Electromyographic and histologic paraspinal abnormalities in polymyositis/dermatomyositis

Area of Science:

• Neurology
• Rheumatology
• Pathology

Background:

• Idiopathic inflammatory myopathies (IIMs), including polymyositis and dermatomyositis, are rare autoimmune diseases.
• Diagnosis often relies on a combination of clinical findings, muscle enzymes, electromyography (EMG), and muscle biopsy.
• The role of paraspinal muscle involvement in IIM diagnosis requires further elucidation.

Purpose of the Study:

• To evaluate the diagnostic significance of paraspinal muscle electromyography (EMG) and biopsy in patients with suspected idiopathic inflammatory myopathy.
• To assess the prevalence and pattern of paraspinal muscle abnormalities in confirmed cases of polymyositis and dermatomyositis.

Main Methods:

• A 2-year retrospective study identified 60 patients with clinical, EMG, or biopsy findings suggestive of myopathy.
• Twelve patients met Bohan and Peter criteria for definite polymyositis or dermatomyositis.
• EMG was performed on proximal, distal, and paraspinal muscles; biopsies were taken contralaterally from electrodiagnostically abnormal muscles.

Main Results:

• All 12 patients exhibited significant paraspinal muscle abnormalities on EMG, including spontaneous potentials.
• Paraspinal muscles showed early and complete volitional recruitment, decreased action potential duration/amplitude, and increased polyphasia.
• Paraspinal muscle biopsy confirmed myositis in 1 of 3 patients biopsied, while biceps biopsy was equivocal in that case.

Conclusions:

• Paraspinal musculature demonstrates significant electrodiagnostic and histologic abnormalities in idiopathic inflammatory myopathy.
• EMG and biopsy of paraspinal muscles are valuable tools for diagnosing polymyositis and dermatomyositis.
• These findings emphasize the importance of including paraspinal muscles in the diagnostic workup for IIMs.