Idiopathic inflammatory myopathy

Idiopathic Inflammatory Myopathy

Idiopathic inflammatory myopathy (IIM) is a group of rare diseases characterized by chronic muscle inflammation and weakness. These conditions are considered idiopathic because their exact cause is unknown. The primary types of idiopathic inflammatory myopathies include polymyositis, dermatomyositis, inclusion body myositis, and necrotizing autoimmune myopathy.

Classification

Idiopathic inflammatory myopathies are classified based on clinical presentation, histopathological findings, and the presence of specific autoantibodies. The main types are:

• Polymyositis: Affects primarily adults and is characterized by symmetrical muscle weakness, particularly in the proximal muscles.
• Dermatomyositis: Similar to polymyositis but also includes a distinctive skin rash. It can affect both adults and children.
• Inclusion Body Myositis (IBM): Typically affects older adults and is characterized by both proximal and distal muscle weakness, often with asymmetric involvement.
• Necrotizing Autoimmune Myopathy: Characterized by severe muscle weakness and elevated muscle enzymes, often associated with specific autoantibodies.

Pathophysiology

The pathophysiology of idiopathic inflammatory myopathies involves immune-mediated muscle damage. In polymyositis and dermatomyositis, T cells and macrophages infiltrate muscle tissue, leading to muscle fiber necrosis. In dermatomyositis, complement-mediated damage to capillaries also plays a role. Inclusion body myositis involves both inflammatory and degenerative processes, with the presence of rimmed vacuoles and amyloid deposits in muscle fibers.

Clinical Features

Patients with idiopathic inflammatory myopathy typically present with:

• Muscle Weakness: Symmetrical weakness of proximal muscles, such as those of the hips and shoulders.
• Skin Rash: In dermatomyositis, patients may have a heliotrope rash on the eyelids or a Gottron's papules on the knuckles.
• Dysphagia: Difficulty swallowing due to involvement of esophageal muscles.
• Systemic Symptoms: Fatigue, fever, and weight loss may occur.

Diagnosis

Diagnosis of idiopathic inflammatory myopathy involves a combination of clinical evaluation, laboratory tests, and imaging studies:

• Laboratory Tests: Elevated serum creatine kinase (CK) levels indicate muscle damage. Autoantibody testing can help identify specific subtypes.
• Electromyography (EMG): Shows characteristic patterns of muscle inflammation.
• Muscle Biopsy: Confirms the diagnosis by revealing inflammatory infiltrates and muscle fiber necrosis.
• Magnetic Resonance Imaging (MRI): Can detect muscle edema and inflammation.

Treatment

Treatment of idiopathic inflammatory myopathy aims to reduce inflammation and improve muscle strength. Common treatments include:

• Corticosteroids: First-line treatment to reduce inflammation.
• Immunosuppressive Agents: Such as methotrexate or azathioprine, used in steroid-resistant cases.
• Intravenous Immunoglobulin (IVIG): May be effective in certain cases, particularly dermatomyositis.
• Physical Therapy: Helps maintain muscle strength and function.

Prognosis

The prognosis of idiopathic inflammatory myopathy varies depending on the subtype and response to treatment. Early diagnosis and treatment are crucial for improving outcomes. Dermatomyositis and polymyositis generally have a better prognosis than inclusion body myositis, which is often resistant to treatment.

Also see

• Autoimmune disease
• Muscle biopsy
• Rheumatology
• Systemic lupus erythematosus

Classification
External resources