Polymyositis may begin abruptly, but the usual presentation is one of progressive muscle weakness over weeks to months. The weakness chiefly involves proximal muscle groups of the upper and lower extremities as well as the neck. Leg weakness (eg, difficulty in rising from a chair or climbing stairs) typically precedes arm symptoms. In contrast to myasthenia gravis, polymyositis and dermatomyositis do not cause facial or ocular muscle weakness. In contrast to PMR, pain and tenderness of affected muscles occur in one-fourth of cases, but these are rarely the chief complaints. About one-fourth of patients have dysphagia. In contrast to scleroderma, which affects the smooth muscle of the lower esophagus and can cause a “sticking” sensation below the sternum, polymyositis or dermatomyositis involves the striated muscles of the upper pharynx and can make initiation of swallowing difficult. Muscle atrophy and contractures occur as late complications of advanced disease. Clinically significant myocarditis is uncommon even though there is often creatine kinase-MB elevation. Patients who are bed-bound from myositis should be screened for respiratory muscle weakness that can be severe enough to cause CO2 retention and can progress to require mechanical ventilation.
The characteristic rash of dermatomyositis is dusky red and may appear in a malar distribution mimicking the classic rash of SLE. Facial erythema beyond the malar distribution is also characteristic of dermatomyositis. Erythema also occurs over other areas of the face, neck, shoulders, and upper chest and back (“shawl sign”). Periorbital edema and a purplish (heliotrope) suffusion over the eyelids are typical signs (Figure 20–6). Coloration of the heliotrope and other rashes of dermatomyositis can be affected by skin tone. In blacks, the rashes may appear more hyperpigmented than erythematous or violaceous. Periungual erythema, dilations of nailfold capillaries, Gottron papules (raised violaceous lesions overlying the dorsa of DIP, PIP, and MCP joints), and Gottron sign (erythematous rash on the extensors surfaces of the fingers, elbows, and knees) are highly suggestive. Scalp involvement by dermatomyositis may mimic psoriasis. Infrequently, the cutaneous findings of this disease precede the muscle inflammation by weeks or months. Diagnosing polymyositis in patients over age 70 years can be difficult because weakness may be overlooked or attributed erroneously to idiopathic frailty. A subset of patients with polymyositis and dermatomyositis have the “antisynthetase syndrome,” a group of findings including inflammatory nonerosive arthritis, fever, Raynaud phenomenon, “mechanic’s hands” (hyperkeratosis along the radial and palmar aspects of the fingers), interstitial lung disease, and often severe muscle disease associated with certain autoantibodies (eg, anti-Jo-1 antibodies).
Bilateral heliotrope rash, which is a pathognomonic sign of dermatomyositis. (Used, with permission, from Richard P. Usatine, MD, in Usatine RP, Smith MA, Mayeaux EJ Jr, Chumley H. The Color Atlas of Family Medicine, 2nd ed. McGraw-Hill, 2013.)
Measurement of serum levels of muscle enzymes, especially creatine kinase and aldolase, is most useful in diagnosis and in assessment of disease activity. Inflammatory myositis can be misdiagnosed as hepatitis because of elevations in serum levels of muscle-derived alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels. Anemia is uncommon. The ESR and CRP are often normal and are not reliable indicators of disease activity. Rheumatoid factor is found in a minority of patients. Antinuclear antibodies can be present, especially when there is an associated connective tissue disease. A number of autoantibodies are seen exclusively in patients with myositis and are associated with distinctive clinical features (Table 20–10). Examples of myositis-specific antibodies include (1) anti-Jo-1 antibody (seen in the subset of patients who have antisynthetase syndrome), (2) anti-Mi-2 (associated with dermatomyositis), (3) anti-SRP (anti-signal recognition particle, associated with rapidly progressive, severe polymyositis, and dysphagia), (4) anti-155/140 (strongly associated with dermatomyositis with malignancy), and (5) anti-MDA5 (melanocyte differentiation-associated protein 5, linked to dermatomyositis with cutaneous ulcerations and rapidly progressive interstitial lung disease). Chest radiographs are usually normal unless there is associated interstitial lung disease. Electromyographic abnormalities consisting of polyphasic potentials, fibrillations, and high-frequency action potentials can point toward a myopathic, rather than a neurogenic, cause of weakness. MRI can detect early and patchy muscle involvement, can guide biopsies, and often is more useful than electromyography. The search for an occult malignancy should begin with a history and physical examination, supplemented with a complete blood count, comprehensive biochemical panel, and urinalysis and should include age- and risk-appropriate cancer screening tests. Given the especially strong association of ovarian carcinoma and dermatomyositis, transvaginal ultrasonography, CT scanning, and CA-125 levels may be useful in women. No matter how extensive the initial screening, some malignancies will not become evident for months after the initial presentation.
Table 20–10.Myositis-specific antibodies. ||Download (.pdf) Table 20–10. Myositis-specific antibodies.
|Antibody ||Clinical Association |
|Anti-Jo-1 and other antisynthetase antibodies ||Polymyositis or dermatomyositis with interstitial lung disease, arthritis, mechanic’s hands |
|Anti-Mi-2 ||Dermatomyositis with rash more than myositis, good prognosis |
|Anti-MDA5 (anti-CADM 140) ||Dermatomyositis with rapidly progressive lung disease, cutaneous ulcers |
|Anti-155/140 ||Cancer-associated dermatomyositis |
|Anti-NXP-2 ||Juvenile dermatomyositis |
|Anti-SAE ||Cancer-associated dermatomyositis, dermatomyositis with rapidly progressive lung disease |
|Anti-signal recognition particle ||Severe, acute necrotizing myopathy |
|Anti-HMG CoA reductase ||Necrotizing myopathy related to statin use |
Biopsy of clinically involved muscle is the only specific diagnostic test. The pathology findings in polymyositis and dermatomyositis are distinct. In dermatomyositis, the cellular infiltrate is mostly perifascicular and perivascular, while in polymyositis, the inflammatory infiltrate involves the fascicle itself. False-negative biopsies sometimes occur in both disorders because of the sometimes patchy distribution of pathologic abnormalities.