Neuro-ophthalmology and strabismus
Neuro-ophthalmology rewards localisation. Every question is essentially: where is the lesion, and is it urgent? Strabismus adds the binocular sensory system, which explains why a child needs fast treatment and an adult with sudden diplopia needs a vascular work-up.
Localisation by field
Optic nerve disease gives monocular loss, an afferent pupillary defect, and disc swelling or pallor. Chiasmal lesions give bitemporal hemianopia. Optic tract lesions give incongruous homonymous hemianopia. Temporal lobe lesions give homonymous superior quadrantanopia, parietal lesions inferior quadrantanopia, and occipital lesions homonymous hemianopia with macular sparing, often without an afferent defect. Papilloedema is bilateral with acuity spared until late; idiopathic intracranial hypertension gives headache, pulsatile tinnitus and an empty sella.
Pupils and cranial nerves
Horner syndrome means ptosis, miosis, anhidrosis and dilation lag; a painful Horner syndrome suggests carotid dissection. Third nerve palsy gives ptosis, a down-and-out eye and mydriasis; pupil involvement needs urgent imaging for aneurysm, while pupil sparing is usually microvascular. Fourth nerve palsy gives vertical diplopia worse on head tilt to the opposite side. Sixth nerve palsy gives horizontal diplopia worse at distance and can be a false localising sign of raised pressure.
Strabismus
Cover and alternate cover tests measure tropia and phoria; prisms quantify. Accommodative esotropia appears at 2 to 3 years with hyperopia and is corrected with glasses. Congenital esotropia needs early surgery to preserve binocularity. Amblyopia is treated with refractive correction, occlusion or atropine penalisation, with best results under 7 years.
Worked example
A 68-year-old has acute diplopia, a painful third nerve palsy and a dilated pupil. Reasoning: pupil involvement with pain points to compression, typically a posterior communicating artery aneurysm, so arrange urgent neuroimaging rather than assuming diabetic microvascular disease, which classically spares the pupil.
Traps
Calling every pupil-sparing third nerve palsy benign; forgetting the afferent pupillary defect in optic neuritis; treating adult-onset diplopia as refractive; missing bilateral sixth nerve palsy as a sign of raised intracranial pressure.