A change in an animal’s ability to sense its environment can be caused by disease in either the central nervous system or the peripheral nervous system. The primary signs of nervous system disorders include behavioral changes, seizures, tremors, pain, numbness, lack of coordination, and weakness or paralysis of one or more legs. The effects of an injury on sensory and motor functions depend on its location and severity.
A spinal cord injury can cause loss of feeling and paralysis below the level of the injury. Mild spinal cord injuries can result in clumsy movement and mild weakness of the limbs. Moderate spinal cord injuries can cause a greater weakness of the limbs. In severe spinal cord injuries, a complete loss of movement (paralysis) and feeling can occur. However, not all spinal cord injuries cause paralysis. For example, injury to the spinal cord in the lower back can result not in limb paralysis but in loss of bladder control.
Brain injuries result in different effects, again depending on which part of the brain is affected. Injuries to the brain stem can cause a loss of balance, weakness of the limbs, hyperactive reflexes, stupor, or coma. Injuries to the cerebellum can result in a lack of coordination of the head and legs, tremors, and a loss of balance. Injuries to the cerebrum can cause complete or partial blindness, loss of the sense of smell, seizures, coma, stupor, pacing or circling behavior, and inability to recognize an owner.
Some injuries to the nervous system can cause damage that is not evident until 24 to 48 hours after the injury occurs. Longterm damage is usually caused by swelling or internal bleeding of the vessels in the brain. Strokes caused by clogged arteries or high blood pressure are rare in pets.
Mechanisms of Disease
In addition to the effects of injuries, nervous system disorders can include birth defects, inherited disorders, infections and inflammations, poisoning, metabolic disorders, nutritional deficiencies, degenerative diseases, cancer, or unknown causes.
Most birth defects, often called congenital disorders, are obvious at birth or shortly after. Some genetic diseases cause the neurons to degenerate slowly and irreversibly in the first year of life. In other inherited diseases, such as epilepsy, the animal may not show any signs for 2 to 3 years.
Infections of the nervous system are caused by specific viruses or microorganisms. Other inflammations, such as certain types of meningitis, can be caused by the body’s own overactive immune system. These are known as autoimmune disorders. Various chemicals can cause a toxic reaction in the nervous system. These include certain pesticides and herbicides, rat poisons, antifreeze, and sedatives. High doses of certain types of medication can also be toxic to the nervous system. Botulism, tetanus, and tick bites, as well as coral and tiger snake venom, can also affect the nervous system and cause paralysis.
Some metabolic disorders affect the function of the nervous system, including low blood sugar, shortness or loss of breath, liver disease, and kidney failure. Thyroid and adrenal gland abnormalities and electrolyte abnormalities can also cause neurologic signs. A lack of thiamine (vitamin B1) in the diet can cause a loss of motor control, stupor, seizures, and coma in cats. Inadequate amounts of B6 can cause seizures.
Tumors of the Nervous System
Tumors of the nervous system are classified by the cell type affected, the behavior of the tumor, the pattern of growth, and any secondary changes seen in and around the tumor. Tumors may grow from cells within or associated with the nervous system, they can form in surrounding tissues, or they can spread to the nervous system from another tumor in the body. Tumors affecting the nervous system are less common in cats than in dogs.
A variety of tests are used to confirm the presence of a brain tumor. Plain x-rays, myelography, scintigraphy, computed tomography (CT) scans, and magnetic resonance imaging are all used to diagnose nervous system tumors. Cerebrospinal fluid analysis may also be useful.
The outlook for animals with nervous system tumors varies and depends on the location, extent of tissue damage, access by surgery, and rate of tumor growth. Recent improvements in treatment have centered on surgical removal, radiation therapy, and chemotherapy.