Seizure disorders in dogs - Australian Veterinary Association

In cats older than 5 years of age (and more especially older than 10 years) intracranial tumourand cerebrovascular disease secondary to hypertension (renal disease, hyperthyroidism) shouldalso be considered.If an underlying cause of seizures is found specific treatment should be given.If idiopathic (primary or acquired) epilepsy is diagnosed the decision on whether or not to starttreatment with anticonvulsant medication depends on seizure frequency and severity. Animalsthat seizure infrequently (less than 1 seizure every 4-6 weeks) do not require medication unlessseizures are long, are accompanied by intolerable behaviour changes, or seizures aredistressing to the owner.Dogs that seizure more frequently than 1 seizure a month and/ or have seizures inclusters (more than 2 seizures in 24 hours) and/or have ever had an episode of statusepilepticus (continuous seizuring) and/or have prolonged seizures (> 3-5 mins) should betreated with anti convulsant medication.It has been suggested that the more seizures an animal has the more likely it is to seizure againas if the brain becomes ‘wired’ to seizure more easily. Early institution of anticonvulsant therapyhas been suggested as a way of achieving better seizure control. However starting therapy aftera single seizure may result in unnecessary treatment of dogs that would have seizured veryinfrequently and there have been no studies to show that early treatment prevents thedevelopment of refractory seizures. Seizures are generally not life threatening howevermisadventure associated with seizure may result in death (falls in swimming pool and drowns)and status epilepticus (state of continuous seizures or repeated seizuring without recoverybetween seizures) is a medical emergency as prolonged hypoxia and hyperthermia are lifethreatening. Some breeds have a known tendency to the development of cluster seizures,increased risk of staus epilepticus and pharmacoresistance to anticonvulsant medicationespecially large breeds (G.Shepherds, Golden retrievers, Labrador retrievers,Boxers) ,BorderCollies and Australian Shepherds.Anticonvulsant medication is a long term and often lifetime commitment. Medication needs to begiven every day at regular intervals and all medications have significant side effects. The mosteffective anticonvulsant in dogs is phenobarbitone. Starting dose of phenobarbitone is 2-4mg/kg q12hours (dogs) and 2mg/kg q12 hours for cats. Animals may be dopey and ataxic forfirst 10-14 days but generally become tolerant to the sedative effects of the drug after this time.Occasionally dogs will become hyperactive and excessively vocal -this effect also subsidesgenerally with continued treatment. It takes 2 weeks for phenobarbitone to reach a steady serumlevel. A serum phenobarbitone level of >150umol/L may be required to manage seizures insome animals. A phenobarbitone level of >170umol/L is more likely to be hepatotoxic. Thephenobarb dose given should be determined by the level of seizure control and persistent sideeffects (sedation). If seizures are not managed at a lower dose a higher dose may achievebetter control) and for dogs receiving a higher mg/kg dose (>5-6mg/kg q12 hours) serumblood levels should be monitored. Individual dose requirements vary considerably. Thetherapeutic level of a drug is a guide only –some dogs will be poorly controlled at a serumdrug level at the lower end of the therapeutic range and better controlled with a higherblood level. The upper end of the therapeutic range is the most useful guide as it determinesthe likelihood of hepatotoxicity. If an animals seizures are well controlled and the blood level islower than the therapeutic range there is probably no need to increase the phenobarb dose. Formany dogs the dose of phenobarbitone given is limited by the side effects especially persistentsedation and polyphagia.Primidone (Mysoline) is metabolised predominantly to phenobarbitone(some PEMA andprimidone) and its effectiveness is similar to phenobarbitone (and due to the anticonvulsanteffects primarily of phenobarbitone) however it may be more effective than phenobarb in theoccasional dog. Sedation and hepatotoxicity is more frequently seen with Primidone thanphenobarbitone alone. Monitoring is as for phenobarbitone.