Neuro-Ophthalmology
Neuro-ophthalmology has a reputation for being the hardest part of the subject, and the reputation is undeserved. It is difficult only when learned as a catalogue of eponymous syndromes.
It is easy when treated as what it actually is, which is a localisation exercise. The visual and ocular motor systems are laid out anatomically, and every sign is a statement about where along that anatomy something has gone wrong.
1. Three Questions
| Question | Answered by |
|---|---|
| Where in the visual pathway? | The shape of the field defect |
| Afferent or efferent? | The pupil |
| Which nerve or muscle? | The pattern of ocular misalignment |
The organising tool is that each of these three systems is anatomically separate, so each gives independent information. A patient can have a dense field defect with normal pupils, or normal fields with a dilated pupil, and the combination localises far better than either alone.
2. The Visual Pathway
Fibres from the nasal half of each retina cross at the chiasm; temporal fibres do not. Since the nasal retina sees the temporal field, crossing fibres carry the temporal fields of both eyes.
Four rules follow, and they answer almost every field question.
A defect confined to one eye means the lesion is anterior to the chiasm, in the retina or optic nerve, because before the chiasm the two eyes are still separate.
A defect respecting the vertical midline in both eyes means the chiasm or behind it, because the vertical midline is where the fibres divide. A bitemporal hemianopia means the chiasm itself, classically compressed by a pituitary adenoma from below.
A homonymous defect, meaning the same side of the field in both eyes, means behind the chiasm.
The further back the lesion, the more congruous the defect. Corresponding fibres from the two eyes travel further apart in the optic tract and converge as they approach the occipital cortex, so an incongruous homonymous hemianopia suggests the tract while a precisely matching one suggests the cortex.
Macular sparing suggests an occipital lesion, because the macular representation at the occipital pole has a dual blood supply from the middle and posterior cerebral arteries.
3. The Pupil
The light reflex has an afferent limb through the optic nerve to the pretectal nucleus, which projects to both Edinger-Westphal nuclei, and an efferent limb through the third nerve to the ciliary ganglion and the sphincter.
Because the pretectal nucleus projects bilaterally, light in one eye constricts both pupils, which is what makes the swinging flashlight test possible.
A relative afferent pupillary defect means asymmetric disease of the afferent pathway, and it is elicited by swinging the light between the eyes: the affected pupil dilates when the light reaches it, because it is receiving less total input than it was from the fellow eye.
A dense cataract does not cause a relative afferent pupillary defect. Media opacities scatter light rather than eliminating it, and the pupil responds to total light energy reaching the retina, not to image quality. This is examined constantly, because it is the point at which candidates confuse vision with afferent function.
Light-near dissociation
When the light reaction is poor but the near reaction is preserved, the lesion lies in the pretectal region, since near fibres approach the Edinger-Westphal nucleus more ventrally.
| Pupil | Features |
|---|---|
| Argyll Robertson | Small, irregular, bilateral, neurosyphilis |
| Adie tonic pupil | Large, unilateral, young women, slow tonic near response, vermiform iris movements |
| Parinaud | Dorsal midbrain lesion, with upgaze palsy and convergence-retraction nystagmus |
The Adie pupil shows denervation supersensitivity, constricting to dilute pilocarpine that leaves a normal pupil unaffected, because the postganglionic parasympathetic supply has been lost.
4. Anisocoria
The first question is not which pupil is abnormal but in which lighting the difference is greater.
Anisocoria greater in the dark means the smaller pupil is at fault, because it is failing to dilate, which points to a sympathetic lesion and Horner syndrome.
Anisocoria greater in bright light means the larger pupil is at fault, because it is failing to constrict, which points to a parasympathetic lesion, a third nerve palsy, a tonic pupil or pharmacological dilation.
Horner syndrome combines miosis, partial ptosis from loss of Muller muscle, and anhidrosis, with apparent enophthalmos. Dilation lag is characteristic, the affected pupil taking several seconds longer to dilate when the lights go out.
A painful Horner syndrome demands imaging of the carotid, because internal carotid dissection is a cause and a stroke may follow. In a child, a Horner syndrome raises the question of neuroblastoma.
5. Optic Neuritis and the Antibody Era
Typical demyelinating optic neuritis presents in a young adult with subacute visual loss over days, pain on eye movement, loss of colour saturation out of proportion to acuity, and a relative afferent pupillary defect.
In retrobulbar neuritis the disc looks normal, which produced the old teaching that the patient sees nothing and the doctor sees nothing.
The Optic Neuritis Treatment Trial established two things still examined. Intravenous methylprednisolone speeds recovery but does not alter final acuity. And oral prednisone alone increased the recurrence rate and is contraindicated.
When it is not typical
The important modern change is that optic neuritis is no longer a single entity. Antibody testing now separates three diseases with different treatments and prognoses.
Atypical features should prompt testing for aquaporin-4 and myelin oligodendrocyte glycoprotein antibodies: simultaneous bilateral involvement, very severe loss, poor recovery, absence of pain, a longitudinally extensive or chiasmal lesion, recurrent episodes, steroid dependence, and onset in children or older adults.
Aquaporin-4 antibody indicates neuromyelitis optica spectrum disorder, which causes severe attacks with poor recovery and requires long-term immunosuppression rather than the observation appropriate after typical demyelinating neuritis.
Myelin oligodendrocyte glycoprotein antibody disease is distinct again, often with marked disc swelling, better recovery, and a strong tendency to relapse on steroid withdrawal.
The 2023 MOGAD diagnostic criteria emphasise that titre matters. Testing must use a cell-based assay, and high-titre results carry a positive predictive value around 90 percent while low-titre results fall to roughly 53 percent, so low positives require supporting clinical and radiological features. Aquaporin-4 antibody, by contrast, is highly specific at any titre.
6. Papilloedema and Disc Swelling
Papilloedema means disc swelling due to raised intracranial pressure specifically, and using the word for any swollen disc is the commonest terminological error in the subject.
| Feature | Papilloedema | Papillitis |
|---|---|---|
| Laterality | Bilateral | Usually unilateral |
| Acuity | Preserved until late | Reduced early |
| Pupil | No relative afferent defect | Relative afferent defect present |
| Field | Enlarged blind spot | Central scotoma |
| Pain | Headache, not ocular | Pain on eye movement |
Transient visual obscurations, seconds of greying out on standing or straining, are characteristic of papilloedema and indicate marginal perfusion of the swollen nerve head.
Idiopathic intracranial hypertension typically affects young obese women with headache, obscurations and sometimes a sixth nerve palsy. That sixth nerve palsy is a false localising sign, produced by the long intracranial course of the nerve being stretched, and it does not indicate a lesion at the sixth nucleus.
Management combines weight reduction, acetazolamide to reduce cerebrospinal fluid production, and surgical intervention if vision is threatened.
7. Ischaemic Optic Neuropathy
Anterior ischaemic optic neuropathy divides into two forms whose distinction is a genuine emergency.
Non-arteritic ischaemic optic neuropathy causes sudden painless loss, characteristically altitudinal, in a patient with vascular risk factors. The fellow disc shows the disc at risk, meaning a small crowded disc with little or no cup, which is the structural predisposition.
Arteritic ischaemic optic neuropathy from giant cell arteritis occurs in older patients with headache, scalp tenderness, jaw claudication, and features of polymyalgia rheumatica. Visual loss is more profound, the disc is characteristically chalky white and swollen, and the erythrocyte sedimentation rate and C-reactive protein are raised.
High-dose corticosteroid is started immediately on clinical suspicion, before biopsy. The fellow eye is at high risk within days, and waiting for histology risks bilateral blindness. Temporal artery biopsy remains valid for one to two weeks after starting steroid, and a long segment is taken because the arteritis has skip lesions.
Tocilizumab has changed maintenance treatment. The GiACTA trial showed higher rates of sustained remission with tocilizumab added to a tapering steroid course, and a cumulative prednisone dose less than half that of the steroid-only arm, which matters in an elderly population highly vulnerable to steroid toxicity.
8. Toxic Optic Neuropathy and Optic Atrophy
Toxic and nutritional optic neuropathies are bilateral, symmetrical, painless and gradual, and they characteristically produce a centrocaecal scotoma with early loss of red-green colour discrimination.
Ethambutol is the cause that matters most in India, because tuberculosis treatment is so widely prescribed. The toxicity is dose and duration related, affects the papillomacular bundle, and is potentially reversible if the drug is stopped promptly, which is why baseline and periodic colour vision testing is part of the regimen.
Methanol poisoning is the acute counterpart. Formic acid, the toxic metabolite, inhibits cytochrome oxidase in the optic nerve, and blindness is accompanied by a severe high anion gap metabolic acidosis. Treatment is with fomepizole or ethanol, alkalinisation and dialysis.
Vitamin B12 deficiency, and the tobacco-alcohol amblyopia that overlaps with it, produce the same painless bilateral centrocaecal picture over months.
Optic atrophy
Optic atrophy is the end state of any optic neuropathy, and the disc becomes pale because axons have been lost and the capillary network they supported has regressed.
Pallor takes four to six weeks to appear after the insult, so a normal-looking disc soon after an acute event does not exclude severe damage.
Primary atrophy follows a lesion behind the eye and gives a chalky white disc with sharp margins. Secondary atrophy follows long-standing disc swelling and gives a dirty grey disc with blurred margins. Consecutive atrophy follows retinal disease.
Band or bow-tie atrophy in one eye indicates a chiasmal lesion, because loss of the crossing nasal fibres leaves atrophy in the nasal and temporal disc but spares the arcuate bundles above and below.
9. The Ocular Motor Nerves
| Nerve | Supplies | Palsy produces |
|---|---|---|
| Third | Medial, superior, inferior rectus, inferior oblique, levator, sphincter | Down and out eye, ptosis, dilated pupil |
| Fourth | Superior oblique | Vertical diplopia worse on downgaze and head tilt to the same side |
| Sixth | Lateral rectus | Horizontal diplopia worse on gaze to that side |
The fourth nerve is anatomically unusual in three ways that explain its clinical behaviour: it is the only cranial nerve to exit dorsally, it has the longest intracranial course, and it decussates before emerging, so a nuclear lesion affects the contralateral eye.
Patients with fourth nerve palsy tilt the head away from the affected side, because tilting towards it forces intorsion that the paretic superior oblique cannot supply.
Myasthenia gravis is the great mimic and can reproduce any pattern of ocular misalignment, because it is a disorder of the neuromuscular junction rather than of a nerve.
Two features give it away. The pupil is never involved, since the iris sphincter is smooth muscle with muscarinic innervation, and the deficit is fatigable, worsening through the day and after sustained upgaze.
10. The Pupil Rule in Third Nerve Palsy
This is the single most important rule in neuro-ophthalmology, and it rests entirely on anatomy.
The parasympathetic fibres travel on the outside of the third nerve, superficially and dorsomedially, while the motor fibres occupy the core.
A compressive lesion therefore involves the pupil early, because compression acts from outside inward.
An ischaemic lesion, as in diabetes or hypertension, spares the pupil, because the vasa nervorum supply the core and the superficial fibres take oxygen directly from surrounding cerebrospinal fluid.
A painful third nerve palsy with a dilated pupil is a posterior communicating artery aneurysm until proved otherwise, and it is imaged the same day. The consequence of being wrong is subarachnoid haemorrhage.
The converse is not entirely safe either. A pupil-sparing palsy in an older diabetic is usually microvascular and recovers over three months, but it must be reviewed, because an incomplete palsy may develop pupil involvement later.
11. Brainstem Gaze Disorders
Internuclear ophthalmoplegia is a lesion of the medial longitudinal fasciculus, which carries the signal from the abducens nucleus on one side to the medial rectus subnucleus on the other.
The result is failure of adduction on the side of the lesion, with nystagmus of the abducting fellow eye, and preserved convergence, because convergence uses a different pathway that does not require the fasciculus.
Bilateral internuclear ophthalmoplegia in a young person suggests multiple sclerosis; a unilateral one in an older patient suggests brainstem infarction.
One-and-a-half syndrome combines a lesion of the paramedian pontine reticular formation with the adjacent fasciculus, producing a horizontal gaze palsy to one side plus an internuclear ophthalmoplegia, so the only remaining horizontal movement is abduction of one eye.
12. Nystagmus
Nystagmus is involuntary rhythmic eye movement, and the distinction that matters is between jerk and pendular forms and between peripheral and central causes.
Jerk nystagmus is named by the direction of the fast phase, though the slow phase is the pathological movement.
Peripheral vestibular nystagmus is unidirectional, horizontal with a torsional component, suppressed by fixation, and accompanied by vertigo. Central nystagmus may change direction with gaze, may be purely vertical, and is not suppressed by fixation.
Downbeat nystagmus suggests a lesion at the craniocervical junction, classically an Arnold-Chiari malformation.
Congenital nystagmus is present from infancy, usually horizontal in all directions of gaze, and often has a null point where it is least marked, which the child adopts a head posture to use.
13. Worked Examples
Example 1. A 45-year-old has sudden painful ptosis with the eye deviated down and out and a dilated unreactive pupil. What is the diagnosis and what is done?
A pupil-involving third nerve palsy, which is a posterior communicating artery aneurysm until proved otherwise. Parasympathetic fibres run superficially on the nerve, so compression involves the pupil early while ischaemia spares it. Same-day angiography is required, because the alternative diagnosis is an aneurysm about to cause subarachnoid haemorrhage.
Example 2. A patient with a dense cataract has no relative afferent pupillary defect despite acuity of 6/60. Is this expected?
Yes. Media opacities scatter light but do not remove it from the eye, and the pupillary reflex responds to total light energy reaching the retina rather than to image quality. A relative afferent pupillary defect therefore indicates disease of the optic nerve or extensive retina, not a cloudy medium, and finding one in a patient with cataract means something else is also wrong.
Example 3. A 70-year-old has sudden loss of vision in one eye, a chalky white swollen disc, jaw pain on chewing and an erythrocyte sedimentation rate of 96. What is done first?
High-dose corticosteroid, immediately and before biopsy. This is arteritic ischaemic optic neuropathy from giant cell arteritis, and the fellow eye can be lost within days. Temporal artery biopsy remains informative for one to two weeks after starting steroid and takes a long segment because of skip lesions. Tocilizumab is added for maintenance, having been shown to increase sustained remission and halve cumulative steroid exposure.
Example 4. A 28-year-old has bilateral simultaneous severe optic neuritis with poor recovery. Why is this not managed as typical demyelinating disease?
Because bilateral simultaneous involvement, severity and poor recovery are all atypical features that mandate testing for aquaporin-4 and myelin oligodendrocyte glycoprotein antibodies. Aquaporin-4 positivity indicates neuromyelitis optica spectrum disorder, which requires long-term immunosuppression rather than observation, and is highly specific at any titre. MOG antibody disease relapses characteristically on steroid withdrawal, and its 2023 criteria require a cell-based assay with attention to titre.
Summary
Three questions: where in the pathway, afferent or efferent, and which nerve.
Monocular means anterior to the chiasm; respecting the vertical midline means chiasm or behind; homonymous means retrochiasmal; congruity increases posteriorly; macular sparing suggests occipital.
A relative afferent pupillary defect indicates asymmetric afferent disease and is never caused by cataract.
Light-near dissociation localises to the pretectum, and the Adie pupil shows denervation supersensitivity.
Anisocoria greater in the dark means the small pupil is at fault; greater in light means the large one is.
A painful Horner syndrome means carotid dissection until excluded.
Intravenous steroid speeds optic neuritis recovery without changing the outcome, and oral prednisone alone is contraindicated.
Atypical optic neuritis requires aquaporin-4 and MOG antibody testing, with titre mattering for MOG and not for aquaporin-4.
Papilloedema is bilateral with preserved acuity, no afferent defect and an enlarged blind spot.
A sixth nerve palsy in raised pressure is a false localising sign.
Giant cell arteritis is treated with steroid before biopsy, and tocilizumab halves cumulative steroid exposure.
Parasympathetic fibres run superficially in the third nerve, so compression takes the pupil and ischaemia spares it.
Internuclear ophthalmoplegia spares convergence, and is bilateral in multiple sclerosis, unilateral in stroke.
