Differential diagnosis of chronic idiopathic polymyositis and neuromyositis.

The term neuromyositis indicates a very rare type of polymyositis where, beside the usual muscular manifestations, there are signs of peripheral neuropathy. Although a lot of patients with neuromyositis has been published, it is a very controversial entity. There may be considerable difficulty in distinguishing chronic polymyositis from neuromyositis, even with the aid of electromyographic and muscle biopsy examinations. The aim of this investigation was to re-establish the clues for distinguishing chronic polymyositis from neuromyositis. Fifty-seven patients took part in this investigation. They were subdivided into two groups according to EMG and muscle biopsy data. First group consisted of 29 patients with chronic polymyositis and second group consisted of 28 patients with chronic neuromyositis. Our data confirm the existence of neuromyositis as an entity with different clinical, electromyographic and biopsy findings than polymyositis. The differentiation between chronic polymyositis and neuromyositis is a difficult task on purely clinical ground. Electromyographic recordings with widespread myopathic features, some large potentials and spontaneous activity could be found in both diseases. The most common and important electromyographic findings in patients with neuromyositis are prolonged sensory and motor nerve conduction velocities. In conclusion electromyography and muscle biopsy may be useful in the differential diagnosis of polymyositis and neuromyositis.