What are the extramuscular symptoms and complications of dermatomyositis, polymyositis, immune necrotizing myositis, inclusion body myositis, and overlap myositis?

Extramuscular Manifestations of Inflammatory Myopathies

All idiopathic inflammatory myopathies are systemic diseases with significant extramuscular involvement affecting multiple organ systems, with pulmonary and cardiac complications being the most critical for morbidity and mortality. 1

Common Systemic Features Across All Subtypes

• Constitutional symptoms including fever, weight loss, and fatigue occur across all inflammatory myopathies 1
• Myositis-specific autoantibodies predict specific extramuscular organ involvement, particularly pulmonary and cardiac systems, and offer prognostic value 1

Dermatomyositis (DM)

Cutaneous Manifestations

• Pathognomonic skin findings include heliotrope rash (periorbital edema with violaceous discoloration), Gottron papules (erythematous papules over extensor surfaces), periungual telangiectasias, and poikiloderma 1
• Photosensitive erythematous rash involving face, neck, torso, fingers, and extensor surfaces of extremities 1
• Cutaneous vasculitis and ulcerations occur infrequently in adult DM but are more common in juvenile DM 1

Pulmonary Complications

• Interstitial lung disease (ILD) is the most common pulmonary manifestation, typically presenting as nonspecific interstitial pneumonitis 1
• Aspiration pneumonia can occur due to weakness of ventilatory muscles and dysphagia 1
• ILD represents a major cause of morbidity and mortality in DM patients 1

Cardiac Involvement

• Cardiac system involvement is recognized as a significant extramuscular manifestation affecting prognosis 1
• Cardiac involvement is associated with poorer outcomes and increased mortality 2

Gastrointestinal Manifestations

• Dysphagia is a common complication and represents a poor prognostic factor 2
• Gastrointestinal vasculopathy can occur, particularly in juvenile DM 1

Malignancy Association

• Elevated malignancy risk persists for several years after myositis onset, requiring vigilant cancer screening 3
• Elderly patients with DM have particularly high malignancy risk 2

Juvenile-Specific Features

• Calcinosis cutis is characteristic of juvenile DM, occurring less frequently in adults 1
• Cutaneous vasculitis with ulcerations is more prominent in children 1
• Gastrointestinal tract vasculopathy can be severe in pediatric cases 1

Polymyositis (PM)

Extramuscular Features

• Joint involvement (arthritis) can occur as part of the systemic disease 3
• Interstitial lung disease affects pulmonary function 3
• Cardiac involvement impacts prognosis 3
• Myositis-associated autoantibodies are frequently present and correlate with distinct clinical manifestations 3

Key Distinction

• Extramuscular skin findings are absent in PM, distinguishing it from DM 3
• Malignancy risk is elevated similar to DM 3

Immune-Mediated Necrotizing Myopathy (IMNM)

Triggering Factors

• Statin exposure is a major trigger, associated with anti-HMGCR antibodies 1, 2
• Viral infections can precipitate disease 1
• Malignant neoplasms are associated triggers 1
• Connective tissue diseases may coexist 1

Clinical Features

• Severe proximal muscle weakness with acute or subacute onset, though chronic courses also occur 1
• Markedly elevated CK levels (>10 times upper limit of normal) are characteristic 1, 2
• Minimal extramuscular manifestations compared to DM/PM, as the disease is primarily myopathic 2

Important Caveat

• Recovery may occur over weeks to months after discontinuing the offending agent (e.g., statins), though many patients require immunosuppression 1

Sporadic Inclusion Body Myositis (sIBM)

Distinctive Pattern

• Extramuscular manifestations are usually absent, distinguishing sIBM from other inflammatory myopathies 3, 4
• Degenerative features predominate with pathology resembling neurodegenerative disease rather than systemic autoimmune disease 1
• Myositis-associated autoantibodies are typically absent 3

Clinical Characteristics

• Dysphagia can occur due to pharyngeal muscle involvement 2
• Disease is refractory to immunosuppressive treatment, reflecting its degenerative rather than purely inflammatory nature 4

Overlap Myositis (OM)

Defining Features

• Coexistence with other connective tissue diseases defines this subgroup 5, 6
• Extramuscular manifestations reflect the overlapping connective tissue disease (e.g., scleroderma, lupus, rheumatoid arthritis features) 6

Antisynthetase Syndrome

• Distinct clinical phenotype associated with antisynthetase antibodies (anti-Jo-1 and others) 3, 7
• Classic triad includes myositis, interstitial lung disease, and arthritis 3
• Additional features may include mechanic's hands (hyperkeratotic, cracked skin on palms/fingers), Raynaud phenomenon, and fever 7
• ILD is particularly prominent and often determines prognosis in antisynthetase syndrome 3

Critical Prognostic Factors Across All Subtypes

Poor prognostic indicators that increase morbidity and mortality include:

• Advanced age at presentation 2
• Presence of dysphagia 2
• Cardiac involvement 2
• Associated malignancy 2
• Severe interstitial lung disease 1

Common Pitfall to Avoid

Do not confuse polymyalgia rheumatica with inflammatory myositis in elderly patients—polymyalgia rheumatica presents with pain but not true weakness and has normal CK levels, whereas inflammatory myositis causes genuine weakness with elevated CK 2. This distinction is critical as treatment approaches differ substantially.