binasal hemianopia - bmjrelationship to the production of visual field defects. it is probable that...

Journal of Neurology, Neurosurgery, and Psychiatry, 1973, 36, 697-709

Binasal hemianopiaJ. E. A. O'CONNELL AND E. P. G. H. DU BOULAY

From St. Bartholomew's Hospital, London

SUMMARY Three patients with nasal visual field defects are described. In each case it is believedthat compression of the lateral fibres of the optic nerve by the anterior cerebral or internal carotidartery was the cause. Binasal hemianopia can thus be produced by a single lesion and is as much a

true hemianopia as the common bitemporal one. The value of careful neuroradiological investiga-tion to display the relationships of a tumour to the chiasma, optic nerves, and related vessels andthus explain the field defects is demonstrated.

Isolated loss of the nasal fields of vision is un-common. In Traquair's Clinical Perimetry (Scott,1957) it is stated that the term hemianopia isbest reserved for bilateral field defects producedby a single lesion-chiasmal or suprachiasmal.This author considered that binasal hemianopiawas not a true hemianopia, since its occurrenceindicated bilateral involvement of the uncrossedfibres at the chiasma, the crossed fibres remainingintact. This could be due to displacement of theterminal parts of both optic nerves against theinternal carotid arteries in internal hydroceph-alus: atherosclerosis or aneurysms of bothinternal carotid vessels could lead to compressionof the nerves in the same area: symmetricalgummata or other lesions could produce a simi-lar result. Duke-Elder (1971) also takes the viewthat bilateral symmetrical lesions at the chiasmaare necessary for the development of binasalhemianopia, though he suggests that bilateraloccipital lesions might produce such a defect.From a list of possible causes those due to pres-sure which are mentioned are atherosclerosis oraneurysms of the internal carotid arteries com-pressing the terminations of both optic nerves,or displacement of the nerves against blood ves-sels by a local tumour, or a distended thirdventricle in internal hydrocephalus. It is hopedto show in this paper that binasal hemianopiacan result from a single intracranial lesion andthat, though rare, it is therefore as much a truehemianopia as is the common bitemporal one.Although the importance of ophthalmoscopy

and visual field examination began to be recog-

nized during the middle of the last century, thepassage of time and increasing experience werenecessary before these methods of clinicalinvestigation could be fully utilized. During thesame period the pathology of compression of theoptic nerves and chiasma attracted considerableattention and a number of descriptions of in-volvement of these structures by the Willisianvessels, observed at postmortem examination,appeared. In 1852 Turck described the necropsyfindings in a patient with a pituitary tumour whohad total blindness in the left eye with someresidual vision in the right one. Both opticnerves were deeply grooved by the anteriorcerebral arteries and on the left the optic tractwas also grooved; at the site of compression theleft optic nerve was reduced to an empty sheath-there being preservation of the medial one-thirdof the right one. Knapp (1873) had stated thatsclerosis of the intracranial internal carotidarteries could cause blindness. Smith (1905)reported his postmortem finding of atrophy ofboth optic nerves secondary to atherosclerosis ofthe supraclinoid portion of the internal carotidarteries. He had seen altogether six such cases indissecting and postmortem rooms in Cairo. Herefers to the second edition of Fuchs's Textbookof Ophthalmology published in the United Statesin 1903 as also describing such cases. In 1905Sachs, cited by Fay and Grant (]928), describedthe necropsy findings in 48 cases of pituitarytumour and noted that in two the optic nerveswere grooved by the anterior cerebral arteries.Bartels (1906) cited Erdheims's account of seven

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J. E. A. O'Connell and E. P. G. H. du Boulay

necropsy specimens in which the optic nervesand chiasma were grooved by the anteriorcerebral or anterior communicating arteries, orthe optic tract by the internal carotid artery.Bartels also described a case of his own in whichthe tracts were grooved by the anterior cerebralarteries. Uhthoff (1911) described another suchcase where the anterior cerebral arteries con-stricted the optic nerves.

Sixty years ago, in a number of papers,Cushing and Walker reported their carefulstudies of the visual fields in patients with intra-cranial tumours; among these was one on binasalhemianopia which appeared in 1912. Theseauthors had noted in patients with optic atrophysecondary to high intracranial pressure that,where some vision had been retained, this wasusually in the temporal fields. They suggestedthat the uncrossed fibres at the chiasma weredamaged not only at the nerve head by develop-ing scar tissue here, but also close to the chiasmaas a result of pressure of the nerves against theinternal carotid arteries occasioned by distensionof the third ventricle. The visual acuity was soreduced in their patients that it is extremelydoubtful if the small islands of temporal visionwhich persisted justified the diagnosis of binasalhemianopia. Hirsch (1921) described groovingof both optic nerves by both anterior cerebralarteries and of the chiasma by the anterior com-municating artery as a necropsy finding in apatient with a pituitary tumour. Fay and Grant(1923) provided a good review of the literatureand described a case of interpeduncular cranio-pharyngioma, which had displaced the chiasmaanteriorly so that the optic tracts on both sideswere grooved by the internal carotid arteries,the left one also being grooved by the anteriorcerebral artery. These authors attributed thenasal portion of the severe visual field disturb-ance in this patient to vascular notching of theoptic tracts. Balado and Malbran (1933a, b)discussed involvement of the optic nerves, tracts,and chiasma by related arteries in the presence ofpituitary tumours, cerebral tumours in varioussituations, and internal hydrocephalus. In addi-tion, they describe three patients with binasalhemianopia and in each case relate this to thepostmortem findings. Rucker and Kernohan(1954) described the necropsy findings in fivepatients who died after surgery for pituitary

tumours; excellent photographs demonstrate thegrooving of the nervous structures by the vascu-lar ones in these postoperative cases. Therelationship of the findings to the field changes isby no means clear and it would seem that post-operative swelling of residual tumour tissueaccounted in some measure for the postmortemfindings.

It will be seen that all these studies have beenbased on the examination of postmortemmaterial. With the development of neurologicalsurgery, and resulting accurate diagnosis andsuccessful surgical treatment of tumours in thesellar region, the opportunities for such post-mortem examinations dwindled. At the sametime the surgeon's goal would be to excise thelesion with minimal trauma and the wide expo-sure possible at postmortem examination wouldnot be obtained. Perhaps for this reason, rela-tively little has been said in recent years concern-ing the involvement of the optic chiasma, nerves,and tracts by related vessels and its possiblerelationship to the production of visual fielddefects. It is probable that many surgeons havenoticed grooving of the optic nerve or chiasmaby the anterior cerebral artery in operating forpituitary tumours and some have related this toan inferior quadrantic nasal field defect on theaffected side (Schneider et al., 1970).Modern methods of investigation and their

skilful interpretation can provide very fullinformation concerning the relationship of theoptic nerves and chiasma, the Willisian vessels,and an associated tumour mass to one another.With the single exception of the bony landmarksof the posterior margin of the optic canal in thelateral skull view, the radiological anatomy iswell known. The pneumographic appearance ofthe chiasmatic cistern was described in detail byDilenge in 1955 and by Liliequist in 1959. Bull(1955) has made a detailed study of the normalvariations in the position of the optic recess ofthe third ventricle and Gado and Bull haverecently (1971) shown the many ways in whichthe anterior cerebral arteries may be displacedby suprasellar masses.

In the normal patient the pre-communicatingparts of the anterior cerebral arteries bear afairly constant relationship to the chiasma andmay, of course, be demonstrated by angio-graphy, but a more precise localization of the

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FIG. 1. Case 1. Fields of vision.

optic chiasma and the intracranial parts of theoptic nerves is possible both in the normal andthe abnormal patient by observation of certainother anatomical features shown during thevarious radiographic examinations. At bothpneumoencephalography and ventriculographythe impression made by the optic chiasma on theanterior end of the third ventricle, between theoptic and infundibular recesses, may, itself,regularly be seen. This is true, also, in abnormalcases when the chiasma and the third ventricleare displaced. It may be seen in the cases underdiscussion here and it locates the transverse limbof the optic chiasma for the observer. Atpneumoencephalography also, that part of thechiasma which extends forwards as well as theintracranial portions of the optic nerves may bedirectly outlined by air in the chiasmatic cistern,and may be seen, when normal, without thebenefit of tomography in lateral views; buttomography is a valuable addition in confirmingthe exact limits ofthe shadows and in distinguish-ing right from left. In anteroposterior projectionstomography is essential in determining the pre-cise positions of the nerves within the normal air-filled chiasmatic cisterns.When the optic nerves are stretched over the

surface of a* tumour it becomes impossible tosurround them with air so that they, themselves,can only rarely be seen for any considerable part

of their course, even after the most carefulpneumography in cases of pituitary adenoma.However, just as the attachment of the chiasmato the third ventricle can virtually always beshown even in abnormal cases (by pneumo-encephalography), so can the anterior point ofattachment of the optic nerves at the skull basebe recognized on lateral plain radiographs andthis localization may be transferred to lateralangiograms and lateral air studies. Accuratelycentred, good quality lateral radiographs of theskull show a short, fine, curved line-shadow justanterior to and approximately on a level withthe lower border of the anterior clinoid process.This line represents either the posterior, superior,lateral or the posterior, superior, medial marginof the optic canal, depending upon the indi-vidual skull. The optic nerve is below it, or insome cases crosses its down-turned curve. Theline-shadow has been used in the present cases tofix the point from which the optic nerve emergesinto the cranial cavity. In anteroposterior andposteroanterior skull views the position of theoptic canal may also be defined even if its detailsare invisible, chiefly by its relationship to theanterior clinoid process. The ophthalmic arteryis almost invariably filled with contrast mediumduring carotid angiography and may be seen inlateral, Towne's, and other anteroposteriorviews. It has a recognizable osseous portion.

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FIG. 2. Case 1. (a) Left carotid angiograms: A.P. (b) Explanatory drawing showing displacenzent of leftanterior cerebral artery across the line of the left optic nerve.

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FIG. 3. Case 1. (a) Left carotid angiogram: lateral. (b) Explanatory drawing: this traces the carotid syphon,ophthalmic artery, andprecommunicating part of the anterior cerebral artery and shows how the line ofthe opticnerve must be altered by fAe medial displacement of the inferiorly situated anterior cerebral artery.

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This also serves to fix the situation of the opticnerve in any angiographic projection.

Thus, by a combination of radiographictechniques it is possible either to predict theexact course of, or actually see, the intracranialportions of the optic nerves and the opticchiasma in patients who undergo neuroradio-logical investigation for tumours in the sellarregion. It should be noted that these samemethods demonstrate and localize the terminal

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FIGS 4 (top) and 5 (bottom). Case 1. Drawings ofmneningioma of falx cerebri and nteurovascularrelationiship.

carotid artery and the circle of Willis as fillingdefects in the air, or as direct contrast-filledshadows and, in this way, the relationships of thenerves and the arteries may be worked outbefore operation. By utilizing such informationit is believed that the mechanism of nasal hemi-anopia has been explained in the three cases nowto be described.

CASE 1

E.P., a 53 year old woman, was admitted to St.Bartholomew's Hospital on 2 November 1962 com-plaining of urinary incontinence. For 38 years shehad been increasingly troubled by hirsutes. She hadhad one normal pregnancy and had menstruateduntil the menopause at 48 years. Endocrine functionhad been investigated in hospital three years beforeand found normal, though the pituitary fossa wasconsidered to be enlarged, and a diagnosis of pitui-tary tumour had been made. Since there was then nosubjective or objective visual disturbance, furtherinvestigation with a view to surgical treatment wasnot considered. For two years before her latestadmission she had suffered from urinary inconti-nence, mainly but not invariably nocturnal. Over thesame period she became forgetful, lethargic, apa-thetic, and unsteady when walking. After exclusionof the diagnosis of adrenal tumour, she was referredfor neurosurgical investigation. There was nothingrelevant in the past or family history.

Examination revealed a retarded woman withdifficulty of comprehension, slowness of thought,disorientation, and dyscalculia; she was euphoric andlacking in insight. Facial hirsutes was well marked.There was no swelling of the optic discs and thevisual acuity was normal; the left visual field showedan inferior quadrantic nasal defect (Fig. 1). Therewas a left central facial paralysis, a spastic weaknessof the right arm, largely proximal, with ataxia and acoarse tremor. In the lower limbs there was spasticity,ataxia, and moderate weakness greater on the rightside. There was no demonstrable sensory disturb-ance, but a grasp reflex on the right with generalhyper-reflexia, reduced right abdominal reflexes, andflexor plantar responses. The gait was ataxic on awide base and Romberg's sign was positive. Skullfilms revealed that the posterior clinoid processes anddorsum sellae had disappeared and there was a largesphenobregmatic sinus. Carotid angiography re-vealed a very large left frontoparietal tumour. It dis-placed the falx cerebri to the right and there was agross displacement of the left anterior cerebralartery far to the right of the midline (Figs 2 and 3).A left frontoparietal flap was made and, when the

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IoddIFIG. 6. Case 2. Fields of vision.

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FIG. 7. Case 2. (a) Pneumoenzcephalogram, lateral, brow up. (b) Explanatory drawing: the third ventricle isdisplaced inferiorly and posteriorly and its optic recess marks the position of the chiasma. The commencementof the intracranial course of the optic nerve is shown by the margin of the anterior clinoid process; posteriordisplacement of the frontal horns of the ventricles is also shown.

tense dura mater had been relaxed by the injection of40 g urea intravenously, reflection of the dural flaprevealed a walnut-sized tumour attached to the con-vexity dura mater beneath the skull peripheral to theanterior limb of the bone flap. This was excised anda firm mass was palpated beneath the wide flatconvolutions in the anterior part of the exposure.Retraction of the cerebrum from the falx exposed the

nodular surface of a globular tumour. This wasgently separated from the cerebrum which it exca-vated deeply. The interior of the mass was removedwith cutting diathermy and suction and the capsulefolded in upon itself. It separated readily from thesuperior sagittal sinus and falx cerebri as far as itsplace of attachment near the free margin of thelatter. The falx was incised at this point and a very

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large right-sided mass was seen excavating the rightcerebrum. After removal of the interior of this por-tion of the tumour, the capsule on the right side wasseparated from the right cerebrum. A large branchof the left anterior cerebral artery entering the cap-sule was clipped and divided, the right artery beingcarefully preserved. The capsule was now peeledfrom its remaining attachments and removed, andthe usual closure effected. Both tumours weremeningiomata, in part fibroblastic and in parttransitional.

Postoperatively, consciousness was not fully re-gained for 15 days. Then the pupils remained largeand poorly reacting and there was a paralysis of up-ward eye movement. These changes were thought tobe due to oedema of the brain-stem which had almostcertainly been distorted preoperatively by this verylarge tumour. The paralysis slowly cleared over anumber of weeks and all evidence of frontal lobedeficit disappeared. Nine years postoperatively she

remained extremely well from the neurosurgicalviewpoint, although after six years she had surgicaltreatment for a fibrosarcoma of the terminal ileum.The tumour here did not reach the skull base and

the chiasma was not seen. Examination of the carotidangiogram permits placement of the left optic nerveand indicates that it lies in the groove between thestretched left anterior cerebral artery and the medi-ally displaced left internal carotid artery. Thus canbe explained the inferior nasal quadrantic defect inthe left visual field (Figs 4 and 5), the superior un-crossed fibres in the nerve having been compressedby the anterior cerebral artery.

CASE 2

A.B., a 53 year old woman, was admitted to theDepartment of Neurological Surgery of St. Bartholo-mew's Hospital on 30 December 1968 complainingof failing vision. She stated that during the previous

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FIG. 8. Case 2. (a) Left carotid angiogram: A.P. (b) Explanatory drawing: the anterior cerebral artery istightly stretched inferior and posterior to the tumour. The position of the ophthalmic artery indicates the com-mencement of the intracranial course of the optic nerve.

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FIG. 9. Case 2. (a) Left carotid angiogram: lateral. (b) Explanatory drawing: the most displaced branches ofthe anterior cerebral artery, the carotid syphon and the ophthalmic artery are shown. The ophthalmic arteryindicates the site of the optic foramen and the position of the precommunicating part of the anterior cerebralartery indicates downward displacement of the structures in this region.

five months sight had progressively deteriorated inboth eyes. Near and distant vision were equallyaffected and she had become unable to distinguishcolours. There had been no associated headache,sickness or diplopia, nor any symptoms of endocrinedisturbance. Indeed, the only other symptom was a

reduction of the sense of smell. In the past history a

point of significance was the occurrence of twogeneralized epileptic attacks two and a half yearsearlier. No abnormality was found at the time apartfrom some left temporal paroxysmal features atelectroencephalography. With anti-convulsant medi-cation there was no recurrence of attacks.

Examination revealed a well-orientated, heavilyfeatured, obese woman with no abnormality on

general examination. The abnormalities on neuro-

logical examination were a bilateral anosmia, somepallor of the left optic disc, and a marked reductionof visual acuity-VAL less than 6/60ths and J 20;VAR less than 6/60ths and J 18 (corrected). The visualfields showed an inferior nasal quadrantic defect on

the left and a less well-defined nasal defect on theright (10 mm test object); with a 3 mm test object theleft-sided quadrantic defect was seen to involvecerrtral vision and there was some diminution of thetemporal field, while on the right a pericentralscotoma was added to the nasal defect (Fig. 6). Theonly other neurological abnormality was a possibleleft central facial paresis. Lumbar puncture revealedthat the cerebrospinal fluid (CSF) had a pressure of200mm CSF and a protein content of 116 mg/100 ml.

Plain skull films showed an area of sclerosis of theskull base in the posterior ethmoidal region, andpneumoencephalography a bilateral subfrontal masslarger on the left side (Fig. 7). Bilateral carotidangiography confirmed that these appearances sug-gested the presence of an olfactory groove menin-gioma (Figs 8 and 9).A right frontal craniotomy was effected, the

tumour palpated through the frontal lobe with acannula, and the right frontal horn tapped of a fewml. CSF. After reflection of the dural flap and eleva-tion of the frontal lobe, a large lobulated tumour wasrevealed. A small (37 g) frontal lobectomy was madeand piecemeal removal of the interior of the tumourcommenced. This was achieved in the case of boththe left and right portions of the mass. Its attachmentto the cranial floor posterior to the cribriform plateswas now separated and, after splitting the falxcerebri to improve the exposure, the capsule waswithdrawn from beneath both frontal lobes toexpose the suprasellar portion. After clipping onebranch of the right anterior cerebral artery, separa-tion was effected here and the capsule removed. Adistinct tumour 1 cm in diameter attached to thedura mater over the left orbital roof was also excised.The enostosis on the cranial floor was heated withcoagulating diathermy and covered with a pericranialgraft and the wound closed in the usual manner.Histologically, the tumours were meningiomata ofsyncitial type.The relationship of the tumour to the optic nerves

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internal carotid artery was tethered medially by thestretched anterior cerebral arteries and had notchedthe lateral aspects of both optic nerves. Postopera-tive recovery was rapid with full restoration of visualacuity, but a persistent inferior nasal quadranticfield defect on the left.

Study of the pneumoencephalogram and the caro-tid angiogram here reveals the relationship betweenoptic nerves and internal carotid arteries seen atoperation. Compression of the lateral portions of thenerves by the internal carotid arteries is believed tobe the explanation of the binasal hemianopia.

FIGS 1O (top) and 11 (bottom). Case 2. Drawings ofolfactory groove meningiolna anid nieurovascularrelationlships.

and chiasma was of particular interest (Figs 10 and1 1). The chiasma was postfixed and the hyphophysealstalk was seen emerging from beneath it and passinganteriorly to penetrate the sellar diaphragm. Theoptic nerves had been arched inferiorly by thetumour and also displaced laterally on to the internalcarotid arteries whose medial margins could be seenmedial to the nerves anteriorly. Posteriorly, each

CASE 3

J.Z., a 50 year old man, was admitted to St. Mary'sHospital, London, under the care of Dr. RogerBannister on 26 October 1968, complaining ofblurred vision. For five weeks vision in his left eyehad progressively deteriorated and for two weeksright vision had been involved. There were no othersymptoms, in particular none of headache or endo-crine disturbance. There was nothing significant inthe past history.He was a fit-looking man without clinical evidence

of endocrine abnormality. The abnormal physicalsigns were confined to visual function, the acuity onthe right being 6/18 and on the left 6/36. There waspallor of the optic discs, greater on the left, and visualfield examination showed a heteronymous hemi-anopia, the inferior nasal quadrants being thoseaffected (Fig. 12). Investigation showed normalthyroid function but reduced 24 hour urinaryexcretion of ketosteroids, and a plasma cortisol atthe lower level of normal. Skull films showed agreatly enlarged sella turcica, and pneumoencephalo-graphy a defect in the anterior third ventricle (Fig.13). Bilateral carotid angiography revealed lateraldisplacement of the termination of the internal caro-tid arteries and elevation of the first part of theanterior cerebral arteries-the right being hypo-plastic (Figs 14 and 15).

Operation was carried out by Mr. L. Walsh.Through a right frontal exposure it was seen that theright optic nerve was elevated and displaced laterally,with the tumour and internal carotid artery lyingbeneath it. An intracapsular excision of the tumour(a chromophobe adenoma) was effected and thechiasma and right optic nerve satisfactorily de-compressed. The posterior end of the left nerve wasseen and appeared normal. Closure was theneffected and the patient subsequently received acourse of irradiation to the sellar region. He madeexcellent progress and on examination one year laterhis visual acuity was normal, though bilateral inferior

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FIG. 12. Case 3. Fields ofvision.

(a) (b)FIG. 13. Case 3. (a) Pneumoencephalogram, lateral, brow up. (b) Explanatory drawing: the anterior end of thethird ventricle, indented and displaced, shows the position of the optic chiasma. The suprasellar portion of thetumour is outlined by air which also reveals the shadows ofparts of the circle of Willis lying on its surface.

nasal quadrantic defects were still present but re-duced.

Study of the pneumogram and the carotid angio-gram here shows that the right optic nerve is crossedon its lateral aspect by the stretched anterior cerebralartery arching over the tumour (Figs 16 and 17). Itis believed that this explains the right inferior nasalquadrantic field defect and that there was a similarexplanation for the same type of field defect on theleft side.

DISCUSSION

As has been described, compression of the opticnerves, tracts and chiasma by the Willisianvessels is a well-documented postmortem finding.It may result from aneurysms or sclerotic changesin the internal carotid arteries or displacement ofthe nerves against vessels, or of the vesselsagainst the nerves. As a result of such displace-ments, the vessels have been shown to cut deep

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FIG. 14. Case 3. (a) Left carotid angiogram: A.P. (b) Explanatory drawing: the precommunicating parts ofboth anterior cerebral arteries are stretched by the tumour beneath them.

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FIG. 15. Case 3. (a) Right carotid angiogram: lateral. (b) Explanatory drawing: the course of the anteriorcerebral artery displaced superiorly and posteriorly is indicated-it is but faintly outlined in the angiogram.The osseous portion of the ophthalmic artery places the optic canal on the film.

grooves in the nervous structures. Neurosurgeons of nerves to blood vessels and tumour, demon-have at intervals remarked upon such groovings stration of the vascular cause of certain fieldobserved at operation and occasioned by the defects is possible. This has been done in theanterior cerebral artery. For the most part they three patients described above and the possi-have not been related to visual field defects bility of binasal hemianopia being the result of awhich may have existed. When the fields have single lesion, and thus a true hemianopia hasbeen charted and preoperative investigation been demonstrated. In case 1 a great medial dis-carried out to display the detailed relationships placement of one anterior cerebral artery by an

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FIGS 16 (top) and 17 (bottom). Case 3. Drawings ofchromophobe pituitary adenoma and neurovascularrelationships.

enormous falx meningioma caused it to groovethe superolateral aspect of the related opticnerve and produce an inferior quadrantic nasaldefect in the field of the ipsilateral eye. In case 2an olfactory groove meningioma displaced theoptic nerves inferiorly and laterally-the post-fixation of the chiasma permitting this displace-

ment without the development of a bitemporalhemianopia. The internal carotid arteries weredrawn medially by the stretched anterior cerebralvessels and the lateral fibres of the optic nervesgrooved by them with the production ofa binasalhemianopia. In case 3 the radiological findingsagain suggest lateral displacement of the opticnerves against the stretched anterior cerebralarteries with grooving of their lateral aspects.Possibly an unusually intimate relationship be-tween the anterior cerebral arteries and the opticnerves caused arterial compression of thembefore the degree of displacement necessary toproduce a bitemporal hemianopia had occurred.The common type of heteronymous hemi-

anopia is a bitemporal one; the binasal one israre and may be produced by the mechanismdescribed. The relative insensitivity of the lateralfibres of the optic nerves and chiasma to thepresence of a suprasellar mass suggests that insome respects they differ from the medial fibres.Observations, which will be discussed in a subse-quent paper, suggest that this difference lies inthe more rapid development of internal tensionin the medial fibres after displacement thanoccurs in the lateral fibres. However, if the nerveis brought into contact with one of the Willisianvessels lying on its lateral aspect, the lax lateralfibres are compressed between the artery and themore tense medial fibres and a sharply localizedpulsatile vascular compression results; this dis-torts nerve fibres acutely and occasions disturb-ance of their function. Conditions such as a post-fixed chiasma and optic nerves which are readilydisplaceable or an existing intimate relationshipbetween nerve and artery will make the develop-ment of such vascular compression the morelikely. Absolute proof of this mechanism of pro-duction of a binasal hemianopia would necessi-tate the demonstration of the neurovascularrelationships with appropriate grooving of thenerve or chiasma at operation or necropsy. It isbelieved, however, that the evidence presentedhere goes some way to demonstrate a cause andpossibly the only common cause of binasalhemianopia.

The authors would like to express their gratitude toDr. Roger Bannister and Mr. L. Walsh for per-mission to examine the records of case 3 and publishan account of the case. They are also much indebted

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to the Department of Medical Illustration of theHospital for the preparation of the photographicplates and to Miss Susan Hales for her carefuldrawings.

REFERENCES

Balado, M., and Malbran, J. (1933a). Contribuci6n a laestructura del quiasma 6ptico humano (hemianopsiabinasal; estudio anatomo-clinico). Semana Medica(Butenos Aires), 2, 797-807.

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