Polymyositis and Dermatomyositis
Polymyositis and dermatomyositis are idiopathic inflammatory diseases of skeletal muscle characterized by progressive weakness affecting proximal muscle groups, particularly those of the shoulders and hips. Women are more commonly affected than men (2:1), with a peak age incidence between 40 and 50 years. Both disorders are associated with polyarthritis, dysphagia, and interstitial pulmonary disease. Ocular involvement is relatively uncommon, apart from the heliotrope rash of dermatomyositis, which is very specific but not often present (Fig 9-7). In rare cases, the extraocular muscles may be involved, resulting in ophthalmoplegia.
Dermatomyositis is distinguished from polymyositis by the presence of cutaneous lesions. These skin lesions appear as an erythematous to violaceous rash variably affecting the eyelids (heliotrope rash; see Fig 9-7), cheeks, nose, chest (V-neck sign), and extensor surfaces (Gottron sign). Pathogenically, dermatomyositis is associated with immune complex deposition in the vessels, whereas polymyositis appears to reflect T-cell–mediated muscle injury.
Figure 9-7 Heliotrope rash in dermatomyositis, with typical reddish-purple appearance and associated eyelid swelling.
(Used with permission from Wolff K, Johnson RA, Saavedra AP. Fitzpatrick’s Color Atlas and Synopsis of Clinical Dermatology. 7th ed. www.accessmedicine.com.)
Laboratory findings in both disorders include elevated serum muscle enzymes, serum and urine myoglobin, and abnormal electromyography results. A wide range of autoantibodies is found in most patients, including several that are specific to myositis. Muscle biopsy may confirm the muscle damage from inflammation.
Glucocorticoids are typically initiated at the time of diagnosis and are usually tapered over a period of 9–12 months. Immunosuppressive agents, such as azathioprine or methotrexate, are sometimes used in patients unresponsive to steroid treatment or in those who develop adverse effects.
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.