Localisation of visual field defects

What it is

The visual route runs from the eyes and the retina to the optic nerve, then the optic chiasm, the optic tract, the lateral geniculate nucleus, the optic radiations, and last the visual cortex in the occipital lobe. Each level carries a different mix of fibers, so the shape of the field loss tells where the lesion sits. Two rules carry most of it. A pre-chiasmal lesion gives an ipsilateral monocular visual field defect, that is one eye only, on the same side as the lesion. A post-chiasmal lesion gives homonymous visual field defects of the contralateral side, that is the same half of the field is lost in both eyes.

How it presents

The named defects run along the route in order.

• A lesion of the optic nerve, before the chiasm, gives monocular blindness, also written as anopsia of that eye.
• A lesion at the optic chiasm gives a bitemporal hemianopia, also called heteronymous. In real life the deficit is usually not as clearly divided or as symmetrical as the diagram shows.
• Anopsia of one eye together with an upper quadrantanopsia of the other is named a junctional scotoma.
• A lesion of the optic tract gives a homonymous hemianopia of the contralateral side, so a right optic tract lesion gives a left homonymous hemianopia.
• A lesion of the inferior, temporal optic radiations, the fibers of Meyer's loop, gives a superior or upper quadrantanopia of the contralateral side, so a right temporal lesion gives a left upper quadrantanopsia.
• A lesion of the superior, parietal optic radiations gives an inferior or lower quadrantanopia of the contralateral side, so a right parietal lesion gives a left lower quadrantanopsia.
• A lesion of the geniculo-calcarine fibers gives a homonymous hemianopia of the contralateral side, and a total hemianopia is less likely there than with lesions of the optic tract.
• A lesion of the calcarine cortex gives a homonymous hemianopia with macular sparing.
• A lesion restricted to the cuneus can also give an inferior quadrantanopia, and one restricted to the lingual gyrus a superior quadrantanopia.
• An ipsilateral nasal hemianopia is listed as well, from a lesion at the side of the chiasm.