Incidence and prevalence of idiopathic inflammatory myopathies among commercially insured, Medicare supplemental insured, and Medicaid enrolled populations: an administrative claims analysis.
BACKGROUND
Idiopathic inflammatory myopathies (IIMs) are a rare group of autoimmune syndromes characterized by chronic muscle inflammation and muscle weakness with no known cause. Little is known about their incidence and prevalence. This study reports the incidence and prevalence of IIMs among commercially insured and Medicare and Medicaid enrolled populations in the US.


METHODS
We retrospectively examined medical claims with an IIM diagnosis (ICD-9-CM 710.3 [dermatomyositis (DM)], 710.4 [polymyositis (PM)], 728.81[interstitial myositis]) in the MarketScan® databases to identify age- and gender-adjusted annual IIM incidence and prevalence for 2004-2008. Sensitivity analysis was performed for evidence of a specialist visit (rheumatologist/ neurologist/dermatologist), systemic corticosteroid or immunosuppressant use, or muscle biopsy.


RESULTS
We identified 2,990 incident patients between 2004 and 2008 (67% female, 17% Medicaid enrollees, 27% aged ≥65 years). Overall adjusted IIM incidence for 2004-2008 for commercial and Medicare supplemental groups combined were 4.27 cases (95% CI, 4.09-4.44) and for Medicaid, 5.23 (95% CI 4.74-5.72) per 100,000 person-years (py). Disease sub-type incidence rates per 100,000-py were 1.52 (95% CI 1.42-1.63) and 1.70 (1.42-1.97) for DM, 2.46 (2.33-2.59) and 3.53 (3.13-3.94) for PM, and 0.73 (0.66-0.81) and 0.78 (0.58-0.97) for interstitial myositis for the commercial/Medicare and Medicaid cohorts respectively. Annual incidence fluctuated over time with the base MarketScan populations. There were 7,155 prevalent patients, with annual prevalence ranging 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.


CONCLUSIONS
We found higher IIM incidence than historically reported. 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.
Thomson Reuters, 4301 Connecticut Ave, Washington, DC 20008, USA. Karen.tomic@pharmagenesis.com
Karen E
KE
Smoyer-Tomic
Anthony A
AA
Amato
Ancilla W
AW
Fernandes
D000293:Adolescent; D000305:Adrenal Cortex Hormones / Q000627:therapeutic use; D000328:Adult; D017677:Age Distribution; D000367:Age Factors; D000368:Aged; D001706:Biopsy / Q000706:statistics & numerical data; D016009:Chi-Square Distribution; D057225:Data Mining; D016208:Databases, Factual; D005260:Female; D006801:Humans; D007166:Immunosuppressive Agents / Q000627:therapeutic use; D015994:Incidence; D008297:Male; D008484:Medicaid / Q000706:statistics & numerical data*; D016345:Medicare Part B / Q000706:statistics & numerical data*; D008875:Middle Aged; D009220:Myositis / Q000175:diagnosis / Q000453:epidemiology* / Q000628:therapy; D015995:Prevalence; D012017:Referral and Consultation / Q000706:statistics & numerical data; D012189:Retrospective Studies; D017678:Sex Distribution; D012737:Sex Factors; D013997:Time Factors; D014481:United States / Q000453:epidemiology; D055815:Young Adult
D000305:Adrenal Cortex Hormones; D007166:Immunosuppressive Agents