Studying patients with inflammatory back pain and arthritis of the lower limbs clinically and by magnetic resonance imaging: many, but not all patients with sacroiliitis have spondyloarthropathy.

OBJECTIVE Clinical and magnetic resonance imaging (MRI) data of 170 consecutive patients with inflammatory back pain (IBP) and/or oligoarthritis of the lower limbs were evaluated in a retrospective study. The aim was to determine the frequency of sacroiliitis and spondyloarthropathy (SpA) in this population, and to assess the significance of HLA B27 measurements for diagnosis in early disease. METHODS Pelvic X-rays were performed in all IBP patients and dynamic MRI of the sacroiliac joints in patients with IBP who had indefinite results on sacroiliac X-rays (n = 32). RESULTS European Spondyloarthropathy Study Group criteria for SpA were fulfilled by 106/170 patients (62.4%); eight additional patients had symptoms suggestive of SpA (4.7%). The most frequent SpA subset was undifferentiated SpA (uSpA), diagnosed in 46/106 patients (43.4%). Sacroiliitis was detected by MRI in 21/32 patients with IBP and unclear X-rays (65.6%). Of those, 14 were diagnosed as SpA and seven females with moderate unilateral sacroiliitis, but no features of SpA, also not on follow-up (at least 1 yr), were classified as undifferentiated sacroiliitis (US). Ten of the 14 SpA (71.4%) and none of the seven US patients were HLA B27 positive. CONCLUSION HLA B27 positivity in IBP patients with MRI-proven sacroiliitis positively predicts SpA. uSpA is a frequent SpA subset. There are HLA B27-negative non-SpA patients with moderate unilateral sacroiliitis whom we propose to be classified as US.