[Phenytoin: paradoxical toxicity; a discussion of 4 cases].

INTRODUCTION: Phenytoin overdosing results in a wide variety of signs and symptoms - ataxia, nistagmus, loss of consciousness. On occasions increased frequence of seizures may be seen in patients with high phenytoin serum levels and no evidence of standard toxicity symptoms - paradoxical toxicity.

OBJECTIVE: To study the frequency of phenytoin paradoxical toxicity, and to analyze patients" clinical status.

PATIENTS AND METHOD: Prospective study of 100% of patients monitored by the Pharmacy Department during August-December, 1998 and who had phenytoin serum levels above therapeutic range and seizures. Laboratory determinations in samples was performed by polarized immunofluorescence analysis. The outcome of each patient was monitored through their pharmacotherapeutic record and clinical history.

RESULTS: The number of patients was 1706; 124 of wich had serum levels above the therapeutic range. Out of this group, 3 males and 1 female, with ages ranging from 17 to 73, a diagnosis of epilepsy, and chronic therapy using phenytoin, come to our Emergency Department because of convulsions. Serum levels of phenytoin were ordered due to suspected lack of compliance or underdosing. In all 4 patients phenytoin was discontinued until the therapeutic range was reached, to be then reset with plasma level-adjusted dosages.

CONCLUSIONS: Paradoxical toxicity may lead to errors, and therefore we should rule out such possibility in patients with exacerbated epilepsy undergoing treatment with phenytoin.