The Neurologic Evaluation of Dogs
Evaluation of the nervous system begins with an accurate history and general physical examination, followed by a neurologic examination. There are a number of specific physical tests that can be carried out to evaluate the functioning of the various components of the nervous system. These include tests of various reflexes, muscle function and control, and posture and gait.
Laboratory tests are often needed to diagnose the specific problem. Common laboratory tests include blood tests, urinalysis, analysis of the cerebrospinal fluid, x-rays, computed tomography (CT) scans, magnetic resonance imaging (MRI), and evaluation of the electrical activity of the brain, peripheral nerves, and muscles.
A neurologic examination evaluates 1) the cranial nerves, 2) the gait, or walk, 3) the neck and front legs, and 4) the torso, hind legs, anus, and tail. Your pet’s reflexes will also be tested to determine, if possible, the location of the injury in the brain, spinal cord, or nerves in the peripheral nervous system.
The 12 pairs of cranial nerves extend from specific segments of the brain stem to the left and right sides of the head (see Table: The Cranial Nerves). They include the nerves that transmit smell, those responsible for vision and the movement of the eyes, those that control facial movements, those responsible for hearing and balance, and those responsible for chewing, swallowing, barking, and movement of the tongue. Testing the reflexes of these nerves can help identify the location of the damage. Your veterinarian will perform specific tests designed to pinpoint any signs of dysfunction in these nerves.
An evaluation of the cranial nerves tests mental activity, head posture and coordination, and reflexes on the head. Signs identified during this evaluation indicate an injury or disease of the brain. Signs of damage to the cerebrum and brain stem can include mental deterioration, constant pacing, seizures, depression or coma, or a head turn or circling in one direction. A head tilt, bobbing, tremors, or other unusual head movements may indicate damage to the cerebellum.
The Cranial Nerves
Evaluation of the neck and front legs will include searching for evidence of pain and loss of muscle size or tone, which may indicate an injury to the upper spinal cord. Various types of tests are done to help detect minor spinal cord injuries.
Some examples of tests that are commonly used to evaluate the neck and front legs include the wheelbarrow test (in which the back legs are lifted slightly and the animal is evaluated while walking on its front legs), the righting test (in which the dog is placed on its side or upside down to see how well it can right itself), and the positioning test (in which a foot or limb is moved from its normal position in order to evaluate how quickly and accurately the dog resumes its normal stance). Spinal reflexes and muscle condition are also evaluated.
The trunk, or torso, is evaluated for abnormal posture or position of the vertebrae, pain, loss of feeling or hypersensitivity to light touch or pinpricking, and loss of muscle mass. Some tests used to evaluate the nerves of the neck and front legs (see above) are also used to evaluate the torso and hind legs. Various reflexes can also be evaluated. Loss of muscle around the torso or hind legs can indicate damage to a nerve associated with that muscle.
Blood tests are often used to detect metabolic disorders, some of which can affect nervous system activity. Blood tests can also identify other conditions, including lead poisoning, certain infections, and myasthenia gravis, an autoimmune disease in which the connections between nerve and muscle are blocked and weakness results.
Analysis of cerebrospinal fluid (the fluid that surrounds the brain and spinal cord) is often useful for diagnosing a central nervous system disorder. Cerebrospinal fluid is collected from the base of the skull or from the lower back in a procedure called a spinal tap. An unusually high amount of protein in the cerebrospinal fluid may indicate encephalitis (inflammation of the brain), meningitis (inflammation of the covering of the brain), cancer, or a compressive injury of the spinal cord. Increased numbers of white blood cells in the cerebrospinal fluid indicate an inflammation or infection. Other disorders that can be identified by cerebrospinal fluid analysis include bacterial or fungal infections, internal bleeding, brain abscesses, and some types of tumors. Cerebrospinal fluid can also be tested for the presence of canine distemper virus, Rocky Mountain spotted fever, and some other infectious diseases.
Several different types of radiographic tests can be used to detect disorders of the nervous system. Plain x-rays of the skull and spine can detect fractures, infections, or bone cancer. However, in most infections or cancers of the brain and spinal cord, plain x‑rays appear normal. In a procedure known as my-elography, a special dye that is visible on x‑rays is injected into the cerebrospinal canal. This dye can highlight specific types of spinal problems, such as herniated (“slipped”) disks and spinal cord tumors. Computed tomography (CT) and magnetic resonance imaging (MRI) scans can also help evaluate changes in bone structure, internal bleeding, abscesses, inflammation, and certain nervous system cancers.
Other tests may be used in some cases. An electroencephalogram records electrical activity in the brain. Results are abnormal in meningitis or encephalitis, head injuries, and brain tumors. An electroencephalogram can sometimes help determine the cause and severity of a seizure. An electromyogram records electrical activity in muscles and nerves. In this test, a nerve is stimulated electrically, and the speed of conduction along the neurons is calculated. This technique can detect nerve injury and myasthenia gravis. A brain stem auditory evoked response (BAER) records electrical activity in the pathway from the sound receptors in the ear to the brain stem and cerebrum. In cases of deafness caused by nerve damage, the BAER generates no response. Brain-stem disorders may also change the BAER.