Treating seizures: The old and the new (Proceedings)

Apr 01, 2008
By staff

Principles and philosophy of therapy

The primary goal is to treat the underlying disorder. Even if an underlying disorder exists, antiepileptic drugs (AED) may be needed for adjunctive therapy (ie. brain tumor, encephalitis). If the primary treatment does not achieve seizure control, or the animal has primary epilepsy, chronic anticonvulsant therapy is warranted. In some disorders, such as reactive seizures (i.e. portosystemic shunt), AED therapy may be contraindicated.

It is difficult to know when to initiate AED therapy. Most clinicians agree that seizures occurring more often than every 4-6 weeks may benefit from chronic AED therapy. It is not warranted to treat with anticonvulsants after 1 mild to moderate generalized seizure. Severe seizures, cluster seizures or status epilepticus do warrant early therapy. It is best to start chronic therapy early in the course of the disease, but starting after the first seizure makes it difficult to know if therapy is truly needed.

Basics and goals of using anticonvulsant drugs

The ability to control seizures will depend upon the serum concentration of the anticonvulsant drug. Serum concentration will depend on the half-life of the drug. The dosage interval should not exceed the half-life of the drug. It will require approximately 5 half-lives to achieve steady state concentrations. Fluctuations in the serum concentration will still occur following steady state. The peak (highest) concentration will usually occur a few hours after oral dosing. The trough (lowest) level will occur just prior to the next dose. Therapeutic control often depends on the trough concentration. This is the serum concentration that should be measured. (For example, if an animal is normally dosed at 8:00 a.m., then blood should be obtained between 7:00-8:00 a.m. and the anticonvulsant should be administered on time.). This is probably most important early in phenobarbital therapy as once the animal has been chronically treated, peaks and trough concentrations will be very similar. Most therapies will require dosing q8-12 hours. Dosing at fixed times will decrease the chance for fluctuations of drug concentrations outside of the therapeutic range and may decrease the sedative properties associated with most anticonvulsant drugs.

With few exceptions, phenobarbital will always be the most efficacious and least costly medication. It is the only anticonvulsant with proven efficacy in dogs. However, Bromide is becoming a more commonly used single agent. There is no proven efficacy to initial multiple drug therapy. Monotherapy is always warranted initially.

The owner must play an integral role in the management of seizures in their pet. The client should be educated about use of the medications, side effects, monitoring, and expectations of the therapy. The owner should be encouraged to keep a journal describing the frequency, duration, and severity of the seizures. Additionally, they should record any suspected precipitating conditions. The client should be aware that anticonvulsant therapy is rarely curative. A seizure-free status without adverse effects of the anticonvulsant therapy is the ultimate goal. This is not always attainable. Control may be considered a 50% decrease in the frequency, duration, and severity of seizures. The owner needs to know that the therapy may be lifelong, and in some instances, costly. The client should not attempt to "help" the dog during a seizure, unless the animal's life is threatened (i.e. potential to fall off a porch). The client needs to know that most seizures are relatively short in duration and the animal will not "swallow its tongue". The client should contact the veterinarian if there is an increase in frequency, severity, or duration of seizures. The animal should be taken to the veterinarian if cluster seizures (several seizures over a short duration) or status epilepticus occurs.

Animals are frequently left for long periods of the day and a seizure may not be seen. However, there is often evidence of seizure activity, such as urination, defecation, salivation, and disrupted environment. Knowing when the animal is most likely to seizure may help in dosing the medication. For example, if a dosage can not be divided equally, and the animal is more likely to seizure in the early morning hours, giving the larger portion of the dose at night is warranted.