Visual Pathway
On this page
Direct answer
Bitemporal hemianopia means one thing anatomically — a lesion at the optic chiasma, usually a pituitary tumour pressing from below — and that is how the visual pathway is examined: by field defect, not by list. The pathway runs from retinal ganglion cells through optic nerve, chiasma, tract, lateral geniculate body, optic radiation and area 17 of the calcarine cortex. At the chiasma the nasal retinal fibres of each eye cross while the temporal fibres stay uncrossed, so behind the chiasma every structure carries the opposite visual field, and each level signs its own characteristic defect.
What you must remember
- Chain of neurons: photoreceptors, bipolar cells, ganglion cells whose axons form the optic nerve — a CNS tract with oligodendroglial myelin, so it cannot regenerate and swells in papilloedema.
- Chiasma: nasal retinal fibres (temporal field) decussate; pituitary compression from below gives bitemporal hemianopia, classically taking the upper quadrants first because the inferior chiasmal fibres are affected first.
- Optic tract and lateral geniculate body: each tract carries the contralateral field; in the six-layered lateral geniculate body, crossed fibres synapse in layers 1, 4 and 6, uncrossed in 2, 3 and 5.
- Optic radiation: upper retinal (lower field) fibres pass through the parietal lobe to the superior calcarine bank; lower retinal (upper field) fibres loop forwards over the inferior horn in the temporal lobe — Meyer's loop.
- Cortex: area 17 in the calcarine sulcus walls, flanked by areas 18 and 19, supplied by the calcarine branch of the posterior cerebral artery with middle cerebral anastomoses at the pole — the basis of macular sparing.
- Defect by level: optic nerve — ipsilateral blindness with lost direct light reflex; chiasma — bitemporal hemianopia; tract — contralateral homonymous hemianopia; temporal lobe — contralateral superior quadrantanopia ("pie in the sky"); parietal — inferior quadrantanopia; occipital cortex — congruous homonymous hemianopia with macular sparing.
- Light reflex fork: pupillary fibres leave the tract for the pretectal nucleus before the lateral geniculate body, then project to both Edinger-Westphal nuclei — hence cortical blindness spares the reflex, and the consensual response exists.
Working through three field charts
Chart one: a 35-year-old with amenorrhoea and galactorrhoea shows bitemporal loss, denser above. The prolactinoma presses the inferior chiasmal fibres — crossing fibres of the lower nasal retina serving the upper temporal fields — so the defect begins as a superior quadrantanopia before completing; naming that sequence is the mark-earning sentence. Chart two: a hypertensive man has a congruous right homonymous hemianopia that spares the macula. Congruity points behind the lateral geniculate synapse, macular sparing to the occipital pole with its middle cerebral collaterals — a calcarine infarct of the posterior cerebral artery. Chart three: after a temporal lobe seizure, a young man loses the left upper quadrant in both eyes — right temporal lobe, Meyer's loop, whose fibres arch far anteriorly around the inferior horn; temporal lobectomy for epilepsy risks exactly this quadrantanopia, so the loop is mapped beforehand.
Then the reflex fork: in cortical blindness both pupils constrict briskly, because the reflex exits at the pretectal nucleus upstream of the lateral geniculate body; in optic nerve disease the affected pupil constricts poorly to direct light — the relative afferent pupillary defect, revealed by the swinging torch test.
Where students slip
The retinal half and the visual half are tangled: the nasal retina sees the temporal field because the lens inverts the image, so "nasal fibres cross" and "temporal field lost" are the same statement — write both once and the mapping stays fixed. Meyer's loop is assigned the lower quadrant; it carries upper-field fibres, giving "pie in the sky". Optic neuritis and papilloedema are used interchangeably: optic neuritis gives pain on eye movement, a central scotoma and a Marcus Gunn pupil; papilloedema is bilateral, painless, and leaves acuity surprisingly intact early. And the light reflex is said to pass "through the cortex" — fibres leave the tract before the lateral geniculate body, which is the entire explanation for intact reflexes in cortical blindness.
Frequently asked questions
What is Meyer's loop, and what is its lesion pattern?
The optic radiation fibres from the lower retina looping forwards over the inferior horn of the lateral ventricle in the temporal lobe, representing the upper field; a lesion gives contralateral superior homonymous quadrantanopia — pie in the sky.
Why is the pupil normal in cortical blindness?
Pupillary reflex fibres leave the optic tract for the pretectal nucleus before the lateral geniculate body and cortex, so bilateral occipital destruction blinds the patient while direct and consensual light reflexes remain intact.
Why is the macula spared in occipital cortex infarction?
The macular representation at the occipital pole receives collateral supply from middle cerebral artery branches, while the rest of the calcarine cortex depends on the posterior cerebral artery.
What is a relative afferent pupillary defect?
A pupil that constricts poorly to direct light but consensually when the other eye is illuminated — the swinging torch test — indicating optic nerve or severe retinal disease, classically optic neuritis.