optic nerves occasions noincrease in tension in the lateral uncrossed fibresof the nerves and chiasma-indeed, it may havethe reverse effect. Only when the displacement is,as a result of progressive tumour growth, ob-structed by the anterior cerebral or internal caro-tid artery, will notching of the superolateralfibres occur and loss of vision in the inferiornasal quadrants develop. The inferolateral fibres,relatively protected from the vessels, will be thelast to be involved. It is, therefore, believed thatfield defects from chiasmal lesions frequentlydepend upon the development of increased ten-sion in particular groups of crossing fibres andnot simply on which group of fibres is mostclosely related to the tumour. Thus a nodule oftumour beneath the lateral chiasma could dis-

place the uncrossed fibres, occasioning no in-crease in tension within them, nor any functionalloss. If the nodule were anteriorly placed, a lossof the temporal field on the affected side couldbe the sole result and would commence with ahemiscotoma, as the most lateral group of cross-sing fibres would be first affected; if more pos-teriorly placed, the crossing fibres from theopposite side would be involved, and a peripheraltemporal defect in the field of the opposite eyewould be added. At operation in a number ofcases the dome of a suprasellar tumour has beenseen in such a situation, under one side of thechiasma, spreading the uncrossed and crossingfibres apart from one another. The tensionhypothesis may thus not only throw light on thefactors responsible for the development of thecommon form of heteronymous hemianopia,but, in addition, possibly that of other less com-mon field defects, the method of production ofwhich is at present ill-understood.

It is possible that this tension hypothesis hasrelevance in certain traumatic lesions of thechiasma. At times a bitemporal hemianopiadevelops after a severe blunt head injury, fre-quently a frontal one; there may be associatedhypothalamic disturbance. In the majority of thereported cases no evidence of a basal skull frac-ture which could directly injure the chiasma hasbeen found. It is likely that the lesion is due totraction occasioned by movement of the brainwithin the skull, and injury to the anatomicallydelicate and physiologically sensitive area in themedian sagittal plane of the chiasma where thecrossing fibres interweave. While gross tearingof the chiasma can occur, at exploration no suchlesion may be found and the damage would thenappear to lie within the pial sheath. The injury tonerve fibres occasioned by such acutely appliedtension would be likely to be severe and lead tothe permanent bitemporal hemianopia whichusually follows. Traquair et al. (1935) favoureda vascular cause for the field defect in these trau-matic cases, taking the same view which he heldin tumour cases as discussed on an earlier page.It was argued that the chiasma is supplied by arich network of blood vessels derived from theinternal carotid arteries and their branches. Thevessels are firmly attached to the skull base andmovement of the brain and chiasma relative to itmight result in these vessels being torn. It can be

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argued against this vascular hypothesis that thesmall chiasmal vessels referred to are far removedfrom the attachment of the internal carotidartery to the skull base and that any stress result-ing from brain movement would fall on themajor vessel itself. Moreover, the chiasmal vesselslie within the pia mater and arachnoid mater,will move with the structures they supply, and,being extremely tortuous (Dawson, 1958), will beprotected from injury by longitudinal stress.Traquair stated that the obvious common factorshared by the traumatic and 'compression'cases was the vascular supply of the chiasma.The anatomy of the chiasma itself is anothercommon factor, however. The chiasma isanchored to the optic canals by the crossed fibresof the optic nerves interweaving in the mediansagittal plane. Any posterior displacement of thechiasma is likely to injure these fibres by themarked increase in tension which will develop inthem with even minimal chiasmal movement.

It is thus believed that the hypothesis whichhas been discussed explains a number of thepuzzling features of heteronymous hemianopia.In the first place the uncrossed fibres of the opticnerve and chiasma, having no firm posteriorattachment, are relatively lax. They can thereforebe displaced without any tension developing inthem and without any consequent field defect.Only when this displacement is impeded by theinternal carotid or anterior cerebral artery willcompression of the lateral fibres develop and anasal field loss result. Secondly, the crossingfibres in the optic nerves are by comparisonresistant to displacement because of their inter-weaving with the crossed fibres from the oppositenerve in the median plane of the chiasma. Dis-placement laterally of the nerves or posteriorlyof the chiasma, whether produced acutely byhead injury or gradually by a tumour mass,applies tension to the crossing fibres and thetemporal visual field loss develops on a readilyunderstandable anatomical basis. This tensionwill develop in the crossing fibres in the medianplane of the chiasma in spite of variations in theexact size, shape and position of the tumourdome.It is a pleasure to express my debt to those who havehelped in the course of this work. Dr. R. Curetonand Dr. A. G. Stansfeld kindly provided me withexcellent facilities in the Department of Histo-

pathology of the hospital. In the Department ofMedical Illustration skilled photography was alwaysavailable. The work of Miss Susan Hales was of thehighest value, as the published drawings reveal.

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Dawson, B. H. (1958). The blood vessels of the human opticchiasma and their relation to those of the hypophysis andhypothalamus. Brain, 81, 207-217.

Duke-Elder, Sir S., and Scott, G. I. (1971). Neuro-ophthal-mology. In System of Ophthalmology. Vol. 12, p. 292.Kimpton: London.

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O'Connell, J. E. A., and du Boulay, E. P. G. H. (1973).Binasal hemianopia. Journal of Neurology, Neurosurgery,and Psychiatry, 36, 697-709.

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Traquair, H. M. (1917). Bitemporal hemiopia: the laterstages and the special features of the scotoma. BritishJournal of Ophthalmology, 1, 216-239, 281-294, and 337-352.

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Wilbrand, H., and Sanger, A. (1904). Die Neurologie desAuges. Bergmann: Wiesbaden. Vol. 3, Abt 1, pp. 98-etc.

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