The Neurologic Evaluation in Horses
Evaluation of the nervous system begins with an accurate history and general physical examination, followed by a neurologic examination. Laboratory tests are often needed to diagnose the specific problem. Common 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. They include the nerves that transmit smell, those responsible for vision and the movement of the eye, those that control facial movements, those responsible for hearing and balance, and those responsible for chewing, swallowing, vocalizing, and movement of the tongue (see Table: The Cranial Nerves). Testing the reflexes of these nerves can help identify the location of damage. Your veterinarian will perform specific tests designed to pinpoint any signs of dysfunction in these nerves.
The Cranial 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, 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.
Your veterinarian will watch your horse as it walks, trots, runs, turns, steps to the side, and backs up. Signs of dysfunction include circling, weakness or complete paralysis of any limbs, falling, stumbling, rolling, or loss of coordination. Walking horses up and down a hill or on and off a curb may make subtle problems easier to notice. Evaluation of gait is especially important in horses because spinal reflexes usually are not tested unless the horse is lying down.
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 are also used to evaluate the torso and hind legs. 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 some poisonings, certain infections, muscle disorders, and the rare (in horses) condition called 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 inflammatory condition 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.
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 myelography, 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 (EEG) records electrical activity on the surface of the brain, which is affected by changes in deeper parts of the brain. Results are abnormal with hydrocephalus (excess cerebrospinal fluid), meningitis or encephalitis, head injuries, and brain tumors. An electroencephalogram can sometimes help determine the cause and severity of a seizure. An electromyogram (EMG) 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.