The Neurological Examination – Part I
Dr. Melissa Mazan
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. Your veterinarian will open your horse's mouth and gently pull on the tongue to assess jaw tone and tongue symmetry and strength, then assess your horse's cranial nerves. 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, swallow food, or pick up even very slight noises.
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, 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 to determine if your horse can see.
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 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, which is 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. It is also 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 will note 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 will observe your horse's leg placement. 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. 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. 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.
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.
Urination and Defecation
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.
Most horses voluntarily allow the veterinarian to assess this during the course of an examination, but some horses will need the encouragement of a freshly bedded stall.