Page 72 - Recognizing axial spondyloarthritis - Janneke de Winter

CHAPTER FOUR
The only means to avoid overdiagnosis of axial SpA as a diagnostician is to act based on thorough knowledge of the clinical syndrome of axial SpA and MRI abnormalities thereof, to ask for diagnostic MRI of the SI joints only in patients in whom there is a moderately high suspicion of axial SpA (e.g., based on clinical algorithms) (18), and to always consider alternative (more likely) diagnoses before making a diagnosis of axial SpA. A highly specific and sensitive gold standard for axial SpA is lacking (a positive MRI finding can definitely not serve as a gold standard) and the diagnosis of axial SpA relies on skillful pattern recognition rather than on diagnostic imaging. Obviously, the ASAS criteria for axial SpA are not meant to differentiate subjects with no back pain from patients with axial SpA in clinical practice. But the purpose of this study was not to validate the ASAS criteria; this study was undertaken to test the specificity of MRI of the SI joints (and not to test the diagnostic value of MRI of the SI joints). A sound starting point then is to compare 2 extreme groups. Therefore, in this study we included patients with chronic back pain who do not meet the ASAS classification criteria as a negative control group, and have contrasted them to patients with axial SpA formerly judged to have positive MRI findings who do meet the ASAS classification criteria as a positive control group.
Our finding that deep (extensive) lesions appeared almost exclusively in axial SpA controls has not been observed before and needs confirmation in another cohort of patients with axial SpA. This finding may help to create a stricter definition of a positive finding on MRI of the SI joints indicating axial SpA, so that overdiagnosis of axial SpA (and associated health care expenses) will be limited.
Our study has important strengths. The three readers who had received standardized training were completely blinded with regard to all of the clinical information and information regarding the hospital of recruitment during the scoring process. The study set was a deliberate mix of previously positive and previously negative control MRIs, derived from the SPACE cohort, in order to constrain bias by reader expectation. These controls were matched for sex and age. Most importantly, though, the experienced readers were instructed (and agreed) to only score according to ASAS guidelines, which indicates that only MRI findings that are highly suggestive of axial SpA should be considered positive, and MRIs of the SI joints with solitary white spots, which may count in the SPARCC scoring system, should be considered negative. The not more than moderate interreader agreement obtained among these experienced and trained readers testifies to the inherent difficulty of interpreting the ASAS definition that starts with “lesions highly suggestive of axial SpA.” Since patients with axial SpA have far more extensive lesions than non-SpA patients or healthy individuals, and this extent is reflected by high SPARCC scores, interreader agreements based on continuous SPARCC scores were far better. But this finding will not help to avoid misdiagnosis since a diagnosis is based on a binomial choice (positive or negative) rather than on a SPARCC score.
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