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, infections and inflammations, poisoning, metabolic disorders, nutritional deficiencies, degenerative diseases, or cancer.
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, chocolate, and sedatives. 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 gland abnormalities can also cause neurologic signs. A lack of thiamine (a vitamin) in the diet can cause a loss of motor control, stupor, seizures, and coma in dogs.
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.
Brain tumors are occasionally found in young animals, but most are found in mature and older animals. In dogs, the brain is the more common site of primary tumors of the nervous system than the spinal cord (see Brain, Spinal Cord, and Nerve Disorders of Dogs: Tumors) or peripheral nerves (see Brain, Spinal Cord, and Nerve Disorders of Dogs: Tumors). Adult dogs of several brachycephalic breeds—Boxers, English Bulldogs, and Boston Terriers—are often reported as having the highest incidence of brain tumors among domestic animals. (Brachycephalic dog breeds are those that have characteristically flattened faces and short noses.)
A variety of tests are used to confirm the presence of a brain tumor. Plain x-rays, myelography, computed tomography 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 is guarded to poor, 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 (see Cancer and Tumors: Cancer Treatment).
Last full review/revision July 2011 by William B. Thomas, DVM, MS, DACVIM (Neurology); Cheryl L. Chrisman, DVM, MS, EDS, DACVIM (Neurology); Charles E. Rupprecht, VMD, MS, PhD; Kyle G. Braund, BVSc, MVSc, PhD, FRCVS, DACVIM (Neurology); Caroline N. Hahn, DVM, MSc, PhD, DECEIM, DECVN, MRCVS; Charles M. Hendrix, DVM, PhD; Karen R. Munana, DVM, MS, DACVIM (Neurology); T. Mark Neer, DVM, DACVIM; Robert Wylie, BVSc, QDA