Investigators at Washington University School of Medicine, St Louis, MO, evaluated retrospectively clinical, laboratory, and muscle histochemistry and oxidative enzyme characteristics in 49 children with suspected mitochondrial disorders. Children (n=18) with CoQ10 deficiency in muscle were compared to 31 with normal CoQ10 values. Motor delay/hypotonia and cognitive/language delay were the most frequent clinical features in CoQ10-deficient and control groups. Seizures or epileptiform EEG occurred in 56%. Type 2C muscle fibers were 5.5-fold more frequent in CoQ10-deficient patients compared with mitochondrial and nonmitochondrial controls (P<0.0001). Type 2C fiber frequency of >5% had 89% sensitivity and 84% specificity for CoQ10 deficiency in these patients. No biopsy showed active myopathy. Multiple abnormalities in muscle oxidative enzyme activities were more frequent in CoQ10-deficient patients than controls. 
COMMENT. An increased frequency of type 2C fibers in morphologically normal muscle is a sensitive and specific histological marker of CoQ10 deficiency in a child suspected of having a mitochondrial disorder. Clinical syndromes associated with CoQ10 deficiency are diverse and include myopathy, cerebellar ataxia, nephrotic syndrome, and encephalopathy including Leigh syndrome. Early diagnosis is important since some cases are responsive to treatment with CoQ10 supplementation.