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Dr. Maria Michailidou



General Considerations

Inflammatory myopathies are rare diseases. Estimates of incidence range from 0.5 to 8.4 cases per million. The incidence does appear to be increasing, although this may simply reflect increased awareness and more accurate diagnosing.  lists the clinical classification of the idiopathic inflammatory myopathies.  

These diseases are seen in all age groups, but overall, the age at onset has a bimodal distribution with peaks observed between ages 10 and 15 years in children and between 45 and 60 years in adults. However, the mean ages for specific groups differ. Both myositis associated with malignancy and inclusion body myositis are more common after age 50 years. The age at onset for myositis occurring with another collagen vascular disease is similar to that for the associated condition. Women are twice as commonly affected as men, with the exception of inclusion body myositis, which affects men more often.

Symptoms and Signs of polymyositis

The clinical features of polymyositis in the adult are representative of all the inflammatory myopathies. Typically, polymyositis begins insidiously over 3–6 months with no identifiable precipitating event. Pelvic and shoulder girdle musculature are most affected, but weakness of neck muscles, particularly the flexors, is also common. Ocular and facial muscles are virtually never involved. Dysphagia may develop secondary to esophageal dysfunction or cricopharyngeal obstruction. Pharyngeal muscle weakness may cause dysphonia and difficulty swallowing. Myalgias and arthralgias are not uncommon, but severe muscle tenderness and frank synovitis are unusual. Raynaud phenomenon is sometimes present, and periorbital edema may be noted.

Pulmonary and cardiac manifestations may precede the onset of muscle weakness or develop at any time during the course of disease. Velcro-like crackles may be heard on chest auscultation of patients with interstitial fibrosis or interstitial pneumonitis. Cardiac involvement is usually limited to asymptomatic electrocardiographic abnormalities. However, supraventricular arrhythmia, cardiomyopathy, and congestive heart failure can occur.

The clinical features of dermatomyositis include all those described for polymyositis plus a variety of cutaneous manifestations. Skin involvement varies widely from patient to patient. Rashes can antedate the onset of muscle weakness or follow its development by more than a year. Furthermore, the characteristics of the rash may change over time. Two cutaneous manifestations are considered pathognomonic. These include Gottron papules (symmetric lacy pink or violaceous raised areas typically found on the dorsal aspect of interphalangeal joints, elbows, patellae, and medial malleoli) and heliotrope (violaceous) discoloration of the eyelids. The latter is often with associated periorbital edema. Other characteristic cutaneous findings include macular erythema of the posterior shoulders and neck (shawl sign), anterior neck and upper chest (V-sign), malar region, forehead, or small joints of the fingers; dystrophic cuticles; and mechanic's hands (darkened or dirty-appearing horizontal lines and fissures that are seen across the lateral and palmar aspects of the fingers). Periungual telangiectasias and nailfold capillary changes similar to those observed in patients with scleroderma or systemic lupus erythematosus and Raynaud phenomenon can be seen.

The inflammatory myopathy that affects children tends to have a highly characteristic pattern, although a disease similar to adult polymyositis does occur. In juvenile dermatomyositis, the skin lesions and weakness are almost always coincidental, but the severity and progression of each varies greatly from patient to patient. In some patients, remission is complete with little or no therapy. The juvenile variant differs from the adult form because of the coexistence of vasculitis, ectopic calcification, and lipodystrophy. Unfortunately, in dermatomyositis accompanied by vasculitis, progression may be devastating despite therapy. Gastrointestinal ulcerations resulting from vasculitis can cause hemorrhage or perforation of a viscus. Ectopic calcifications may occur in the subcutaneous tissues or in the muscles.

Some patients with biopsy-confirmed, classic cutaneous findings of dermatomyositis have normal motor function, muscle enzyme, electromyograms (EMGs), and muscle histology. The terms "amyotrophic dermatomyositis" and "dermatomyositis sine myositis" have been used to describe these patients. Although there is no evidence of myopathy, fatigue may be a dominant complaint. Some patients with this presentation continue to have skin disease only, whereas others progress over time, becoming weak and developing typical dermatomyositis. There may be an increased prevalence of neoplasia associated with this presentation.

Muscle weakness is a common finding in patients with collagen vascular diseases. The features of idiopathic inflammatory myopathy may dominate the clinical picture in some patients with scleroderma, systemic lupus erythematosus, mixed connective tissue disease, and Sjögren's syndrome, but the classic picture of polymyositis is less common in rheumatoid arthritis, Wegener granulomatosis, polyarteritis nodosa, and adult Still disease. Weakness in these latter conditions is more commonly the result of vasculitis.

Muscle weakness associated with an underlying malignancy develops in a subset of patients with inflammatory myopathies. The true incidence of this relationship is not clear. Although malignancy may precede, or develop after, the onset of muscle weakness, usually the two are diagnosed within a 1-year period. The association occurs in patients of all ages but is rare in childhood. Although an associated malignancy may be more common with dermatomyositis, cancer can be found in association with each subset. The sites or types of malignancy that occur in association with myositis are those that are expected for the age and gender of the patient. Ovarian cancer may prove the exception; it appears to be overrepresented in women with dermatomyositis.

Inclusion body myositis mainly affects older persons. The symptoms begin most insidiously and progress slowly. Symptoms are often present for 5–8 years before the diagnosis is made. The clinical picture in some patients is identical to that of typical polymyositis. In others, however, focal, distal, or asymmetric weakness is present. Dysphagia is not infrequent in this disease. As the muscle weakness becomes severe, it can be accompanied by atrophy and diminished deep tendon reflexes. In some patients, inclusion body myositis follows a slow, steadily progressing course. In others, the weakness seems to plateau, leaving the person with fixed weakness and atrophy of the involved musculature.

Laboratory Findings of polymyositis

An abnormal creatine kinase (CK) level is possibly the most sensitive indicator of skeletal muscle damage. The serum level of this enzyme is elevated at some time during the course of an inflammatory myopathy and, in most instances, the serum CK level reasonably correlates with disease activity. Normal levels of CK may be found very early in the course of polymyosits or dermatomyositis, in advanced cases with significant muscle atrophy, or in myositis associated with a malignancy. CK levels are often only minimally elevated or can be normal in inclusion body myositis. Other enzymes derived from diseased skeletal muscle include aldolase, aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH). Accordingly, these enzymes may also be elevated in the course of the disease.

Tests of acute phase reactants, the erythrocyte sedimentation rate (ESR) and C-reactive protein levels, are abnormal in only some patients with myositis. The ESR is normal in about half of patients with polymyositis and is elevated above 50 mm/h (Westergren method) in only 20%.

Antinuclear antibodies (ANAs) may be found in the serum of over 50% of patients with inflammatory muscle disease. The presence of a high-titer ANA may indicate the presence of an associated collagen-vascular disease (for example, anti-Sm or anti-dsDNA in systemic lupus erythematosus or anti-Scl-70 antibodies in scleroderma). In the other forms of myositis, the ANA tends to be present in low titer and is nonspecific in nature.

Certain autoantibodies are found almost exclusively in patients with idiopathic inflammatory myopathies and, therefore, are termed myositis-specific autoantibodies (). With extremely rare exceptions, an individual patient will have only one myositis-specific autoantibody, and the particular autoantibody present appears to identify relatively homogeneous groups of patients with regard to clinical manifestations and prognosis.

 Most myositis-specific autoantibodies are directed against amino acyl-tRNA synthetase activities. The most common of these is anti-histidyl-tRNA synthetase, called anti-Jo-1. Patients with these autoantibodies typically manifest myositis (polymyositis more commonly than dermatomyositis) plus several extramuscular features including interstitial lung disease, arthritis, mechanic's hands, and Raynaud phenomenon. The combination of these features and an inflammatory myopathy has been termed "antisynthetase syndrome." Patients with this syndrome have a variable response to therapy and often are difficult to treat because they tend not to sustain complete remission. Anti-Mi-2 antibodies are directed against helicase activities. These autoantibodies are found almost exclusively in patients with dermatomyositis and most respond very well to treatment. In contrast, but with some exceptions, polymyositis of sudden onset develops in patients with antibodies to signal recognition particle (SRP); these patients are relatively resistant to treatment. Cardiomyopathy and distal muscle weakness are also associated with the presence of anti-SRP antibodies.