A seizure is the result of a sudden, excessive discharge of neurons in any part of the cerebral cortex, and likely subcortical neurons as well. These hyperexcitable neurons can depolarize irregularly 700-1000 times a second, can increase in intensity to exceed the inhibition of adjacent neurons, inducing them to discharge, and the seizure can spread (generalize) to other areas of the brain.
When looked at from a biochemical point of view, hyperexcitability of neurons can be caused by either decreased inhibition or increased excitation from the synaptic input and neurotransmitters. Alterations in cell membrane and/or of cell metabolism can also influence the neuronal discharge. Any alteration of the extracellular environment, whether structural, chemical, or unknown can lead to spontaneous discharge.
Every animal has a certain threshold of stimulation for a seizure to occur that is established by the environment of the neuron. If the threshold is exceeded, a seizure occurs. Seizures may be stimulated in animals with lower seizure thresholds by such conditions as fatigue, fever, photic stimulation, estrus, etc. A seizure discharge can occur in a normal cerebral cortex, as is seen with certain drug administration or withdrawal. One in 20 people will have a seizure at some point in their life. Estimates of seizure incidence are up to 5.7% of all dogs and up to 1% of all cats. Some animals have spontaneous seizures without a detectable stimulus. This is common in “true” epileptics.
Because seizures can be a sign of anything from organic brain disease to genetically determined neuronal morphology/physiology that allows spontaneous depolarization, the treatment and prognosis is dependent on the underlying cause. It is the responsibility of the veterinarian to determine this to the best of his/her ability. Educating the client as to the significance of this clinical sign is also an important responsibility of the clinician and may determine the survival of the patient and the success of the therapy. Because the external manifestation of seizures can vary considerably, any unusual involuntary phenomenon that is episodic and recurrent in nature should be evaluated as a potential seizure disorder.
Seizures are most often managed with anti-seizure medications, where the goal is to decrease the likelihood of the abnormal neurons firing and to prevent the seizure focus spreading throughout the brain. There is a subset of veterinary patients that are refractory to medical management, and UC Davis Neurology/Neurosurgery Service is developing deep brain stimulating techniques to help manage the severe seizures seen in some of our patients.