Ankylosing Spondylitis and Nonradiographic Axial Spondyloarthritis: Part of a Common Spectrum or Distinct Diseases?

Objective.To investigate the features of ankylosing spondylitis (AS) and nonradiographic axial spondyloarthritis (nr-axSpA) in a Canadian cohort of 639 patients with AS and 73 patients with nr-axSpA.Methods.Clinical and laboratory data were compared for patients with AS and nr-axSpA enrolled in a longitudinal SpA cohort.Results.The proportion of male patients was higher in AS than in nr-axSpA (76.2% vs 47.9%; p < 0.0001). There was no difference in the presence of HLA-B27 between AS (78.9%) and nr-axSpA (72.5%) patients, nor in age at the time of diagnosis, although AS patients were younger at the time of symptom onset (23.9 yrs vs 26.4 yrs; p = 0.03). Disease duration at the time of last clinic visit was longer for AS than for nr-axSpA patients (17.7 yrs vs 12.1 yrs; p = 0.0002). Acute-phase reactants were higher in AS than in nr-axSpA (C-reactive protein 11.4 vs 5.2, p < 0.0001; erythrocyte sedimentation rate 13.7 vs 9.9, p = 0.02). The Bath Ankylosing Spondylitis Metrology Index was higher in patients with AS (2.84 vs 1.35, p < 0.0001).Conclusion.Patients with nr-axSpA were more likely to be female and to have lower inflammatory markers than patients with AS. When restricted to female patients only, acute-phase reactants did not differ significantly between AS and nr-axSpA. The evidence provides indirect support for the concept that nr-axSpA may represent an early form of AS, but that also has features of a distinct disease entity with significant burden of symptoms.