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Study Authors: Christine G. Parks, Jesse Wilkerson, et al.

Target Audience and Goal Statement: Rheumatologists, dermatologists, family physicians, primary care physicians

The goal of this study was to understand whether personal exposure to environmental ultraviolet (UV) radiation may be a modifiable risk factor for dermatomyositis.

Question Addressed:

• Which environmental factors and individual sun sensitivity might modify the relationship of personal sun exposure with the development of dermatomyositis?

Synopsis and Perspective:

A person's exposure to UV radiation through job- and hobby-related sun exposure may influence the risk of developing , a rare type of inflammatory myopathy characterized primarily by inflammatory and degenerative changes of the muscles and skin, reported Lisa G. Rider, MD, of the National Institute of Environmental Health Sciences' Environmental Autoimmunity Group in Bethesda, Maryland, and colleagues.

Sunburn and moderate to high job-related sun-exposure in the year prior to diagnosis was associated with the development of dermatomyositis versus the other two major subtypes of idiopathic inflammatory myopathies (IIMs), polymyositis (PM), and inclusion body myositis (IBM), they wrote in .

These results were obtained using a wide range of self-reported information about both U.S. adult and pediatric patients with IIM, including demographics, clinical manifestations, medications, and environmental exposures that may be associated with these diseases that were collated in the National Myositis Registry ().

Consistent with prior findings, the retrospective study showed that ambient UV radiation exposure was associated with dermatomyositis in females. Collectively, the results suggested that personal exposure to environmental UV radiation might be a modifiable risk factor for dermatomyositis.

IIMs are rare, systemic autoimmune diseases that can affect all ages and have the hallmarks of muscle weakness and inflammation. Serious extramuscular manifestations, such as severe skin rash, interstitial lung disease, and arthritis can complicate the management of IIMs. Distinctive (anti-Mi-2, transcription intermediary factor 1 gamma [TIF-1γ], and melanoma differentiation-associated protein 5 [MDA5]) and the presence of photosensitive rashes are features unique to dermatomyositis when compared to PM and IBM. IBM tends to be more common in men over the age of 55, whereas dermatomyositis and PM subtypes predominate in females.

Evidence from the literature identified a link between dermatomyositis and geospatial differences in UV radiation exposure, and clinical data suggesting that patients with the condition and frequent photosensitive rashes had a greater sensitivity to UV exposure. However, neither body of evidence addressed the role of individual determinants of UV exposure prior to diagnosis.

Rider's group examined the likelihood of dermatomyositis versus PM and IMB among 1,350 U.S. adults, in relation to self-reported sunburn history and job- and hobby-related sun exposure in the year prior to diagnosis. Researchers restricted their analysis to adults, ages 18 to 65 at the time of diagnosis, and comparisons were made with job and hobby patterns in adults in their prime working years.

After application of exclusion criteria, the final sample consisted of 638 persons with dermatomyositis (106 males, 532 females), 290 (175 males, 115 females) with IBM, and 422 (104 males, 318 females) with PM. Participants completed questionnaires detailing demographics, residence, sun exposure related to occupation and/or recreation, skin tone and sensitivity, and number of sunburns in the previous year.

Nimbus‐7 Total Ozone Mapping Spectrometer () were used to determine maximum ambient UVB irradiance. This method collects ozone and reflectivity data to estimate surface exposure intensity as Joules/m². The geographic location of each case was determined using a zip code of each participant's residence at the time of diagnosis. The proportion of DM cases in participants with adult IIM was predicted by estimating the maximum UV daily dose in the year prior to diagnosis, averaged by geographic region.

National estimates of sunburn in the past 12 months in the U.S. population from 1999, 2003, and 2004 based on data from the Behavioral Risk Factor Surveillance System (BRFSS) surveys served as a reference for population sunburn prevalence. "Any time that even a small part of your skin stayed red for more than 12 hours," served as the definition of sunburn in the BRFSS.

Persons who reported sunburn tended to be younger regardless of IIM phenotype, and sunburn was more common in females with dermatomyositis. About 42% of patients with dermatomyositis reported a history of sunburn in the year prior to diagnosis versus 28% of those with PM/IBM. While more than two thirds of participants with dermatomyositis had recreational sun exposure, only 15% of respondents had high or moderate level UV exposure (irrespective of IIM phenotype).

After adjusting for age, sex, and skin tone, the dermatomyositis association was somewhat stronger for reporting two or more sunburns (odds ratio 1.77, 95% CI 1.28-2.43 vs PM/IBM). This link remained statistically significant for one sunburn (OR 1.44, 95% CI 1.06-1.95). Dermatomyositis was also associated with having elevated job- or hobby-related sun exposure (high OR 1.64 95% CI 1.08 -2.49 or moderate exposure OR 1.35, 95% CI 1.02-1.78 vs low or no exposure). A link was also observed between the disease and UV intensity in females (beta=3.97, P=0.046), but not overall.

Study limitations included the retrospective design, use of self-reported data, the lack of clinical evidence such as testing for myositis-specific autoantibodies previously associated with dermatomyositis, and misclassification/recall biases. For instance, patients with dermatomyositis may have been counseled to avoid sun exposure and use sunscreens, which could have influenced their recall and reporting of sunburn.

Bohan and Peter combine clinical, laboratory, and pathologic features to define polymyositis and dermatomyositis. These muscle-related criteria may miss cases with classic skin symptoms that do not meet the full criteria. The authors also acknowledged that patients who were likely to be more photosensitive and representative of the full spectrum of UV exposure (and IIM) were also not included in the study sample, such as patients with or dermatomyositis. Researchers also did not have information on the time from symptom onset to diagnosis.

Source Reference: 2019; DOI: 10.1002/acr.24059

Study Highlights: Explanation of Findings

Using the largest registry of myositis patients in the U.S., Rider's group performed a blinded and systematic assessment to identify patients with personal sun exposure to identify individuals with higher potential for intense UV exposure in the year prior to their diagnosis. Compared to other myositis phenotypes, the authors found that individual-level factors related to personal sun exposure were associated with the probability of developing dermatomyositis. Results also supported the hypothesis that UV exposure modulates the IIM phenotype and influences the risk of developing DM.

Previous studies have observed sex differences in the incidence of sunburn and sun-protective behaviors, but there has been increasing evidence that these differences also relate to immune-related effects of UV radiation. have been reported as having higher levels of sensitivity to UV-related immunosuppression, and an of melanoma demonstrated a greater response to lower levels of UV.

"A larger sample is needed to disentangle sex differences in sun exposure from differences in responsiveness to ambient UV," the researchers wrote.

There also has long been interest in a potential role -- positive or negative -- for UV radiation in various types of autoimmune diseases. It has been suggested as having a protective role in rheumatoid arthritis and multiple sclerosis, but has been implicated as a risk factor for systemic lupus erythematosus. Moreover, "there are multiple mechanisms that may contribute to acute effects of intense UV exposure on the development of an autoimmune response, including increased reactive oxidative species, damage to DNA, and apoptosis leading to cellular debris and other changes," Rider's group explained.

"Our findings support the hypothesis that UV exposure modulates idiopathic inflammatory myopathy phenotype and influences the risk of developing DM, and they provide evidence that individual-level factors related to personal sun exposure are associated with the probability of developing DM compared to other myositis phenotypes," they observed.

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