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Examining the horse for nervous system diseases

Carrie Finno, DVM, PhD, University of Minnesota

A healthy nervous system is essential for equine performance. However, when do you need a complete neurologic examination performed on your horse?

If you or your veterinarian suspects that your horse has neurological disease, it is necessary to have a full neurologic examination performed. Neurologic disease can be difficult to sort out in the horse and it is important to have a veterinarian experienced in neurologic examinations examine your horse. Similar to physicians with specialization in neurology, there are board certified large animal internal medicine specialists at the University of Minnesota that specialize in neurologic disease. We begin asking, first, does a neurological problem exist? Second, what and where in the neurological system is the problem located? A detailed examination can provide answers to these questions.

What does a neurologic examination involve? Questions in a complete medical history often include:

It is often very useful if owners keep a journal of any changes noted in their horse's behavior or gait. Digital videos showing the abnormal behavior are great to have, especially if the horse only does the behavior under certain conditions. Sometimes, horses won't show the abnormality when the veterinarian is examining them, either because they are more stressed than usual or in an unusual environment, such as a referral vet hospital. It can be very frustrating to go through all the trouble of scheduling an appointment only to have your horse not show the strange behavior or abnormal gait so taking the time to make a video is worthwhile. The examination used by specialists in internal medicine is described below.

Step 1: Examine in the stall/paddock. We begin by assessing your horse in a stall or small paddock, wherever the horse is more comfortable. One of the first things to evaluate is your horse's mentation, which means is your horse behaving normally for the environment that he is in. If your horse is in a new environment, he should be bright and alert and looking around. If your horse is in their stall at home, he/she may be quieter, but still appear bright when someone enters the stall. Next, is looking to see how your horse moves around the area. Does he stumble or bump into things? We assess how your horse stands at rest, or his posture. Horses do not need to stand squarely at all times but they shouldn't rest with one or more limbs out from under their trunk for extended periods of time (Figure 1). Then the resting respiratory rate is taken while your horse is in the stall or paddock before being handled, which may excite him. During this time, your horse's body condition score, which is a score assigned based on your horse's weight (is he too skinny, in good condition, or too fat?) is recorded.

Step 2: Full physical examination at rest. After the initial observation, a routine physical examination is performed, which includes assessing your horse's cardiovascular, respiratory, gastrointestinal and musculoskeletal systems. Liver or kidney disease can have neurologic signs, and some abnormalities might be apparent on a general physical examination. It is always important to perform a basic lameness examination at the same time that the neurologic examination is performed because they are often difficult to distinguish. In addition, in many horses, there may be both a lameness and a neurologic component to the problem. While your horse is standing for his physical examination, we pay careful attention to his muscle symmetry to determine if there is any atrophy, or loss. Muscle atrophy may be symmetrical or asymmetrical and provides useful information (Figure 2). The hooves are examined for abnormal wear. Horses that drag their hind toes often square the toe off. A basic ophthalmologic examination may be useful if your horse is running into objects as he may have a primary eye problem.

Step 3: Full neurologic examination: cranial nerves. The next thing to perform is an examination of your horse's cranial nerves. We begin by performing a menace response, where a hand is "menacingly" moved towards your horse's eye. The correct response is to blink. This response is tested from both the inside (medial) and outside (lateral) approaches to fully evaluate this reflex. While examining the eyes, the pupillary light reflex is tested, where a bright light is shone in your horse's eye and the pupil constricts. While performing this test, pupil position is also evaluated. The pupil should be in the center of the horse's eye and should not be moving when the horse's head is held still. Next your horse's facial sensation is tested (does he have sensation up his nostril, along his eyelid and in his inner ear?) and muscle tone to the face (is there any evidence of facial asymmetry, such as a droopy ear or eyelid or a muzzle that deviates to one side?). We also assess muscle symmetry of the masseter (cheek) muscles and the temporalis muscles under forelock. Your horse is carefully evaluated for a head tilt and a temporary blind-fold might be used, where a towel is placed over the horse's eyes to see if this makes it worse. Lastly, your horse gets a treat for being such a good patient and in order to assess his ability to chew and swallow. Your horse thinks this is the best part of the exam!

Cervical (neck) region: A horse's neck is felt on both sides of its spine from its head to its withers for pain or swelling. Then flexibility of the neck is tested by offering treats on each side at the shoulder and determining how far the horse can stretch. It is important to remember that, although abnormalities in these tests might indicate neck pain, they are often very subjective. The cervicofacial reflex is tested by pushing a blunt object, such as a pen cap, into the skin on his neck and noticing if the horse twitches its ear or grimaces on the same side, which is a normal response.

Thoracolumbar (trunk) region: The cutaneous trunci reflex, which is the skin rippling reflex that horses use to keep flies off, is tested by looking for a twitch response to pressure.

Sacrocaudal (tail) region: A horse's tail and anal tone are examined to determine if there are any abnormalities that could be localized to the sacrocaudal region. If a horse has had difficulty urinating or passing manure, a rectal examination is used to further assess this region.

Gait exam: Assessing a horse's gait is an important part of the exam. The most common series of maneuvers include walking and trotting in a straight line, walking in serpentine figure, spiraling in on a circle at the walk, backing, going up and down an incline and on or off curbs or steps, walking with your horse's head elevated, and the tail pull. All of these tests are aimed at evaluating the gait and coordination when your horse is asked to do complex movements. When circling, it is important to pay careful attention to a horse's leg position. Does the horse make mistakes and step on himself? Does the horse drag its forelimbs when backing? Sometimes, in subtle cases, it is necessary to perform a full gait examination, then lunge the horse for 15 to 20 minutes, and repeat the gait examination. Subtle abnormalities that were not noticeable before exercise may become apparent after some work.

Interpreting the neurologic exam: grading system: Once the exam is completed, a veterinarian will have an answer to the original two questions: (1) Does a horse have neurologic abnormalities? (2) If the horse does, where do they localize to and how severe are they?

To answer question #2, a veterinarian will localize a horse's clinical signs to either a region of the brain, spinal cord or to a peripheral nerve (i.e. nerve outside the spinal cord). If a horse demonstrates a spinal ataxia (i.e. in-coordination originating from the spinal cord), a grading scale to determine how severe a horse's gait deficits are is assigned. Each limb is graded separately with regard to four aspects: ataxia (incoordination), weakness, dysmetria (a combination of overshooting and weakness commonly observed with disease of the cerebellum) and spasticity. Grade 0: Normal; no deficits observed; Grade 1: Mild intermittent gait deficits; Grade 2: Mild consistent gait deficits; Grade 3: Moderate consistent gait deficits; Grade 4: Severe consistent gait deficits; or Grade 5: Recumbent.

A great deal of experience is required to interpret the signs, localize them to the proper region, and to select the best diagnostic tests. As we continue this series, we will discuss the four most commons causes of spinal ataxia , and the diagnostic tests that can be used to try and determine which of these diseases is responsible for a horse's clinical signs.  

The neurologic exam answers two questions: does your horse demonstrate neurologic abnormalities? If yes, what specific part of the nervous system is affected and to what severity? Localizing the clinical signs to specific regions within the nervous system is important because different diseases affect different regions. The following region/categories are often used:

Diseases that affect the spinal cord are the most common neurologic abnormalities encountered in the horse. If your veterinarian suspects neurologic abnormalities that localize to other regions, the diagnostic tests would differ from what are discussed here.

The predominant clinical sign involved in all the diseases we will be discussing is ataxia. Ataxia is defined as the inability to control voluntary movement, and most frequently results from disorders in particular regions of the brain or spinal cord. There are different types of ataxias:

Sensory ataxias due to spinal cord disease is common. Many horses with sensory ataxia will also present with signs of weakness and a veterinarian will classify the type of weakness as upper (hyperactive, signs of spasticity) or lower motor neuron (diminished reflexes, muscle atrophy). Your horse's neurologic signs will be graded. Each limb is graded separately with regard to four aspects: ataxia, weakness, dysmetria (combination of overshooting and weakness commonly observed with disease of the cerebellum) and spasticity.

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