Epilepsy is the most common neurological disorder seen in dogs and affects nearly 1% of the canine population. The condition can be inherited (genetic epilepsy), due to structural changes in the brain (structural epilepsy) or due to an unknown cause. Diagnosis of the type and cause of the seizures aids in the determination of a suitable therapeutic regime.
Seizures are primarily classified according to where they start in the brain. There are 2 main types of seizure - generalised and focal. Generalised seizures involve both sides of the brain and clinical signs are apparent on both sides of the body. This manifests as a bilateral involuntary muscle movement with increase of loss of tone. An individual’s awareness of the environment is lost and often salivation, urination and defaecation can occur.
Focal seizures originate in a specific part of the brain and result in a localised abnormality. This can be muscular such as facial twitches or leg paddling, it can be behavioural such as fear or attention seeking, or it can be a dysfunction of the autonomic nerves resulting in salivation or vomiting. Dogs can be completely aware during a focal seizure. Focal seizures can spread to both sides of the brain and become general.
If a dog has a seizure the vet will ask detailed questions to gain as much information as possible. Vets will often ask owners to keep a diary of information detailing parts of the body affected, timing of seizures, length and frequency of the seizures. Details of the pre-ictal behaviour will help owners predict a seizure. Following a seizure, some dogs will quickly return to normal, however many will behave abnormally in the post ictal period and demonstrate anxiety, blindness or sedation.
Idiopathic epilepsy is generally seen in dogs from 1-5 years of age. There are repeated seizures with no identifiable structural, toxic, or metabolic cause. The exact cause of the seizure is unknown and is assumed to be genetic in origin. Certain dog breeds are predisposed.
Structural epilepsy is the result of damage to the brain from inflammation, trauma or a tumour. These can be diagnosed with an MRI scan or with analysis of the fluid around the spine and brain. Reactive seizures occur due to a specific stimulus such as toxin exposure or a high temperature. Reflex seizures occur occasionally following exposure to a specific stimulus such as flashing lights or loud noises.
Seizures result from dysfunction of the brains electrical activity where there is either excessive brain activity or opposing activity that is unusually depressed. In the absence of structural damage or metabolic insults the causes of such dysfunction are unknown.
Anti-epileptic drugs (AED) work by inhibiting excitatory neurotransmitters, increasing inhibitory pathways, or altering electrical activity along nerves in the brain. Some drugs work better in some animals than others. AED are rarely started in dogs that have had a single seizure but tend to be introduced if a dog has over 2 seizures in a 6-month period or a 24-hour period with multiple generalised seizures.
Phenobarbital is a widely used AED. It is very effective, relatively inexpensive, and generally well tolerated. Initially it can cause sedation and ataxia, but these generally wear off over the first few days. Its main side effect is liver damage so dogs receiving treatment should have regular blood tests to assess this.
Potassium Bromide is often used in conjunction with Phenobarbital if seizures are not well controlled on a single drug therapy. It is also useful to allow a reduction in dosage of Phenobarbital in dogs where liver damage is a worry. Its main disadvantage is a long time to reach steady effective drug levels in the blood.