When less is more: a case of phenytoin toxicity.

The authors present a case of an 87-year-old gentleman who presented with general deterioration, increased confusion, recurrent falls and unsteadiness. He was treated for a urinary tract infection but was found to deteriorate rapidly, developing bilateral nystagmus, marked pastpointing, dysarthria and central ataxia. He had a complex medical history including epilepsy controlled with long-term phenytoin. Phenytoin is 90% protein bound and displaced by bilirubin. At the time of deterioration his total phenytoin concentration was within the limits of the laboratory's recommended therapeutic range. The biochemistry report also denoted the patient was hypoalbuminaemic and hyperbilirubinaemic. His symptoms completely resolved with phenytoin dose reduction. The combination of low albumin and high bilirubin may cause an increase in the free phenytoin concentration, resulting in toxicity, despite the measured total phenytoin concentration being within the therapeutic interval.