Electromyographic and histologic paraspinal abnormalities in polymyositis/dermatomyositis.

A 2-year study was undertaken which identified 60 patients who had an abnormal electromyogram (EMG), muscle biopsy, and/or clinical examination suggestive of myopathy. The clinical records were reviewed, and 12 patients were found to fulfill the criteria of Bohan and Peter for definite diagnosis of polymyositis or dermatomyositis. Electromyography was performed on distal and proximal muscles including the paraspinals on 1 side of the body. Biopsies were taken contralaterally from muscles determined to be abnormal from electrodiagnostic tests. The most significant finding was the degree of involvement of the paraspinal musculature. These muscles showed 1 to 4 plus spontaneous potentials in all 12 patients. Volitional recruitment was early and complete. Mean action potential duration and amplitude decreased, and polyphasia increased. Comparing the degree of electrodiagnostic abnormalities in different muscle groups revealed sole involvement of paraspinals in 1 patient and predominant involvement in 8. Confirmative biopsies were taken from paraspinal muscles of 3 patients. In 1 patient, the biopsy of the paraspinal muscle was positive for myositis while the biceps brachii muscle biopsy proved to be equivocal. Our study emphasizes the EMG and histologic significance of the paraspinal musculature in the diagnosis of idiopathic inflammatory myopathy.