Possible atypical cross-sensitivity between phenytoin and carbamazepine in the anticonvulsant hypersensitivity syndrome

Anticonvulsant hypersensitivity syndrome (AHS) is a rare but potentially life-threatening reaction that occurs in response to common anticonvulsants in predisposed individuals. It is often characterized by fever, rash, lymphadenopathy, hepatitis, and laboratory abnormalities. Consequently, it often is overlooked or even misdiagnosed by practitioners unfamiliar with AHS. Cross-sensitivity manifests frequently between phenytoin, phenobarbital, and carbamazepine as an exacerbation of presenting signs and symptoms. We report a case of AHS in a patient whose clinical features changed significantly when switching from phenytoin to carbamazepine. Physicians and pharmacists must become aware of the extreme variability in AHS manifestation so that the offending anticonvulsant regimen can be discontinued in a timely manner.