Figure 2.

Percentage of adult and juvenile onset patients with undifferentiated SpA (u-SpA) progressing to ankylosing spondylitis (AS) throughout the course of the disease as reported in retrospective studies. Note: Variations are probably related to the characteristics of the population included in the study, entry criteria, and the type of assessments carried out. Interestingly, most references related to adult-onset patients include a moderately high proportion of patients with peripheral arthritis in combination or not with axial symptoms. Most studies consider AS the outcome measure, but some others referred to other parameters, for example radiographic sacroiliitis.