Methods A population-based IIM cohort of patients diagnosed between 2002 and 2011 and general population comparators were identified using healthcare registers. They were linked to the cause of death register for follow-up.

Results 224 (31%) of the 716 patients with IIM and 870 (12%) of the 7100 general population died during follow-up. This corresponded to a mortality rate of 60/1000 person-years in IIM and 20/1000 person-years in the general population. The cumulative mortality at 1 year after diagnosis was 9% in IIM and 1% in the general population, and increased in both IIM and the general population with time. The overall hazard ratio (HR) 95%CI of death comparing IIM with the general population was 3.7 (3.2 to 4.4). When we stratified on time since diagnosis, we noted an increase in mortality already within the first year of diagnosis compared with the general population, HR 9.6 (95% CI 6.9 to 13.5). This HR then plateaued around 2 after >10 years with the disease, although the estimates were not statistically significant. Malignancies, diseases of the circulatory and respiratory system were common causes of death.

Conclusion Mortality is increased in patients with contemporary IIM. The increased mortality was noted within a year of diagnosis, which calls for extra vigilance during the first year of IIM diagnosis.