Fatal neonatal lactic acidosis with respiratory insufficiency due to complex I and IV deficiency.

We present a newborn boy who died after 53 days of life in respiratory failure with lactic acidosis. Analysis of skeletal muscle mitochondria demonstrated a combined defect in complexes I and IV of the respiratory chain. The boy had severe muscle hypotonia but no signs of encephalopathy, illustrating the variation in multi-organ presentation of mitochondrial defects.