Polymyositis is a systemic connective tissue disease characterized by inflammatory and degenerative changes in muscle tissue (skin is often similarly involved, i.e., dermatomyositis), resulting in symmetric muscle weakness and some degree of muscle atrophy, primarily in the limb girdle muscles. The classification of myositis includes primary idiopathic polymyositis; childhood-type dermatomyositis or polymyositis; adult-type primary idiopathic dermatomyositis; inclusion body myositis (IBM); dermatomyositis or polymyositis with malignancy; and overlapping syndromes of polymyositis or dermatomyositis with various connective tissue diseases, including mixed connective tissue disease (see below) and sclerosing dermatomyositis. Some of the clinical manifestations of polymyositis and dermatomyositis are identical to those of progressive systemic sclerosis, and sometimes to SLE or vasculitis. Etiology and morbidity The etiology is unknown. It may be due to an autoimmune reaction. IgM, IgG, and C3 deposits have been found in the skeletal muscle vasculature, especially in children with dermatomyositis. Cell-mediated immune responses play an important role in muscle. Viruses may also be involved in pathogenesis: micro RNA virus-like structures have been found in myocytes, and tubular inclusions resembling paramyxovirus nucleosomes have been identified by electron microscopy in myocytes and endothelial cells of skin and muscle vessel walls. The association of malignant tumors with dermatomyositis (and much less frequently with polymyositis) suggests that tumors can cause myositis as a result of an immune response against common antigens of muscle and tumor. This disease is not uncommon and its incidence is lower than that of SLE and progressive systemic sclerosis, but higher than that of polyarteritis nodosa. It can develop in any age group, but the highest incidence is in people aged 40-60 years or in children aged 5-15 years. Pathology The dermoscopic findings are non-specific, such as epidermal atrophy, basal cell liquefaction, degeneration, vasodilation and lymphocytic infiltration. The lesions of the affected muscle tissue are highly variable. Common changes include necrosis, phagocytosis, regeneration of myofibers, basophilic myocytes, hypertrophic and vacuolated nuclei, prominent nucleoli, atrophy and degeneration of myofibers, especially around the myofasciculus in patients with dermatomyositis; inward migration of nuclei and formation of vacuoles; variation in myofiber size; lymphocytic infiltration (especially around blood vessels); and increased connective tissue in the endomysium and later in the myofasciculus. Inclusion body myositis – another subtype of inflammatory myopathy, muscle biopsy shows myofiber necrosis and perivascular inflammatory cell infiltration is not obvious, common myofiber hypertrophy with basophilic granules in peripheral vacuoles. Inclusion bodies are most specifically identified by electron microscopy, and in children (dermatomyositis is more common than polymyositis), ulcers and infarcts are widely formed in the gastrointestinal tract due to necrotizing arteritis. Symptoms and signs The onset of the disease can be acute or insidious. Acute infections can be the precursor or trigger for the onset of the disease. Adults and children have similar symptoms, but the onset is often acute in children and insidious in adults. Early symptoms are usually proximal muscle weakness or a rash. (In inclusion body myositis, the distal muscles are also involved even more than the proximal muscles). Muscle tenderness and pain are significantly less than muscle weakness. Rash, polyarthralgia, Raynaud’s phenomenon, dysphagia, pulmonary disease, general malaise, fever, malaise, weight loss, etc. may be present. Myasthenia gravis can occur suddenly and continue to progress for weeks to months or more. It can destroy up to 50% of the muscle fibers and cause muscle weakness (muscle weakness indicates progressive myositis). Patients have difficulty raising the upper extremities above the shoulders, walking up steps and standing from a seated position, and are wheelchair bound or bedridden due to weakness of the pelvic girdle and shoulder girdle muscles. The cervical flexors may be so severely involved that the head cannot be lifted from the pillow. Weakness of the laryngeal muscles is the cause of dysphonia. Involvement of the chest wall muscles and diaphragm can cause acute respiratory insufficiency. Involvement of the upper pharyngeal and transverse esophageal muscles causes dysphagia and reflux. Weak and dilated peristalsis of the lower esophagus and small intestine is indistinguishable from that seen in progressive systemic sclerosis. The muscles of the hands, feet, and face are not usually involved. Limb contractures may occur in the late chronic phase of the disease. The skin rash usually occurs in dermatomyositis and is characterized by dark erythema with lavender periorbital edema. The lesions are slightly elevated, smooth or scaly, and may occur on the forehead, cervical triangle, shoulders, chest, back, forearms, lower legs, elbows, inner ankles, and the dorsal surface near the proximal interphalangeal joints. The nail bed and nail edge are congested. A characteristic desquamative dermatitis with skin splitting often involves the radial side of the fingers. The skin lesion often resolves completely, but brownish hyperpigmentation, atrophy, scarring, or white spots may remain. Calcification of the skin may also occur, especially in children. Its distribution is similar to that of progressive systemic sclerosis, but tends to be more widespread (generalized calcium deposits), especially in those patients who are untreated or inadequately treated. About 30% of patients with polymyositis and dermatitis often develop polyarthralgia with other manifestations such as joint swelling, joint effusion, and nonteratogenic arthritis. The incidence of Raynaud’s phenomenon is particularly high in patients with polymyositis in combination with other connective tissue diseases. Compared with the incidence of visceral lesions in other connective tissue diseases (e.g., systemic lupus erythematosus, progressive systemic sclerosis), visceral complications (other than pharynx and esophagus) are less common in polymyositis, but occasionally manifestations of visceral involvement precede myasthenia gravis. Interstitial pneumonia (manifested by dyspnea and cough) can occur and can be the main clinical manifestation. A progressive increase in the incidence of cardiac involvement has been reported, mainly in the form of electrocardiographic abnormalities, such as arrhythmias, conduction disturbances, and inter-systolic abnormalities. In some cases, acute renal failure with myoglobulinuria (crush syndrome) has been reported as a result of severe rhabdomyolysis. Some patients develop Sjogren’s syndrome. Abdominal symptoms are more common in children and may include vomiting of blood or black stools, which are caused by gastrointestinal ulcers that can progress to perforation and require surgical treatment. Nearly 15% of men and a few women over 50 years of age have a malignant tumor. There are no distinctive characteristics of the type and site of origin. Laboratory tests are clinically helpful, but lack specificity. Blood sedimentation is often increased. A few patients have antinuclear antibodies or lupus cells, especially in combination with other connective tissue diseases. About 60% of patients are positive for anti-thymus nuclear antigen (RM-1) or anti-whole thymus nuclear extract (Jo-1) antibodies. The relationship of these antibodies to disease pathogenesis is unclear, although Jo-1 antibodies are an important marker associated with fibroalveolitis and pulmonary fibrosis. Serum muscle enzymes are elevated, especially transaminases, creatine kinase (CR) and aldolase. Regular checks of CK levels can help guide treatment; effective therapy can bring down elevated enzymes. However, in patients with chronic myositis and extensive muscular atrophy, muscle enzyme levels can be normal even during the active phase. Diagnosis There are five main diagnostic criteria: (1) proximal muscle weakness; (2) characteristic rash; (3) elevated serum myosin levels; (4) muscle biopsy changes (often decisive); and (5) a specific EMG triad: increased spontaneous fibrillation potentials and positive sharp waves and insertional provocation; polyphasic short time limit potentials during active contractions; and repeated disordered high frequency discharges in response to mechanical stimulation. Electromyography is usually monometric, so the muscles with EMG abnormalities should be selected for biopsy, usually the deltoid and quadriceps, but to avoid areas that have been stimulated by EMG probes, they should be taken from the contralateral limb. Even in patients with typical dermatomyositis and polymyositis, a muscle biopsy is still necessary for the final diagnosis and to differentiate from some of the rarer diseases such as inclusion body myositis and rhabdomyolysis after viral infection, and to provide a clear histologic diagnosis before the physician can give immunosuppressive therapy to some hormone-insensitive patients. Repeat biopsies are sometimes needed to distinguish recurrent polymyositis from corticosteroid-induced myopathy. MRI can sometimes differentiate edematous areas of inflammation to give a positive result, especially in children. Each adult patient with dermatomyositis must be carefully examined for malignancy, but a systemic, invasive, blind examination is not recommended. Prognosis It has been reported that the disease can achieve a relatively satisfactory prolonged remission, especially in children. Adult patients may die from severe progressive muscle weakness, dysphagia, malnutrition, and respiratory failure due to aspiration pneumonia or recurrent intrapulmonary infections. Polymyositis complicated by cardiac and pulmonary pathology is often severe and poorly treated. Children usually die from vasculitis of the intestinal tract. The prognosis of patients with polymyositis in combination with malignant tumors generally depends on the prognosis of the malignant tumor. Treatment The patient must be limited in activity until the inflammation subsides. Adrenocorticosteroids are the drug of choice. Prednisone 40-60 mg/d or higher is given to acute patients. Continuous measurement of serum muscle enzyme activity is the best way to monitor the efficacy. In most patients, myokinase decreases to near normal levels within 6 to 12 weeks after treatment. This is followed by an improvement in muscle strength. Once myoenzymes return to normal, the dose of prednisone should be slowly reduced; conversely, if myoenzymes are elevated, the dose should be increased. In adults, prednisone 10-15 mg/d is usually required for long-term maintenance. Discontinuation has been attempted but has rarely been successful, however, some patients can be maintained for several years after effective treatment and can be tapered under close monitoring. The initial dose in children should be high (30-60 mg/m2 per day of prednisone). Prednisone can be discontinued after 1 year of remission. Some patients who have been treated with large amounts of hormones for a long time become increasingly debilitated due to adrenocortical myalgia, in which case the hormones must be stopped or reduced and replaced with other drugs such as immunosuppressive drugs. In patients with myositis or inclusion body myositis in which the tumor cannot be removed or has metastasized, hormonal therapy is generally ineffective. Immunosuppressive drugs, including methotrexate, cyclophosphamide, benzopyridine, azathioprine, and cyclosporine, are beneficial in patients who have failed to respond to hormonal therapy alone. Some patients have been on methotrexate alone (usually at higher doses than for RA) for more than 5 years to control the disease. The effect of intravenous immunoglobulin infusions is being evaluated, and initial data are encouraging, but the high cost has prevented further controlled trials. Patients with myositis in combination with a malignant tumor may spontaneously resolve if the tumor is removed.