What is Idiopathic Epilepsy?
Idiopathic epilepsy is a group of epileptic disorders that are believed to have a strong underlying genetic basis.
It is diagnosed through ruling out all other possible causes for epilepsy.
How is Idiopathic Epilepsy diagnosed?
Idiopathic epilepsy is diagnosed by ruling out all other acquired diseases that can cause seizures.
At minimum, this usually requires a complete blood count, biochemical analysis and urinanalysis. MRI scans are the preferred way to scan the brain and rule out other causes for epilepsy, such as brain tumours, or swelling.
A brain that appears normal on an MRI is more likely to have idiopathic epilepsy.
Analysis of cerebrospinal fluid can be used to rule out inflammation, and other tests such as heart testing can also be performed.
Roughly 2/3rds of dogs who have their first seizure between 1-5 years of age will be diagnosed with Idiopathic epilepsy, and around 1/3rd will be diagnosed with a structural or metabolic cause.
EEG scans are used to find problems related to electrical activity in the brain. They can be used to help diagnose the type of epilepsy, or to check whether someone is in non-convulsive status epilepticus.
This is a kind of epilepsy where there might not be external signs of epilepsy, but they still experience issues with awareness and confusion during a seizure.
However, if nothing comes up on an EEG scan, it does not mean the dog does not have epilepsy. It can only pick up unusual electrical activity, which might not be present at the time of testing.
This study found that 45% of patients with Idiopathic Epilepsy tested normal on their first EEG, and only 33% showed abnormalities typical of epilepsy.
This means, that it is not a reliable method of diagnosing idiopathic epilepsy on its own.
Testing a dog is expensive, time-consuming, and stressful. While I say these steps are typical of how idiopathic epilepsy is diagnosed, we cannot expect all owners to go through extensive testing to rule out other causes.
We also cannot assume that any dog that isn’t fully tested is not epileptic. Because diagnosis does not change genetics, and because the majority of dogs experiencing seizures between 1-5 years of age have Idiopathic Epilepsy, we could be breeding from dogs that carry genes for epilepsy that we have dismissed.
There is little to lose through caution, but a lot to lose if we ignore potential risk in our lines.
Reference:
http://vhc.missouri.edu
https://www.epilepsy.org.uk/info/diagnosis/eeg-electroencephalogram
The genetics of Idiopathic epilepsy
Idiopathic epilepsy is a very heritable disease. This means inherited factors contribute a lot to the variation in how often it is expressed.
Heritability is measured on a scale from 0-1. Above 0.5 is considered to have a significant genetic component.
For comparison, the heritability of hip dysplasia in Labradors was found to be 0.34 in this study from the UK. Of course, other studies provide different heritabilities for Hip Dysplasia, but this was more to give as a reference.
This study analysed the heritability of epilepsy in Poodles, English Mastiffs, Giant Schnauzers, Belgian Sheepdogs, and Belgian Tervuren, involving 400 dogs exhibiting repeated seizure activity.
They found the following heritabilities:
Poodles: 0.59
Giant Schnauzers: 0.78
English Mastiffs: 0.69
Belgian Sheepdog and Tervuren: 0.77-0.83
“Our work in Belgians suggest the predicted mode of inheritance is polygenic but with a single gene of large effect influencing the expression of the epileptic phenotype.”
However, it’s worth mentioning that other studies have come to different conclusions regarding the genetics behind epilepsy.
They also provided sample pedigrees in the study, with black circles or squares representing epileptic dogs.
Poodles: Giant Schnauzer:
English Mastiff: Belgian Tervuren:
So, as you can see, even though it’s highly heritable, it does not express consistently. Just because a dog does not produce an epileptic itself, does not mean it didn’t carry the disease.
Another study looked at Irish Wolfhounds, and found a heritability of 0.87, and while unspecific, this study mentioned familial epilepsy as having high heritability in boxers
From the Irish Wolfhound study, it mentions that "No simple mode of inheritance explains the pattern of affected dogs in pedigrees. Hallmarks of dominant and sex‐linked inheritance were notably absent, and the segregation ratio was less than would be expected for simple autosomal recessive inheritance. "
Epilepsy has been studied across many different breeds for many decades. However, the lack of genetic tests available for most forms of epilepsy suggests that it will be very difficult to develop a test for the disease, and there could likely be multiple genes at play.
There could also be different genes involved depending on the breed, or even multiple kinds of epilepsy in the same breed.
This video on Dermatomyositis in Collies shows some of the complexity that comes when multiple genes are involved.
https://www.youtube.com/watch?v=hlFSqISfzek
Conclusion:
Idiopathic Epilepsy is a highly heritable disease with a strong genetic component. It is diagnosed through extensive testing, and thus there are many reasons an owner might not proceed with getting an official diagnosis of IE.
How it passes on each generation is not consistent, and may skip generations. So we must be careful when breeding to relatives of an affected dog, even if they themselves do not show symptoms.
