Plasma carnitine insufficiency and effectiveness of L-carnitine therapy in patients with mitochondrial myopathy.

Plasma carnitine "insufficiency," (plasma esterified carnitine to free carnitine ratio above 0.25) was found in 21 of 48 (43.8%) patients with mitochondrial myopathy, of whom 4 also showed both total and free carnitine deficiencies in plasma. In addition, plasma levels of SCAC and LCAC were higher in patients with mitochondrial myopathy than in controls (P < 0.001 and P < 0.01, respectively). Patients diagnosed as having plasma carnitine insufficiency or deficiency were treated with L-carnitine (50-200 mg/kg per day in four daily doses). Muscle weakness improved in 19 of 20 patients, failure to thrive in 4 of 8, encephalopathy in 1 of 9, and cardiomyopathy in 8 of 8 patients. Plasma carnitine "insufficiency" provides an additional clue to the diagnosis of mitochondrial myopathy and an indication for L-carnitine therapy.