Classical dermatomyositis (CDM) patients display the hallmark cutaneous manifestations of dermatomyositis (DM), proximal muscle weakness, and laboratory evidence of myositis. The epidemiology and management of both adult-onset and juvenile-onset CDM has been well characterized. However, the clinical significance of the hallmark inflammatory cutaneous manifestations of DM occurring in individuals who have no clinically significant muscle weakness and normal muscle enzymes for prolonged periods of time (ie, 6 months or longer) has not been clear. The term amyopathic DM (ADM) (synonymous with DM siné myositis) has been proposed to draw attention to such individuals. A related form of DM, “hypomyopathic DM” [HDM], is the presence of DM skin disease for 6 months or longer in individuals who have no muscle weakness but who are found to have some evidence of muscle inflammation upon testing (muscle enzyme levels, electromyogram, muscle biopsy, muscle magnetic resonance imaging [MRI]). Clinically amyopathic DM (CADM) is a designation that has been proposed for patients having either ADM or HDM. The clinically amyopathic component of this designation was coined to emphasize the fact that the only clinical problem being experienced by these patients at the time of diagnosis is their DM skin disease. Our personal experience suggests that the CADM subphenotype might be more prevalent in adults than has been thought previously. To test this hypothesis and address questions relating to the optimal management and prognosis of such patients, we have systematically reviewed the published literature in this area.

We carried out a systematic review of the published literature on adult-onset CADM as defined in Table I through May 1, 2004.

We identified 291 adult-onset CADM cases (18 years or older) reported from over 19 countries. The average duration of DM skin disease was 3.74 years (range, 6 months [by definition] to > 20 years), and 73% were female. Among 37 patients with HDM who were identified, the average duration of disease was 5.4 years, and none had developed clinically significant weakness at the time of the reports. Thirty-seven of the reported CADM patients developed muscle weakness greater than 6 months after onset of their skin disease (15 months to 6 years). For the sake of this discussion, such patients have been analyzed under the designation of “CADM → CDM.” Somewhat surprisingly, 36/291 (13%) of the identified published CADM patients developed interstitial lung disease. Incidental to our review, we also identified 10 published cases of individuals having DM skin disease and interstitial lung disease without muscle weakness, 7 of whom died from interstitial lung disease less than 6 months after onset of their DM skin disease (the term pre-myopathic DM coined by others has been used here to refer to such patients). In addition, an associated internal malignancy was found in 41/291 (14%) of the identified CADM cases. A positive antinuclear antibody was reported in 63% and myositis-specific autoantibodies (eg, Jo-1, Mi-2) in only 3.5% of the reported CADM patients in which such data were available.

The results of this analysis suggests that the CADM subphenotype is more common than has been thought previously and that such patients may comprise a relatively high proportion of DM patients followed by dermatologists. Some CADM patients also have been observed to develop overt proximal muscle weakness years after onset of their DM skin disease. In addition, CADM patients appear to be at risk of developing the same potentially fatal disease associations/complications for which CDM patients are at risk (eg, interstitial lung disease and internal malignancy). Population-based studies of the epidemiology and optimal management of CADM patients, including efforts to identify risk factors associated with potentially fatal outcomes such as late-onset muscle weakness, interstitial lung disease, and malignancy, are needed. As an incidental finding to this literature review, we also identified a small number of reported cases of often-fatal interstitial lung disease occurring shortly after the onset of DM skin disease (<6 months) in the complete absence of muscle weakness. This subphenotype, referred to as “pre-myopathic DM,” is one with which dermatologists should be aware as early diagnosis and aggressive management can be lifesaving.