Phenytoin, an anticonvulsant extensively used to control generalized tonic-clonic seizures as well as simple and complex partial seizures, has a stabilizing influence on neuronal membrane. Phenytoin acts by prolonging the inactivated state of sodium channel, thereby extending the refractory period of neurons and limiting the repetitive firing of action potentials. Bioavailability of phenytoin fluctuates widely on the account of pharmacokinetic variability. Consequently, the narrow therapeutic window of phenytoin frequently causes adverse effects and toxicity. Phenytoin at higher doses is associated with various vestibular and cerebellar side effects like motor ataxia, muscle spasms, psychoses and visual disturbances. Long term use of phenytoin at therapeutic and toxic levels can lead to cerebellar atrophy. However, phenytoin-induced cerebellar syndrome is reversible on timely withdrawal of the medication. Regular monitoring of plasma concentrations, accurate dosing, and medication adherence to treatment regimens are important. Here, we report a case of phenytoin-induced cerebellar syndrome presenting with behavioral abnormalities, paraparesis and visual impairment. The patient’s condition improved upon gradually tapering the dose, followed by termination of phenytoin therapy and substitution with carbamazepine. This report emphasizes on the importance of regular monitoring of plasma drug concentration, accurate dosing of drugs having a narrow therapeutic index, and identification of noncompliance in patients treated with phenytoin.
Key words: Phenytoin, Cerebellar syndrome, Epilepsy, Adverse drug reaction.