In order to help us understand seizures it’s important to answer some quick questions first
What is a seizure?
A seizure is an involuntary change in behaviour or neurological status due to the abnormal activity of central nervous system (CNS) neurons.
What does the CNS consist of?
Brain and spinal cord.
What is a neuron?
Neurons are excitable cells of the CNS that process and transmit information through electrical and chemical neurotransmitters; an imbalance in excitatory and inhibitory signals is what causes a seizure.
What is a neurotransmitter?
Endogenous chemicals that allow neurotransmission; a type of chemical messenger that transmits signals (carries messages) across a synapse like a neuromuscular junction from one neuron to another.
For the purpose of the topic of seizures you will need to know the following neurotransmitters;
GABA is the inhibitory neurotransmitter
Glutamate is the excitatory neurotransmitter
Types of seizure
Seizures can be classified into two types; focal and generalised.
Focal seizures (Also known as a partial)
-They affect one hemisphere of the brain; therefore, motor manifestations are asymmetric.
-Simple focal seizures include facial twitching and there is thought to be no impairment of consciousness
-Complex focal seizures there seems to be an impairment of consciousness.
Clinical signs in dogs:
Fly catching, tail chasing, aggression, running in circles and/or vocalisation
Clinical signs in cats:
Drooling, excessive vocalisation and/or rapid running inside the house
Generalised seizures can be described as either tonic, myoclonic, atonic, tonic-clonic.
The term myoclonus can be described as involuntary brief jerks of the body – usually without loss of consciousness (can be seen in metabolic abnormalities such as renal or liver failure.
Atonic – Brief lapse in muscle tone
Tonic-clonic formally known as grand mal seizure
Tonic phase – patient will quickly lose consciousness, skeletal muscles become tense often causing extremities to be pulled towards the body or pushed away from it causing the patient to fall if sitting or standing at the time. Brief vocalisation may be heard which is air being forcefully expelled from the lungs.
Clonic phase – Patients muscles will start to contract and relax rapidly, causing convulsions. Range from exaggerated twitches of limbs to violent shaking. Patient may roll or stretch as seizure spreads across brain. Eyes typically roll back or even close, tongue often suffers bruising or lacerations from strong jaw contractions. Autonomic signs like incontinence occurs in this phase.
-The seizure activity starts in one hemisphere and spreads to both cerebral hemispheres; therefore, motor manifestations are bilateral.
-There is an altered consciousness and the animal is unaware of its surroundings.
-Autonomic signs such as urination, defecation and salivation are portrayed with generalised seizures.
-They are more common in dogs
It is possible for focal seizures to progress to generalised seizures
More than 3 convulsive seizures in 24 hours
2 or more seizures without recovering or 1 seizure lasting more than 5 minutes
Seizures originate from the forebrain, classification can be intracranial or extracranial in origin, here are some examples:
Intracranial causes (within the brain)
-GME (granulomatous meningoencephalitis)
-Infarct (like a stroke)
Extracranial causes (outside the brain)
-Metabolic e.g. toxin ingestion; rat poison, ivermectin, moxidectin
-Electrolyte imbalances such as hypocalcaemia, hypoglycaemia, hypernatremia
To help create a differential diagnosis list, neurologists use a checklist to be able to rule out causes of seizures in the form of the following mnemonic:
It is a very useful tool and can also be applied to spinal disorders. The mnemonic is something I try to use when a new patient comes in to help determine and understand why the patient is displaying the clinical signs.
From a nursing perspective it also provides an understanding as to why the chosen diagnostic procedures have been selected to aid with a diagnosis.
This is just a brief summary of what I believe to be important background information on understanding the complex subject of seizures in the companion animal. I hope you find it an interesting read, if you have any questions just head over our Facebook page and ping us a message
For more information I’ve written the following article:
Seizures in companion animals, VNJ, 2018 vol 33
Lowrie, M., 2013. Canine status epilepticus. Companion animal,18(5), pp. 198-204.
Platt, S and Olby, N., 2013. Neurological emergencies In: Platt and N.Olby, eds. 2013. BSAVA manual of canine and feline neurology. Gloucester: BSAVA. Ch.20.
Podell, M., 2013. Seizures In: Platt and N.Olby, eds. 2013. BSAVA manual of canine and feline neurology.Gloucester: BSAVA. Ch.8.