All about seizures

One night you hear kicking and crashing coming from the barn and think you have a cast horse. You run to see if you can help, and what you find is a horse down on his side paddling as if galloping on the ground. His head and neck are extended and his eyes are slightly rolled back in their sockets, showing more white than normal and moving back and forth in a quick side-to-side motion.

The horse does not respond to touch or sound and is oblivious to his surroundings. You can do nothing to help except to stay out of the way and ensure your own safety. The thrashing stops after 10-15 minutes that seem like hours. The horse stays down, is depressed, has a decreased response to stimulation, and acts somewhat blind. You call your veterinarian, who arrives and confirms that your horse had a seizure.

What I have described is the classic "grand-mal seizure." This article will discuss the various types of seizures and the disease states that can cause them.

What's a Seizure?

The most common seizure disorder is epilepsy, which is fortunately rare in the horse (it is actually controversial if "true" epilepsy actually occurs in the horse). There is little that can safely be done to help a 1,200-pound animal having a seizure.

To understand seizures, we need to review anatomy and physiology. Seizures occur when specific anatomical locations of the brain (forebrain and thalamus) are affected by some type of pathology. By definition, a seizure (also known as a fit, ictus, or convulsion) is considered abnormal behavior; seizures are physical expressions of abnormal electrical discharges in the forebrain neurons that initiate spontaneous, paroxysmal (spastic), involuntary movements.

The seizure event is broken into three phases--the pre-ictal, ictal, and post-ictal periods. Many people who suffer from epilepsy know they are going to have a seizure just before it happens. There might be a strange feeling or focal muscle twitching. The ictal phase or the seizure can be as mild (only muscle twitching), or it can be a full grand-mal seizure.

Typically the ictal phase only lasts 10-20 minutes unless there is an ongoing factor (such as a seizure occurring as a result of a severe allergic reaction) that prolongs the event. The post-ictal phase can last minutes to hours and generally consists of depression, an out-of-touch feeling with your surroundings (often called the "aura" in people), and blindness. The blindness is temporary and can last from a few hours in adults to a few days in babies.

What Causes a Seizure?

The pathology that causes the abnormal electrical discharges of a seizure can be varied and includes genetic abnormalities in the brain tissue (epilepsy), structural defects (tumors), traumatic insults, metabolic insults, and toxic insults.

The syndrome of inherited, recurrent seizures that continue throughout life without an underlying cause (disease process) is "true" epilepsy. Technically this has not been demonstrated in the horse, but it can be difficult to follow genetic lines in some cases and to diagnostically rule out all other potential causes in older horses.

What has been called "benign" epilepsy is a seizure condition of foals up to 12 months old and occurs in all breeds, but has a higher incidence in Arabians. If a seizure occurs in very young foals, it is very important to rule out other causes, as seizures in foals can be caused by various diseases (such as low blood sugar, septicemia, and hepatitis) that require specialized treatment. If there are no other abnormalities and the diagnosis is "benign seizure disorder of foals," the news is good as they typically outgrow the problem.

Adult horses have been reported to develop seizure disorders following a lightning strike.
Any structural abnormality, i.e. tumor, can cause seizures if it's in the correct location within the brain. A common tumor that grows in the forebrain (front section of the brain) of the horse is called a cholesteatoma. These are slow-growing tumors of the older horse and are frequently found incidentally at necropsy in horses that were neurologically normal, although they can cause seizures and other neurologic abnormalities. Unfortunately, the diagnosis can be difficult and expensive to confirm, requiring an MRI or CAT scan.

Another common brain tumor affecting the horse is a pituitary adenoma, which in addition to the familiar signs of Cushings (long, non-shedding hair coat, founder, excessive thirst and urination, etc.) can occasionally cause blindness, but not typically seizures because of the location of the pituitary gland.

Another cause of seizure is the accidental injection of various drugs into the carotid artery in the neck when attempting an intravenous (IV) injection. This is one reason why it can be very dangerous to attempt IV injections unless you have been trained by a veterinarian in the correct procedure. Within the neck just beneath the jugular vein is the carotid artery, which carries oxygen-rich blood from the heart directly to the brain.

Therefore, anything injected into the carotid goes directly and relatively undiluted to the brain. The intensity of the reaction can vary from a mild seizure to instantaneous death or prolonged coma, depending on the type of drug and how much of it went into the artery.

Another reason for untrained people to stay away from IV injections is that in addition to the vein and artery, there is a nerve bundle in close proximity. If caustic drugs such as phenylbutazone are injected outside the vein, they can cause nerve damage that can lead to paralysis of the larynx on the right side of the neck. For most situations involving drug injection into the carotid artery, there is no specific treatment. So, it is best to avoid the situation altogether and leave IV injections to your veterinarian.

There is also a variety of metabolic disturbances that can cause seizures. For example, altered concentrations of sodium in the blood can affect the electrical activity of the brain. A situation leading to this is when a horse has not had access to water for a few days--be sure to check the heated water tanks and automatic waterers in winter. Also, as the horse becomes progressively more dehydrated without loss of electrolytes (as when sweating), the blood concentration of sodium increases (hypernatremia).

The real trouble starts when the dehydrated horse is discovered and allowed free access to water--you do not want this horse to be rehydrated quickly. In this case, the brain tissue is slowly compensating for the increasing sodium concentration in the blood to maintain normal "electrical" function. If the dehydration is rapidly corrected, the blood concentration of sodium is rapidly lowered, but it takes more time for the brain to adjust. Now the brain has a higher sodium content than the blood.

Going back to a little basic chemistry, the osmotic (water) balance is mainly determined by the sodium concentration, with water typically going where sodium concentration is higher along the osmotic gradient. So water moves into the brain, causing cerebral edema (fluid swelling) and potentially seizures.

Very low blood concentrations of sodium (hyponatremia) can also cause seizures. This is most commonly observed in foals with severe kidney (renal) disease or water overload (water toxicity). Sometimes very young foals discover water and for whatever reason decide to drink enormous quantities, which can affect their electrolyte/water balance.

Low blood concentrations of calcium (hypocalcemia) can be associated with seizures. If your horse's diet is not properly balanced with an adequate amount of calcium, he can go into a hypocalcemic state. Equine diets with little roughage and a lot of grain are typically low in calcium.
Another cause of seizures to consider is toxins, including organophosphates, chlorinated hydrocarbons, strychnine, lead, bracken fern, locoweed, and tetanus.

Salt can also be a toxic element when eaten in extremes. Free-choice salt is mandatory for the good health of your horse, but every once in a while you have a horse that digs right in and polishes off a salt block in a few minutes. You have to take note and limit salt block access with such a horse. This situation could lead to a hypernatremic state and cause seizures by affecting the osmotic/water balance as previously discussed.

Treatments

If the seizures are being caused by a specific disease or toxicity state, then treatment will focus on that and the seizures will resolve with the problem. If they are mild and/or infrequent, your veterinarian might elect not to directly treat them unless they become more frequent or dramatically worsen.

The common treatment for seizures when the primary cause cannot be treated is the drug phenobarbitol (a powerful sedative). The response to this drug is variable; the goal is to figure out a daily dose that will prevent the seizures without sedating the horse to a point where he has a poor quality of life (a very fine line in some cases). With over-sedation, a horse can become so lethargic that it is hard to get him to leave the stall. Another difficulty with therapy is that the drugs become less effective over time, requiring adjustment of the dosage.

Other human anti-seizure drugs have been used on horses, but have difficulties associated with expense, maintaining an effective dose, and toxicity. In addition, these drugs and phenobarbitol can cause liver disease, so frequent monitoring of liver function is necessary.

Dangers to Humans

Another aspect of having a horse that seizes in the barn is that they can be extremely dangerous to be around. They can become quite violent and have no control over a flailing 1,200-pound body, and neither do you. The safest thing to do is to get out of the stall and shut the door. Depending on where they are when having a seizure, the horse can bang himself up quite badly, and it can be very alarming to watch.

Some owners maintaining a horse who has seizures will heavily pad the stall in an effort to limit self-injury. The seizures typically happen unpredictably, but some owners can tell when one is approaching by subtle behavioral changes in the pre-ictal phase (often only minutes before the actual seizure). Every case is different, but caring for a horse that has seizures can be very taxing and place you in a situation with some liability should he hurt someone while having a seizure.

Take-Home Message

Seizures are not normal--if your horse has one, contact your veterinarian immediately. There are causes that can be reversed, and disease processes that can be treated. Above all, try to protect yourself and your horse from injury during and after the seizure.