Purpose of review: Acquired immune and inflammatory myopathies (IIMs) are typically subdivided into dermatomyositis, polymyositis and inclusion body myositis. However, many types of IIMs do not fit well into this scheme. Several myopathologic and autoantibody features of IIMs, that are not considered in standard classifications, are useful for defining individual disorders. We will review one set of myopathologic features that occur in some IIMs, mitochondrial abnormalities, and consider its diagnostic, treatment-related and pathogenic implications.
Recent findings: Myopathologic changes that indicate mitochondrial disorders are often widespread in regions of muscle fiber abnormality in dermatomyositis. They distinguish dermatomyositis with vascular pathology from other inflammatory myopathies with skin changes that have prominent perimysial connective tissue lesions, but no mitochondrial, abnormalities. Mitochondrial abnormalities in scattered muscle fibers occur in sporadic inclusion body myositis and clinically similar disorders. Mitochondrial abnormalities in scattered nonnecrotic muscle fibers in IIM biopsies predict a poor response to immunosuppression.
Summary: Muscle biopsy, including evaluation of mitochondrial stains, is important for the correct diagnosis of inflammatory myopathies. By recognizing the full range of distinctive myopathologic changes in the diverse group of IIMs, the clinician can improve diagnostic accuracy and apply appropriate treatment.