Diffuse abnormal electromyographic insertional activity: a preliminary report.

The authors observed 10 patients referred for electromyographic examination in whom EMG findings were normal except for the presence of provoked positive sharp waves in essentially every muscle tested. The positive sharp waves were present only following needle electrode insertion. No fibrillation or fasciculation potentials were seen. Motor unit action potentials were normal, as were nerve conduction and repetitive stimulation studies. The quantity of provoked positive sharp waves was found to vary from time to time, from muscle to muscle and from 1 region of the muscle to another. Detailed clinical evaluations for neuromuscular disorders, electrolyte and metabolic abnormalities and underlying carcinomas were negative. Muscle biopsy in 2 patients, including histochemical analysis in one of them, was normal. Results of single-fiber electromyography were normal in 1 patient. The abnormality was found in family members and appears to be of an autosomal dominant inheritance.