Prognosis

Although the prognosis of idiopathic inflammatory myopathy (IIM) has improved in the past two decades through major advances in understanding the pathophysiology and advent of disease-modifying therapies other than corticosteroids, overall mortality from IIM is still considerably higher than that of the general population.​[1][4]​​

In the first large, long-term, population-based analysis of a well-documented IIM cohort linked to reliable mortality data in Sweden, the overall hazard ratio for death when comparing IIM with the general population was 3.7 (95% CI 3.2 to 4.4).[132]

Reported survival rates vary quite considerably in the literature, likely as a result of case selection.[1]​ In one study, a 5-year survival rate of 95% was reported, but without the inclusion of patients with malignancy or inclusion body myositis, thereby selecting a younger cohort (mean age 38.9 years) without cancer-related mortality risk.[133]​ In contrast, a cohort with a median age of 52 years and cancer recorded in 22% of patients, the 5-year survival rate was 77%.[134]

The prognosis varies between IIM subtypes and are poorest among patients with cancer-associated myositis and anti-MDA5 dermatomyositis with rapidly progressive interstitial lung disease, mortality as high as approximately 60% has been reported for these patients.[135]​ Among histologically defined IIM subgroups, dermatomyositis had the highest standardized mortality ratio (3.04) in an Australian cohort, followed by inclusion body myositis (2.25) and polymyositis (1.65).[136]

Other potential prognostic factors for mortality for patients with IIM include:[1]​​

  • Advancing age

  • Inflammatory markers at baseline

  • Late diagnosis

  • Seronegativity for autoantibodies

  • Upper and lower limb involvement

  • Smoking

  • Cardiac involvement

  • ILD

  • Non-white ethnicity

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