Idiopathic epilepsy

What it is

Epilepsy is a brain disease characterised by an enduring predisposition to generate epileptic seizures, defined in practice as at least two unprovoked seizures more than 24 hours apart. The International Veterinary Epilepsy Task Force (IVETF) classifies it by cause as idiopathic (genetic, suspected genetic or unknown cause, with no structural lesion) or structural (caused by a brain lesion such as a tumour, inflammation, malformation or vascular event). Seizures caused by extracranial problems — hypoglycaemia, hepatic encephalopathy, electrolyte disorders, toxins — are reactive seizures and are not epilepsy.

Who it affects

Typically dogs with a first seizure between about 6 months and 6 years of age. A genetic basis has been shown or is suspected in many breeds, including the Belgian shepherd, Border collie, Australian shepherd, Labrador and golden retrievers, beagle, boxer, Bernese mountain dog, Lagotto Romagnolo, Irish wolfhound, standard poodle and Dalmatian.

Anatomy involved

• Brain: the seizures originate in the cerebral cortex (prosencephalon), as abnormal, excessive, synchronous neuronal activity. In idiopathic epilepsy the brain is structurally normal on imaging and gross examination. Prolonged or clustered seizures can cause secondary damage, for example to the hippocampus and piriform lobe.

Clinical signs

Seizures may be generalised from onset (most commonly tonic-clonic: loss of consciousness, limb paddling or rigidity, jaw chomping, salivation, urination and defecation) or focal (motor signs on one part of the body, such as facial twitching, autonomic or behavioural signs), with or without later generalisation. A prodrome (restlessness, attention-seeking) may precede the seizure and a post-ictal phase follows (disorientation, apparent blindness, ataxia, hunger or thirst) lasting minutes to hours. Between seizures the dog is neurologically normal. Cluster seizures (two or more in 24 hours) and status epilepticus (a seizure lasting more than 5 minutes, or repeated seizures without recovery) are emergencies.

Diagnosis

• History and video of the events, to distinguish seizures from syncope, movement disorders or vestibular attacks.
• Normal physical and inter-ictal neurological examination: a neurological deficit between seizures points to a structural cause.
• Laboratory tests: blood count, biochemistry (including glucose and electrolytes), bile acids or ammonia and urinalysis to exclude reactive seizures.
• IVETF confidence levels: tier I is based on the above together with age at onset; tier II adds normal brain MRI and cerebrospinal fluid analysis; tier III adds characteristic EEG abnormalities.

Treatment

Antiseizure medication is usually started when a dog has had two or more seizures within six months, any cluster or status epilepticus, or severe or worsening post-ictal signs. First-line drugs are phenobarbital, imepitoin and potassium bromide; levetiracetam and other drugs are added when control is insufficient. The aim is to reduce the frequency and severity of seizures with acceptable side effects; complete seizure freedom is reached in only some dogs. Monitoring includes seizure diaries and serum drug concentrations. At home, owners can be given rectal or intranasal diazepam or midazolam for clusters. Affected dogs should not be used for breeding.