The visual pathway and visual field defects

1. From retina to cortex

Each optic nerve carries one eye. At the chiasm the nasal fibres cross, so each optic tract carries the opposite half of the visual world from both eyes. The tracts synapse in the lateral geniculate bodies of the thalamus; the optic radiation fans out through the temporal and parietal lobes to the visual cortex along the calcarine sulcus. Tap a lesion in the panel.

Surgeon’s corner: The rule: a lesion in front of the chiasm affects one eye (a lesion where the optic nerve joins the chiasm can add an upper temporal defect in the other eye — junctional scotoma); at the chiasm, both temporal fields; behind it, the same half of the field in both eyes (homonymous).

2. Optic nerve

Damage to one optic nerve blinds or blurs that eye: optic neuritis (often a young woman with pain on eye movement and faded colours — think of multiple sclerosis), ischaemic optic neuropathy in older patients (over 50: exclude giant cell arteritis the same day — ESR, CRP — and start steroids if suspected, to protect the other eye), or compression by a tumour. Optic neuritis typically gives a central scotoma rather than total loss.

Surgeon’s corner: Swinging flashlight test: both pupils dilate when the light swings onto the affected eye (relative afferent pupillary defect).

3. Optic chiasm

Pressure on the chiasm from below or above cuts the crossing nasal fibres, which carry both temporal fields: bitemporal hemianopia. A pituitary adenoma pushing up from below takes the upper temporal quadrants first; a craniopharyngioma, more often from above, the lower ones.

Surgeon’s corner: Bitemporal loss is easily missed — patients bump into things on both sides. Test the fields by confrontation in anyone with chronic headache, amenorrhoea and galactorrhoea, or acromegalic features.

4. Optic tract and lateral geniculate body

Behind the chiasm, the left optic tract carries the right half of the visual world from both eyes to the left lateral geniculate body. A lesion here gives a right homonymous hemianopia, often incongruous (the two fields not identical) because the fibres from corresponding points are not yet aligned.

Surgeon’s corner: An optic tract lesion can also give a relative afferent pupillary defect in the opposite eye, because more fibres cross than stay.

5. Meyer’s loop: the temporal lobe

The lower fibres of the optic radiation, carrying the upper field, loop forwards round the temporal horn of the lateral ventricle (Meyer’s loop) before running back. A left temporal lesion gives a right upper homonymous quadrantanopia: "pie in the sky".

Surgeon’s corner: Anterior temporal lobectomy for epilepsy can cut Meyer’s loop; the remaining field decides whether the patient may drive.

6. The parietal radiation

The upper fibres of the optic radiation, carrying the lower field, run straight back through the parietal lobe. A left parietal lesion gives a right lower homonymous quadrantanopia: "pie on the floor", often with other parietal signs.

Surgeon’s corner: With a right parietal lesion, the patient may also neglect the left side altogether — test fields and neglect separately.

7. The visual cortex

The left visual cortex, along the calcarine sulcus, sees the right half of the world. An occipital infarct gives a right homonymous hemianopia, usually congruous and often sparing the macula, whose large representation at the occipital pole may also receive blood from the middle cerebral artery.

Surgeon’s corner: Commonest cause: a posterior cerebral artery stroke. Both occipital lobes lost: cortical blindness with preserved pupil reflexes.