Epilepsy is a non progressive brain disorder that induces recurrent seizures. It is very important to understand that there are numerous disorders that can cause seizures, and in order to make a diagnosis of true epilepsy, many of these other causes must be ruled out.
What is a Seizure?
A seizure is the clinical manifestation of a cerebral disorder that can appear suddenly out of a background of normality, and then disappear with equal abruptness. The term fit is often used colloquially when referring to a seizure.
In humans, an epileptic seizure is a definable event that can be classified according to categories set by the International League Against Epilepsy. A formal classification system specifically for dogs and cats does not exist, but seizures can nevertheless be classified as either partial or generalized episodes.
These are seizures in which a localized onset may be determined. They usually have a congenital (present at birth) or an acquired (cancer, encephalitis) cause, and can sometimes evolve into a secondary generalized seizure.
Generalized seizures are the most common type of seizures in dogs and cats. There are several variations of these seizures:
1. Absence seizures (petit mal): sudden brief loss of consciousness, rare in animals
2. Myoclonic seizures: muscle jerk of one or more muscles
3. Clonic seizures: rhythmic muscle contractions
4. Tonic seizures: increase in muscle tone in all skeletal muscles
5. Tonic Clonic seizures (grand mal): the most common form of seizure in pets
Tonic Clonic Seizures
Tonic Clonic (grand mal) seizures account for 60% of seizures in cats and 80% of seizures in dogs. They are usually accompanied by a loss of consciousness, and consist of a tonic phase, where the increased muscle tone causes the animal to fall on its side with its limbs extended, and a clonic phase, consisting of intense muscle jerking or paddling movements.
Causes of Seizures
In order to diagnose true epilepsy, other causes of seizures must be first ruled out. The questions a veterinarian must answer are:
Is the cause inside the brain or outside the brain?
If inside the brain, is the cause progressive (e.g. cancer, infection) or non progressive (epilepsy)?
If outside the brain, does the cause originate inside the body (e.g. low blood glucose, kidney failure, a liver problem) or outside the body (e.g. lead poisoning, organophosphate toxicity)?
Making a Diagnosis
If an animal has a solitary seizure, most veterinarians will advise against expensive and time consuming tests for something that may never happen again. If an animal has more than one seizure though, tests are indicated.
A veterinarian will start by taking a detailed case history, focusing on the description of the animals seizures, their frequency and duration, and the animals behavior between seizures. Ideally the seizure should be videotaped by the owner, and then shown to the veterinarian. It is important to differentiate a proper seizure from other causes of collapse such as an episode of syncope (where the heart misses several beats), narcolepsy or weakness due to another neurological problem (e.g. myasthenia gravis). Features of the description which indicate a seizure are:
1. Tonic (increased muscle tone) and clonic (jerky) muscle movements
2. Loss of consciousness (though not always)
3. Increased salivation
4. Spontaneous urination or defaecation
The veterinarian will then conduct a full clinical examination, followed by a more specific neurological examination. Blood will then be taken for full hematology and biochemistry profiles. A fasting blood glucose is useful to rule out hypoglycaemia, which is a frequent cause of seizures in dogs less than 6 months old and dogs with an insulinoma, an insulin secreting pancreatic tumor. If liver enzymes are raised, a further liver function test (bile acid stimulation) may be done. Blood lead levels should be measured in animals that may have had access to lead containing materials.
Xrays of the chest and abdomen are sometimes performed to check for any other significant disease, and fecal analysis is recommended in puppies with seizures as heavy parasite burdens have been implicated.
If all of the above tests are normal, and no disease outside the brain has been detected, many animals will be diagnosed with epilepsy as further tests are of the expensive variety. If further tests for intracranial (within the brain) disease are to be performed, cerebrospinal fluid (CSF) analysis is the first step. CSF is tested for the presence of viral (e.g. Canine Distemper, Rabies), bacterial, fungal (e.g. Cryptococcus) and protozoal (e.g. Toxoplasmosis) infections. Electroencephalography (EEG) is useful for diagnosing malformations present since birth, such as hydrocephalus. Brain tumors are often only able to be diagnosed by advanced imaging techniques such as Computed Tomography (CT) Scanning or Magnetic Resonance Imaging (MRI). These must be done under general anesthesia as the animal must be completely motionless during the scan.
Treatment of Epilepsy
Once a tentative diagnosis of epilepsy has been made (by excluding all the other known causes of seizures), the animal can be prescribed anticonvulsant drugs. These drugs are not appropriate for animals with seizures caused by a problem outside the brain. The overall goal of anticonvulsant therapy is to eradicate all seizure activity, but this is rarely achieved. Most pets benefit from anticonvulsant drugs by reducing the frequency, severity and duration of their seizures. A more realistic goal is to reduce the frequency of the seizures to a level that is acceptable for the owner, without having negative side effects for the animal. A minority of animals require such high doses of anticonvulsant drugs to suppress their seizures that the adverse effects caused by it outweigh the benefits.
Since epilepsy is not curable, the owner must be prepared to give the medication for the rest of the animals life. However, if after being on anticonvulsant medication for over a year no seizures have occurred, a cautious slow reduction in dose can be attempted.
The three most commonly used drugs in the treatment of epilepsy in cats and dogs are:
Known to many by its trade name Valium, diazepam is used in emergency short term situations to treat animals in status epilepticus, i.e. during a seizure. It is best administered intravenously so that it is delivered to the brain quickly, though finding a vein or placing a catheter in a fitting animal can be a challenge. In those situations, it is sometimes given rectally, where it is fairly rapidly absorbed across the rectal wall. If status epilepticus continues despite the repeated administration of diazepam, phenobarbitone can be given intravenously.
Phenobarbitone is the most commonly prescribed anticonvulsant drug for both canine and feline epilepsy, owing to its efficacy and its low cost. Over three quarters of epileptic dogs have their seizures controlled by phenobarbitone alone. Sometimes it can take several weeks for the level of phenobarbitone in the blood stream to reach a high enough level to work properly, so many vets favor using a higher loading dose initially, reducing it for the maintenance phase. However, care must be taken as high doses of phenobarbitone can cause liver damage, and liver enzymes should be periodically checked in animals on long term therapy. Annual blood tests are also advised to check serum concentration levels of the drug, to calibrate the dose effectively. High doses can also cause sedation.
3. Potassium Bromide
A proportion of dogs are resistant to phenobarbitone. In such dogs, the addition of potassium bromide, i.e. a second drug, can be tried. The potassium bromide may also be tried alone as an alternative to phenobarbitone, but generally only after phenobarbitone has been thoroughly investigated at various doses and found to be inadequate. Phenobarbitone is also deemed unsuitable if the dog has liver dysfunction of any kind, in these cases potassium bromide (often referred to simply as bromide as that is the therapeutic part) can be used. Potassium bromide is not used in cats, only diazepam and phenobarbitone are recommended in this species.