[Chronic phenytoin intoxication occurred below the toxic concentration in serum and its pathological findings]

We reported on autopsy case of chronic phenytoin intoxication with cerebellar degeneration. The serum concentration of phenytoin were always within nontoxic range. This patient was diagnosed as treatment resistant epilepsy with severe mental and physical disorders. The remarkable histological changes were confined to the cerebellum in autopsy. Those lesions were discontinuous and focal, on the basis of morphometrical study of Purkinje cells and granular cells. Furthermore, there was discrepancy between degeneration of Purkinje cell and granular layers, which has not been described before. This case underlines that toxic cerebellar degeneration could be induced even by low level of serum phenytoin in case of combined antiepileptic drug therapy. The measurement of the concentration of a drug or drugs per se is a mere postsign, therefore a patient should be carefully followed.