Normal muscle respiratory chain enzymes can complicate mitochondrial disease diagnosis.

This report presents a case of mitochondrial respiratory chain deficiency in a neonate with elevated plasma lactate, hypotonia, developmental delay, and dysmorphic features. The initial biochemical analyses of muscle tissue for mitochondrial function were normal. Additional testing on skin fibroblasts performed owing to a high clinical suspicion of a possible mitochondrial disorder indicated a deficiency of mitochondrial complex I. Western blotting of samples obtained both from muscle and fibroblast tissues also revealed an extensive defect in mitochondrial respiratory chain complex I, confirming the diagnosis. These observations underscore the fact that both enzymatic and immunological assays should be undertaken in alternate tissues when muscle biopsies are inconclusive in highly suspected cases.