Incidence and prevalence of idiopathic inflammatory myopathies among commercially insured, Medicare supplemental insured, and Medicaid enrolled populations: an administrative claims analysis

Autoimmune idiopathic inflammatory myopathies (IIMs) consist of a rare group of autoimmune syndromes characterized by chronic muscle inflammation and muscle weakness. They are classified as autoimmune disorders [1] and have no known cause, although research has implicated infectious agents, medications, and ultraviolet radiation in their etiology [2]. Three main types of IIMs have been proposed based on clinical and histopathologic features: (a) polymyositis (PM), (b) dermatomyositis (DM), and (c) sporadic inclusion body myositis (sIBM). Recent reports have described two new types: nonspecific myositis, and immune-mediated necrotizing myopathy [3‐5]. Older reports [6] also describe an “interstitial myositis” that has not been reported extensively in the literature. Dermatomyositis is the most common of the myositis disorders [2, 7]. Polymyositis is often associated with other connective tissue disorders such as lupus [6, 8, 9]. Treatments for IIMs typically consist of medications such as corticosteroids as first line therapy, (e.g., high doses of oral or intravenous prednisone) and immunosuppressants and immunomodulators (e.g., methotrexate, mycophenolate mofetil, tacrolimus, rituximab) as second line [2]. Physical therapy is also used [9].

There are few epidemiological studies on IIMs. Until recently, many of the studies of incidence and prevalence of the disease have been on relatively small population studies, often using both medical records and muscle biopsy to ascertain the disease [8]. These types of studies have identified annual myositis (including its sub-types) incidence in the range of 0.1 to 1 per 100,000 person-years (py) [2, 10‐15].

One recent study using administrative claims data for Quebec (approximately 7.5 million beneficiaries) estimated prevalence of 21.5 (95% CI: 19.4, 23.9) per 100,000 patients for dermatomyositis and polymyositis, using an IIM case definition of ≥2 outpatient claims with an ICD-9-CM diagnosis recorded for either condition ≤2 months apart and within a 2 year time span, or ≥1 inpatient claim with either ICD-9-CM code, as well as an alternate algorithm with one or more billing code from a rheumatologist [16]. The authors reported higher prevalence for women and older patients. Other studies reporting rates by age and/or sex also have found that incidence rates increase with age (although dermatomyositis affects children as well as adults) and are approximately twice as high among women as among men [2, 6‐8, 12]. Separate community-based epidemiologic studies identified a range of prevalence for IIMs, including IIM prevalence of 5.1 per 100,000 in the U.S. [17], PM prevalence of 3.45 per 100,000 in Olmsted County, Minnesota, USA [10] and DM prevalence of 21.42 [18].

Recent work using an U.S. privately insured managed care administrative claims database for 2003–2008 identified a 65/35 female to male ratio of patients with myositis (polymyositis, dermatomyositis, or interstitial myositis) with a median age of 49 years. The overall age- and sex-adjusted myositis incidence rate was 6.57 cases (95% CI, 6.20–6.94) per 100,000 py and was 3.79 for polymyositis, 1.38 for dermatomyositis, and 1.69 for interstitial myositis per 100,000 py[19], notably higher than the 0.1 to 0.5 per 100,000 myositis incidence reported using smaller, fixed populations [13‐15]. Annual myositis prevalence identified in the same study for the period 2003–2008 ranged from 13.99 in 2003 to 17.37 in 2008 per 100,000 py[19]. These prevalence rates are in the range of those reported by Bernatsky [16] in an administrative claims database in Quebec, suggesting prevalence of myositis, as identified in administrative claims databases, may be in the range of 14–22 cases per 100,000 py.

The aim of the present study is to examine age and sex adjusted incidence and prevalence of IIM using commercial, Medicare and Medicaid populations. This approach offers several advantages. First, it uses a large, geographically diverse U.S. administrative claims database. Since IIMs are rare, the large sample available in the database allows more robust estimates of incidence and prevalence than in many previous studies, which examined IIM incidence and/or prevalence in smaller populations. Second, this approach provides incidence and prevalence estimates by sex and age group, and includes populations with diverse payers (both commercial and Centers for Medicare & Medicaid Services [CMS]). Third, this study includes outpatient as well as inpatient claims, which limits bias in case identification by providing greater sensitivity in incidence and prevalence estimates than would be possible if limited to hospitalization data only. Many of the published incidence and prevalence studies of IIMs that utilized only hospital data [8] may have under-estimated incidence and prevalence, since myositis often does not require hospitalization.

IIMs are a rare condition, and little is known about the extent of their incidence and prevalence in the U.S. population. In this study, we used the MarketScan Commercial, Medicare, and Medicaid administrative claims databases to identify incidence and prevalence of IIMs based on medical claims with an ICD-9-CM diagnosis indicating DM, PM, or interstitial myositis. Administrative claims analyses, such as the one performed here, have the benefit of data contributed by very large populations seeking medical care, which can be used to identify rare conditions and shed additional light into incidence and prevalence. The limitation of these methods is the flip side of the benefit coin, in that unless patients seek care for their IIM, they will not be correctly identified as incident or prevalent. In addition, misdiagnosis or recording errors can result in under- or over-reporting of the condition. Another limitation in using administrative claims databases for identifying incidence and prevalence is that the results are sensitive to the methods used to identify the population denominator.

Due to the dynamic nature of the employer-based and Medicaid administrative claims databases used here, the annual values are most informative as snapshots providing a range of estimates among a fluctuating population, rather than indicative of trends in incidence or prevalence of myositis in the U.S. population. Another consideration particular to employer-based databases is that employees with chronic conditions may be less likely to change employers and risk changes to their healthcare coverage, and therefore may stay in the data longer than other employees at the same employer.

Both the current study and one using very similar methods (1 inpatient or 2 outpatient claims for PM, DM, or interstitial myositis) in another U.S. managed care administrative claims database [19] reported notably higher incidence of IIMs than the 0.1 to 1 per 100,000 py reported in community-based studies using smaller populations [2, 10‐15]. This study identified age- and sex-adjusted incidence rates of 4.27 (commercial/Medicare) and 5.23 (Medicaid) per 100,000 py for the period 2004–2008, while Furst et al. [19] reported adjusted incidence rates of 6.57 (95% CI, 6.20–6.94) per 100,000 py for combined IIMs. For DM, we identified adjusted incidence rates of 1.52 and 1.70 per 100,000 py in the commercial/Medicare and Medicaid databases respectively, which were relatively comparable to the 1.38 per 100,000 py reported by Furst et al. [19]. Incidence rates for PM were also comparable at 2.46 (commercial/Medicare) and 3.53 (Medicaid) in this study versus 3.79 per 100,000 py in Furst et al [19]. These rates were similar to the prevalence of PM of 3.45 per 100,000 py identified by Wilson et al. [10]. Interstitial myositis varied among the two studies, in that it was the least common of the three types of IIM, with adjusted incidence of 0.73 and 0.78 per 100,000 py in the two databases compared to 1.69 per 100,000 py reported in Furst et al.[19], in which DM rather than interstitial myositis was the least common type of myositis. Due to very low counts of interstitial myositis, along with limited consistent clinical information about the condition and its diagnosis, incidence and prevalence for that sub-type should be interpreted with caution.

In contrast to incidence estimates, the prevalence of IIMs identified by medical claim (20.62 to 25.32 per 100,000 py in commercial/Medicare and 15.35 to 32.74 in Medicaid) in this study is quite comparable to that identified using a contemporary administrative claims database in Quebec (combined DM and PM annual prevalence in 2003 of 21.5 (95% CI 19.4-23.9) per 100,000 py [16]. The prevalence of IIMs identified in this study is, however, somewhat higher than prevalence of combined DM, PM and interstitial myositis given in Furst et al. [19] of 9.54 in 2003 and 13.61 in 2008, albeit the lower prevalence in Furst et al. could be associated with a younger population (9% of prevalent cases were ≥ 65 years in their IIM cohort vs. 24% ≥ 65 years in the commercial/Medicare and 16% of prevalent cases in the Medicaid IIM cohorts). Notably, all three administrative claims studies reported annual prevalence that was nearly 2- to 6-fold higher than that reported in one earlier study from 2003 [17]. It is possible that with increased awareness of IIMs, the condition may be increasingly recognized and diagnosed, and thus showing up more frequently in administrative claims databases over the past decade than in previous decades.

When the 2004–2008 age- and sex-adjusted incidence rates for the two databases are projected to the U.S. population ages 18 years and older, the expected number of incident cases per year ranges from 8,919 to 10,937 (based on the commercial/Medicare and Medicaid incidence rates respectively, standardized to the 2000 U.S. Census). Using the combined sensitivity analysis (e.g., IIM incidence along with one or more of the three sensitivity criteria), the expected number of incident cases per year among adults in the U.S. ranges from 6,871 (commercial/Medicare incidence rates) to 7,312 (Medicaid incidence rates).

Extrapolating the annual prevalence of persons seeking medical care for IIMs reported here to the U.S. population (as of the 2000 U.S. Census) aged 18 years and older from the commercial/Medicare cohort results in a range of an estimated 43,000 to 53,000 treated cases per year, or, after employing any one of the three sensitivity criteria, an estimated 38,000 to 45,000 treated cases per year. If the Medicaid annual prevalence is used, estimated annual U.S. IIM prevalence ranges from approximately 32,000 to 68,000 using IIM claims only, or 26,000 to 60,000 treated cases using any of the three sensitivity criteria.

This study is unique in its incorporation of three payer groups: commercially insured, Medicare enrollees (with employer paid supplemental insurance), and Medicaid enrollees. The different incidence and prevalence identified in the commercial/Medicare versus Medicaid populations in this analysis may be a result of differences in patients’ healthcare-seeking behaviors as well as in reimbursement and practice patterns rather than the underlying difference in disease incidence and prevalence among the two populations. The relative similarity in prevalence and incidence identified in separate studies conducted in different administrative claims databases indicates that these variations are common in these types of databases. Another consideration unique to the current study is the high incidence and prevalence among African Americans identified in the Medicaid database. Since race/ethnicity is unknown in the commercial and Medicare cohorts, it is unknown to what extent differences in racial composition between the two cohorts may influence the observed incidence and prevalence of myositis. Additional studies of IIM incidence and prevalence by race/ethnicity are needed.

The incidence and prevalence estimates reported here are in the range of those reported in other studies using administrative claims databases. Like those studies, the estimates reported here are based on the presence of a medical claim with the ICD-9-CM diagnoses of interest and are a function of individuals seeking medical care and physician reporting of diagnosis. This study used an algorithm to exclude rule-out diagnoses; nevertheless, administrative claims are not a substitute for a complete medical record for the study population and thus mis-diagnoses are possible. Since a one year “clean” period was required to qualify as incident, incidence may have been over-estimated if patients already had an IIM but did not receive medical care in the preceding year. In addition, prevalence was expected to be under-estimated as it was defined by evidence of a qualifying medical claim with an IIM diagnosis in each year. Patients with the condition who did not seek medical care in a given year would not be captured in the prevalence estimates. Finally, employer turnover, miscoding and misdiagnosing, care seeking behavior, and fluctuations in database membership over time can influence the results. Further studies are needed to confirm the incidence and prevalence of IIM.

Among 2,990 incident IIM patients between 2004 and 2008, overall adjusted IIM incidence for 2004–2008 for commercial and Medicare supplemental groups combined were 4.27 cases per 100,000 py, and for Medicaid were 5.23 cases per 100,000 py. The highest incidence among disease sub-types was for PM, at 2.46 (commercial/Medicare) and 3.53 (Medicaid) new cases per 100,000 py. Annual IIM prevalence ranged from 20.62 to 25.32 per 100,000 for commercial/Medicare (83% of prevalent cases) and from 15.35 to 32.74 for Medicaid. Although this study found higher IIM incidence than historically reported based on smaller, localized populations, the similarity of the findings of the current study with others in the literature using administrative claims databases suggests that IIM incidence in the U.S. population is in the range of 3 to 7 new cases per 100,000 py and that prevalence as measured by claims for IIM treatment is in the range of 32,000 to 68,000 per year. Future studies using alternate administrative claims databases will be helpful in narrowing down the range of incidence and prevalence estimates, and additional analyses of the cohorts identified as incident and prevalent using chart review would be beneficial in confirming IIM diagnoses, particularly for the three sub-types.

^aImmunosuppressants included methotrexate, azathioprine, hydroxychloroquine, intravenous immunoglobulin, cyclosporine, mycophenolate mofetil, leflunomide, thalidomide, cyclophosphamide, tacrolimus, and rituximab.

^bMuscle biopsy codes included ICD-9-CM Procedure 83.21 or CPT codes 10021, 10022, 20200, 20205, 20206, 21550, 21925, 23066, 24066, 25066, 27041, 27324, or 27614.