What can cause epileptic seizures in dogs and cats?
Seizures can be caused by a problem outside the brain, by a lesion in the brain, or by a tendency of the brain itself, with no visible lesion. The international expert group IVETF calls these three categories reactive seizures, structural epilepsy and idiopathic epilepsy. Each is treated differently, so the first aim of the investigations is to find out which category your pet falls into.
What are reactive seizures?
Reactive seizures are a healthy brain's response to a problem elsewhere in the body, usually metabolic or toxic, that can be corrected. They are not epilepsy. If the cause is treated, the seizures can stop completely. The most common causes are:
- hypoglycaemia, meaning blood glucose (blood sugar) that is too low;
- hypocalcaemia (low calcium) and other imbalances of the blood electrolytes, such as sodium or potassium;
- liver disease, especially a portosystemic shunt, an abnormal blood vessel that lets blood bypass the liver. Toxins that are no longer filtered out reach the brain and cause hepatic encephalopathy, a disturbance of brain function;
- poisoning: insecticides (organophosphates, carbamates), slug pellets containing metaldehyde, antifreeze (ethylene glycol), lead, strychnine. In cats, a common cause is permethrin flea and tick products meant for dogs.
What does structural epilepsy mean?
Structural epilepsy is caused by a visible lesion in the brain. Seizures can appear at any age, but this cause becomes more likely in an animal whose first seizures come after the age of 6, or that also has other neurological signs between seizures. Possible causes are:
- brain tumours;
- inflammation and infection of the brain (encephalitis, meningoencephalitis);
- head injuries;
- problems with the blood vessels, for example a stroke;
- malformations, such as hydrocephalus, and degenerative diseases.
Most of these lesions can only be seen on MRI and, for inflammation, through CSF analysis.
What is idiopathic epilepsy?
Idiopathic epilepsy means repeated seizures with no lesion in the brain and no disease in the body to explain them. A genetic cause is presumed, particularly in predisposed breeds. It is the most common form of epilepsy in dogs and usually starts between 6 months and 6 years of age. It is diagnosed by ruling other causes out: there is no test that confirms it directly, only a set of investigations that eliminate everything else. You will find more about its course and treatment in the article on epilepsy in dogs.
How is the diagnosis made, step by step?
The IVETF diagnostic guideline sets out three tiers of confidence for idiopathic epilepsy. Each tier adds investigations to the one before.
| Tier | What it includes | What it rules out |
|---|---|---|
| Tier I | At least 2 unprovoked seizures more than 24 hours apart, onset between 6 months and 6 years of age, normal clinical and neurological examination between seizures, blood and urine tests with no significant abnormalities | Most reactive seizures |
| Tier II | Tier I, plus normal bile acids, a normal MRI scan of the brain and normal CSF | Brain lesions and inflammation |
| Tier III | Tier II, plus EEG changes typical of epileptic seizures | Confirms that the brain activity is epileptic |
1. The history and the video
The consultation starts with the story of the seizures: when they began, how often they happen, what they look like, how long they last and how your pet recovers. It also matters which medicines or products your pet has been given, whether it has had access to anything toxic and whether any relatives have had seizures. A video of the episode helps identify the type of seizure and tell it apart from fainting or a balance problem.
Age at the first seizure matters a great deal. In a dog whose seizures start between 6 months and 6 years of age, with a normal examination, idiopathic epilepsy is the most likely explanation. In a puppy under 6 months, the vet thinks first of malformations, infections or metabolic problems such as a portosystemic shunt. In a dog over 6 years old, a tumour or another brain lesion becomes more likely. Breed can also point the diagnosis in a direction, because some breeds have a known predisposition to idiopathic epilepsy.
2. The clinical and neurological examination
The vet checks your pet's gait, posture, reflexes, cranial nerves and responses. An abnormal neurological examination between seizures suggests a lesion in the brain.
3. Blood and urine tests
The recommended minimum includes a complete blood count, biochemistry (with blood glucose and electrolytes), bile acids or ammonia for liver function, and a urine test. In cats, blood pressure is added and, in older cats, thyroid tests.
4. MRI of the brain and cerebrospinal fluid
The MRI scan is done under general anaesthesia, with a protocol designed specifically for epilepsy. CSF is usually collected under the same anaesthetic. The guidelines recommend these mainly when seizures start before 6 months or after 6 years of age, when the neurological examination is abnormal, with cluster seizures or status epilepticus, and when the seizures do not respond to treatment.
A neurological consultation tells you where the problem comes from and what to do next.
What does EEG add?
Electroencephalography (EEG) records the electrical activity of the brain directly, through electrodes placed on the head. It is the only investigation that can show the epileptic activity itself, not just a possible cause of it. In the IVETF guideline, typical EEG changes represent the highest level of diagnostic confidence.
In practice, EEG is most useful when it is unclear whether the episodes are epileptic seizures or something else, for example tremors, sleep disorders or behavioural episodes. It can pick up epileptic discharges between seizures as well. A normal EEG does not, however, rule out epilepsy, because the recording only covers a limited period. The guidelines also note that EEG protocols for animals are still being standardised, which is why it is not a routine investigation at every centre. You will find details of how it is done on the veterinary EEG page.
Why does an early diagnosis matter?
Because treatment depends on the cause. A reactive seizure stops once the cause is corrected, inflammation of the brain needs different treatment from idiopathic epilepsy, and in idiopathic epilepsy starting treatment at the right time helps prevent cluster seizures and status epilepticus. If your pet has had seizures and you do not yet know why, an epilepsy consultation will establish which investigations are still needed.
Frequently asked questions
Is an MRI scan essential to diagnose epilepsy?
Not in every case. In a dog whose first seizures came between 6 months and 6 years of age, with a normal neurological examination and normal blood and urine tests, idiopathic epilepsy can be diagnosed at a basic level of confidence. MRI is recommended mainly when seizures start before 6 months or after 6 years of age, when the neurological examination is abnormal, with cluster seizures and when treatment is not working.
Which tests should I have done before the neurology consultation?
Usually a complete blood count, biochemistry, blood glucose, electrolytes, bile acids or ammonia, and a urine test are useful. Once the vet has seen the documents you already have, they will tell you exactly what is missing.
The test results are normal. Does that mean there is nothing wrong?
Normal results rule out many causes outside the brain, but they do not rule out a lesion in the brain or idiopathic epilepsy. They are only one step in the diagnosis.
How soon after a seizure can the MRI scan be done?
It can be done whenever it is needed. Recent seizures can leave temporary changes on the images, and if these appear, the guidelines recommend repeating the MRI after a seizure-free period of about 16 weeks, to tell them apart from a real lesion.
Sources
- Berendt M. et al. 2015, International veterinary epilepsy task force consensus report on epilepsy definition, classification and terminology in companion animals, BMC Veterinary Research 11:182
- De Risio L. et al. 2015, International veterinary epilepsy task force consensus proposal: diagnostic approach to epilepsy in dogs, BMC Veterinary Research 11:148
- Podell M. et al. 2016, 2015 ACVIM Small Animal Consensus Statement on Seizure Management in Dogs, Journal of Veterinary Internal Medicine 30(2):477-490
- Pakozdy A., Halasz P., Klang A. 2014, Epilepsy in cats: theory and practice, Journal of Veterinary Internal Medicine 28(2):255-263
This article is for information only and does not replace a veterinary consultation.