Respiratory chain complex I deficiency: an underdiagnosed energy generation disorder.

OBJECTIVE: To define the spectrum of clinical and biochemical features in 51 children with isolated complex I deficiency.
BACKGROUND: Mitochondrial respiratory chain defects are one of the most commonly diagnosed inborn errors of metabolism. Until recently there have been technical problems with the diagnosis of respiratory chain complex I defects, and there is a lack of information about this underreported cause of respiratory chain dysfunction.
METHODS: A retrospective review of clinical features and laboratory findings was undertaken in all diagnosed patients who had samples referred over a 22-year period.
RESULTS: Presentations were heterogeneous, ranging from severe multisystem disease with neonatal death to isolated myopathy. Classic indicators of respiratory chain disease were not present in 16 of 42 patients in whom blood lactate levels were normal on at least one occasion, and in 23 of 37 patients in whom muscle morphology was normal or nonspecific. Ragged red fibers were present in only five patients. Tissue specificity was observed in 19 of 41 patients in whom multiple tissues were examined, thus the diagnosis may be missed if the affected tissue is not analyzed. Nine patients had only skin fibroblasts available, the diagnosis being based on enzyme assay and functional tests. Modes of inheritance include autosomal recessive (suggested in five consanguineous families), maternal (mitochondrial DNA point mutations in eight patients), and possibly X-linked (slight male predominance of 30:21). Recurrence risk was estimated as 20 to 25%.
CONCLUSION: Heterogeneous clinical features, tissue specificity, and absence of lactic acidosis or abnormal mitochondrial morphology in many patients have resulted in underdiagnosis of respiratory chain complex I deficiency.