International Journal of Medical and Pharmaceutical Case Reports

Introduction: Phenytoin is a widely used antiepileptic drug with a narrow therapeutic index and dose-dependent non-linear pharmacokinetics. Even minor changes in metabolism or protein binding may precipitate toxicity and neurological manifestations may mimic cerebellar or cerebrovascular disorders, creating diagnostic confusion.

Case Presentation: A 26-year-old woman with a 15-year history of seizure disorder on phenytoin and levetiracetam presented with one week of progressive gait instability, bilateral lower limb weakness and impaired coordination. She had a history of posterior circulation stroke one month earlier. Neurological examination revealed reduced power in both lower limbs (3/5) and impaired finger–nose testing bilaterally. Laboratory investigations were largely unremarkable except for an elevated serum phenytoin level of 35 µg/mL (therapeutic range 10–20 µg/mL). Phenytoin was discontinued, and the patient was managed with levetiracetam, valproate, supportive care and monitoring, resulting in marked clinical improvement.

Conclusion: Phenytoin toxicity should be suspected in patients on chronic therapy who develop new-onset cerebellar signs or neurological deterioration. Therapeutic drug monitoring and early drug withdrawal are crucial to prevent complications and avoid misdiagnosis.