Visual pathway

Only the nasal-retinal fibres cross in the chiasm, and the nasal retina of each eye sees the temporal field. A midline lesion therefore removes the temporal field of both eyes while the uncrossed temporal-retinal fibres, which carry each eye's nasal field, pass by intact. A pituitary adenoma pushing up from below hits the inferior nasal fibres first, so the superior temporal quadrants go first; a craniopharyngioma from above starts with the inferior quadrants.

A tract lesion is incongruous (the defect differs between the two eyes), gives a mild RAPD in the contralateral eye because the pupillary fibres are still travelling with the tract, and shows no macular sparing. An occipital lesion is congruous, spares the macula after a posterior cerebral artery stroke because the occipital pole has middle cerebral collateral supply, and leaves the pupils entirely normal because the reflex fibres left the pathway before the geniculate.

The afferent limb leaves the optic tract through the brachium of the superior colliculus to the pretectal nucleus, then reaches the Edinger-Westphal nuclei bilaterally through the posterior commissure. It never passes through the lateral geniculate nucleus, optic radiation or striate cortex, so bilateral occipital infarction abolishes vision but leaves both direct and consensual reflexes intact, which is the key bedside clue against a functional or ocular cause.

Each pretectal nucleus projects to both Edinger-Westphal nuclei, so whatever light reaches the midbrain from either eye constricts both pupils equally. A damaged optic nerve delivers less afferent signal, so when the torch swings to that eye both pupils dilate relative to their response to the good eye, but at rest the two pupils remain the same size. Anisocoria therefore always means an efferent (CN III, ciliary ganglion or sympathetic) problem, never a purely afferent one.

Meyer's loop in the temporal lobe carries fibres from the inferior retina, which sees the superior visual field, so a temporal lesion gives a contralateral superior homonymous quadrantanopia ('pie in the sky'). The parietal radiation carries the superior retina and therefore the inferior field, giving a contralateral inferior quadrantanopia ('pie on the floor'). The same rule applies in V1: the lingual gyrus below the calcarine sulcus holds the superior field and the cuneus above it the inferior field.

The preganglionic parasympathetic fibres from the Edinger-Westphal nucleus run superficially on the dorsomedial surface of the oculomotor nerve. External compression by a posterior communicating artery aneurysm or a herniating uncus crushes them first, producing a fixed dilated pupil that demands urgent imaging. Microvascular ischaemia in diabetes or hypertension damages the core of the nerve where the vasa nervorum run, so the eye is ptotic and immobile but the pupil reacts normally.