Brain Tumor
History. You examine a I3-year-old Boston terrier. The owner states that during the past 3 weeks the dog has had seizures of increasing frequency, characterized by turning the head to the right, rigidity of the right front and right hind legs, collapsing to the ground, and urination.
More recently, the dog has seemed weak, drowsy, and confused. He tends to walk in circles and seems weak on the right front leg.Clinical Examination. Important physical examination deficits are referable to the nervous system. The dog seems weak, drowsy, confused, and unsteady in gait. He tends to walk in counterclockwise circles. Cranial and spinal segmental reflexes are within normal limits. Proprioceptive placing reaction is abnormal in the right front leg and normal in the other three legs (see Chapters 6 and 10). An EEG reveals that the dominant frequency is slower and the amplitude higher over the left parietal cortex than over the rest of the brain. Occasional bursts of electrical spiking activity can also be seen from the area of the left parietal cortex. A computed tomography (CT) or magnetic resonance imaging (MRl) scan is warranted to determine the presence and nature of a tumor suspected from the EEG patterns. MRI provides the best imaging of intracranial lesions to determine whether this is a primary tumor (originating from the brain tissue) or a secondary tumor (originating from other tissue; e.g., osteosarcoma, lymphosarcoma).
Comment. This is an old dog, with a recent history of progressive, asymmetric brain disease. The history suggests a focal intracranial lesion, perhaps a brain tumor. A focal lesion is further confirmed by the EEG and brain imaging. Brain tumors within the cerebral hemispheres often cause focal slowing of the EEG frequency with increased amplitude. This is called a slow-wave focus. The tumor itself is electrically silent, but its effects on the surrounding cerebral cortex are slowing, and the intermittent bursts of electrical spikes represent seizure activity within the cortex.
Between clinical seizures, these spikes can still be seen with the EEG, but they do not spread widely enough within the cortex to cause a clinical seizure. During a clinical seizure, this abnormal electrical activity spreads more widely to incorporate normal brain, causing the various motor and other events of the seizure. Why such spikes only occasionally spread to incorporate more distant parts of the brain to cause seizures, and why seizures stop, is still unknown.Treatment. Many forms of seizure disorders can be managed successfully by removing the underlying cause, or the frequency of the seizures can be reduced with antiepileptic medication. In this dog the cause is likely a brain tumor. Depending on the nature of the tumor, surgery and radiation therapy may be possible and may extend the dog’s life. However, the prognosis is likely poor. Antiepileptic and steroid medication may improve the quality of the dog’s remaining life.
Acknowledgment
The authors sincerely thank Dr. John Rossmeisl, Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, for his suggestions and valuable guidance in the revision of this chapter.