Page 65 - Recognizing axial spondyloarthritis - Janneke de Winter

INTRODUCTION
Axial spondyloarthritis (SpA) is among the most prevalent forms of chronic inflammatory arthritis (1,2). In recent decades, magnetic resonance imaging (MRI) has been increasingly used to visualize inflammation in the sacroiliac (SI) joints. Inflammation on MRI facilitates the early identification of patients with axial SpA since it precedes structural damage on radiography (3,4). Of all patients with axial SpA, 20–42% have active sacroiliitis on MRI (5–8), but signs of presumed sacroiliitis may also occur in individuals without axial SpA. One study by Weber et al showed that of 59 healthy volunteers, up to 22% had signs of sacroiliitis on MRI (9). Arnbak and colleagues showed that of 1,020 unselected patients with chronic back pain, 21% had sacroiliitis on MRI according to the Assessment of SpondyloArthritis international Society (ASAS) definition (10). The specificity of MRI of the SI joints for SpA-specific sacroiliitis is not well known and physicians may rely too much on a positive finding (11).
In order to obtain insight into the prevalence and extent of SI joint inflammation in healthy individuals, and in those with known mechanical strain acting upon the SI joints, we compared MRIs of the SI joints in 1) healthy individuals without any signs of current or past back pain, 2) patients with classic axial SpA with a documented positive MRI of the SI joints after central reading, 3) patients with chronic back pain, 4) frequent runners, and 5) women with postpartum back pain.
The primary hypothesis was that the difference between axial SpA and non–axial SpA with regard to the presence of sacroiliitis on MRI was quantitative (extent) rather than qualitative (present versus absent).
METHODS
Selection of the study participants
The present study included participants from the Maasstad MRI project and the Spondyloarthritis Caught Early (SPACE) cohort. The Maasstad MRI project included healthy participants, frequent runners, and women with postpartum back pain (12). Healthy participants were employees of Maasstad Hospital and Erasmus University Medical Center (Rotterdam, The Netherlands). Male and female participants between 18 and 45 years of age were included. Runners who were running at least 5 km twice per week were recruited via an athletics club. Healthy participants and runners with any form of current or past acute or chronic back pain and those with contraindication to MRI were excluded. Women with postpartum back pain that presented ≤3 months after pregnancy were included in the Maasstad MRI project and additionally selected from the SPACE cohort.
We selected positive and negative controls from the SPACE cohort. The SPACE cohort is an ongoing, prospective, longitudinal, multicenter cohort that started in 2009 and has been described in detail elsewhere (13). In short, the SPACE cohort includes patients ages ≥16 years with chronic back pain (with a duration of ≥3
MRI IN HEALTHY INDIVIDUALS
63
FOUR