Idiopathic inflammatory myopathies

Idiopathic Inflammatory Myopathies
Synonyms	N/A
Pronounce	N/A
Specialty	N/A
Symptoms	Muscle weakness, fatigue, myalgia
Complications	Dysphagia, respiratory failure
Onset
Duration
Types	Polymyositis, Dermatomyositis, Inclusion body myositis
Causes	Unknown
Risks	Genetic predisposition, environmental factors
Diagnosis	Clinical evaluation, muscle biopsy, MRI, EMG
Differential diagnosis	N/A
Prevention	N/A
Treatment	Corticosteroids, immunosuppressants, physical therapy
Medication	N/A
Prognosis	Variable
Frequency	N/A
Deaths	N/A

Idiopathic inflammatory myopathies (IIM) are a group of rare autoimmune diseases characterized by chronic muscle inflammation and muscle weakness. The term "idiopathic" indicates that the exact cause of these conditions is unknown. The primary types of idiopathic inflammatory myopathies include polymyositis, dermatomyositis, and inclusion body myositis.

Classification

Idiopathic inflammatory myopathies are classified into several subtypes based on clinical presentation, histopathological findings, and associated autoantibodies:

• Polymyositis: A condition characterized by symmetric proximal muscle weakness and inflammation, primarily affecting adults.
• Dermatomyositis: Similar to polymyositis but with additional skin manifestations such as a heliotrope rash and Gottron's papules.
• Inclusion body myositis: A condition that typically affects older adults, characterized by both proximal and distal muscle weakness and the presence of rimmed vacuoles in muscle biopsy.

Pathophysiology

The pathophysiology of idiopathic inflammatory myopathies involves an autoimmune attack on muscle fibers. The exact mechanisms are not fully understood, but it is believed that both cell-mediated immunity and humoral immunity play roles. In polymyositis, CD8+ T cells are thought to directly attack muscle fibers, while in dermatomyositis, complement-mediated damage to capillaries leads to muscle fiber necrosis. Inclusion body myositis involves both inflammatory and degenerative processes.

Clinical Features

The clinical presentation of idiopathic inflammatory myopathies varies depending on the subtype:

• Muscle Weakness: The hallmark of these conditions is symmetric proximal muscle weakness, affecting muscles such as the deltoids, hip flexors, and neck flexors.
• Skin Manifestations: In dermatomyositis, patients may present with a heliotrope rash, Gottron's papules, and a shawl sign.
• Systemic Symptoms: Fatigue, weight loss, and low-grade fever may be present.
• Complications: Dysphagia, interstitial lung disease, and cardiac involvement can occur.

Diagnosis

The diagnosis of idiopathic inflammatory myopathies involves a combination of clinical evaluation, laboratory tests, and imaging studies:

• Laboratory Tests: Elevated serum creatine kinase levels indicate muscle damage. Autoantibodies such as anti-Jo-1 may be present.
• Electromyography (EMG): Shows myopathic changes with increased insertional activity and fibrillations.
• Muscle Biopsy: Essential for diagnosis, showing inflammatory infiltrates and muscle fiber necrosis.
• Magnetic Resonance Imaging (MRI): Can detect muscle edema and inflammation.

Treatment

The treatment of idiopathic inflammatory myopathies aims to reduce inflammation and improve muscle strength:

• Corticosteroids: First-line treatment, often starting with high-dose prednisone.
• Immunosuppressants: Such as methotrexate or azathioprine are used as steroid-sparing agents.
• Physical Therapy: Important for maintaining muscle strength and function.
• Biologic Agents: Such as rituximab may be considered in refractory cases.

Prognosis

The prognosis of idiopathic inflammatory myopathies varies widely. Factors influencing prognosis include the specific subtype, age of onset, and presence of extramuscular involvement. Early diagnosis and treatment are crucial for improving outcomes.

See Also

• Autoimmune disease
• Muscle biopsy
• Rheumatology

External Links

• The Myositis Association