Seizures are the most common neurological disorder encountered in veterinary neurology. An epileptic seizure is the outward physical manifestation of excessive and/or hypersynchronous electrical activity in the prosencephalon (cerebral hemispheres and Thalamus). The exact pathophysiology is uncertain. They could arise due to excessive excitation through aspartate and glutamate neurotransmitters, inadequate inhibition mediated by GABA and glycine neurotransmitters, or a combination of the two. The only way to definitively prove a paroxysmal episode is a seizure is by way of electroencephalography (EEG), which is more difficult to perform and interpret in veterinary patients for a variety of reasons. Not all veterinary neurologists perform EEG and, in many cases, EEG is not necessary for diagnosis. However, EEG should be considered for any paroxysmal episode that is not clearly an epileptic seizure. Convulsive seizures are generally easy to diagnose with an accurate description from the owner and/or veterinary observation. It is important to remember that not all paroxysmal episodes are seizures. Other disorders that can mimic seizures include transient vestibular episodes, narcolepsy/cataplexy, idiopathic head tremors, dyskinesias, neuromuscular weakness, pain episodes, etc. How do you tell if an episode is a seizure? In general, with a seizure, there is an acute, transient, self-limiting episode with a definable post-ictal period (see below), whereas the disorders listed above frequently have a sudden termination with no post-episode abnormalities. Try to get the owner to video an episode for review rather than relying on their assessment that their pet is having seizures.
Types of seizures
- Generalized tonic-clonic seizures – These are the most common seizures identified in veterinary neurology. The patient first goes into a tonic phase with sustained contraction of all muscles. The patient loses consciousness and falls over the side. Opisthotonus, irregular breathing, cyanosis, salivation, urination or defecation may be observed. The average duration of the tonic phase is 1-2 minutes. This is followed by rhythmic contraction of the limbs and/or jaw chomping for another 1-2 minutes. These have traditionally been called “grand mal” seizures, but the other terms listed here are preferred because they’re more descriptive.
- Generalized tonic seizures – Only the tonic phase is noted.
- Generalized clonic seizures – There is rhythmic paddling/limb jerking, but no tonic phase.
- Atonic seizures – These are rare seizures characterized by a sudden loss of muscle tone with possible collapse.
- Myoclonic seizures – These are seizures characterized by brief, shock-like contractions.
- Absence seizures – These seizures are characterized by a brief loss of consciousness with a specific EEG pattern.
Focal seizures are characterized by asymmetric clinical signs that indicate abnormal electrical activity in one region of the forebrain.
Simple focal seizures – These are seizures in which consciousness is maintained. Any portion of the body may be affected. There may be rhythmic contraction of the facial muscles, one limb, eyelids, jaw chomping movements, lip smacking, licking one region of the body, fly-biting, etc.
Complex focal seizures – These are seizures in which consciousness is impaired. Bizarre behaviors are common, such as unprovoked aggression, extreme irrational fear, or running fits.
Prodrome – This is a long-term (hours to days) indication of an impending seizure. These are often not observed / reported by owners, but clinical signs of a prodrome include restlessness, vocalizing, clinginess, or hiding. If an owner describes a possible prodromal stage, try to get an idea of how frequently they occur and how often the owner can accurately predict the seizure.
Aura – The aura is essentially the beginning of the abnormal electrical activity in the brain and lasts for seconds to minutes. Patients may hide, seek out their owner, vocalize, or appear agitated.
Ictus – This is the actual seizure in which there is clinical manifestation of abnormal EEG activity in the brain.
Post-ictal – This is the phase after the abnormal EEG activity has stopped, but the patient has yet to return to normal. It can last for minutes to days, although post-octal signs lasting more than one day should increase the suspicion for an underlying cause other than Idiopathic Epilepsy. Common clinical signs include disorientation, ataxia, and blindness. Some patients will eat or drink ravenously, vomit, urinate, or defecate. Post-ictal aggression is possible.
Any disorder in which there is chronic, recurrent seizures is a form of epilepsy. Too often, the term is loosely used to indicate Idiopathic Epilepsy (see below); however, this is inaccurate terminology. Additionally, the seizures must be recurrent for epilepsy. Transient seizures that occur secondary to a underlying condition that resolves (e.g., toxicity) or brain insult (e.g., traumatic brain injury) are called provoked seizures (a.k.a., reactive seizures).
Types of epilepsy
Idiopathic epilepsy – Also known as primary epilepsy, this is the form of epilepsy that many people think of when they hear or use the term epilepsy by itself, but this is inaccurate and the entire term Idiopathic epilepsy should be used. There is no underlying cause or structural abnormality that can be identified and is considered to have a genetic cause.
Symptomatic epilepsy – Also known as secondary epilepsy, this type of epilepsy is present when an underlying extracranial or intracranial cause can be identified. Examples include brain tumors, encephalitis, hydrocephalus, hepatic encephalopathy, etc.
Cryptogenic epilepsy – This term is used infrequently, but describes any disorder of recurrent seizures in which there is a presumed underlying cause that has not yet been identified. An example would be a geriatric dog that suddenly starts having seizures and is thought to have a brain tumor, but MRI has not been performed.
Epilepsy in human medicine
The definition of epilepsy continues to evolve in human medicine. A Task Force of the International League Against Epilepsy has recently published a new operational (practical) clinical definition for epilepsy. These new definitions (see below) may eventually be adopted in veterinary neurology.
Epilepsy is a disease of the brain defined by any of the following conditions:
- At least two unprovoked (or reflex) seizures occurring > 24 hours apart
- One unprovoked (or reflex) seizure and a probability of further seizures similar to the general recurrence risk (at least 60%) after two unprovoked seizures, occurring over the next 10 years
- Diagnosis of an epilepsy syndrome
Epilepsy is considered to be resolved for individuals who had an age-dependent epilepsy syndrome but are now past the applicable age or those who have remained seizure-free for the last 10 years, with no seizure medications for the last 5 years.
Source: Fisher RS, Acevedo C, Arzimangohglou A, et al. A practical clinical definition of epilepsy. Epilepsia 2014;55:475-82.