A 30-year-old woman has had gradually increased muscle weakness with myalgia for the past year. She now has difficulty getting up from a chair and climbing stairs. She does not have weakness in her hand muscles. Physical examination reveals a fine violaceous rash on her face, predominantly palpebral. Dusky, flat, red patches are present on her elbows, knees, and knuckles. Laboratory studies show serum creatine kinase of 620 U/L. A deltoid biopsy specimen is obtained, and on microscopic examination shows a mononuclear inflammatory cell infiltrate around small blood vessels and groups of atrophic myofibers at the periphery of fascicles. What mechanism is most likely responsible for her disease?
High-Yield Explanation
Dermatomyositis is an immunologically mediated form of inflammatory myopathy. In these patients, antibodies, and complement damage to small capillaries, resulting in characteristic perifascicular myofiber atrophy. The expansion of CTG repeats is seen with myotonic dystrophy. Mutations in ion channel genes give rise to various channelopathies, including hypokalemic periodic paralysis and malignant hyperthermia. The CD8+ T cells are believed to be important in the pathogenesis of polymyositis. T cell-mediated myelin injury is seen with Guillain-Barre syndrome, causing an acute ascending paralysis.