tournal ofNeurology, Neurosurgery, and Psychiatry 1996;60:275-280

NEUROLOGICAL INVESTIGATIONS

Investigation of visual loss: neuro-ophthalmologyfrom a neurologist's perspective

Christian J Lueck

A large proportion of the human nervoussystem is devoted to receiving and processingvisual information.' This means that thepotential for CNS disease to produce visualdisturbance of one sort or another isenormous. The great advantage to theclinician dealing with a visual disturbance isthat the visual pathways are organised veryprecisely, with preservation of topographicrelation from retina through optic nerves,chiasm, tracts, geniculate nuclei, radiations,and on into the visual cortex.2 Hence, carefulattention to visual field disturbance is likely togive a fairly precise clue as to what part of thevisual system is being affected.

Because certain diseases are more likely toaffect some parts of the visual system than oth-ers, it is often possible to narrow down a dif-ferential diagnosis simply on the basis of site oflesion. This process of restricting the differen-tial is further enhanced by knowing the timecourse over which visual disturbance hasdeveloped. These two considerations make theinvestigation of such disturbances much morestraightforward.

Differential diagnosis ofvisual lossVisual loss has a wide differential diagnosis(table 1). Visual loss secondary to trauma, orlongstanding, non-progressive visual loss (forexample, amblyopia),3 is not consideredfurther here; nor is visual loss secondary todiseases of the eye itself, to visible disturbanceof the retina, or secondary to psychogeniccauses.4

VISUAL FIELD LOSSAssessment requires accurate visual fields.'

Department of ClinicalNeuroscience, WesternGeneral HospitalsTrust, Crewe Road,Edinburgh EH4 2XU,UKC J Lueck

Retinaloptic discOphthalmoscopically visible lesions of theretina or choroid are likely to produce focalscotomatous field loss. Patterns of visual fieldloss likely to be encountered by a neurologistand referable to disorders of the retina or opticdisc include central visual field loss (macularlesion), concentric visual field loss, sectoralvisual field loss, altitudinal field loss, and arcu-ate scotomata. Bilateral macular sparinghomonymous hemianopias may masqueradeas bilateral concentric field reduction. There isusually a small step at the vertical meridian if

visual fields are performed carefully.

Optic nerveldiscAlthough the classic pattern of visual field lossis a central scotoma, optic nerve disease cangive rise to various field defects including arcu-ate scotoma, centrocaecal scotoma, paracen-tral scotoma, and altitudinal field defects. Thepattern of monocular visual field loss is of lim-ited utility in distinguishing one disease fromanother.6 Swelling of the optic nerve head maygive rise to an enlarged blind spot.

Optic chiasmCompression of the central chiasm typicallygives rise to a bitemporal hemianopia. Thereare, however, considerable variations on this,including bitemporal partial field defects(which may be considerably asymmetric), andjunctional scotomata. It is important to exam-ine the upper temporal visual field of the"good eye" in a patient presenting with appar-ent unilateral visual loss. A binasal visual fielddefect has been reported to occur with side-ways compression of the chiasm (in theorypicking out the uncrossed fibres of each side),but this is rare.7 Damage to the whole opticchiasm will, of course, give rise to total bilat-eral visual field loss.

Retrochiasmal lesionsCongruity of homonymous defects tends toincrease as the lesion becomes more posterior,but this is by no means an absolute rule.Lateral geniculate lesions tend to give rise to"wedge hemianopias". Lesions in the opticradiation can give rise to homonymous upperor lower quadrantanopias. Occipital lobelesions typically produce congruous hemi-anopias which may or may not be macularsparing. If the lesion in the occipital lobe isvery posterior, it may spare the cortical repre-sentation of the far lateral visual field, therebyproducing sparing of the temporal crescent ofthe contralateral eye.

TIME COURSEThe temporal evolution of visual loss may bedivided into four groups: transient (reversible),irreversible onset over seconds/minutes,progressive onset over hours/days, or progres-sive onset over days to months or longer.

It may be difficult to be confident in allocat-

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Table 1 Differential diagnosis of visual loss based on site and timing of lesion

Site of lesion Transient Seconds to minutes Hours to days Days to months +

Retinal Amaurosis fugax: CRAO/BRAO "Phlebitis" Macular degenerationsThromboembolic CRVO/BRVO Ischaemia Hereditary retinal disease:Benign (young) Retinitis pigmentosa

Retinal migraine Cone/rod dystrophiesPhotostress (ischaemia) Storage disordersHemeralopia (dystrophies) Neurodegenerative conditionsAngle closure glaumoma Uveomeningeal syndromes

Carcinoma associated retinopathyOptic nerve/ AION (vasculitic) Abnormal fundus: Typical ON Compressive:optic disc Obscurations AION: Atypical ON: Neoplasm

(papilloedema) Vasculitic Immune Thyroid eye diseaseNon-vasculitic Infective Aneurysm

Leber's HON Leber's HON GranulomaDisc haemorrhage Neuroretinitis Paget's diseaseDiabetic papillopathy Paraneoplastic Infective/inflammatory:Big blind stop syndrome Toxic SyphilisNeuroretinitis HTLV 1 (TSP)Optic nerve head drusen Tuberculosis

Normal fundus: Orbital/paranasal sinus infectionPION Sarcoidosis(compressive lesion) Leber's HON

InfiltrativeToxic/nutritional (B12)Hereditary neuropathyChronic papilloedemaPost irradiationAVMOptic disc dysplasia

Optic chiasm Cystic tumours Pituitary apoplexy Pituitary tumour Compressive:Craniopharyngioma Ruptured AVM (pregnancy) NeoplasmMucocele Pituitary abscess Sphenoidal mucocele

Sphenoidal abscess Dilated IlIrd ventricleDemyelination Granuloma (sarcoid, TB)Adenohypophysitis Aneurysm

Primary hypothyroidismThalassaemia

Postradiation damageSubacute/chronic meningitisSepto-optic dysplasiaEmpty sella syndrome

Retrochiasmal TIA CVA: Demyelination Tumour:pathways Migraine Thromboembolic Cerebral abscess Intrinsic

Epilepsy Haemorrhage Tumour ExtrinsicTrauma (children) (tumour) Poisoning AVM

Spasm (angiography) Meningitis Creutzfeldt-JakobMigraine Encephalitis Pelizaeus-MerzbacherImpaired cerebral perfusion Metachromatic leukodystrophy

Progressive multifocal leukoencephalopathySubacute sclerosing panencephalitisSchilder's disease

AION = anterior ischaemic optic neuropathy; AVM = arteriovenous malformation; BRAO = branch retinal artery occlusion; BRVO = branch retinal vein occlu-sion; CRAO = central retinal artery occlusion; CRVO = central retinal vein occlusion; CVA = cerebrovascular accident; HON = hereditary optic neuropathy;HTLV 1 = human T lymphocytic virus, type I; ON = optic neuritis; PION = posterior ischaemic optic neuropathy; TB = tuberculosis; TIA = transient ischaemicattack; TSP = tropical spastic paraparesis.

ing the patient's symptoms to one specificgroup, and more than one differential diagnos-tic category may therefore have to be consid-ered. A typical example of this would be whena patient awoke with visual loss in one eye-this could have arisen suddenly over secondsto minutes during the night, or, alternatively,have developed over several hours. All possiblediagnoses in both groups would then have tobe considered.

Investigation of visual lossTRANSIENT VISUAL LOSS IN ONE EYEBy definition, the visual fields will be normalin relation to a transient attack. It is thereforecrucial to try to determine the nature of thevisual field disturbance on the basis of the his-tory. Patients are notoriously unable to tell thedifference between visual disturbance in onevisual hemifield versus that in one eye (amaurosisfugax), and it can be very difficult to sort thisout. Apart from asking the patient whetheralternate eye closure was attempted, it is oftenhelpful to ask them what their vision was likeduring the attack: I usually ask them if, werethey to look directly at my face during an

attack, would the whole face be normal orwould one half of it be affected?

Whether or not the transient visual field losswas referable to the eye or to the occipital cor-tex, preliminary investigations are similar,looking for causes of transient ischaemicattack.89 Such tests include haematological(full blood count, erythrocyte sedimentationrate, clotting) and biochemical (glucose, urea,and electrolytes, cholesterol) blood tests, chestradiography, ECG, and cranial CT.Depending on circumstances, other more spe-cific tests may be indicated.89

Carotid ultrasound (Doppler or duplex)examination is appropriate if the visual lossaffected one eye, if the patient would entertainthe idea of an endarterectomy. The degree ofstenosis must be more firmly established withfurther imaging, such as selective carotidangiography, but some centres are increasinglyusing MR angiography.9

Amaurosis fugax may occasionally be theharbinger of irreversible visual loss in patientswith giant cell arteritis.10 If there is any sugges-tion of headache, jaw claudication, or generalmalaise, an erythrocyte sedimentation rateshould be performed as an emergency. If it is

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Investigation of visual loss: neuro-ophthalmology from a neurologist's perspective

not raised, but the story is suggestive, strongconsideration should be given to temporalartery biopsy because up to 17% of cases arenot associated with raised erythrocyte sedi-mentation rate.'0

In young people, recurrent monocularvisual loss is often a benign condition, unasso-ciated with an increased risk of stroke. It isprobably due to vascular spasm, and may bepathogenetically related to migraine." It isusually regarded as a diagnosis of exclusion,and only applicable after the above investiga-tions and imaging of the optic nerve or visualpathways have come back negative."I 12

Obscurations are transient disturbances ofvision which occur in association with papil-loedema secondary to raised intracranial pres-sure of whatever cause. This usually results inbilateral visual symptoms, but if the papil-loedema is unilateral, monocular symptomscan result. Typically patients do not lose visionentirely (vision does not go "black"), but com-plain of greying of vision, often in associationwith change in posture. In this situation, it isobviously mandatory to image the head (MRIor CT13) to look for a cause of raised pressure.If imaging shows no space occupying lesion,lumbar puncture is indicated. There are manycauses of a raised CSF pressure in the absenceof a space occupying lesion (pseudotumourcerebri),3 but one important condition is sagit-tal sinus thrombosis. Nowadays, this can oftenbe detected by MR angiography, but formalangiography may still be required, dependingon local facilities. This condition and its man-agement have been dealt with in a recent edit-orial in this Journal.'4

There are a few unusual causes. Ocularischaemia can present with transient visualloss provoked by exposure to bright light (pho-tostress). There are usually ophthalmoscopicchanges to suggest the diagnosis, includingperipheral exudates and haemorrhages, micro-aneurysms, and new vessels. A photostresstest3 is useful in this circumstance, and furtherinvestigation will include carotid ultrasoundwith tests as for amaurosis fugax. Transientvisual disturbance provoked by changes inambient lighting can also be produced by reti-nal diseases such as cone dystrophies, anddiagnosis can be aided by the use of elec-troretinography.5 1' Finally, if the diagnosisremains obscure, an attempt should be madeto measure intraocular pressure during anattack as acute closed angle glaucoma canrarely present with transient visual loss.16

SUDDEN, IRREVERSIBLE VISUAL LOSS IN ONEEYEConditions referable to primary ocular struc-tures such as acute angle closure glaucoma orvitreous haemorrhage will not be discussedhere. Nevertheless, it is likely that there will beabnormalities on ophthalmoscopy to aid diag-nosis, especially if the patient is seen within aday or so of the acute event. If possible, pupildilatation should be performed as failure toperform this often results in missing ophthal-moscopic clues to diagnosis. If ophthal-moscopy is completely normal, then the

possibility of posterior ischaemic optic neu-ropathy should be considered, although thiscondition is rare.2

Usually, however, ophthalmoscopy showschanges which suggest a diagnosis, providedthe patient is seen within 24 hours of visualloss. Such diagnoses include retinal arterialinfarction, venous occlusion, optic nerve headinfarction (anterior ischaemic optic neuropa-thy), haemorrhage, or acute disc swelling. Thelast may be associated with sudden visual lossin acute papilloedema, or Leber's hereditaryoptic neuropathy. Visual disturbance with fieldloss usually confined to an enlarged blind spotcan also be seen in diabetic papillopathy,'7 18 orthe big blind spot syndrome,219 although theseconditions more commonly present with mini-mal visual symptoms. Table 2 lists the variousophthalmoscopic findings seen in each of theabove diagnoses.

Table 2 also lists appropriate investigations.Certain points are amplified below.(1) If there are no abnormalities in the retina,posterior ischaemic optic neuropathy is a pos-sibility, but imaging the orbit would berequired to exclude a compressive lesion.Other investigations would be similar to thosefor anterior ischaemic optic neuropathy(AION).2(2) The investigation of central retinal arteryocclusion is similar to that for amaurosisfugax,89 although local retinal causes such asradiation retinopathy28 may contribute to itsaetiology.(3) In central retinal vein occlusion, search foranticardiolipin antibodies is probably notworthwhile unless the patient has other fea-tures of systemic lupus erythematosus.2930(4) Anterior ischaemic optic neuropathy maybe arteritic or non-arteritic. An erythrocytesedimentation rate is required as an emer-gency, and, if any other feature suggests tem-poral arteritis, a temporal artery biopsy shouldbe performed, even if the sedimentation rate isnormal (see above). The role of ophthalmiccolour Doppler is as yet undetermined, but itmay be of use in differentiating arteritic fromnon-arteritic AION."32 In the absence of clini-cal features of temporal arteritis or a raisederythrocyte sedimentation rate, biopsy isunlikely to be positive, and is therefore unnec-essarily invasive. The exception to this is inbilateral simultaneous AION in which a largeproportion of patients have evidence of sys-temic connective tissue disease.2 In youngerpatients or those with bilateral disease, investi-gation for coagulation abnormalities is appro-priate.24 The history may point to one of therarely-encountered associations betweenAION and other diseases,2 but routine investi-gation beyond that indicated in table 2 is prob-ably not worthwhile unless it does.(5) Neuroretinitis is generally thought of as arelatively benign condition in which centralvisual loss occurs over hours and is associatedwith a central scotoma and a macular star onophthalmoscopy.25 33 However, it may presentacutely, and have a poor prognosis.26 In thissituation, investigation should include tests forvasculitic and infective diseases (particularly

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Lueck

Table 2 Ophthalmoscopicfindings and investigations in acute monocular visual loss

Condition Ophthalmoscopic findings Appropriate investigations

Central retinal Opacification of nerve fibre layer ESRartery occlusion Cherry red spot Fluorescein angiography

(Cholesterol) embolus/microemboli (as for amaurosis fugax)Attenuation of arterial tree Carotid ultrasound/angiography

Central retinal Retinal venous dilatation FBC, ESR, fibrinogen, glucosevein occlusion20 21 Scattered retinal haemorrhages (several Lipid profile

disc diameters from optic disc) (fluorescein angiography)(BP, intraocular pressure)

AION Sectoral/complete optic disc swelling ESR, ? temporal artery biopsyDisc haemorrhages Fluorescein angiographyExudates/macular star FBC, fibrinogen, glucose(Small optic discs22) Protein electrophoresis(Buried drusen23) Lipid profile

(coagulopathy screen24)Neuroretinitis25 26 Disc swelling, peripapillary exudates (As for AION)

Macular star formation VDRL, ANA, serum angiotensin converting enzymePossibly vitreous cells25 Viral titres, Lyme serology

Leber's hereditary Circumpapillary telangiectatic Fluorescein angiographyoptic neuropathy microangiopathy Mitochondrial DNA

(Non-oedematous) elevation of optic discInfiltrative optic Optic disc swelling B mode ultrasonography

neuropathy Haemorrhages/exudates MRI/CT, lumbar puncture; chest radiographyBence-Jones proteinSerum angiotensin converting enzymeBone marrow, abdominal CT

Diabetic Disc swelling, usually bilateral Glucose, haemoglobin Al cpapillopathy'7 18 Haemorrhages/exudates Fluorescein angiography

Big blind spot Swollen optic disc (?absent) MRI/CT, lumbar puncturesyndrome2 19 Venous overfilling, occasional haemorrhage Fluorescein angiography

Multiple evanescent white dots27

ESR = erythrocyte sedimentation rate; AION = anterior ischaemic optic neuropathy; FBC = full blood count; VDRL = venerealdisease research laboratory test; ANA = antinuclear antibody.

cat scratch fever) as well as those tests listedfor AION, but in most cases investigation isnegative.26(6) In Leber's hereditary optic neuropathy,fluorescein angiography shows peripapillarytelangiectasia and an apparently swollen discwhich paradoxically does not leak,34 althoughthese findings are not always present.35 Thisfinding should be followed by mitochondrialDNA analysis.35 Interestingly, there may besurprisingly little by way of relative afferentpupillary defect.36(7) Occasionally optic nerve head drusen canpresent as acute visual loss, often with inferiornasal field defect.23 This diagnosis is generallytaken to be a diagnosis of exclusion after fullinvestigation in the form of CT/MRI, lumbarpuncture, and investigations as for AION. Thedrusen themselves may be more specificallydetected by CT, MRI, fluorescein angiogra-phy, or ocular ultrasound. 13 37

OPTIC NERVE PATHOLOGY DEVELOPING OVERHOURS TO DAYSIf a patient presents with the typical historyand signs of optic neuritis, standard wisdomhas been that there is no need to image thepatient, provided there are no clinical featuresto suggest a diagnosis of multiple sclerosis.3Recently, however, this view has been changedsomewhat by two developments. Firstly, therecent optic neuritis trial suggested thatpatients with typical optic neuritis should betreated with intravenous methyl prednisolone(whatever the severity) as this significantlyreduced the likelihood of progressing to diag-nosable multiple sclerosis at two years38(although the most recent report from theoptic neuritis study group suggests that thetwo year benefit is not maintained at threeyears39). Secondly, recent studies have sug-

gested that the prognosis for going on todevelop multiple sclerosis is much less ifMRI isotherwise normal than if it shows multiplelesions.40 For these reasons, it has been advo-cated that all cases of typical optic neuritisshould have MRI,4' but this remains contro-versial. There is no role for visual evokedpotentials, CT, or lumbar puncture in theclinical management of typical isolated opticneuritis.

If any of the features of typical optic neuritisis missing, both optic nerves are affected, thereis no evidence of recovery after four weeks, orthere are additional features, then furtherinvestigation is warranted. In the first instance,it is appropriate to image the orbits, paranasalsinuses, sphenoid, and pituitary fossa to detectcompressive lesions (which may have acutelydecompensated) or infective processes such asparanasal sinus disease. As a first line investi-gation, contrast enhanced orbital CT is proba-bly still the investigation of choice,'342 butintrinsic and inflammatory optic nerve lesionsare better shown on MRI,43 and both may berequired. Skull radiography is not useful in theinvestigation of visual loss.44 Lumbar punctureis indicated if imaging does not yield a diagno-S1S.

Previous or concurrent symptoms of infec-tive illness raise the possibility of viral, paravi-ral, or postviral syndromes, or active bacterialor fungal infection. It is thus worth consider-ing screening blood and CSF for viruses, bac-terial infections, and fungal infections asappropriate to the clinical picture. Optic neu-ritis has been reported in systemic lupus ery-thematosus, and other immune mediatedconditions such as Sjogren's syndrome andulcerative colitis.'2 If the optic neuritis is atypi-cal in any way, antinuclear antigen and anti-dsDNA antibodies, along with extractable

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nuclear antigen should be checked.Perhaps the most common "lookalike" of

optic neuritis is sarcoidosis affecting the opticnerve. Suspicion should be raised by failure ofthe visual loss to improve, or any evidence ofpast or present iritis or uveitis. Initial investi-gation in the form of a chest radiograph,serum angiotensin converting enzyme, andlumbar puncture is appropriate. Unfortu-nately, Kveim testing is no longer available as a

diagnostic test. In the absence of any otherclinical features, my experience has been thatfurther tests such as pulmonary function testsor gallium scans are unlikely to be rewarding,but could be considered.

About 20% of simultaneous bilateral opticneuritis in adults turns out to be due toLeber's hereditary optic neuropathy, even inthe absence of affected relatives,45 and in thiscircumstance, mitochondrial DNA analysisshould be performed. A further 20% turn outto be due to multiple sclerosis.45 Other possi-bilities include toxic neuropathy. This couldbe due to medication the patient is taking, or

possibly to an external agent such as lead.Serum lead, B 12, and a toxicology screen

should be added to the above tests, along withcareful questioning of the patient regardingdrugs, diet, and work exposure.

OPTIC NERVE PATHOLOGY DEVELOPING OVER

DAYS OR MONTHS

In this situation, imaging is mandatory, andshould include good views of the optic nerves

(intraorbital, intracanalicular, and intracra-nial) including the pituitary fossa region. Asmentioned, there is some debate as to whetherhigh quality enhanced orbital CT, or gadolin-ium enhanced MRI is superior as a first lineinvestigation,'34243 but it is not uncommon forboth to be required eventually. Practically, itdepends on local facilities. Most lesions largeenough to cause visual loss by optic nerve

compression will be visible, but it is notuncommon for optic nerve sheath menin-giomas to be missed: repeated scans may benecessary, and are especially indicated if thereis evidence of optic disc swelling or optociliaryshunt vessels on fundoscopy.46

Further investigation of any lesion foundwill, of course, depend on the nature of thelesion. Imaging in the form of angiographymay be necessary, or it might be appropriate toproceed to biopsy. Likewise, thyroid functiontests may be appropriate, but these are oftennormal in thyroid eye disease; the use of a

TRH test considerably improves the diagnos-tic yield.47

If imaging of the optic nerve fails to show a

reason for the visual loss, infective and inflam-matory causes should be screened for withVDRL, HTLV I titres, serum angiotensin con-

verting enzyme, and autoantibody screen, andconsideration should be given to an HIV test.Nutritional optic neuropathy can be screenedfor by checking folate and B12, in combina-tion with a careful dietary history. Smoking,alcohol, drugs (prescribed, "recreational", or

otherwise-for example, methanol) are causesof toxic amblyopia and must be looked for,

possibly with the help of a toxicology screen.If visual field testing suggests a lesion of the

optic nerve and there is disc swelling on fun-doscopy, further thought is required.Papilloedema (disc swelling due to raisedintracranial pressure) does not typically causecentral visual field loss. Therefore, if there israised intracranial pressure, this must be dueto a structural cause which is associated withoptic nerve disease/compression, but imagingshould already have detected such a cause. Ifimaging is normal and the discs are swollen,the next test should be a lumbar puncture. Inthis circumstance, raised pressure should notbe regarded as the cause of the visual loss. Analternative cause in the form of inflammatorydisease such as sarcoidosis must be consideredand investigated as above. Failing this, carci-nomatous meningitis or direct optic disc infil-tration by haematological malignancy shouldbe considered, and investigated appropriately.

Hereditary optic neuropathies may be diag-nosed on the basis of a positive family history,but occasionally one can be surprised by theway Leber's hereditary optic neuropathy pre-sents, and searching for the known mitochon-drial DNA mutations is worthwhile at thispoint.35Two other reasons for progressive anterior

pathway visual failure may have to be consid-ered. Dysplasia of the optic disc may be associ-ated with visual field disturbance. This is oftenlongstanding, but occasionally comes to theattention of the patient and requires explana-tion. Typical examples include tilted opticdiscs, which are usually associated with super-otemporal field loss,2 or optic nerve headdrusen (hyaline bodies).48 Diagnosis of thesecond is suggested by anomalous optic discvasculature, and can be difficult, particularly ifthe drusen are buried. Help may be requiredin the form of fluorescein angiography (look-ing for autofluorescence), or B mode ultra-sonography.'3 37 Occasionally calcification ofthe optic nerve head can be seen on unen-hanced CT. 13 37

Finally, it is not unheard of for a neurologistor neuro-ophthalmologist to be referred apatient who actually has an ophthalmologicaldiagnosis, even if the source of referral is anophthalmologist. The most common situationin which this arises is that of a maculopathybeing misdiagnosed as possible optic nervedisease. It is always worth re-examining theocular fundus in the case of monocular visualdisturbance, particularly if there is no associ-ated relative afferent pupillary defect, so as notto be led into inappropriate investigations.

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