OVERVIEW
Polymyositis (PM) is a systemic connective tissue disease characterized by inflammation, degeneration, and other changes in muscle tissue (the skin is often similarly affected, i.e., dermatomyositis), which results in symmetric muscle weakness and some degree of muscle atrophy, primarily in the limb-girdle muscles. The etiology is unknown and may be due to an autoimmune response. Deposits of IgM, IgG, and C3 have been found in the vasculature of skeletal muscle, with a particularly high rate of positivity in childhood forms of dermatomyositis.
Etiology
The etiology is unknown. Cell-mediated immune responses play an important role in muscle. Viruses may also be involved in pathogenesis: tiny RNA virus-like structures have been found in myocytes, and tubular inclusion bodies resembling paramyxovirus nucleocapsids have been found in myocytes and endothelial cells of the skin and muscle vessel walls using electron microscopy. The phenomenon of correlation between malignant tumors and dermatomyositis suggests that tumors can cause myositis as a result of an immune response against common antigens in muscle and tumor.
The disease is not uncommon and has a lower incidence than systemic lupus erythematosus and progressive systemic sclerosis, but a higher incidence than polyarteritis nodosa. The male-to-female ratio is 1:2. It can occur in any age group, but the highest incidence is in persons aged 40 to 60 years, or in children aged 5 to 15 years.
In dermatomyositis, histologic changes are similar to those of lupus erythematosus. The epidermis is thinned, the basal lamina is liquefied, the dermal papillae are puffy, and there is a lymphocytic infiltrate around the blood vessels in the superficial layers of its dermis. There are scattered melanophagocytes . There is no invasion into adnexal structures as in lupus erythematosus. There are more characteristic changes in the muscles. The deltoid, trapezius and quadriceps are almost always involved and are the best sites for biopsy.
Symptoms.
Polymyositis can have an acute or insidious onset. Acute infections can be the precursor or trigger for the onset of the disease. Symptoms in adults are similar to those in children, but the onset in children tends to be acute, whereas in adults the onset is usually insidious. Early symptoms are usually proximal muscle weakness or rash. (In inclusion body myositis, the distal muscles are also involved, even more so than the proximal muscles.) Muscle tenderness and pain symptoms are significantly less common than muscle weakness. Rash, polyarthralgia, Raynaud’s phenomenon, dysphagia, lung disease and general malaise, fever, malaise and weight loss may be present.
1. Muscle weakness
Myasthenia gravis can occur suddenly and continue to progress for weeks to more than a few months. It can destroy 50% of the muscle fibers, thus causing muscle weakness (muscle weakness indicates progressive myositis). Patients have difficulty lifting their upper limbs above their shoulders, walking up steps and standing up from a seated position, and are wheelchair bound or bedridden due to weakness in the pelvic girdle and shoulder girdle muscle groups. The cervical flexors can be severely involved and the head cannot be lifted from the pillow. Weakness of the laryngeal muscles is responsible for dysphonia. Involvement of the chest wall muscles and diaphragm can cause acute respiratory insufficiency. Involvement of the transverse muscles of the pharynx and upper esophagus causes dysphagia and reflux. Decreased peristalsis and dilatation of the lower esophagus and small intestine are indistinguishable from progressive systemic sclerosis. Muscles of the hands, feet, and face are generally not involved. Limb contractures may occur in the late chronic phase of the disease.
2. Rash
Rash usually occurs in dermatomyositis, mostly dark erythema, with mauve periorbital edema as the characteristic skin changes of this disease. The lesions are slightly elevated above the skin, with a smooth or scaly surface, and may occur on the forehead, cervical triangle, shoulders, chest, back, forearms, calves, elbows, inner ankles, and the dorsal surface near the proximal interphalangeal joints. Nail beds and nail margins are congested. A characteristic desquamative dermatitis with skin cracking, often involving the radial side of the fingers. The skin damage often subsides completely, but brownish pigmentation, atrophy, scarring, or leukoplakia may remain. Calcification of the skin may also occur, especially in children, and its distribution is similar to that of progressive systemic sclerosis, but tends to be more widespread (generalized calcification), especially in those patients who are untreated or inadequately treated.
3. Arthralgia
Polyarthralgia occurs in about 30% of patients with polymyositis and dermatitis and is accompanied by other manifestations such as joint swelling, joint effusion, and non-tremorigenic arthritis. These rheumatologic pains are usually mild and occur more frequently in Jo-1 antibody-positive individuals. The incidence of Raynaud’s phenomenon is particularly high in patients with polymyositis and other connective tissue diseases.
4. Fundus changes
Fundoscopy may show cotton-wool-like exudative spots, discoid and punctate hemorrhages near the retina.
Examination
Laboratory tests are helpful clinically but lack specificity. Blood sedimentation is often increased. A few patients have antituberculous antibodies or lupus cells, which are especially positive in combination with other connective tissue diseases. About 60% of patients are positive for anti-thymic nuclear antigen (RM-1) antibodies or anti-whole thymus nuclear extract (Jo-1) antibodies. The relationship of these antibodies to the pathogenesis of the disease is unclear, although Jo-1 antibodies are an important marker associated with fibrosing alveolitis with pulmonary fibrosis.
Serum muscle enzymes are elevated, particularly transaminases, creatine kinase (CK), and aldolase. Regular checking of CK levels can help guide treatment; the use of effective therapy can lead to a decrease in elevated enzymes. However, in patients with chronic myositis and generalized muscular dystrophy, myosin levels may be normal even during the active phase.
Complications
Compared with the incidence of visceral lesions in other connective tissue diseases (e.g., systemic lupus erythematosus, progressive systemic sclerosis), visceral complications (other than pharyngeal and esophageal) are less common in polymyositis, although occasional manifestations of visceral involvement precede muscle weakness. Interstitial pneumonitis (manifested by dyspnea and cough) may occur and can be the predominant clinical manifestation. The incidence of cardiac involvement has been reported to be progressively increasing, mainly in the form of electrocardiographic abnormalities such as arrhythmias, conduction disturbances, and intersystolic interval abnormalities. Some cases have developed acute renal failure due to severe rhabdomyolysis with myosinuria (crush syndrome). Sjogren’s syndrome occurs in some patients. Abdominal symptoms are more common in children and may include vomiting of blood or black stools, which are caused by gastrointestinal ulcers and can progress to perforation, thus requiring surgical treatment.
Malignant tumors complicate nearly 15% of men and a few women over 50 years of age. The type and site of development are not clearly characterized.
Diagnosis
1. Muscle weakness
Proximal muscle weakness.
2. Rash
Characteristic rash.
3. Myosin
Elevated serum myosin levels.
4. muscle biopsy
Myocardial biopsy changes are often decisive.
5. Electromyography
Specialized EMG triad.
Differential Diagnosis
Polymyositis must be differentiated from salpingitis, systemic lupus erythematosus, angioneurotic edema, and erythema multiforme. It is difficult to differentiate from scleroderma. Thus, the disease occurs well in the scapular region, while the muscles are often not involved. Trichinosis can cause muscle weakness similar to dermatomyositis, and some others to consider are myasthenia gravis, myotonic dystrophy, dengue, and angioneurotic edema.
Aldosterone with adrenal and pancreatic adenomas and hypokalemia can also cause vesicular purple coloration of both eyelids and face. Systemic scleroderma changes are often concomitant, whereas in dermatomyositis, systemic involvement is uncommon except in malignant disease.
Treatment
1. Restriction of activity
The patient’s activities must be restricted until the inflammation subsides.
2. Hormones
Adrenocorticotropic hormone is the drug of choice. Prednisone is given to acute patients. Serial measurement of serum myosin activity is the best way to observe the efficacy of treatment. In most patients, myosin decreases within 6 to 12 weeks after treatment and approaches the normal level. Improvement in muscle strength ensues. Once the muscle enzyme levels return to normal, the dose of prednisone should be slowly reduced; conversely, if the muscle enzymes are elevated, the dose should be increased. In adults, prednisone is usually required for long-term maintenance. Discontinuation has been attempted but is rarely successful, however some patients can be maintained for several years with effective treatment and the dose can be tapered under close monitoring. Higher initial doses are appropriate in children, and prednisone can be discontinued after 1 year or more of remission. Some patients who have been treated with large amounts of hormones for long periods of time have become increasingly debilitated due to adrenocorticotropic myalgia, in which case the hormones must be discontinued or reduced and replaced with other drugs such as immunosuppressants. For patients with myositis or inclusion body myositis whose tumors cannot be removed or have metastasized, hormone therapy is generally ineffective.
3. Immunosuppressants
Immunosuppressive agents, including methotrexate, cyclophosphamide, azathioprine phenylbutyrate, azathioprine, and cyclosporine, are beneficial for patients who have failed hormone therapy alone. The effectiveness of intravenous immunoglobulin infusion is being evaluated, and initial information is encouraging, but the high price has prevented further controlled research trials. Patients with myositis in combination with malignant tumors may resolve spontaneously if the tumor is removed.