3.4 NEUROLOGICAL DISEASES OF OLD AGE CHRONIC 'OLD DOG' ENCEPHALITIS SEIZURES
Seizures can begin at any age and if they occur at a frequency greater than once every 6 weeks, or if the animal has clusters of seizures more than once every 8 weeks, anticonvulsant therapy is indicated.

The onset of seizures in old animals should prompt a search for an extracranial cause (e.g. hepatic disease) or an intracranial structural lesion (e.g. brain tumour).

Phenobarbitone and primidone are the drugs of choice for managing seizures in dogs, and phenobarbitone and diazepam for cats.

In 20-25% of dogs seizures are reported to be refractory to treatment with phenobarbitone and 40% and 48% of cases are refractory to primidone. Similar results have been reported for cats (Schwartz-Porsche 1992).

The most common cause of failure in treatment is inadequate dosage either by the clinician or due to owner non-compliance. The therapeutic range of serum concentration of phenobarbitone in the dog is 20-40 mg/ ml and for the cat 10-30 mg/ml. The recommended dose rate is 1.5-5.0 mg/kg body weight but if adequate serum concentrations are achieved but seizure control does not occur even higher doses are recommended by some authors. Intervals between doses should be less than the half-life of the drug in the body to minimise fluctuations in serum concentrations.

The serum concentration of a drug is determined not only by dose but also by its bioavailability, metabolism and elimination. In older animals the objective should be to reduce the dose to the minimum needed to maintain serum concentrations within the recognised therapeutic range.

The recommended dose of anticonvulsants varies from one author/ reference to another. During the initial treatment of refractory seizures the more rapidly the therapeutic dose is reached the greater the success so a high initial loading dose of phenobarbitone may be beneficial particularly for the most difficult seizures to control in dogs (clusters of generalized tonic-clonic seizures (GTCS)) and cats (complex focal seizures). For phea gradually increasing dose rate of up to 10-15 mg/kg body weight orally has been used for these cases (Schwartz-Porsche 1992).

Care is needed when using anticonvulsants in old animals - particularly if high doses are needed. The patient should be screened for evidence of impaired liver function and should be carefully monitored to ensure early detection of hepatotoxicity.

Primidone is not recommended at high dose rates for out-patients because it causes sedation. It is recommended to be given at 25 mg/kg body weight twice daily orally for both cats and dogs, though some authors advise administration at least three times daily.

Diazepam is the drug of choice for the initial control of status epilepticus in cats and dogs at a dose rate of 5-50mg given i.v. in 5-10mg doses followed by slow intravenous infusion at 2-5 mg/h in 5% glucose intrafluid. Orally diazepam is only 2-3% bioavailable but a dose of 0.5-2.0 mg/kg t.t.d. has been recommended for cats.

Potassium bromide and mephenytoin have been used successfully as adjuvants to conventional treatment but combination therapy should only be tried if drugs by themselves have proved to be unsuccessful. See Schwartz-Porsche 1992 for a review of adjunctive therapy.

In all cases a rapid reduction in dose rate or too sudden a change in treatment can result in relapse and recurrence of seizures.

Drug interactions are common between anticonvulsants and antibiotics, antacids, theophylline, cardiac drugs, steroids and antirheumatics. Phe(e.g. acepromazine), anthelminthics (e.g. piperazine and mebendazole) and metoclopramide administration may lower the seizure threshold and precipitate seizures in a stable case.

The incidence of seizures may be altered by the presence of concurrent disease - see Table 3.5. For this reason routine screening is advisable in geriatric patients with seizures.