Homonymous Hemianopia
History. You examine a 10-year-old male German shepherd whose owner reports that the dog has recently begun to bump into objects with the left side of his face and has had two seizures.
The seizures were characterized by turning of the head to the left and stiffening of the left front leg.Clinical Examination. Physical examination abnormalities are limited to the nervous system. When presented with a maze of unfamiliar objects in the examination room, the dog collides with objects as if he does not see from the left side. He seems somewhat weak in the left front leg. The dog is otherwise bright, alert, and responsive. Cranial nerve and spinal segmental reflexes are within normal limits, as are intersegmental, proprioceptive placing responses for the right front and right rear legs. However, the proprioceptive placing responses for the left front and left rear legs are quite prolonged.
Comment. This dog’s history and neurological examination abnormalities are common in dogs with brain tumors. This dog has a tumor (neoplasm) arising from the meninges over the right posterior cerebral cortex. It is in this posterior (occipital) cortex that the visual image is interpreted from the visual field of the left side (see Figure 14-7). Lesion-induced functional damage to the right occipital cortex results in loss of vision from the left visual field (homonytnous hemianopsia). Also in the right cerebral cortex, the conscious proprioception response for the left legs is interpreted. This dog's seizures feature turning the head to the left and displaying transient rigidity of the left front leg, because the seizure activity arose from the cerebral cortex at the site of the tumor and spread to the right motor cortex but remained limited to the cerebral cortex on the right side. Because the pyramidal system’s corticospinal tract controlling the muscles of the left neck and left front leg arise in the right motor cortex (see Chapter 10), seizure activity causes the transient head turning and left leg stiffness.
Treatment. This dog had a meningioma of the right posterior cerebral cortex. Surgical removal was not attempted in this case.