Neuro-ophthalmology
Concept anchors
Afferent disease is localised by the pattern of visual loss and the presence of a relative afferent pupillary defect. Optic neuritis is painful, subacute and unilateral with reduced colour vision, a relative afferent pupillary defect and often a swollen disc; it may herald multiple sclerosis. Anterior ischaemic optic neuropathy is painless, altitudinal, with sector disc edema and a small crowded disc, while giant cell arteritis should be suspected in anyone over 55 with headache, jaw claudication, scalp tenderness or raised inflammatory markers - give high-dose steroid immediately and arrange temporal artery biopsy.
Motility and pupils
Third nerve palsy with pupil involvement suggests compression (aneurysm), while pupil sparing suggests microvascular ischemia. Sixth nerve palsy is the commonest false localising sign. Horner syndrome shows ptosis, miosis and anhidrosis with a dilation lag; localise with topical apraclonidine or hydroxyamphetamine. Bilateral disc swelling means raised intracranial pressure until proven otherwise.
Field localisation
Chiasmal lesions give bitemporal hemianopia; optic tract and radiation lesions give homonymous defects, with congruity increasing posteriorly. Cortical blindness may spare the pupillary light reflex.
Traps
Distinguish papilledema from papillitis; recognise Leber hereditary optic neuropathy (young male, painless sequential loss); myasthenia (fatigable ptosis, ice test); thyroid eye disease (lid retraction, restricted elevation).
Rule of thumb
Suspected giant cell arteritis is a medical emergency - treat before biopsy, never after.