Neuro-ophthalmology
Core concepts
Neuro-ophthalmology links afferent and efferent pathways. Afferent: visual acuity, colour vision, visual fields, pupil reactions (RAPD), and optic disc appearance. Efferent: pupillomotor, oculomotor, and lid function. Know the visual field defects produced by lesions at each level-optic nerve (central scotoma), chiasm (bitemporal hemianopia), optic tract (incongruous homonymous), radiation and cortex (congruous homonymous, macular sparing). Understand pupil pathways: light reflex (afferent limb optic nerve, efferent limb Edinger-Westphal and ciliary ganglion) and near reflex. Recognise optic neuritis, ischaemic optic neuropathy, papilloedema, and cranial nerve palsies.
Traps
Questions test laterality: a chiasmal lesion gives bitemporal defects; a tract lesion gives incongruous homonymous defects. RAPD indicates afferent disease, not efferent. Papilloedema is bilateral; unilateral disc swelling suggests local disease. Distinguish Horner syndrome (ptosis, miosis, anhidrosis) from third nerve palsy (ptosis, down-and-out eye, dilated pupil).
Worked example
A patient has a right RAPD and reduced colour vision in the right eye. Localise to the right optic nerve. Rule of thumb: the swinging flashlight test detects asymmetric afferent conduction; the abnormal eye dilates when light swings to it.