Clinical Examples
| Example 3 |
Symptoms: At his annual physical exam, the patient complains of a general malaise and changes in his vision that he noticed while playing soccer. He said he was often "blindsided" on the playing field because he "couldn't see players approaching him from the side". Ophthalmoscope examination does not reveal abnormalities in either eye2. Confrontation field testing indicates a constriction of the temporal hemifields of both eyes. The patient is referred for neuroradiographic tests and perimetry testing.
Perimetry Test Results: The results indicate a bitemporal hemianopia, i.e., loss of vision in the temporal hemifields of both eyes (Figure 15.17).
Side & Retinotopicity of damage: The visual loss
- is not related to changes in the retina of either eye
- involves vision in both eyes
- encompasses only the temporal hemifields
You conclude that the visual field defect is related to damage that
- is retrobulbar (beyond the retina)
- involves the optic chiasm (Figure 15.14, Lesion 3)
Neural imaging results (Figure 15.18) indicate a pituitary adenoma that is compressing the optic chiasm. Compression of the decussating nerve fibers prevents action potentials from the nasal hemiretina to reach the contralateral lateral geniculate nucleus of the thalamus. As the tumor grows larger it will crush the optic chiasm, destroying it and eventually compromising the remaining optic nerve fibers.
Figure 15.18 |
Optic Chiasm Damage: The fibers of the optic nerve that originate from ganglion cells in the nasal half of the retina decussate in the optic chiasm to the opposite optic tract (Figure 15.1). The crossing fibers of the optic chiasm may be crushed by a pituitary tumor. Damage to the optic chiasm produces a unique form of visual field deficit, a bitemporal hemianopia (Figure 15.17). Recall that the fibers of the optic chiasm carry information about objects in the temporal hemifields of both eyes (i.e., the right hemifield of the right eye and the left hemifield of the left eye). Consequently section of the optic chiasm produces a visual loss in only the temporal half of the visual field of each eye. When the patient views the world out of both eyes, the boundary of his binocular visual field is narrower than normal.
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