The Neurological Examination - Part I
Your horse has had subtle, but distinct performance problems over the past year - you and your trainer can't decide if he is lame or not. Your veterinarian suspects that your horse has neurological disease - and is going to do a full neurological examination. What does this mean? What is your veterinarian going to do? What is she trying to find out?
When your veterinarian does a neurological examination, she is trying to sort out two things. First, does a neurological problem exist? Second, what and where in the neurological system is the problem? It can be very difficult to sort out whether a subtle gait abnormality is a neurological problem or not. The ingredients to successfully diagnosing neurological problems are a thorough knowledge of how the normal horse behaves and moves, a systematic examination, and experience.
What should I expect my veterinarian to do during the neurologic examination?
Your veterinarian will start with a complete medical history. Details such as the age, the breed, and the sex of the horse can be very helpful, because some diseases preferentially affect certain groups of horses. It is very important for your veterinarian to know what you use your horse for, and how hard the horse is being used. For instance, if your horse is a pasture potato, he may have had subtle signs for quite some time that you haven't noticed because you haven't challenged your horse with difficult maneuvers (such as going up and down hills) and you haven't spent as much time with your horse. Your veterinarian will want you to try to pinpoint exactly when the abnormal gait or behavior began, and whether it has progressed rapidly or remained relatively static. Your veterinarian is like a detective, who needs as many clues as possible to solve the mystery - your horse's problem.
Your veterinarian will start by assessing your horse in its usual environment. That usually translates into quietly observing the horse in his stall or paddock. First of all, your veterinarian wants to know if the horse's mentation is normal. If you think about it, this is something that you are probably good at assessing, yourself. Ask yourself, how does a normal horse act in this situation, and then observe your horse. Any time that your horse is in a strange situation, he should be alert and bright. His ears should be pricked forward toward any strange sounds, his head should be up, and he should be ready to assess the situation and decide if flight is the right course of action. For many horses, this includes a veterinarian's visit! Your veterinarian will also want to observe the way that your horse moves in the paddock or stall. For instance, does he seem balanced? Does he trip or stumble? Does he bump into things? All of these things could indicate neurological disease.
After the initial observation, your veterinarian will want to do a complete physical examination. This is important for many reasons. There are some systemic diseases, such as liver or kidney disease, that can have neurologic manifestations, and you might pick up some indicators on a general physical examination. It is also possible that some of the abnormalities are due to things that have no neurologic connections at all. Perhaps your horse is tripping because his toes are too long or his shoes are loose. There may be other indicators of neurologic disease on physical examination. Some neurological diseases cause muscle loss that is asymmetrical. Your veterinarian will look to see if your horse is well muscled, and if that musculature appears the same on both the left and right sides. Some neurological diseases cause foot dragging and stumbling - this will often manifest itself as abnormal wear on the toes of your horse's hooves. Some neurologic diseases may cause the horse to seizure - your veterinarian may suspect this if she sees signs of unexplained trauma - scrapes and abrasions that aren't the result of a pasture fight. As part of that complete physical examination, your veterinarian will often wish to perform a complete ophthalmologic examination. Perhaps he is bumping into things because he is developing a cataract in one of his eyes. It is also possible to directly examine one of the nerves of the central nervous system (CNS) by looking at the back of the eye - your veterinarian can look for any abnormalities in this nerve.
Depending on what your veterinarian has found at this point, she may choose to do bloodwork - usually a complete blood count (CBC) and a serum chemistry profile. The CBC may tell you that there is infection or inflammation in the body, as may be found with meningitis (and infection of the coverings of the brain) or encephalitis (an infection of the brain itself). The serum chemistry profile may tell you that there is liver disease, which can result in abnormal mentation, or that there are serum electrolyte abnormalities, which can also interfere with the normal function of the brain.
The Neurological Examination Proper:
Most veterinarians choose to start with examination of the head. First, the veterinarian will assess the symmetry of your horse's head, especially the area of the muscles of mastication (chewing), and the muscles that give your horse facial expression, such as those governing the movement of the ears, the mouth, and the eyes. The veterinarian will open your horse's mouth and gently pull on the tongue to assess jaw tone and tongue symmetry and strength. Then, your horse's cranial nerves will be assessed.
These are twelve nerves that govern the function of various processes in the head, such as your horse's ability to cock his ears forward when he hears something interesting, your horse's ability to swallow food, or his ability to pick up even very slight noises. Why is it so important to assess these nerves? Well, your veterinarian has knowledge of the anatomy of the brain, which includes the position of the cranial nerves. Assessing these nerves is like a game of elimination - by mapping out which nerves work and which don't, your veterinarian can eventually pinpoint where in the brain the problem is located - or, conversely, can conclude that the brain is intact. Your veterinarian will assess various reflexes - the most important ones are the menace, palpebral, pupillary light, and gag reflexes.
- To perform the menace reflex, the examiner abruptly moves a hand toward the horse's eye. The appropriate response is to blink the eye, and perhaps move the head away. This helps us to determine if your horse can see. (See Figure 2)
- To perform the palpebral reflex, the examiner lightly touches the eyelid, and the horse should close his eye. This helps to determine if the horse has skin sensation, and if he has control of the muscles of the face.
- To perform the pupillary light reflex, the examiner brings the horse into dim lighting, and shines a penlight first into one eye, then the other. The pupils of the eye should constrict. This helps to assess pathways in the brain that control eye function.
- To perform the gag reflex, the examiner can reach into the back of the mouth, attempt to pass a nasogastric tube, or simply watch the horse eat.
The examiner will next examine the neck. Many diseases affect the portion of the spinal cord that runs through the neck (the cervical spine, or C-spine). Your veterinarian will assess whether the neck is symmetrical, and will palpate it for any lumps or bumps, or for signs of pain. Then, it is important to determine how flexible your horse's neck is. Most examiners will use a treat such as a piece of apple to encourage the horse to bring his neck down to the ground, up in the air, and to each side - the normal horse should be able to touch his flank with his muzzle. An inability to perform these maneuvers could indicate trauma, arthritis, or Wobbler's syndrome - all problems involving the bones, or vertebrae of the C-spine.
Your veterinarian will then examine the rest of the body for symmetry, strength, and signs of muscle loss. Neurological disease can cause loss of muscle mass, which can be especially noticeable in the hind end. Your veterinarian will look for other signs such as tremors or abnormal contraction of the muscles. He will also perform the panniculus reflex - he will take a blunt object, such as a ballpoint pen, and gently but firmly touch the skin from the level of the neck all the way down the backbone. The normal horse will respond by twitching the skin - as if a fly was bothering him. The nerves coming from the spinal cord govern this reflex, which assesses whether the horse has skin sensation and control of the muscles under the skin, and it offers a way for the veterinarian to map out where in the spinal cord a problem might exist. Your veterinarian will also test the strength of the tail, and will gently stimulate the anus - the normal horse's response is to constrict the anus and clamp his tail down. These maneuvers can help to determine if the problem is in the sacrocaudal, or very tail-most section of the spinal cord.
Now, your veterinarian will be ready to assess your horse's gait. The most common series of maneuvers include backing, large and small circles, going up and down an incline, and the tail pull. When your horse is being backed, your veterinarian is noting whether your horse interferes with his feet, or, in some severe cases, whether this maneuver makes him lose his balance or even fall down. While watching your horse perform circling maneuvers, your veterinarian is observing your horse's leg placement. (See Figure 3) Does he swing a leg out (circumduction), pivot on one leg, drag his toes, or interfere? Going up and down an incline, your veterinarian is looking for stumbling, interfering, or dragging the toes. (See Figure 4) Backing All these maneuvers allow your veterinarian to assess whether the problem may be in the spinal cord, and where in the spinal cord that problem might reside. (See Figure 5) In order for your horse to have a normal gait, the signals must be able to get from the brain, to the spinal cord, and to the nerves that govern the muscles. If the circuit is interrupted at any point along this line, then your horse will likely exhibit gait abnormalities. Finally, your veterinarian will assess your horse's strength by pulling on his tail as he walks. The normal horse may take one or two steps to realize what is going on, and then should easily resist a pull on his tail.
Urination and Defecation:
Most horses voluntarily allow the veterinarian to assess this during the course of an examination. Some horses will need the encouragement of a freshly bedded stall. The ability to urinate and defecate normally is governed by nerves coming from the spinal cord - abnormalities of these functions can further help to localize the problem.
At the end of the examination, your veterinarian may feel that she has adequate information to diagnose the problem, or she may feel that she still needs additional information. The most common ancillary tests include x-rays of the cervical spine, a myelogram, and a cerebrospinal fluid analysis.
The portions of the horse's body that are easily accessible to x-rays include the skull and the C-spine. Other areas are too thick for even the most penetrating x-ray beam, or will require general anesthesia to obtain the correct exposure. With x-rays, your veterinarian can look for signs of trauma, developmental bone disease that impinges on the spinal cord (Wobbler's Syndrome), or arthritis.
The myelogram must be done under general anesthesia. In this procedure, x-rays of the horse's spine are taken while a contrast agent is placed in the space surrounding the spinal cord. This does involve some risk to the horse, as the contrast agent must be placed into the atlanto-occipital space - a little bit behind the level of the horse's ears. There is a portion of the brain at this area, so the veterinarian must be very careful not to hit the brain with the needle that is used to place the contrast agent into the space. After the contrast agent is placed, x-rays are taken while the neck is manipulated in order to see if the spinal cord becomes pinched during these maneuvers - thus confirming a diagnosis of Wobbler's Syndrome.
Cerebrospinal Fluid Analysis:
In this procedure, a small amount of fluid is removed from the area surrounding the spinal cord. This can be done in the standing, sedated horse, from the lumbosacral space (roughly at the level of the highest point of the croup) or in the anesthetized horse from the atlanto-occipital space. It is less risky to the horse to retrieve the fluid from the lumbosacral space, but more risky to the person performing the maneuver - every once in a while, a horse will have a strong reaction to the 'tap'. The fluid can be analyzed for abnormal protein, cells, or evidence of diseases such as equine protozoal myeloencephalopathy (EPM).
Other, less common ancillary tests include the EEG, which measures the electrical activity of the brain, or brain waves, the electromyelogram, which tells us about the electrical activity of muscle cells and the nerves that control those cells, nerve conduction tests, which tell us about the way that signals are going through peripheral nerves, and computed tomography (CAT scan), which can give us a more detailed picture of the brain, skull, vertebrae, and spinal cord than an x-ray can.
Neurologic disease can be puzzling, and difficult for even experienced veterinarians to diagnose. However, with a careful, methodical neurologic examination and selected ancillary tests, the detective can find the right clues, and the mystery can be solved. The goal? A correct diagnosis and appropriate therapy to return your horse to his usual functions.