Page 71 - Recognizing axial spondyloarthritis - Janneke de Winter

axial SpA, or runners, 4) deep (extensive) lesions are highly specific for axial SpA– associated sacroiliitis, and 5) BME lesions in healthy participants are preferably but not exclusively located in the lower iliac bone.
Our finding of positive MRIs in healthy individuals is consistent with the findings of Weber and Maksymowych, who showed that 25% of healthy participants have BME lesions on MRIs of the SI joints (17). Our findings of positive MRIs in women with postpartum back pain are also consistent with a previous study showing that 60% of women with postpartum back pain have BME lesions on MRI of the SI joints (18). Differently than we hypothesized, our data suggest that the proportion of runners with BME lesions on MRI of the SI joints is not higher than the proportion of healthy participants with BME lesions on MRI of the SI joints. However, little demographic or lifestyle data were collected, and our group of healthy participants might have included runners. Further research should correct for possible confounding factors such as body mass index and general health status.
In this study, BME lesions in MRIs scored positive for sacroiliitis and in the MRIs of healthy participants were most frequently found in the posterior lower ilium, which is consistent with data from Weber et al showing a similar preferential location in amateur and professional athletes (17). In patients with axial SpA, lesions occur throughout the SI joints. BME was seen in the anterior lower ilium in 16.7% of the MRIs in which lesions were not highly specific for SpA.
The percentage of patients with chronic back pain with BME lesions was lower than the percentage of healthy volunteers with BME lesions (6.4% versus 23.4%, respectively). This is an artifact caused by channeling patients with an MRI negative for sacroiliitis toward the (non–axial SpA) chronic back pain group, a rational diagnostic approach since a clinical diagnosis of axial SpA (or the exclusion thereof) relies on clinical signs and laboratory findings as well as imaging results (MRI). Similarly, the 91.5% positive MRI rate in the axial SpA group is the consequence of channeling. Prior MRI studies have been used to classify these patients as having axial SpA. As such, the low prevalence of MRI positivity found in the chronic back pain group—and the high prevalence found in the axial SpA group—adds to the credibility of our findings (internal validity).
One of the most important reasons to perform this study is that misclassification of MRIs of the SI joints as positive is a real threat, which may lead to a spuriously high number of patients being diagnosed as having axial SpA. Evidence of such a spurious mechanism has been reported by Arnbak et al (10) and Van Hoeven et al (20), showing that nearly 25% of patients with chronic back pain could be classified as having axial SpA when MRI of the SI joints was the leading factor in the diagnostic consideration. Since we and others have demonstrated that MRIs that are highly suggestive of axial SpA may be seen frequently in unaffected individuals, relying too much on a positive MRI finding will result in overdiagnosis, and consequently in overtreatment, of these patients who may have nonspecific chronic back pain rather than axial SpA.
MRI IN HEALTHY INDIVIDUALS
69
FOUR