What is myositis?

What is Myositis?

Myositis, also known as idiopathic inflammatory myopathy (IIM), is a heterogeneous group of autoimmune disorders characterized by muscle inflammation and muscle weakness, with the most common adult subtypes being dermatomyositis, polymyositis, and inclusion body myositis, and juvenile dermatomyositis in children. 1

Core Defining Features

Muscle inflammation and weakness are the hallmark features that distinguish myositis from other muscle disorders. 1 The condition presents with:

• Symmetric proximal muscle weakness developing over weeks to months, affecting the ability to stand from a seated position, climb stairs, and lift arms overhead 1, 2
• Persistently elevated creatine kinase (CK) levels, often markedly elevated, reflecting ongoing muscle damage 1, 2
• Muscle inflammation visible on biopsy and advanced imaging 1

Major Subtypes

Dermatomyositis (DM)

• Characterized by distinctive cutaneous manifestations including photosensitive erythematous rash, Gottron papules (over knuckles), heliotrope rash (purple discoloration of eyelids), periorbital edema, and periungual telangiectasias 1, 2
• Symmetric proximal muscle weakness coupled with elevated muscle enzymes 1
• Juvenile dermatomyositis (age <18 years) frequently presents with calcinosis cutis, cutaneous vasculitis, ulcerations, and gastrointestinal vasculopathy 1, 2

Polymyositis (PM)

• Mediated by CD8+ cytotoxic T-cell invasion of non-necrotic muscle fibers 2
• Subacute onset over weeks to months with symmetric proximal weakness and persistently elevated CK 2
• No characteristic skin manifestations, distinguishing it from dermatomyositis 1

Inclusion Body Myositis (IBM)

• Shows degenerative neuromuscular features with muscle fiber vacuolization and abnormal accumulation of amyloid-β and phosphorylated tau proteins analogous to Alzheimer disease 1, 2
• Most common in adults, particularly older individuals 1

Immune-Mediated Necrotizing Myopathy (IMNM)

• Biopsy reveals severe myopathic changes with minimal inflammatory infiltrate, distinguishing it from polymyositis 2
• Triggered by viral infections, statin exposure, or underlying malignancies 2
• Presents with markedly elevated CK levels (often >10 times upper limit of normal) and acute to subacute proximal weakness 1, 2

Diagnostic Laboratory Features

Myositis-specific autoantibodies define distinct clinical phenotypes and have prognostic significance, including anti-Jo-1, anti-Mi-2, anti-MDA5, anti-TIF1-γ, and anti-NXP2 2, 3, 4. Approximately 60% of patients with IIM have detectable myositis-specific autoantibodies 3.

Inflammatory markers (ESR and CRP) are typically markedly elevated, reflecting systemic inflammation 2.

Electromyography shows polyphasic motor unit action potentials of short duration and low amplitude, with increased insertional and spontaneous activity including fibrillation potentials and sharp waves 1, 2.

Important Etiologic Distinctions

Infectious Myositis

Acute viral myositis typically resolves within 3-7 days with normal to mildly elevated CK levels that normalize rapidly, representing a self-limited course distinct from autoimmune myositis 2, 5.

Drug-Induced Myositis

• PD-1/PD-L1 immune checkpoint inhibitors provoke myositis more frequently than CTLA-4 blockade, with median onset approximately 4 weeks after therapy initiation 2
• ICI-related myositis can present as fulminant necrotizing myopathy with rhabdomyolysis and potentially fatal myocardial involvement 2
• Statin therapy causes drug-induced muscle disorders considerably more frequently than antipsychotic medications 2
• Persistent CK elevation exceeding 10 times the upper limit of normal suggests necrotizing myopathy rather than benign reversible myopathy 2

Malignancy-Associated Myositis

Myositis can arise as a paraneoplastic syndrome linked to occult malignancy, necessitating dedicated oncologic evaluation beyond immunomodulatory therapy 2.

Classification Criteria

The 2017 EULAR/ACR classification criteria employ a weighted scoring system where each clinical, laboratory, and pathologic item receives a score, with a total score corresponding to probability of IIM 1.

• A probability cutoff of 55% (score ≥5.5 without biopsy, ≥6.7 with biopsy) defines "probable IIM" with sensitivity/specificity of 87%/82% without biopsies and 93%/88% with biopsies 1
• These criteria allow classification of "definite," "probable," and "possible" IIM, plus major subgroups including juvenile IIM 1
• The criteria have been endorsed by international rheumatology, dermatology, neurology, and pediatric groups and generally perform better than older Bohan and Peter criteria 1

Common Pitfall

Polymyalgia rheumatica-like presentations feature severe myalgia and fatigue without objective muscle weakness, with normal CK levels and no evidence of myopathy on MRI/EMG, distinguishing them from true myositis 2. The key distinction is the presence of true weakness versus pain alone.