Virtually every woman with epilepsy who has an increase in seizure frequency during pregnancy will be found to have subtherapeutic levels of AEDs, which can be explained on the basis of malabsorption, decreased plasma protein binding, increased medication clearance, and noncompliance from fear of inducing congenital malformations.29 For that reason, unbound (biologically active) levels of AEDs (when available) should be measured before conception as a baseline and then at the beginning of every trimester and in the last 4 weeks of pregnancy in women with well-controlled seizures, and more often for women with recurrent seizures.30,38 More frequent monitoring should be performed when the patient is taking lamotrigine, since it has been recently shown that the serum level of this drug decreases by 60% to 90% during pregnancy.39 Ultimately, however, the decision to change the dose of an AED should be based on clinical grounds. More frequent AED monitoring should occur in the first few weeks postpartum, since therapeutic levels of an AED may become toxic with the reversal of the physiologic changes seen in pregnancy.38
Discontinuation of AEDs before pregnancy is not always practical but can be considered in women with epilepsy with well-controlled seizures. The American Academy of Neurology states that AED withdrawal can be considered in a patient who has been seizure-free for 2 to 5 years, with a normal electroencephalogram and normal findings on neurologic examination.30 The risk of recurrent seizures is cumulative, but greatest within the first 6 months of discontinuing AEDs; therefore, conception should not be planned for at least 6 months after complete withdrawal of AEDs.
Reviewed February 2004 by Cynthia Harden, MD, Weill Cornell Medical College.
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