The differential diagnosis of Reye syndrome: muscle biopsy evaluation.

To evaluate the usefulness of muscle biopsy to obtain a diagnosis of Reye syndrome (RS), muscle biopsies were performed in 10 patients, with definite RS (1 patient) and mimicking RS (9 patients) (Table 1). The muscle biopsy results were compared with those of liver biopsies in 7 patients including one patient with definite RS and 6 with mimicking RS. In 3 clinical RS patients, only a muscle biopsy was performed. In the patient with definite RS, oil red O positive lipid droplets were mildly increased in type 1 fibers, with no morphological abnormality of muscle mitochondria, whereas the liver biopsy specimen showed both centrinuclear fatty metamorphosis and mitochondrial abnormality, i.e., a typical RS pathology. Marked lipid accumulation in type 1 fibers and morphological abnormality of muscle mitochondria were observed in two patients with systemic carnitine deficiency and one of two patients with fulminant hepatitis without jaundice who showed mimicking RS clinically. Therefore, it is difficult to make a diagnosis of RS simply on the basis of the clinical findings and muscle morphology.