AN ABNORMAL EXERCISE TEST RESPONSE REVEALING A RESPIRATORY-CHAIN COMPLEX-III DEFICIENCY

A 29-year-old man with a progressive exertional muscle intolerance sin ce childhood was referred for incremental exercise test on a bicycle e rgometer. The response pattern suggested a mitochondrial myopathy: tha t is, a greatly reduced maximum oxygen consumption with appropriate he art rate increase and an anaerobic threshold point reached early. The metabolic investigation in plasma revealed an abnormal oxidoreduction status (hyperlactataemia and high lactate/pyruvate ratio) at rest and after a carbohydrate rich meal. The histochemical examination of a mus cle biopsy revealed red granular deposits under the sarcolemma for all type 1 fibers. Oxypolarographic and enzymological studies of the mito chondrial respiratory chain in both isolated mitochondria and muscle h omogenate demonstrated a marked deficiency of ubiquinol cytochrome c r eductase (complex III) activity.