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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 |
