Lesion site determines the pattern, from monocular loss to homonymous hemianopia
Mnemonic
Work back along the pathway, and the rule is “the further back, the more congruous and the more macular sparing”:
| Defect | Lesion |
|---|---|
| Monocular visual loss | Optic nerve or eye |
| Bitemporal hemianopia | Optic chiasm, classically pituitary |
| Homonymous hemianopia | Contralateral optic tract or radiation |
| Upper quadrantanopia | Temporal lobe, Meyer’s loop |
| Lower quadrantanopia | Parietal lobe |
| Homonymous hemianopia with macular sparing | Occipital cortex |
“PITS”: Parietal Inferior, Temporal Superior, for the quadrantanopias.
Macular sparing in occipital lesions occurs because the occipital pole has a dual blood supply from the middle cerebral artery as well as the posterior.
Bitemporal hemianopia is the one to act on: it means chiasmal compression, usually a pituitary adenoma pressing on the decussating nasal fibres, and it starts in the upper quadrants because the compression comes from below.
Expansion
| Lesion site | Defect |
|---|---|
| Optic nerve | Monocular visual loss |
| Optic chiasm | Bitemporal hemianopia (pituitary tumour) |
| Optic tract | Contralateral homonymous hemianopia |
| Temporal radiation (Meyer loop) | Contralateral superior quadrantanopia (pie in the sky) |
| Parietal radiation | Contralateral inferior quadrantanopia |
| Occipital cortex | Homonymous hemianopia with macular sparing |
The chiasm is the pivot: nasal retinal fibres, carrying the temporal visual fields, cross there, so a compressing pituitary adenoma or craniopharyngioma produces bitemporal loss.
Meyer loop sweeps forward into the temporal lobe carrying superior field fibres, which is why temporal lobe lesions and temporal lobectomy give a superior quadrantanopia.
Macular sparing in occipital infarction is attributed to dual supply of the occipital pole by the middle cerebral artery.
A useful bedside rule: congruous (identical) defects in both eyes suggest a lesion close to the occipital cortex, while incongruous defects suggest a more anterior lesion.