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Seizures
Seizures in cats remain difficult to control

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by Arnold Plotnick MS, DVM, ACVIM, ABVP

Introduction

There are few things more upsetting for a pet owner than witnessing their beloved companion in the throes of a seizure. Luckily for cat owners, feline seizure disorders are fairly uncommon. Whereas primary epilepsy affects up to 3% of the canine population, cats are much less susceptible. Unfortunately, the number of drugs that have been developed or recommended for seizure control in cats is limited.

Causes of seizures in cats: primary epilepsy vs. secondary epilepsy

The term “primary epilepsy” implies that the cat is having recurrent episodes of seizure activity that is associated with a primary brain disorder. Trying to prove that a cat has primary epilepsy can be difficult. They appear normal on physical examination, as well as on neurological examination, and all tests show no abnormalities, including advanced imaging tests like CT or MRI as well as examination of the cerebrospinal fluid (CSF). As such, the diagnosis of primary epilepsy is usually achieved by exclusion of other causes of seizures.

“Secondary epilepsy” suggests that the recurrent episodes of seizure activity are associated with an underlying structural disorder such as inflammatory disease, trauma, or cancer. This is seen more commonly in cats vs. dogs. Metabolic diseases and toxicities can lead to seizures, but these are less commonly seen in cats as compared to dogs. Infectious causes that should be considered in cats with seizure disorders include feline infectious peritonitis (FIP), feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), Cryptococcus, and rabies virus. Toxoplasmosis is a rare cause of seizures in cats, unless there is concurrent immunosuppression. Up to 20% of cats presenting with acute onset of seizure activity may have cerebral ischemic encephalopathy, a condition in which the brain is damaged due to decreased blood flow to a part of the brain (similar to a “stroke”). Causes of acute cerebral ischemia in cats are presently unknown. Cancer is a possible cause of seizures in cats, with the most common brain tumor being a meningioma. However, lymphoma should not be overlooked. Another possible cause of acute seizure activity in cats is the larva of the parasite Cuterebra, migrating through the brain.

Diagnosis

A thorough history and a comprehensive physical exam and neurologic exam, including a funduscopic exam (evaluation of the retinas) must be done in all cats with a history of seizures. A complete blood count, serum biochemistry panel, urinalysis, and evaluation of infectious disease (FeLV, FIV, FIP, Toxoplasma, Cryptococcus) should be performed or considered. Often, these tests are normal. If neurologic deficits were detected on examination, a CSF tap and advanced imaging tests (CT scan or MRI) are advised.

Treatment

Phenobarbital remains the first-choice anticonvulsant in cats. Numerous studies and abundant clinical experience have proven this drug to be effective in cats, and it is generally considered safe, however, cats may occasionally experience adverse effects such as sedation, excessive thirst and urination, and incoordination. Signs consistent with an allergic reaction (low platelet count, low white blood cell count, temporary facial swelling) have been reported in cats receiving this drug, and phenobarbital has also been implicated in blood clotting disorders in cats. Other potential negative attributes of phenobarbital include its potential to cause liver damage in dogs, although this is rarely reported in cats. Cats receiving phenobarbital can be difficult to regulate, as minor changes in dosage can result in large fluctuations in blood levels, either rendering the drug ineffective (blood level too low) or resulting in excessive sedation (blood level too high).

For cats that do not tolerate phenobarbital well, diazepam (Valium) is usually the second choice of most veterinarians. Unlike epileptic dogs who become refractory to treatment with Valium over time, epileptic cats remain responsive to the drug, and seizures become fairly well-controlled. Unfortunately, up to 20% of cats show minimal response. Adverse effects in some cats include unacceptable sedation and increased appetite and weight gain. Several reports have also documented severe liver toxicity in a few cats receiving Valium. Acute hepatic necrosis has been seen as early as 5 days after initiating the recommended doses of oral diazepam. Therefore, liver enzymes should be evaluated 5 to 7 days after prescribing the drug, and monitoring should continue at least every 6 months.

Dogs that do not respond well to phenobarbital are sometimes treated with primidone, a drug that gets converted into phenobarbital and other metabolites by the liver. The phenobarbital has anticonvulsant activity, and some of the metabolites are believed to possess anticonvulsant properties as well. Cats cannot take this drug, however, because the liver is not very good at metabolizing the primidone. The feline liver in general is not very efficient at metabolism (that’s why Tylenol and aspirin can be deadly to cats). Phenobarbital, Valium, and primidone are all metabolized by the liver.

The first anticonvulsant used to treat seizures in humans was bromide, in the mid- 1800s. In the early 1900s, however, phenobarbital was introduced as a more effective anticonvulsant, and bromide fell out of favor. In the early 1990s, bromide was rediscovered, and was used in conjunction with phenobarbital in dogs unresponsive to phenobarbital alone. Many veterinarians are reaching for bromide as their first choice, and many are using it as a sole anticonvulsant, rather than combining it with phenobarbital. Bromide has a number of characteristics that make it a favorable choice as an anticonvulsant. The drug is eliminated from the body via the kidneys rather than the liver, minimizing any deleterious effects on the liver as compared to other anticonvulsants. Another characteristic of bromide is that it has a very long half-life. The half-life of a drug is the amount of time it takes for the blood level of a drug to decrease 50%. This means that once an established therapeutic drug level is reached in the blood stream (the so-called “steady state” level), drug concentrations barely fluctuate when the drug is given as prescribed (every 12 or 24 hours). This is very different from phenobarbital, where accidentally missing one dose (or even being late by several hours) can lead to a brief period of sub-therapeutic blood levels, rendering the pet much more susceptible to a seizure. Adverse effects in dogs are uncommon, with sedation, incoordination, and gastrointestinal disturbance (vomiting, diarrhea, decreased appetite) being the most often reported.

Potassium bromide is quickly becoming the second-choice anticonvulsant in cats. It tends to be used when Phenobarbital on its own isn’t sufficient to control the frequency or severity of the seizure activity. While it takes dogs 4 to 5 months to reach therapeutic (“steady state”) blood levels, in cats this is achieved within 2 months. Side effects of bromide are uncommon, although there has been an association made between the use of bromide in cats and the onset of respiratory disease, most likely due to an allergic reaction to the bromide. This has been reported in humans as well. Withdrawing the medication usually causes the respiratory signs to resolve.

Conclusion

Treatment of seizures in cats remains a frustrating challenge for most practitioners. Many seizure events in cats are due to secondary epilepsy, and the use of antiepileptic medication is only a symptomatic therapy. Unless the primary disease is addressed, the response to anticonvulsant therapy will always be somewhat unpredictable.
 

        

Updated 3/30/06