DERMATOLOGY FOR THE USMLE AUTOIMMUNE SKIN DISORDERS (3) 3. DERMATOMYOSITIS

DERMATOLOGY FOR THE USMLE

AUTOIMMUNE SKIN DISORDERS (3)

3. DERMATOMYOSITIS

→	General: Complex, multisystemic autoimmune inflammatory myopathy of unknown etiology characterized by prominent cutaneous involvement. It is thought that autoantibodies and other unknown factors activate the complement cascade in muscle capillaries and arterioles leading to muscle microinfarction. Polymyositis is another inflammatory myopathy with similar clinical presentation but without cutaneous involvement. The main organs affected in dermatomyositis are: » GI tract: Dysphagia and GERD. » Heart: Dilated cardiomyopathy, arrhythmias and heart block. » Lung: Interstitial lung disease (ILD). » Constitutional symptoms: Fatigue, weakness and weight loss. » Musculoskeletal: Symmetric proximal muscle involvement leading to inability to climb stairs, brush hair, rise from sitting position or reach objects above shoulder level.
→	Clinical: Cutaneous manifestations vary. They may be the initial and only sign of dermatomyositis. » Heliotrope rash: Symmetric and edematous purplish-red periorbital rash. » Shawl sign: Erythematous and pruritic rash with telangiectasias predominantly affecting the upper chest, upper back, shoulders and extensor surface of arms in a “cape-like” distribution. » Gottron papules: Red-to-purple, flat-topped papules on the interphalangeal joints (knuckles). Gottron sign is a violaceous discoloration on the knees, elbows or feet. » Mechanic’s hands: Roughened, thickened and cracked palmar and lateral aspects of hands. » Ragged nail cuticles
→	Diagnosis » Best initial test: Clinical + labs (creatinine kinase, aldolase, ESR and CRP) + autoantibody assays to detect anti-Jo-1 (anti-histidyl t-RNA synthetase) and anti-Mi-2 (very specific). Pulmonary function tests (PFTs) to identify underlying lung disease. Electromyography (EMG) or MRI are useful to investigate specific muscle damage and guide biopsy site. » Most accurate test: Muscle biopsy showing a perifascicular and perivascular lymphocytic infiltrate and muscle fiber degeneration.
→	Treatment » First line: Systemic steroids. » Second line: Methotrexate, mycophenolate mofetil or IVIG.
→	USMLE Pearls: Approximately 20 to 40% of patients diagnosed with dermatomyositis have an underlying malignancy (eg, lung, ovarian, breast). Treating the cancer may result in symptomatic resolution of dermatomyositis. Screen for malignancies at the time of diagnosis and yearly during the first 3 years with: » Urinalysis and FOBT or colonoscopy » Serum PSA and CA-125 » Mammogram and pelvic ultrasound » CT scan of chest, abdomen and pelvis