This article is from the Canine Epilepsy FAQ, by Alicia Wiersma-Aylward with numerous contributions by others.
Primary epilepsy: also known as idiopathic, genetic, inherited, or
true epilepsy. There are no positive diagnostic findings that will
substantiate the diagnosis. It is a case of ruling out every other
possibility. The first seizure in a dog with primary epilepsy usually
occurs between the ages of 6 months and 5 years. (Oliver, Seizures).
However, a diagnosis of primary epilepsy is not proof of a genetic
defect; only careful breeding studies could prove that. The breed, the
age, and the history may suggest a genetic basis for primary epilepsy
if there is a familial history of seizures.
Secondary epilepsy refers to seizures for which a cause can be
determined, and there are many. In dogs less than one year of age, the
most commonly-found causes of seizures can be broken down into the
following classes: degenerative (storage diseases); developmental
(hydrocephalus); toxic (lead, arsenic, organophosphates, chlorinated
hydrocarbons, strychnine, tetanus); infectious (distemper,
encephalitis, and others); metabolic (such as transient hypoglycemia,
enzyme deficiency, liver or kidney failure); nutritional (thiamine,
parasitism); and traumatic (acute injury). In dogs 1-3 years of age, a
genetic factor is most highly suspected. In dogs 4 years of age and
older, seizures are commonly found in the metabolic (hypoglycemia,
cardiovascular arrhythmia, hypocalcemia, cirrhosis) and neoplastic
(brain tumor) classes. (Oliver, Seizure). Dr. Jean Dodds has mentioned
that seizures are also associated with hypothyroidism, which is a
familial (inherited) autoimmune disease of purebred dogs.