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Polymyositis / Dermatomyositis

BASICS

  • Definition: Systemic connective tissue disease causing inflammatory and degenerative changes in proximal muscles. Characteristic skin findings = dermatomyositis.
  • Key Skin Signs (dermatomyositis):
  • Gottron sign: Symmetric, scaly, violaceous, erythematous eruption over dorsal MCP and IP finger joints
  • Heliotrope rash: Reddish-violaceous eruption on upper eyelids
  • Subtypes:
  • Idiopathic polymyositis
  • Idiopathic dermatomyositis
  • Overlap myositis (with SLE/systemic sclerosis/mixed CTD)
  • Malignancy-associated myositis
  • Necrotizing autoimmune myositis (statin-associated)
  • Inclusion body myositis (IBM) – atypical weakness, unique pathology
  • Antisynthetase syndrome: Subset with certain antibodies and ILD
  • Systems affected: Musculoskeletal, skin, pulmonary, cardiovascular

EPIDEMIOLOGY

  • Incidence: 1.2–19 per million/year
  • Sex: Female > Male (2:1)
  • Age peaks: 5–15y (juvenile), 40–60y (adults, peak mid-40s)
  • Prevalence: 2.4–33.8/100,000
  • Elderly: Myositis/dermatomyositis—↑ malignancy risk
  • Pediatric: Childhood dermatomyositis—cutaneous vasculitis, muscle calcifications

ETIOLOGY & PATHOPHYSIOLOGY

  • Autoimmune T-cell mediated inflammation
  • Muscle damage: Primarily skeletal, degenerative in IBM
  • Genetics: Mild HLA-DR3, HLA-DRw52 association
  • Triggers: Viral, genetic, environmental factors

RISK FACTORS

  • Family history of autoimmune disease (SLE, vasculitis, myositis)

COMMONLY ASSOCIATED CONDITIONS

  • Malignancy (esp. dermatomyositis/older adults)
  • SLE, progressive systemic sclerosis, mixed connective tissue disease, vasculitis

DIAGNOSIS

History

  • Symmetric proximal muscle weakness (shoulders, hips)
  • Difficulty rising, climbing stairs, lifting arms
  • Joint pain/swelling
  • Dysphagia, dyspnea
  • Rash (face, eyelids, hands, arms)

Physical Exam

  • Proximal muscle weakness: shoulder/hip girdle
  • Muscle swelling, stiffness, induration
  • Distal weakness (only IBM)
  • Rash: Face (eyelids, nasolabial folds), upper chest, dorsal hands/knuckles, “mechanic’s hands,” periorbital edema
  • Calcinosis cutis (children)
  • Arrhythmias, heart failure

Differential Diagnosis

  • Vasculitis, SLE, systemic sclerosis, RA, muscular dystrophy
  • ALS, Lambert-Eaton, sarcoidosis
  • Endocrine: thyroid, Cushing
  • Drug-induced myopathies: statins, colchicine, steroids, alcohol, chloroquine, zidovudine
  • Infectious myositis
  • Electrolyte/metabolic myopathies

Diagnostic Tests & Interpretation

Key Diagnostic Criteria:
Muscle component diagnosis = 4 findings: - Weakness - CK and/or aldolase elevation - Abnormal EMG - Muscle biopsy findings

Skin rash = dermatomyositis

Emerging: Myositis-specific antibodies (MSA) for prognosis/workup (malignancy, ILD).

Initial Labs/Imaging

  • ↑ CK, aldolase
  • ↑ AST, LDH, myoglobinuria
  • ↑ ESR, leukocytosis, anemia, hyperglobulinemia, ANA+
  • Anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase & anti-SRP: necrotizing autoimmune myositis
  • Anti-Jo-1, anti-synthetase: antisynthetase syndrome/ILD
  • Anti-MDA-5: severe ILD, overlap myositis
  • CXR: evaluate for ILD, malignancy

Other/Follow-up

  • Muscle MRI: Edema/inflammation, biopsy guidance, response to therapy
  • EMG: muscle irritability, low amplitude/polyphasic/fibrillations
  • Biopsy: Muscle fiber degeneration, phagocytosis, perifascicular atrophy (dermatomyositis), inflammatory infiltrates (adult form), inclusion bodies (IBM)
  • Monitor muscle enzymes for activity

TREATMENT

General Measures

  • Multidisciplinary: rheumatology, neurology, dermatology input
  • Physical therapy for ROM; avoid strenuous activity in active disease

First Line

  • Prednisone: 0.5–1 mg/kg/day PO (divided); pulse IV methylprednisolone 1g x 3–5d for severe cases
  • Taper when muscle enzymes normalize; often need 5–10 mg/day maintenance
  • Add Immunosuppressant: (usually needed for maintenance)
  • Azathioprine 1–2 mg/kg PO daily
  • Methotrexate 10–25 mg PO weekly
  • Mycophenolate mofetil 2–3 g/day
  • Dermatomyositis rash: Topical steroids or hydroxychloroquine
  • IBM: Poor response to standard therapy

Second Line/Refractory

  • Other immunosuppressants: cyclophosphamide, cyclosporine, tacrolimus, ACTH gel
  • IVIG or rituximab (autoantibody-positive, severe, or refractory disease)
  • Combination MTX + AZA for refractory
  • Cyclophosphamide for severe lung/heart involvement

Contraindications

  • Methotrexate: liver disease, alcohol use, pregnancy, renal dysfunction (Cr >1.5)
  • Prednisone: caution in CHF, diabetes, infection

Precautions

  • Steroids: infection, osteoporosis, cataracts, hypertension, diabetes
  • Azathioprine: bone marrow suppression, ↑ LFTs, infection
  • Methotrexate: stomatitis, marrow suppression, lung/liver toxicity

Referral

  • Diagnostic uncertainty
  • Poor response to steroids
  • Need for immunosuppressants/steroid-sparing agents

No role for surgery (except biopsy)

ONGOING CARE

Follow-up

  • Monitor: Muscle enzymes, strength, functional status
  • Watch for steroid complications: osteoporosis, glucose, potassium, blood pressure, infections
  • Labs for immunosuppressants: CBC, LFTs, creatinine
  • Bone density testing, calcium/Vitamin D, bisphosphonates as needed
  • Attempt steroid taper as able

Diet

  • Calorie/sodium moderation to avoid weight gain

Patient Education

  • Avoid overexertion during active disease
  • Range of motion exercises; gradually advance as disease quiesces
  • Explain chronicity, risk of malignancy (esp. older adults, dermatomyositis)
  • Encourage follow-up and monitoring for complications

PROGNOSIS

  • Residual weakness: 30%
  • Persistent disease: 20%
  • 5-year survival: 65–75% (most mortality in year 1)
  • Worse prognosis: Women, African Americans, dermatomyositis, IBM, malignancy, ILD
  • Full recovery: 20–50%
  • IBM: Poor response, slow progression
  • ILD: Poor outcome

COMPLICATIONS

  • Infection, pneumonia, MI, malignancy (breast, lung), dysphagia, respiratory failure, aspiration, ILD, steroid myopathy/diabetes/osteoporosis/hypertension

ICD-10

  • M33.20 Polymyositis, organ involvement unspecified
  • M33.90 Dermatopolymyositis, unspecified
  • M33.92 Dermatopolymyositis with myopathy

CLINICAL PEARLS

  • Rapid CK/aldolase rise may be transient after injury/fever—repeat labs.
  • In persistent enzyme elevation + weakness, proceed to EMG and biopsy.
  • IBM: Elderly, slow onset, asymmetric or distal weakness, poor steroid response.
  • Most patients require steroids + immunosuppression.
  • Watch for malignancy in adults with dermatomyositis.