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 consec utive patients with inflammatory back pain (IBP) and/or oligoarthritis of t he lower limbs were evaluated in a retrospective study. The aim was to dete rmine the frequency of sacroiliitis and spondyloarthropathy (SpA) in this p opulation, and to assess the significance of HLA B27 measurements for diagn osis in early disease. Methods. Pelvic X-rays were performed in all IBP patients and dynamic MRI o f 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 ful filled 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 detecte d by MRI in 21/32 patients with IBP and unclear X-rays (65.6%). Of those, 1 4 were diagnosed as SpA and seven females with moderate unilateral sacroili itis, but no features of SpA, also not on follow-up (at least 1 yr), were c lassified 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 pro pose to be classified as US.