Systemic juvenile idiopathic arthritis: frequency and long-term outcome in Western Australia.

Systemic juvenile idiopathic arthritis (S-JIA) is a rare but potentially life threatening autoinflammatory condition of childhood. Given the limited data on S-JIA from the Australasian region, we investigated the epidemiological characteristics and long-term disease outcome in S-JIA. All hospitalised patients under the age of 16 years registered with ICD-10-AM code M08.2 in in the period 1999-2014 were identified in longitudinally linked administrative health data across all Western Australian (WA) hospitals. Incidence and point prevalence estimate were per 100,000 population with Poisson regression to analyse the incidence trend. Readmissions with S-JIA as primary diagnosis were considered flares with rates for flare and other complication reported per 100 person years with 95% confidence intervals (CI). Annual S-JIA incidence was 0.61/100,000 (CI 0.28-1.25) (46 incident cases, 71.7% girls, median age 6.5 years) and stable over time as S-JIA point prevalence reached 7.15/100,000 (CI 5.29-7.45) at the end of study. Most incident cases were diagnosed in winter and spring, but documented preceding infections were rare. During a median follow-up of 8 years, disease flares occurred in 24% of patients with higher flares rate in boys (58.3; CI 44.5-74.9) than girls (14.7; CI 9.9-20.9). No deaths occurred and arthroplasty was the main, but uncommon S-JIA complication (4%). However, readmission (86.3; CI 76.4-97.2) and ED visit (73.3; CI 64.2-83.4) rates for illnesses other than S-JIA were substantial. S-JIA is as rare in WA as in other regions and while s-JIA incurred no deaths in the era of biologics, it associated with a significant long-term burden of (co-) morbidity.