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Idiopathic Inflammatory Myopathies (IIM) - Autoimmune Muscle Diseases

Idiopathic inflammatory myopathies are a heterogeneous group of rare autoimmune diseases that primarily affect skeletal muscle, causing symmetric and progressive weakness. The estimated annual incidence is approximately 1–10 cases per million people, with a slightly higher prevalence in women and a peak onset between the ages of 40 and 60.

The main clinical forms include:

Dermatomyositis;
Immune-mediated necrotizing myopathies;
Overlap myopathies (associated with other systemic connective tissue diseases), which include anti-synthetase syndrome;
Inclusion body myositis.

Forms that do not fit into these specific categories are generically referred to as polymyositis.

Diagnosis is based on clinical findings, laboratory data (elevated CK levels and antibody positivity), electromyographic results, and histopathological analysis (muscle biopsy). Therapeutic strategies are based on corticosteroids and immunosuppressants, with the option of using intravenous immunoglobulins (IVIG) or biological drugs in refractory cases.

Idiopathic inflammatory myopathies are autoimmune diseases in which the immune system mistakenly attacks muscle tissue. The pathogenesis varies according to the subtype: in dermatomyositis (DM), the predominant mechanism is an immune-mediated vasculopathy involving complement activation and damage to endomysial capillaries, whereas in polymyositis (PM), cytotoxic CD8+ T cells directly attack the muscle fibers. In inclusion body myositis (IBM), an inflammatory process coexists with a degenerative process characterized by the accumulation of abnormal proteins (TDP-43, p62, ubiquitin) within the fibers, suggesting a more complex pathogenetic mechanism. In immune-mediated necrotizing myopathy (IMNM), autoantibodies (anti-SRP and anti-HMGCR) mediate muscle fiber necrosis in the absence of significant inflammatory infiltrate. Recognized triggers include viral infections (EBV, Coxsackievirus), drug exposure (particularly statins for anti-HMGCR IMNM), and ultraviolet radiation (relevant in DM).

Manifestations vary significantly by subtype but share common features:
Muscle Presentation

Symmetric proximal weakness: Involving the shoulder and pelvic girdles, progressing over weeks to months.
Characteristic pattern: Acute/subacute onset of difficulty climbing stairs, rising from a chair, or lifting arms (e.g., combing hair, getting dressed).
Specific involvement: Swallowing (30% of cases, risk of aspiration), respiration (diaphragmatic involvement in severe forms), and cardiac involvement.
Muscle pain: Variable (present in about 50% of cases), more frequent during acute phases.

Dermatomyositis (DM)

Pathognomonic cutaneous manifestations: Heliotrope rash (violaceous erythema of the eyelids), Gottron papules (over the finger joints), shawl sign/V-sign (erythema on the shoulders/décolletage).
Subtypes: Classic (both muscle and skin involvement), amyopathic (skin only), juvenile, and paraneoplastic.

Polymyositis (PM)

Isolated muscle involvement without cutaneous manifestations.
Diagnosis: Muscle biopsy is always required for confirmation.

Inclusion Body Myositis (IBM)

Age of onset >50 years (male-to-female ratio 3:1), with an asymmetric distal and proximal pattern.
Characteristic distribution: Finger flexors, quadriceps, neck flexors; frequent dysphagia (80%).
Progression: Slowly progressive with limited response to therapy.

Systemic Manifestations

Pulmonary: Interstitial Lung Disease (ILD, 40-60% of cases), dyspnea, dry cough; represents the main prognostic factor.
Cardiac: Arrhythmias, cardiomyopathy (15-30%), more frequent in the anti-SRP IMNM form.
Articular: Non-erosive arthritis (common in anti-synthetase syndromes).
Paraneoplastic: Mandatory cancer screening (adult DM, PM >40 years).

Diagnostic Evaluation

Clinical assessment: Targeted medical history (medications, infections, malignancies), physical examination (strength rating scales, skin examination, joint, lung, and heart assessment).
Muscle enzymes: CK (elevated in 90% of cases, reaching 5–100x normal values), aldolase, LDH, AST/ALT (of muscular origin), troponins (for cardiac involvement).
Myositis-specific autoantibodies (MSA): anti-Jo1 (anti-synthetase syndrome), anti-Mi2 (classic DM), anti-SRP and anti-HMGCR (often secondary to statin therapy), anti-TIF1γ (paraneoplastic DM), anti-MDA5 (amyopathic DM + ILD).
Electromyography (EMG): Myopathic pattern (small, low-amplitude motor unit potentials), spontaneous activity (fibrillation potentials/positive sharp waves).
Muscle MRI: Edema (detectable on T2-STIR sequences), fatty replacement in chronic forms (detectable on T1 sequences); used to guide muscle biopsy site selection.
Muscle Biopsy: The gold standard for diagnosis; it highlights specific features:
- DM: Perifascicular atrophy.
- PM: Endomysial infiltrates.
- IBM: Rimmed vacuoles and protein deposits.
- IMNM: Diffuse necrosis with minimal inflammatory infiltrate.
Systemic screening: High-resolution CT (HRCT) of the chest (for ILD), ECG/Echocardiogram (cardiac involvement), and comprehensive cancer screening.

The Centro Dino Ferrari provides:

- Dedicated outpatient pathway: Myasthenia and Myositis Clinic (accessible following an initial visit at the Dino Ferrari Center’s Neuromuscular Diseases Clinic).
- Availability of key diagnostic tests: Blood tests with autoantibody screening, electromyography, muscle MRI, muscle biopsy, as well as cardiac and pulmonary imaging and functional diagnostics.
- Expertise in treatment: Management using conventional drugs and biological therapies.
- Clinical trials: Opportunities for inclusion in therapeutic clinical trials.

Available therapies

Immunosuppressive Therapy

First-line Corticosteroids: Intravenous methylprednisolone pulses or intravenous immunoglobulins (IVIG) for acute forms, especially with bulbar involvement; alternatively, in less severe cases, prednisone 1–2 mg/kg/day. Initial “attack” phase for 6–8 weeks, followed by a slow tapering.
Intravenous Immunoglobulins (IVIG): 2 g/kg per cycle, repeatable at intervals of at least 4 weeks; indicated for severe dysphagia or refractory forms.
Conventional Immunosuppressants: Primarily used as steroid-sparing agents: methotrexate (15–25 mg/week), azathioprine (2–3 mg/kg/day), cyclosporine, mycophenolate, cyclophosphamide (for progressive ILD).

Biological Therapies

Rituximab: B-cell depletion; 375 mg/m² x 4 (weekly administration) or 1000 mg x 2 (administered 2 weeks apart); indicated for refractory forms or anti-synthetase syndrome.
Emerging Therapies: Abatacept, tocilizumab, JAK inhibitors, anti-interferon agents.

Management of Complications

Respiratory Failure: Monitoring of pulmonary function tests (PFTs), ventilatory support, specific therapies (cyclophosphamide, rituximab).
Dysphagia: FEES (Fiberoptic Endoscopic Evaluation of Swallowing), diet consistency modification, PEG placement in case of aspiration risk, speech therapy.
Cardiac Involvement: ECG/Echocardiogram, antiarrhythmics, intensification of immunosuppression.

Rehabilitative Support

Physical Therapy: Passive mobilization in the acute phase, progressive active exercises, low-intensity aerobic training.
Occupational Therapy: ADL (Activities of Daily Living) assistive devices, environmental adaptations.

Management of Dermatomyositis (DM)

Sun Protection: Essential for photosensitivity, SPF >30, physical barriers (clothing).
Topical Therapies: Corticosteroids, calcineurin inhibitors, hydroxychloroquine 200–400 mg/day.

Research in progress

Development of novel biological therapies targeting specific inflammatory pathways.
Precision medicine with therapeutic stratification based on autoantibody profiling.
Innovative biomarkers for monitoring disease activity and therapeutic response.
Advanced muscle imaging, including quantitative MRI and MR spectroscopy.
Studies on pathogenetic mechanisms and phenotypic modifier factors.
Combination therapies to optimize personalized immunosuppression strategies.