An unusual case of phenytoin intoxication

A case of a ten years old boy is reported who developed a severe intoxication after changing the phenytoin treatment from the calcium salt to the free acid. The difference in bioavailability reduced the drug requirement to 55% of the previous dose. The child was receiving also sodium valproate and clonazepam. Serial measurements revealed a significant inverse correlation between the toxic phenytoin serum concentration and the serum level of valproic acid. The measurements of phenytoin as well as of valproic acid in this case have shown again the value of routine monitoring of anticonvulsant therapy.