These diseases are rare, with prevalence rates of approximately 1 per 100,000 in the general population. The annual incidence ranges from 2 to 10 cases per million. Although the disease peak incidence is seen in childhood and adults between ages of 40 and 50, it can occur at any age.

The female-to-male ratio is 2 : 1. Disease incidence is greater in blacks than whites (3 : 1).

ETIOLOGY AND PATHOGENESIS

Although the etiology of the idiopathic inflammatory myopathies remains unknown, the pathogenesis is better understood. The major pathologic feature is focal inflammation (myositis), injury, and death of myocytes. Regeneration and hypertrophy, atrophy of myocytes, and replacement of muscle by fibrosis and fat are observed. The inflammatory infiltrate comprises lymphocytes and macrophages and is typically localized and focal.

Both cellular and humoral immune systems are involved in the pathogenesis. In the perivascular regions there is a predominance of B lymphocytes, whereas T lymphocytes are most frequent in the endomysial areas. In DM, perivascular B lymphocytes are numerous with C5b-9 membrane attack complex preceding the inflammatory infiltrate. CD8⁺ T lymphocytes and macrophages invade the individual muscle fibers, within the fascicle in PM. Expression of MHC Class I is increased in myocytes in both PM and DM. Levels of IL-1 and TNF-α are also elevated in patients with active PM and DM.

Although autoantibodies may be seen in idiopathic inflammatory myopathies, these are more common in patients with an associated connective tissue disease. A group of autoantibodies against cytoplasmic RNA synthetases, other proteins, ribonucleoproteins, and certain nuclear antigens, called myositis-specific autoantibodies, occur in approximately 30% of patients with idiopathic inflammatory myopathies. The pathogenic role of these antibodies is not known, but myositis-specific autoantibodies are associated with specific clinical features and prognosis. The three main autoantibodies include anti-Mi-2, anti–signal recognition particle (anti-SRP), and Jo-1 (anti-synthetase), which is typically associated with interstitial lung disease, fevers, arthritis, Raynaud phenomenon, and mechanic’s hands. Anti-SRP is associated with severe PM, and anti-Mi-2 is seen in classic DM.

CLINICAL PRESENTATION

Patients with idiopathic inflammatory myopathies usually present with an insidious progressive painless muscle weakness over a course of several weeks to several months. Acute onset is less common. Patients infrequently report myalgias, and these tend to be mild. Weakness is usually proximal and symmetric, less frequently involving the distal muscles. Rising from a chair, climbing stairs, and combing hair may be difficult. Gait may become unsteady and waddling. Pharyngeal weakness may lead to hoarseness and difficulty swallowing, causing lung aspiration. Fatigue occurs in most patients.

Skin lesions differentiate DM from PM. Cutaneous manifestations typically occur in the upper eyelids, malar areas involving the nasolabial folds, anterior chest, neck, upper back, extensor surfaces of elbows, hands, knees, and periungual areas. Gottron papules are raised plaques over the finger joints, and Gottron’s sign is a macular rash over these areas, elbows, and knees. The rash is often photosensitive and may precede, develop simultaneously, or occur after muscle disease. Patients may have DM without muscle involvement, which is referred to as amyopathic dermatomyositis or dermatomyositis sine myositis. In DM, skin lesions may ulcerate and be difficult to heal.

Extramuscular disease may occur, with polyarthralgias or polyarthritis most frequently affecting the hands, wrists, and knees in a rheumatoid-like distribution. The arthritis is usually mild and nondeforming, but in PM associated with anti–Jo-1 antibodies the arthritis may lead to joint damage.

Calcinosis is more common in late stages of chronic DM and can be disabling. Sites of trauma are usually affected, and periarticular calcification can result in joint contractures. Overlying skin may ulcerate and may be complicated by infections and draining sinuses.

Alveolitis or interstitial fibrosis causes dyspnea and cough. Severe alveolitis may be rapidly progressive in patients with anti-tRNA synthetase autoantibodies. Patients with idiopathic inflammatory myopathies usually present with slow progression of lung disease. Dyspnea and cough may also be a result of respiratory muscle weakness or aspiration pneumonia due to pharyngeal weakness.

Dysphagia occurs in up to a third of patients and is due to weakness of the oropharyngeal muscles or striated muscles of the upper third of the esophagus. Patients may have difficulty swallowing, nasal regurgitation, dysphonia, and lung aspiration. In juvenile DM, gastrointestinal bleeding from vasculitic lesions may occur.

Cardiac involvement is frequently asymptomatic with electrocardiographic abnormalities. It rarely causes congestive heart failure, myocarditis, or symptomatic pericarditis.

Raynaud phenomenon may be seen and is often associated with the anti-synthetase syndrome.

An association between idiopathic inflammatory myopathies and malignancy exists, with the risk of cancer appearing to be higher in DM, often occurring within 1 year of diagnosis. Genitourinary sites are more commonly affected, but any neoplasm is possible.

DIFFERENTIAL DIAGNOSIS

The idiopathic inflammatory myopathies are rare diseases that must be distinguished from other conditions affecting skeletal muscle. Other conditions that cause proximal weakness include the muscular dystrophies, metabolic myopathies, myasthenia gravis, myopathies of endocrine causes, and toxic myopathies. Inclusion-body myositis typically causes distal weakness but may be clinically difficulty to distinguish from PM. Muscle weakness in inclusion-body myositis is asymmetric, is greater in distal than proximal muscles, and occurs at an older age (>50 years).