The visual fields for the right and left eye have significant but not complete overlap with one another. Because the temporal visual field for each eye is larger than its nasal field, there is a portion of the visual field at the horizontal extremes—the “temporal crescent”—served by the temporal field of one eye but extending beyond the nasal field of the other eye. An anterior occipital lesion may therefore produce a monocular visual field defect affecting the temporal crescent in the eye contralateral to the lesion.
At the optic chiasm, fibers from the nasal aspect of the optic nerves (which subserve the temporal visual fields of each eye) decussate and continue in the contralateral optic tracts. Lesions of the optic chiasm, especially when compressive, tend to affect these crossing fibers at the center of the chiasm first, resulting in a bitemporal hemianopia. Note that because fibers in the superior aspect of the optic nerve and chiasm represent the inferior visual fields and vice versa, compression of the optic chiasm from below produces an upper bitemporal hemianopia (often seen with pituitary adenomas), whereas compression from above produces a lower bitemporal hemianopia (as seen, for instance, with craniopharyngiomas).
An afferent pupillary defect may be seen in optic tract lesions, usually ipsilateral to the side of field loss and thus contralateral to the optic tract lesion. The temporal visual field (served by the nasal fibers of the optic nerve) is larger than the nasal visual field (served by the temporal fibers of the optic nerve). This leads to a relative overrepresentation of the nasal optic nerve fibers within the optic tract—that is, since only the nasal fibers decussate at the chiasm, the optic tract contains a greater proportion of nasal fibers from the contralateral eye than it does temporal fibers from the ipsilateral eye. Therefore, in an optic tract lesion, there can be more pupillary constriction when light is shone in the ipsilateral eye (since only the temporal fibers have been affected) than in the contralateral eye (since more nasal fibers have been affected), resulting in a contralateral afferent pupillary defect. Hemianopias due to optic tract lesions also tend to be incongruous (one eye is affected more than the other).
Macular sparing is occasionally seen in ischemic stroke affecting the occipital lobe and reflects the fact that blood supply to the occipital pole is variable and may be from branches of either the middle or posterior cerebral arteries. While not always present in hemianopia due to occipital lobe lesions, when present it is highly localizing.
Quadrantanopsias due to occipital lobe lesions tend to respect the horizontal meridian, whereas those due to lesions of the temporal radiations typically do not extend fully to the horizontal meridian, and those due to lesions of the parietal radiations can extend past the horizontal meridian.
The lateral geniculate nucleus consists of the medial horn, lateral horn, and hilum. The medial and lateral horns are supplied by the anterior choroidal artery (a branch of the internal carotid artery) and the hilum is supplied by the lateral choroidal artery (a branch of the posterior cerebral artery). Anterior choroidal artery infarctions can result in a quadruple quadrantanopsia, and lateral choroidal artery infarctions in a horizontal sectoranopia.
Visual Field Loss
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