Muscle biopsy in primary fibromyalgia. Light-microscopical and histochemical findings.

Seventy-seven muscle biopsies from 57 patients with primary fibromyalgia and 17 biopsies from 9 healthy controls were examined by routine histopathological and histochemical methods. All patients fulfilled strict diagnostic criteria. 42 biopsies from patients were deemed normal or borderline, while 35 showed discrete pathological changes (degeneration, regeneration, inflammatory infiltrates, ragged red fibres, and 'moth-eaten' fibres in muscles other than m. trapezius). Biopsies from controls were normal or borderline in 14 and showed mild focal abnormalites in 3 cases. 'Moth-eaten' fibres were found in 35 of 41 trapezius biopsies taken from patients and in 9 of 10 trapezius biopsies from controls. Ragged red fibres were found in 15 of 41 trapezius biopsies taken from patients. In the deltoid muscle, 'moth-eaten' fibres were found in 3 and ragged red fibres in 2 of 9 biopsies from patients. Frequency of type I and type II fibres and the area of muscle fibres were the same in patients as in controls. Capillary density was the same in both groups. These changes, although not sufficient to constitute a diagnostic test for PF, indicate that this condition has an organic basis.