High frequency of inflammatory back pain and other features of spondyloarthritis in patients with rheumatoid arthritis.

The aim of this study was to investigate the frequency of patients with rheumatoid arthritis (RA) who have inflammatory back pain (IBP) and meet the existing classification criteria for ankylosing spondylitis (AS) and spondyloarthritis (SpA). We included 167 patients fulfilling the ACR 1987 revised criteria for RA. After obtaining a medical history and performing a physical examination, standard pelvic X-rays for examination of the sacroiliac joints (SIJ) were ordered in all patients. A computed tomography (CT) or magnetic resonance imaging (MRI) of SIJ was performed in patients with suspected radiographic sacroiliitis and MRI of SIJ in those who have IBP but no radiographic sacroiliitis. IBP was defined according to both Calin and experts' criteria. The modified New York (mNY) criteria were used to classify AS, both ESSG and Amor criteria for SpA and ASAS classification criteria for axial SpA. There were 135 female and 32 male patients with a mean age of 54.8 years. The mean disease duration was 9.8 years. RF was positive in 128 patients (79.2 %) and anti-CCP in 120 patients (81.1 %). Twenty-eight patients with RA (16.8 %) had IBP (Calin criteria), and four (2.4 %) had radiographic sacroiliitis of bilateral grade 3. Three patients (1.8 %) fulfilled the mNY criteria for AS, 31 (18.6 %) ESSG and 26 (15.6 %) Amor criteria for SpA. Nine patients (five with MRI sacroiliitis) (5.3 %) were classified as having axial SpA according to new ASAS classification criteria. This study suggests that the prevalence of SpA features in patients with RA may be much higher than expected.