Visual Pathways; Anatomy and Field Defects

Abstract

The shape and distribution of visual field loss closely reflect the site of a lesion affecting the optic pathways. Nerve fiber bundle defects are field abnormalities in which at least part of the border coincides with the course of the retinal nerve fiber layer. Peripheral nerve fiber bundle defects in the nasal field tend to have an arcuate shape when they are secondary to retinal or optic nerve disease. Although monocular visual field defects are usually due to retinal or optic nerve disease, in the early stages of a chiasmatic lesion the loss may be restricted to the temporal portion of the field corresponding to the ipsilateral eye. Bitemporal field defects are most often due to a compressive mass lesion affecting the optic chiasm, such as pituitary tumors. A central defect in one field with a superior temporal defect in the opposite field points to the involvement of the anterior angle of the chiasm, with damage of the ipsilateral optic nerve and of the loop made by the fibers from the inferonasal retina of the other eye (Wilbrand's knee). Because of its localizing implications, this type of visual field defect has been termed junctional scotoma. Homonymous hemianopias appear with lesions in the retrochiasmatic pathways. Homonymous hemianopia may thus be caused by lesions affecting the optic tract, lateral geniculate body, optic radiations, or occipital lobe. Homonymous hemianopias affecting the tract and lateral geniculate body tend to be incongruous, but the more posteriorly the lesion is located in the optic pathways, the greater the congruity of the defect in either field. Superior homonymous quadrantic defects (pie-in-the-sky field defects) may result from a lesion in the temporal (Meyer's) loop of the optic radiations or in the inferior bank of the calcarine fissure. Involvement of the optic radiations in the depth of the parietal lobe gives rise to an inferior quadrantic defect (pie-on-the-floor defect). Medial occipital lesions cause highly congruous homonymous hemianopias. Bilateral occipital lobe lesions may occur from a single or consecutive events and may cause bilateral homonymous scotomas, usually with some macular sparing (ring scotomas), that respect the vertical midline.