Page 12 - Recognizing axial spondyloarthritis - Janneke de Winter

CHAPTER ONE
radiograph) plus ≥ 1 SpA feature. Patients with sacroiliitis on plain radiograph according to the modified New York (mNY) criteria are defined as radiographic axial SpA, patients fulfilling the clinical arm or having sacroiliitis on MRI are defined as non-radiographic axial SpA.
B The ASAS criteria for peripheral SpA. Patients are classified as peripheral SpA in case of peripheral arthritis and/or enthesitis and/or dactylitis, plus at least one or two other SpA features, depending on the nature of the feature.
Diagnostic delay
Classifying axial SpA according to its phenotypic presentation might justify the similarities of the different subtypes, it doesn’t necessarily facilitate an early diagnosis. Back pain, frequently the presenting symptom of axial SpA, and even inflammatory back pain, is very frequent in the general population. Early studies suggested that only 5% of patients presenting with back pain has some form of axial SpA (18). Although enhanced referral techniques (19–21) and the use of MRI to show active SI joint inflammation shortened the time to diagnosis in the recent years, the diagnostic delay is still estimated to be 5 to 11 years (22–25). Shortening this delay is of major importance for two reasons. Firstly, axial SpA has a significant burden of disease, even in its initial phase (26). Secondly, treating axial SpA in an early phase might modify the disease course and limit structural damage (27,28), although there is also evidence suggesting that structural damage Is not influenced by early treatment.
The identification of biomarkers might shorten the current diagnostic delay. In 2001, the National Institutes of Health defined a biomarker as ‘a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention’ (29). Initiatives to study and diagnose axial SpA in an early phase as the SPondyloArthritis Caught Early (SPACE) cohort (30), the early axial SpA cohort DESIR (31) and the early AS and axial SpA cohort GESPIC (32) will hopefully lead to the identification of robust biomarkers, capable to distinguish between axial SpA and resembling diseases. Such biomarkers will most likely be derived from serum, genetic and imaging characteristics.
Serum biomarkers
Despite numerous attempts to show additional value, studies failed to prove the existence of robust serum biomarkers yielding enough sensitivity and specificity to ease diagnosing axial SpA (33). Recent studies provided preliminary evidence that anti-CD74 antibodies are elevated in axial SpA (34,35), results that were by our knowledge not confirmed in a representative diagnostic setting.
Genetic biomarkers
The common origin of SpA phenotypes, particularly of axial SpA, is suggested by its shared genetic background. Axial SpA is considered an oligogenetic disease.
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