Muscular fatigability in mitochondrial myopathies. An electrophysiological study.

A frequent occurrence of ophthalmoplegia and muscle fatigability in mitochondrial myopathy (MAM) often makes its differential diagnosis from myasthenia rather difficult. Neuromuscular transmission was investigated in 9 patients with MAM, presenting marked fatigability. The aim of the study was to see whether there were any other causes of muscle fatigability in addition to the metabolic factors. Classical electrostimulation as well as the SFEMG, which is very sensitive in detecting neuromuscular transmission disorders, were used. The findings were far from uniform: we found normal neuromuscular transmission in 5 cases, in 3 patients we observed slight abnormalities of neuromuscular transmission, in 1 case neuromuscular transmission disturbances seemed to be of neurogenic origin. Our results allow an assumption that the causes of muscle fatigability in MAM are of a much more complex nature than it has been anticipated. They might depend not only on the metabolic disorders within the muscle fibre itself but also on the impaired function of the peripheral nerve or of the neuromuscular junction. All the mechanisms combined may also play a role, though in individual patients the contribution of particular factors responsible may vary.