Retrochiasmal Lesions

Etiology: Retrochiasmal lesions may result from a wide variety of neurologic disorders such as tumors, vascular insults, basal meningitis, aneurysms of the posterior communicating artery, abscesses, injuries (such as a contrecoup injury to the occipital lobe), and vasospasms (in an ocular migraine).

Symptoms, diagnostic considerations, and clinical picture: Visual field testing in particular will provide information on the location of the lesion. Perimetry is therefore a crucial diagnostic study. Bilateral simultaneous visual field defects are common to all retrochiasmal lesions of the visual pathway. Often these defects will be incongruent.

H Homonymous visual field defects are the result of a retrochiasmal lesion.

Lesions of the optic tract and the lateral geniculate body. Because the nerve fibers are concentrated in a very small space, the visual field defect that occurs typically in these lesions is homonymous hemianopsia. Lesions on the right side produce visual field defects in the left half of the visual field and vice versa. Partial primary atrophy of the optic nerve may occur as the third neuron is affected, which extends from the retina to the lateral geniculate body. An afferent pupillary defect on the side opposite the lesion will be present. The cause of this defect is not known.

Lesions of the optic radiations. The visual field defects assume many different forms due to the wide spread of the optic radiations. Injuries to both the temporal and parietal lobes typically produce homonymous hemianopsia. Injuries primarily involving the temporal lobe produce homonymous superior quad-rantic anopsia; injuries primarily involving the parietal lobe produce homo-nymous inferior quadrantic anopsia. Pupillary findings are normal because the lesion affects the fourth neuron. Approximately 30% of all cases involve an afferent pupillary defect on the side opposite the lesion. The cause of this defect is not known.

Lesions of the visual cortex. The visual field defects, like the lesions of the visual pathway, are homonymous and hemianoptic. The macula may or may not be affected depending on the extent of the lesion.

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