Progression of Nonradiographic Axial Spondyloarthritis to Ankylosing Spondylitis: A Population-Based Cohort Study

Objective The long-term outcome of patients with nonradiographic axial spondyloarthritis (SpA) is unclear, particularly whether few or most progress to ankylosing spondylitis (AS). Our objective was to examine the progression to AS in a population-based inception cohort of patients with nonradiographic axial SpA. Methods The Rochester Epidemiology Project (REP) is a longstanding population-based study of health in the residents of Olmsted County, Minnesota. We searched the REP from 1985 to 2010 using diagnostic and procedural codes for back pain, HLA-B27, and magnetic resonance imaging of the pelvis, and we performed detailed chart reviews to identify subjects who fulfilled the Assessment of SpondyloArthritis international Society classification criteria for axial SpA but did not have AS. We followed these subjects from disease onset to March 15, 2015, and used survival analysis to measure the time to progression to AS. Results After screening 2,151 patients, we identified 83 subjects with new-onset nonradiographic axial SpA. Over a mean follow-up of 10.6 years, progression to AS occurred in 16 patients. The probability that the condition would remain as nonradiographic axial SpA at 5, 10, and 15 years was 93.6%, 82.7%, and 73.6%, respectively. There was more frequent and more rapid progression among subjects in the imaging arm (n = 18) than among those in the clinical arm (n = 65) (28% versus 17%; hazard ratio 3.50 [95% confidence interval 1.15-10.6], P = 0.02). Conclusion Progression to AS occurred in a minority (26%) of patients with nonradiographic axial SpA over as long as 15 years of follow-up. This suggests that the classification criteria for nonradiographic axial SpA identifies many patients in whom the condition is unlikely to progress to AS or that nonradiographic axial SpA represents a prolonged prodromal state that takes longer to evolve to AS and thus requires longer follow-up.