Seizures: Examining and Diagnosing

Equine seizures can be disturbing to witness, and they can be puzzling for a veterinarian to diagnose, simply because the veterinarian isn't always there to watch the horse as it has a seizure. Diagnostic tests can be expensive and time-consuming, but are necessary to determine the cause of the seizures and help identify ways to prevent them in the future. Stephanie Kube, DVM, neurology resident at the University of California, Davis, explained how to work up a horse with seizures at the 50th annual American Association of Equine Practitioners (AAEP) Convention in Denver, Colo., Dec. 4-8, 2004.

Seizures are uncontrolled discharges of neurons, and they are fairly uncommon in horses. They can be "partial" (focused in a localized area) or "generalized" (all-over) seizures, and can be caused by extracranial (outside the head) and intracranial (inside the head) abnormalities. Extracranial causes can include metabolic or toxic causes, while intracranial causes can include trauma, neoplastic growths (caused by a neoplasm, an abnormal growth of tissue whether benign or cancerous), infections, inflammation, or possibly inherited problems.

Elevated estrogen levels in mares have been known to cause seizures in mares and their foals; hypoxia (low oxygen levels), sepsis (systemic infection), and/or hypoglycemia (low blood sugar) are common causes of seizures. (Hypoglycemia can also cause seizures in adults.) True epilepsy is characterized by recurrent seizures of an undetermined cause and is thought to occur in Arabians (suspected familial inheritance), some ponies, and Paso Finos.

The diagnostic plan can include, but is not limited to, a thorough history collected on the horse, physical and neurologic examinations, development of a list of possible causes, a complete blood count (CBC), urinalysis (UA), electroencephalography (EEG, a non-invasive way to analyze brain activity), cerebrospinal fluid (CSF) analysis, and imaging, including skull radiographs, computed tomography (CT), or magnetic resonance imaging (MRI).

Complete History

Most important in the diagnostic plan is a complete history of the horse, including environment, nutritional status, diet, vaccinations, deworming, prior illnesses or trauma, travel history, and history of exposure to other animals, plants, pesticides, and contaminants.

"Autonomic disturbances such as defecation, urination, or salivation, and tonic (tone)/clonic (rapid succession of contraction and relaxation) muscle movements are more suggestive of a seizure," noted Kube. In a partial seizure, a horse "may turn its head to one side, have abnormal tonus/clonus in facial musculature or a limb, or show paroxysmal (spasmodic or seizure-like) behavior changes. A partial seizure may progress to a generalized seizure."

The owner should carefully describe the seizures, their frequency and duration, and how the horse behaves between seizures, since this information can help the veterinarian determine a cause. Seizures can be confused with other disorders such as narcolepsy (a sleep disorder) or cataplexy (a sudden loss of muscle tone). "A video recording of events is ideal, if possible," she said.

An extremely important differential diagnosis to keep in mind is rabies; the possibility of exposure highlights the necessity of the veterinarian to be cautious and to consider all signs and history.

Physical Exam

The thorough physical exam (with CBC and UA) will look for other extracranial causes, because sometimes cardiac or respiratory signs can help pinpoint a cause--abnormalities in this realm could indicate a potential syncopal (a temporary suspension of consciousness) episode rather than a seizure. Episodic weakness, such as myasthenia gravis, may also be confused with a seizure. Skull radiographs might be useful for identifying fractures or deformities.

Upon examining the eyes, the veterinarian might detect chorioretinitis (inflammation of the choriod and adjacent retina) associated with viral, fungal, or protozoal diseases, or papilledema resulting from increased intracranial pressure. By examining the ears, the veterinarian might detect leakage of CSF caused by skull fracture or trauma. Additionally, elevated body temperature (thought to contribute to or be a result of seizures), body condition, and posture should be noted, and integumentary (skin), gastrointestinal, lymphatic, and urogenital systems should be examined for anomalies.

Neurologic Exam

Kube explained, "In the neurologic exam, we look at mentation (mental state), gait and posture, cranial nerves, and postural reactions." She said neurologic exams could be misleading post-seizure, as blindness or behavioral changes could occur between seizures and can last from minutes to hours. Day- to week-long blindness has been noted in some Arabians with familial epilepsy. Be aware of the effects of anticonvulsants and sedatives that might have been administered before evaluating the horse.

Rule-outs with laboratory tests include liver or renal disease, hyperlipidemia (elevated lipids in the plasma), or hyperkalemia (as hyperkalemic periodic paralysis or HYPP, the muscle disorder found in certain lines of Quarter Horses, could be confused with seizure activity).

CSF analysis should be completed unless cerebral edema or an increase in intracranial pressure is suspected. In that case, it should be approached cautiously or avoided altogether. Skull radiographs might be useful for identifying fractures or deformities, although CT scan and MRI are preferable, albeit not readily available.

"With the very little we know about seizures in horses, EEG can help us understand them better," said Kube. By looking at the brain activity, veterinarians might be able to identify a seizure focus or a diffuse (widely distributed) disease such as encephalitis. She added that values for a normal EEG are currently being established so this can be validated as a useful test.

Treatment recommendations are based on pharmacokinetic properties and toxicological effects, because data on treating equine seizures with medication is limited. Diazepam is used to control seizure recurrence, but it is not a good choice for long-term control of seizures because of its short half-life (it doesn't last very long in the blood). Typically, long-term anticonvulsants aren't prescribed for a single seizure because of the difficulty in knowing if it was a true seizure that occurred, but if seizures recur (or if there is no doubt that a seizure occurred without an obvious reason), phenobarbitol is recommended. Anecdotally, potassium bromide has been used with some success, but there is limited research on its use as an anticonvulsant in horses. Kube mentioned that phenytoin is no longer favored as an anti-convulsant. Pharmacologic treatment is considered effective if the seizures stop.

Kube said, "A history of more than one seizure warrants admitting [the horse] to a horse hospital," where he should be kept in a padded stall with a padded helmet.

"Client education is important in managing and treating seizures," she added. "I strongly discourage assistance of the horse (during a seizure), and that horse should not be ridden or worked."

She recommends the owner keep a seizure log of the horse and his response to therapy. More EEGs would be ideal, but this isn't always possible in the ambulatory environment. Removal from seizure medication should be gradual, after there has been no evidence of seizure activity for a significant period of time and there has been no underlying cause (such as intracranial space-occupying mass) shown to be causing the seizures. When a horse has gone a period of time without the use of anticonvulsants without a seizure and the veterinarian has given his or her blessing, the horse should be safe to ride.

Kube presented several case studies, with causes ranging from a neoplasia to a non-progressive (not worsening) inherited epilepsy.

"The key points are to understand the causes of seizures," she concluded. "If you understand, (the veterinarian) can help guide you through the diagnostic plan. Treatment should be considered carefully. It is important to treat the underlying cause as well as treating the seizures with an anticonvulsant. Client communication and follow-up are extremely important."

About the Author

Stephanie L. Church, Editor-in-Chief

