Coexistence of guanidinoacetate methyltransferase (GAMT) deficiency and neuroleptic malignant syndrome without creatine kinase elevation.

We describe the first child with guanidinoacetate methyltransferase (GAMT) deficiency who developed neuroleptic malignant syndrome (NMS) after the treatment of risperidone without elevated creatine kinase (CK) levels. The patient presented with lethargy, hyperthermia, generalized tremor and rigidity with normal serum CK levels. After cessation of risperidone and adding clonezepam to the supportive treatment, symptoms of NMS were ameliorated. We conclude that although serum CK elevation is a useful indicator for the early detection of NMS, normal serum CK levels may be seen during the NMS course in the presence of GAMT deficiency.