Dermatomyositis and Polymyositis

Contributed by: Eman Bahrani, Baylor College of Medicine, 1 Baylor Plaza, Houston, Tx 77030
Overview
  • Dermatomyositis (DM) and Polymyositis (PM) are rare idiopathic inflammatory myopathies. Both manifest with proximal skeletal muscle weakness, though DM has associated cutaneous findings. They display a two fold female predominance and affect all ages with peak incidence after 40; however, DM displays a second peak in a juvenile population, termed Juvenile DM.
Etiology and Pathogenesis
  • The etiologies of DM and PM are unknown, though they are believed to represent autoimmune responses to outside triggers (e.g. malignancy, drugs, infection) in genetically susceptible individuals. In adult patients with DM, the presence of an occult malignancy is an especial concern.
  • While previously thought to represent a spectrum of disease, new evidence shows that DM and PM likely have distinct pathogeneses. DM appears to involve humoral generation of autoantibodies that promote complement deposition and injury of vasculature within muscles. In contrast, PM appears to involve production of autoreactive CD8+ T-cells that directly injure myocytes.
Morphology
  • Dermatomyositis
    • Muscle biopsies in DM show perivascular and perimysial lymphocytic infiltrates as well as perifascicular atrophy. Skin biopsies show subtle changes including epidermal atrophy, damage to keratinocytes of the basal layer, and a sparse dermal lymphocytic infiltrate.
  • Polymyositis
    • Muscle biopsies in PM show endomysial infiltration of muscle fibers with CD8+ T-cells. A mix of necrotic and regenerating muscle fibers can be seen. In contrast to DM, there is no vascular inflammation and no perifascicular atrophy.
Clinical Manifestations
  • Overview
    • DM and PM share a virtually indistinguishable proximal muscles weakness whereas skin involvement is limited to DM. In some patients these diseases are associated with systemic sequelae which we briefly discuss.
  • Muscle Symptoms
    • Weakness develops subacutely, over weeks to months, is symmetric and primarily involves the proximal muscles of the neck, shoulders, and pelvic girdle. Myalgias are also often present with involved muscles tender to palpation.
    • The striated muscles of the oropharyx and esophagus can also be affected, causing dysphagia.
  • Skin Manifestations
    • Cutaneous changes are only found in DM and are characterized by poikiloderma a term referring to a pink-violet color change to the skin with small mixed areas of hyper- and hypo-pigmentation as well as telangiectasis. Poikilodermatous changes in different characteristic areas of skin have developed specific names.
    • Heliotrope Rash: Seen around the eyes, which can be accompanied by eyelid edema
    • V-sign and Shawl Sign: Rash around the photo-exposed chest (V-sign) and upper back (Shawl Sign).
    • Gottrons Papules: Raised lesions seen overlying the knuckles, highly characteristic of DM
    • Patients with DM also often have ragged cuticles with characteristic telangiectasias at the nail folds.
    • Patients with juvenile DM can develop calcinosis cutis or painful depositions of calcium within their skin.
  • Systemic Disease
    • A minority of patients develop interstitial pulmonary disease characterized by fibrosis. These diseases can also affect the heart, typically causing conduction abnormalities, and thus arrhythmias. Finally, polyarthritis is also sometimes seen.
    • As mentioned, adult DM is often associated with an occult malignancy and thus the presence of a visceral cancer should be considered.
Diagnosis
  • Overview
    • A variety of tests can aid in the diagnosis of DM and PM when clinically suspected. Skin and muscle biopsies provide confirmation.
  • Muscle Enzymes
    • Myositis yields leakage of muscle enzymes which can be detected by elevated levels of creatinine kinase (CK), aldolase, AST, and LDH.
  • Auto-antibodies
    • A variety of auto-antibodies are associated with the disease and include Antinuclear Autoantibodies (ANA), Rheumatoid Factor (RF), and more specific myositis-associated antibodies (e.g. anti-Jo-1, anti-Mi-2, anti-SRP).
  • Electromyography (EMG)
    • EMGs are often positive although these studies, along with muscle biopsies, are increasingly being replaced by MRIs.
Management
  • The mainstay of therapy for both DM and PM is systemic corticosteroids with very slow tapers. Further immunosupressants or immunomodulators can be considered for resistant cases.
  • Adult patients with DM should have screening for occult malignancy.
Further Reading
  • Dimachkie MM, Barohn RJ, Amato AA. Idiopathic inflammatory myopathies. Neurol Clin. 2014;32(3):595-628.
  • Femia AN, Vleugels RA, Callen JP. Cutaneous dermatomyositis: an updated review of treatment options and internal associations. Am J Clin Dermatol. 2013;14(4):291-313.