Magnetic resonance imaging (MRI) is increasingly used to detect sacroiliitis earlier. This study was undertaken to investigate what proportion of patients with MRI-evident sacroiliitis develop ankylosing spondylitis (AS) in the long term and whether there are predictors of outcome.
Consecutive undiagnosed patients with early inflammatory back pain (IBP) (of <2 years' duration) were assessed clinically and radiologically. Baseline imaging assessments included fat-suppressed MRI sequences of the sacroiliac joints and lumbar spine that were scored for active bone marrow edema representative of acute inflammation, and anteroposterior radiographs of the pelvis and lateral radiographs of the lumbar spine, which were scored using the Stoke Ankylosing Spondylitis Spine Score. Patients were reassessed clinically and radiographically after 8 years. The primary outcome was the modified New York criteria for AS at followup.
Fifty patients were assessed at the beginning of the study, and 40 patients were followed up after a mean of 7.7 years. Of these 40 patients, 58% were HLA–B27 positive, and 98% met the European Spondylarthropathy Study Group criteria. At baseline, 33 (83%) of the 40 patients followed up had MRI-evident sacroiliitis, and 6 (12%) had unequivocal AS according to the modified New York criteria. At followup, despite significant improvements in clinical outcomes, 13 of 39 patients (33.3%) had AS according to the modified New York criteria. The combination of severe sacroiliitis seen on MRI with HLA–B27 positivity was an excellent predictor of future AS (likelihood ratio [LR] 8.0, specificity 92%), while mild or no sacroiliitis, regardless of HLA–B27 status, was a predictor of not having AS (LR 0.4, specificity 38%).
Our findings indicate that in patients with early IBP, a combination of severe sacroiliitis and HLA–B27 positivity has a high specificity for development of AS, compared with mild or no sacroiliitis, regardless of HLA–B27 status, which confers a low likelihood of developing AS. This has implications for the diagnosis of “early” AS and possibly for selection of more aggressive therapies.