Homonymous hemianopia · Grey Matter

Homonymous hemianopia is the loss of the same half of the visual field in both eyes (the left half or the right half of everything seen) caused by damage behind the optic chiasm on the opposite side.

Homonymous hemianopia. Homonymous hemianopia is the loss of the same half of the visual field in both eyes (the left half or the right half of everything seen) caused by damage behind the optic chiasm on the opposite side.

Behind the optic chiasm each optic tract, radiation and visual cortex carries the opposite visual field from both eyes, so a lesion anywhere along that path on the left blinds the right half of the field of each eye, and the reverse. The border of the defect follows the vertical midline of the field. In adults most cases come from strokes of the posterior cerebral artery, which supplies the occipital visual cortex; tumours, trauma and surgery cause most of the rest.

An occipital stroke often spares the centre of the field (macular sparing). The occipital pole, where central vision is mapped, often receives blood from the middle cerebral artery as well, so it can survive the loss of the posterior artery.

Partial lesions give partial defects. Damage to the fibres that loop through the temporal lobe takes the upper quarter of the field, and fibres through the parietal lobe the lower quarter.

Patients may not notice it at first and bump into things on the blind side; reading and driving are the activities most affected.

Questions: Why does a stroke in the left occipital lobe blind the right half of both eyes? After the chiasm, the left visual pathway carries the right half of the visual field from both eyes, so destroying the left visual cortex removes that half from each eye's view: a right homonymous hemianopia. Covering one eye does not change the defect, which is how it is told apart from damage to an eye or an optic nerve. With an occipital stroke the central few degrees are often spared, because the occipital pole where central vision is mapped often also receives blood from the middle cerebral artery.