Polymyositis and Dermatomyositis

The prevalence of PM/DM is 1 per 100,000 adults, with a female to male ratio of 2 : 1; there is also a childhood form of dermatomyositis. Presenting symptoms include shoulder and pelvic-girdle muscle weakness, malaise with fatigue, and occasionally myalgia and muscle tenderness gradually worsening over weeks to months. Patients report difficulty arising from a chair, combing hair, and climbing stairs. A defining symptom of dermatomyositis is skin rash over the face, hands, neck, and extensor aspects of the extremities.

Clinical examination discloses proximal muscle weakness of the arms and legs and neck flexor muscles. Distal muscle weakness, typically, is mild. In DM, and not PM, cutaneous manifestations include Gottron’s sign (scaly erythematous rash over hand and finger joints and elbow and knee extensor areas), heliotropic rash (violaceous discoloration with upper eyelid edema), shawl sign (V-shaped flat erythematous rash over the anterior neck and chest), erythematous rash over the face (malar region and forehead), periungual telangiectasia (affecting fingernail bed capillaries), mechanic’s hands (cracking of skin over hands and fingers), and calcinosis cutis (subdermal calcium deposits). In both PM and DM, dysphagia (esophageal weakness), cardiac disease (heart block or congestive heart failure), and constitutional findings (fever, weight loss, arthritis) may occur.

Dermatomyositis and polymyositis need to be distinguished from other conditions that may cause muscle weakness, including other muscle disorders, that is, muscular dystrophies, toxic (drug-induced), metabolic, and endocrine myopathies; chronic inflammatory demyelinating neuropathy (CIDP); myasthenia gravis; and, rarely, motor neuron disease.

Diagnosis of DM and PM is based on clinical findings supported by elevated CK and an abnormal electromyogram (EMG; at rest, fibrillation potential activity, with low levels of contraction; short-duration, low-amplitude, polyphasic motor unit potentials firing with an increased number, designated “early recruitment pattern”). Twenty percent of patients have an antibody to histidyl transfer ribonucleic acid (tRNA), called anti-Jo1, defining a patient subgroup with interstitial lung disease and mechanic’s hands.

Muscle biopsy of a weak proximal muscle is the definitive study. Histopathologic features of both DM and PM include inflammatory cell infiltration and muscle fiber necrosis, with degenerating and regenerating muscle fibers scattered throughout the fascicle. In DM, the inflammatory cell infiltrate, composed mainly of B cells and CD4⁺ cells, is predominantly perifascicular and perivascular. Muscle atrophy, occurring at the periphery of muscle fascicles, is known as perifascicular atrophy (especially prominent in childhood DM). Another characteristic finding in DM is deposition of the terminal component of the complement cascade—the membrane attack complex (MAC)—in capillary walls. In contrast, PM is characterized by inflammatory cell infiltration of T cells and macrophages occurring diffusely within the endomysium surrounding and invading non-necrotic muscle fibers within the fascicles. Cell-mediated immune mechanisms are important, having increased numbers of cytotoxic CD8⁺ T cells and increased expression by muscle fibers of major histocompatibility complex (MHC) antigens.

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