A new mitochondrial myopathy. Biochemical studies revealing a deficiency in the cytochrome b-c1 complex (complex III) of the respiratory chain.

A 20-year-old Chilean girl presented with lifelong ptosis and fatiguable weakness which was initially thought to be due to a congenital myasthenic syndrome. Studies of an intercostal muscle biopsy showed normal endplate morphology, abundant acetylcholinesterase activity and a normal number of junctional acetylcholine receptors as determined by radiochemical assay, but a high proportion of the muscle fibres contained large peripheral aggregations of abnormal mitochondria. Biochemical investigation of mitochondria isolated from the vastus lateralis muscle demonstrated a hitherto unreported respiratory chain deficiency localized to complex III.