to the Optic Tract
Definition
Optic atrophy after damage to the optic tract is a characteristically bilateral, though asymmetric, form of optic atrophy that develops in patients with chronic lesions of the optic tract.
Lesions affecting this last portion of the third neuron (see Chap. 3) produce a characteristic pattern of optic atrophy that is ophthalmoscopically manifest within several weeks or months after the damage (also see Chap. 19). Corresponding to the course of the nerve fibers, the atrophy in the disc ipsilateral to the damaged tract is vertically oriented, causing pallor in the superior and inferior quadrants of the optic disc. The contralateral eye, which suffers loss in the temporal half of the visual field, develops the opposite pattern of atrophy, preserving the superior and inferior quadrants, but causing a horizontal band of pallor extending across the disc. Its shape has been likened to that of a bow tie. The asymmetry of the atrophy is most evident in the nasal quadrants: The eye ipsilateral to the lesion has preserved color in the nasal quadrant, while the disc of the partner eye has a pale color in the nasal quadrant (see â– Fig. 8.23).
Fig. 8.23. The characteristic appearance of bilateral, asymmetric partial optic atrophy associated with a lesion of the left optic tract, causing a homonymous hemianopia to the right (n.b.: The drawing of the visual field defects in the lower third of the picture is drawn from the examiner's perspective, as if seeing the patient's visual field charts transparently from behind). The drawings of the neuronal pathways (across the middle third of the picture) portray the damaged axonal fibers as white lines. In the boxed insets are magnified diagrams of the optic discs' appearance. Note the asymmetry of the disc atrophy, which is particularly evident when comparing the nasal quadrants of the discs. The atrophy in the nasal quadrant of the eye contralateral to the lesion is markedly atrophic, while the nasal quadrant in the ipsilateral eye retains a healthy color. (Compare the zones marked by yellow ovals in the disc photos at the top of the figure)
Conclusion
Optic neuropathies play an important role among neuro-ophthalmic disorders. The physician faces a significant challenge when trying to differentiate ischemic, toxic, and degenerative disorders from compressive diseases of the optic nerve. â– Table 8.2 p. 114 reviews some of the clinical elements that provide clues to the correct diagnosis.
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