Key Points
Question
Does dermatomyositis sine dermatitis exist, and if yes, what are the serologic features of patients with dermatomyositis sine dermatitis?
Findings
In this cohort study of 182 patients with dermatomyositis who were positive for dermatomyositis-specific autoantibodies and myxovirus resistance protein A expression in myofibers, 14 (8%) had dermatomyositis sine dermatitis. Anti–nuclear matrix protein 2 autoantibodies were significantly associated with dermatomyositis sine dermatitis.
Meaning
Dermatomyositis sine dermatitis does exist and is significantly associated with anti–nuclear matrix protein 2 autoantibodies.
Abstract
Importance
Reports on dermatomyositis (DM) sine dermatitis (DMSD) are scarce, and the concept of the disease has not been widely accepted.
Objective
To confirm the existence of DMSD, determine its prevalence, and characterize its serologic features.
Design, Setting, and Participants
This is a cohort study that reviewed clinical information, laboratory data, and muscle pathology slides from January 2009 to August 2019. We further assessed the follow-up data of 14 patients with DMSD. The median (interquartile range) follow-up period was 34 (16-64) months.
Muscle biopsy samples, along with clinical information and laboratory data, were sent to a referral center for muscle diseases in Japan for diagnosis.
Of patients whose myopathologic diagnosis was made at the National Center of Neurology and Psychiatry between January 2009 and August 2019, 199 patients were eligible for inclusion. These patients underwent full investigation for DM-specific autoantibodies (against transcriptional intermediary factor γ, Mi-2, melanoma differentiation–associated gene 5, nuclear matrix protein 2 [NXP-2], and small ubiquitin-like modifier activating enzyme ); however, 17 patients were excluded because their muscle fibers did not express myxovirus resistance protein A, a sensitive and specific marker of DM muscle pathology.
Main Outcomes and Measures
Diagnosis of DMSD was based on the absence of a skin rash at the time of muscle biopsy.
Results
Of the 182 patients, 93 were women (51%) and 46 were children (25%) (<18 years). Fourteen patients (8%) had DMSD and none were clinically diagnosed with DM. Among the 14 patients with DMSD, 12 (86%) were positive for anti-NXP-2 autoantibodies, while the remaining 2 were positive for anti–transcriptional intermediary factor γ and anti-Mi-2 autoantibodies, respectively. Only 28% of patients (47 of 168) with a skin rash were positive for anti-NXP-2 autoantibodies, indicating a significant association between anti-NXP-2 autoantibodies and DMSD (86% [12 of 14] vs 28% [47 of 168]; P < .001). This association was also supported by multivariable models adjusted for disease duration (odds ratio, 126.47; 95% CI, 11.42-1400.64; P < .001).
Conclusions and Relevance
Dermatomyositis sine dermatitis does exist and accounts for 8% of patients with DM confirmed with muscle biopsy. Dermatomyositis sine dermatitis is significantly associated with anti-NXP-2 autoantibodies, which contrasts with anti-MDA5 DM, which is typically clinically amyopathic in presentation. It is essential to distinguish DMSD from other types of myositis because DM-specific therapies that are currently under development, including Janus kinase inhibitors, may be effective for DMSD.
This study confirms the existence of dermatomyositis sine dermatitis, determines its prevalence, and characterizes the serologic features of dermatomyositis sine dermatitis.
Introduction
Myositis does not manifest in a subset of patients with dermatomyositis (DM) and is termed clinically amyopathic DM (CADM).1 Clinically amyopathic DM is associated with anti–melanoma differentiation-associated gene 5 (MDA5) antibodies, which are among the DM-specific autoantibodies (DMSAs).2 In contrast, DM without dermatitis, known as DM sine dermatitis (DMSD),3,4 has been documented in only a few case reports5,6,7,8 without large cohort studies, leading to doubts about its existence.
To date, 5 DMSAs, antibodies against transcription intermediary factor 1 γ (TIF1-γ), Mi-2, MDA5, nuclear matrix protein 2 (NXP-2) and small ubiquitin-like modifier activating enzyme (SAE), have been identified. Mounting evidence indicates that each DMSA is associated with a specific clinicopathologic feature.9 However, no DMSA is known to be associated with DMSD.
The aim of this study was to confirm the existence, determine the prevalence, and characterize the serologic features of DMSD in a cohort of patients who were pathologically confirmed to have DM.
Methods
Clinical Information
This cohort study was conducted at the National Center of Neurology and Psychiatry (NCNP), a major referral center for muscle disease in Japan, between January 2009 and August 2019. Among 8848 patients whose muscle biopsies were pathologically examined for diagnostic purposes, 199 eligible consecutive patients were fully investigated for DMSAs (against TIF1-γ, Mi-2, MDA5, NXP-2, and SAE); patients were excluded if myxovirus resistance protein A (MxA) expression was not observed in myofibers on muscle biopsy (see the following paragraphs). Finally, we enrolled 182 patients in this study. The diagnosis of DMSD was made when patients were without any skin rash at the time of muscle biopsy as previously described.10
Histological Information
A battery of routine histochemical stains and immunohistochemical analyses were performed for diagnostic purposes for all patients. The conditions for immunohistochemistry are summarized in eTable 1 in the Supplement. The pathological slides were reevaluated. Positive MxA expression was determined by sarcoplasmic staining, except in necrotic or regenerating fibers.11
Serologic Information
Testing for the autoantibodies against TIF1-γ, Mi-2, and MDA5 is covered by the national insurance program in Japan and is conducted at 3 authorized commercial laboratories using the same MESACUPTM kit (Medical & Biological Laboratories). All patients who tested negative for these 3 DMSAs and anti-NXP-2 and anti-SAE autoantibodies were evaluated with immunoprecipitation and Western blotting.12,13
Standard Protocol Approvals, Registrations, and Patient Consent
This study was approved by the institutional review boards of the NCNP and Tsukuba University. All the materials used in this study were obtained for diagnostic purposes. The patients provided written informed consent for the use of the samples for research.
Statistical Analysis
We compared the clinical features, myopathologic findings, and DMSAs of the patients with and without a skin rash with the χ2 test for categorical variables and the Mann-Whitney U test for continuous variables that did not have a normal distribution. We included the time elapsed from the onset of the disease to the time of muscle biopsy as well as a few potential confounding variables, including age, sex, and perifascicular atrophy in multivariable logistic regression analyses (LRA), because disease duration could influence the development of skin rashes. We classified the patients according to disease duration and divided the cohort into 3 quartile-based categories: short (≤25th percentile), middle (>25th and ≤75th percentile), and long (>75th percentile). The percentage of patients with DMSD in each category was compared with the short category and multivariable LRAs were performed with the short category as a reference. Each variable, adjusted for several potential confounders, was first examined individually. Two-sided P values less than .05 were considered statistically significant. Statistical analyses were performed with R, version 3.5.3 (the R Foundation).
Results
Patient Characteristics
Of 199 patients with DM who were fully investigated for DMSAs, 84 (42%) had perifascicular atrophy, 101 (51%) had MHC class I expression with perifascicular enhancement, 165 (83%) had MAC deposition on capillaries, and 182 (91%) had MxA expression on myofibers. Among the total of 182 patients with MxA expression on myofibers, 93 (51%) were women and 46 (25%) were children (<18 years). The median age at biopsy was 56 years (interquartile range [IQR], 18-68 years).
Skin rashes were absent in 14 patients (8%). Therefore, these patients were diagnosed as having DMSD. Among them, 12 exhibited anti-NXP-2 autoantibodies, 1 patient had anti-TIF1-γ autoantibodies, and another patient had anti-Mi-2 autoantibodies. Anti-NXP-2 autoantibodies were observed in 86% of patients with DMSD (n = 12 of 14), while 28% of patients (n = 47 of 168) with skin rashes tested positive for anti-NXP-2 autoantibodies (86% vs 28%; P < .001). In contrast, anti-TIF1-γ, anti-Mi-2, and anti-MDA5 autoantibodies were considerably less frequent in DMSD compared with DM with skin rash (Table 1). No other clinical or pathological characteristics were associated with DMSD except increased probability of developing perifascicular atrophy (71% vs 44%; P = .04).
Table 1. Clinical Features, Myopathological Findings, and Autoantibodies.
| Characteristic | No. (%) | P value | |
|---|---|---|---|
| With skin rash (n = 168) | Sine dermatitis (n = 14) | ||
| Female | 85 (51) | 8 (57) | .64 |
| Age at biopsy, median (IQR), y | 55 (46) | 60 (30) | .44 |
| Clinical features | |||
| Rash of typical dermatomyositis | 132 (79) | 0 | NA |
| Gottron sign | 119 (71) | 0 | NA |
| Heliotrope | 81 (48) | 0 | NA |
| Shawl sign | 48 (29) | 0 | NA |
| V-sign | 36 (21) | 0 | NA |
| Periungual erythema | 33 (20) | 0 | NA |
| Other skin rasha | 147 (88) | 0 | NA |
| Muscle weakness | 150 (89) | 14 (100) | .20 |
| Myalgia | 119 (71) | 13 (93) | .08 |
| Dysphagia | 59 (35) | 7 (50) | .27 |
| Interstitial lung disease | 28 (17) | 1 (7) | .35 |
| Malignancy | 35 (21) | 2 (14) | .56 |
| Examination results | |||
| CK, median (IQR), U/L | 826 (262-4477) | 1179 (750-1987) | .52 |
| KL-6, median (IQR), U/mLb | 278 (203-400) | 305 (243-444) | .32 |
| Myogenic change in electromyographyb | 94 (87) | 12 (100) | .18 |
| Myopathologic findings | |||
| Perifascicular atrophy | 73 (43) | 10 (71) | .04 |
| Microinfarction | 26 (15) | 2 (14) | .91 |
| Perifascicular MHC class I reinforcement | 89 (53) | 9 (64) | .42 |
| MHC class II expression | 22 (13) | 3 (21) | .38 |
| MAC deposition on capillaries | 151 (90) | 13 (93) | .72 |
| Autoantibodies positivity | |||
| TIF1-γ | 64 (38) | 1 (7) | .02 |
| Mi-2 | 25 (15) | 1 (7) | .43 |
| MDA5 | 22 (13) | 0 | .15 |
| NXP-2 | 47 (28) | 12 (86) | <.001 |
| SAE | 4 (2) | 0 | .56 |
Abbreviations: CK, creatinine kinase; IQR, interquartile range; KL-6, Krebs von den Lungen-6; MAC, membrane attack complex; MDA5, melanoma differentiation-associated gene 5; MHC, human major histocompatibility complex; NA, not applicable; NXP-2, nuclear matrix protein 2; SAE, small ubiquitin-like modifier activating enzyme; TIF1-γ, transcription intermediary factor 1 γ.
SI conversion factor: To convert creatinine kinase to microkatals per liter, multiply by 0.0167.
Other skin rashes are all kinds of rashes except for typical DM skin rashes, such as nailfold punctuated hemorrhage, seborrheic eczema, butterfly rash, and nonspecific rash.
The denominator of the percentage indicates the total number of nonmissing responses.
All patients with DMSD demonstrated perifascicular atrophy and/or the deposition of membrane attack complex in the capillaries, further confirming the diagnosis of DM (Table 2). However, none of these patients were clinically diagnosed with DM; instead, most (n = 11 of 14; 79%) had been diagnosed as having polymyositis. All patients with DMSD underwent a muscle biopsy owing to muscle weakness, and were tested for DMSAs after the diagnosis of DM was made histopathologically. We further assessed the follow-up data of 14 patients with DMSD. The median follow-up period was 34 months (IQR, 16-64 months); the follow-up period of each patient is listed in Table 2. All patients were treated with oral prednisolone with or without additional immunotherapy after muscle biopsy. Two patients had subcutaneous edema, and calcification was never seen in the available follow-up period. One patient with DMSD with anti-NXP-2 autoantibodies had severe interstitial lung disease and needed noninvasive positive-pressure ventilation support.
Table 2. Features of Patients With Dermatomyositis Sine Dermatitis.
| Patient No. | Clinical diagnosis | DMSA | Disease duration (from onset to biopsy), mo | Skin rash at biopsy | Skin rash after biopsya | Follow-up period, mo | Sarcoplasmic expression of MxA | Perifascicular atrophy | MAC deposition on capillaries |
|---|---|---|---|---|---|---|---|---|---|
| 1 | PM | NXP-2 | 3.5 | No | No | 133 | Yes | Yes | Yes |
| 2 | PM | NXP-2 | 1.5 | No | No | 83 | Yes | Yes | Yes |
| 3 | PM | NXP-2 | 4.0 | No | No | 66 | Yes | Yes | Yes |
| 4 | Myositis | NXP-2 | 3.0 | No | No | 42 | Yes | Yes | Yes |
| 5 | Myositis | NXP-2 | 3.0 | No | No | 26 | Yes | Yes | Yes |
| 6 | PM or IMNM | NXP-2 | 3.0 | No | No | 25 | Yes | Yes | Yes |
| 7 | PM | NXP-2 | 1.0 | No | No | 15 | Yes | No | Yes |
| 8 | PM | NXP-2 | 1.5 | No | No | 10 | Yes | Yes | Yes |
| 9 | PM | NXP-2 | 1.0 | No | No | 7 | Yes | No | Yes |
| 10 | PM | NXP-2 | 2.0 | No | Yes (32) | 76 | Yes | No | Yes |
| 11 | PM | NXP-2 | 13.0 | No | Yes (31) | 44 | Yes | Yes | Yes |
| 12 | Myositis | NXP-2 | 1.5 | No | Yes (0.5) | 7 | Yes | No | Yes |
| 13 | PM | TIF1-γ | 3.0 | No | No | 58 | Yes | Yes | Yes |
| 14 | PM | Mi-2 | 3.0 | No | Yes (1.5) | 20 | Yes | Yes | No |
Abbreviations: DMSA, dermatomyositis-specific autoantibodies; IMNM, immune-mediated necrotizing myopathy; MAC, membrane attack complex; MxA, myxovirus resistance protein A; NXP-2, nuclear matrix protein 2; PM, polymyositis; TIF1-γ, transcription intermediary factor 1 γ.
The numbers in parentheses represent the duration (months) between muscle biopsy and the appearance of the skin rash.
Factors Associated With DMSD on Multivariable Analyses
We performed separate multivariable LRA14 for anti-NXP-2 and anti-TIF1-γ autoantibodies because these antibodies do not coexist in a single patient,9 after adjusting for confounding variables that might affect the association with DMSD. Anti-NXP-2 autoantibodies were significantly associated with DMSD (odds ratio [OR], 126.47; 95% CI, 11.42-1400.64; P < .001), while anti-TIF1-γ autoantibodies were negatively associated with DMSD (OR, 0.07; 95% CI, 0.01-0.64; P = .02) (Table 3). It is noteworthy that disease duration was not associated with DMSD, according to these analyses.
Table 3. Factors Associated With Sine Dermatitis in Patients With Dermatomyositis on Logistic Regression Analysis.
| Variable | Unadjusted (n = 182) | Adjusted (n = 182)a | ||
|---|---|---|---|---|
| OR (95%CI) | P value | OR (95%CI) | P value | |
| NXP-2 | ||||
| Age | 1.01 (0.99-1.03) | .42 | 1.05 (1.01-1.08) | .01 |
| Female | 1.30 (0.43-3.92) | .64 | 0.43 (0.08-2.22) | .31 |
| Disease duration | ||||
| Short (≤25th percentile) | 1 [Reference] | NA | 1 [Reference] | NA |
| Middle (>25th and ≤75th percentile) | 1.08 (0.33-3.50) | .90 | 1.03 (0.19-5.75) | .97 |
| Long (>75th percentile) | 0.23 (0.03-2.02) | .18 | 0.06 (0.003-1.12) | .06 |
| Perifascicular atrophy | 3.25 (0.98-10.79) | .05 | 13.65 (2.25-82.93) | .005 |
| NXP-2 | 15.45 (3.33-71.63) | <.001 | 126.47 (11.42-1400.64) | <.001 |
| TIF1-γ | ||||
| Age | 1.01 (0.99-1.03) | .42 | 1.02 (0.99-1.04) | .20 |
| Female | 1.30 (0.43-3.92) | .64 | 1.07 (0.30-3.84) | .91 |
| Disease duration | ||||
| Short (≤25th percentile) | 1 [Reference] | NA | 1 [Reference] | NA |
| Middle (25th and 75th percentile) | 1.08 (0.33-3.50) | .90 | 0.48 (0.11-2.12) | .33 |
| Long (>75th percentile) | 0.23 (0.03-2.02) | .18 | 0.09 (0.01-0.95) | .05 |
| Perifascicular atrophy | 3.25 (0.98-10.79) | .05 | 7.23 (1.65-31.62) | .009 |
| TIF1-γ | 0.13 (0.02-0.98) | .05 | 0.07 (0.01-0.64) | .02 |
Abbreviations: NA, not applicable; NXP-2, nuclear matrix protein 2; OR, odds ratio; TIF1-γ, transcription intermediary factor 1 γ.
Adjusted for autoantibodies (NXP-2 or TIF1-γ) and disease duration. Each category of the disease duration includes following period: short (≤25th percentile, ≤1.5 months), middle (>25th and ≤75th percentile, >1.5 months and ≤6 months), and long (>75th percentile, >6 months).
Discussion
To our knowledge, this study is the first to report the prevalence of DMSD (8%; n = 14 of 182) in a large cohort of patients with DM confirmed by biopsy and the significant association between anti-NXP-2 autoantibodies and DMSD.
In our study, 4 of 14 patients with DMSD developed skin rashes after muscle biopsy. Similarly, a patient with DMSD was reported to have developed a skin rash 2 years after muscle biopsy.7 Interestingly, this particular patient was reportedly positive for anti-NXP-2 autoantibodies. To our knowledge, this is the only patient with DMSD whose DMSA was characterized among all the case reports on DMSD,5,6,7,8 further supporting the association between anti-NXP-2 autoantibodies and DMSD. The positivity of anti-NXP-2 autoantibodies may raise the possibility of DMSD even before muscle biopsy. There are possibilities that such patients may eventually develop skin rashes or that immunotherapy may mask skin rashes, especially in patients with a shorter disease duration. Nevertheless, there was no statistically significant difference in disease duration in patients with and without skin rash on unadjusted and adjusted multivariable LRAs (Table 3), suggesting that few untreated patients with DMSD develop skin rashes. We speculate that not only anti-TIF1-γ but also anti-Mi-2 and anti-MDA5 autoantibodies are negatively associated with DMSD, although statistical significance was not reached in this study, likely owing to the sample size.
There are no established criteria for the duration of the adermopathic period necessary for the diagnosis of DMSD. Thus, we diagnosed DMSD based on the absence of skin rash at the time of muscle biopsy. A diagnosis of CADM is made if the amyopathic period lasts for 2 years or more.1 Even if we excluded patients with DMSD with an adermopathic period of less than 2 years (by applying the same concept), we cannot discount the evidence that unadjusted LRA revealed a statistically significant association (OR, 20.33; 95% CI, 2.48-166.96; P = .005) between anti-NXP-2 autoantibodies and DMSD in 9 patients with DMSD (eTable 2 in the Supplement). Also, it is controversial as to whether we can classify patients who eventually (>2 years) develop a skin rash as having DMSD. However, even if we apply this concept to this study, the association between anti-NXP-2 autoantibodies and DMSD in 10 patients with DMSD remains significant (eTable 3 in the Supplement).
Selection bias may have influenced our study results because the cohort was muscle biopsy–oriented. However, it is critical to confirm DM-specific muscle damage for the diagnosis of DMSD because skin rashes are absent. Indeed, none of our patients with DMSD had been clinically diagnosed as having DM before muscle biopsy. Therefore, we only included patients with confirmed expression of MxA in this study. As shown in eTables 4 and 5 in the Supplement, the results were still statistically significant regarding the association between DMSD and anti-NXP-2 autoantibodies, even if we selected perifascicular atrophy (DMSD 80% [n = 8 of 10] vs with rash 16% [12 of 74]; P < .001) or MAC deposition on capillaries (DMSD 92% [n = 12 of 13] vs with rash 30% [46 of 152]; P < .001) instead of MxA. Myxovirus resistance protein A is a surrogate marker of the type I interferon signature in muscle fibers, which is highly characteristic of DM.11 Several studies have reported the efficacy of Janus kinase inhibitors, which block the type I interferon pathway, in patients with refractory DM.15 These therapies may be effective for DM but not for other types of myositis. Therefore, differentiating patients with DMSD from those who are clinically diagnosed as having polymyositis is more essential than ever.
Limitations
Our study has some limitations. First, we did not follow up with all of the patients with DM, except for patients with DMSD, thus leaving the detailed clinical features of DMSD elusive. In addition, the possibility of double positivity of myositis-specific autoantibodies cannot be excluded in our study, although such cases are very rare. Lastly, there is a possibility that other potential confounding variables, which were not included in the multivariable analysis, may affect the association between DMSD and autoantibodies.
Conclusions
In this cohort study, DMSD accounted for 8% of patients with DM confirmed by muscle biopsy. Dermatomyositis sine dermatitis was significantly associated with anti-NXP-2 autoantibodies. Our study indicates the substantial existence of DMSD. Further studies with several ethnic groups are necessary to confirm our results.
eTable 1. Autoantibodies used for immunohistochemistry
eTable 2. Predictors of dermatomyositis sine dermatitis in patients with dermatomyositis on logistic regression analysis (adermopathic period > 2 years)
eTable 3. Predictors of dermatomyositis sine dermatitis in patients with dermatomyositis on logistic regression analysis (for dermatomyositis sine dermatitis without a rash throughout available follow-up period)
eTable 4. Predictors of dermatomyositis sine dermatitis in patients with dermatomyositis
eTable 5. Predictors of dermatomyositis sine dermatitis in patients with dermatomyositis on logistic regression analysis (with MAC deposition on capillaries)
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
eTable 1. Autoantibodies used for immunohistochemistry
eTable 2. Predictors of dermatomyositis sine dermatitis in patients with dermatomyositis on logistic regression analysis (adermopathic period > 2 years)
eTable 3. Predictors of dermatomyositis sine dermatitis in patients with dermatomyositis on logistic regression analysis (for dermatomyositis sine dermatitis without a rash throughout available follow-up period)
eTable 4. Predictors of dermatomyositis sine dermatitis in patients with dermatomyositis
eTable 5. Predictors of dermatomyositis sine dermatitis in patients with dermatomyositis on logistic regression analysis (with MAC deposition on capillaries)