P183 Osteitis condensans ilii or axial spondyloarthritis: a diagnostic challenge!

Background/Aims Osteitis condensans ilii (OCI) is a rare cause of chronic non-inflammatory axial lower back pain. It has become an important differential diagnosis of axial spondyloarthritis. Bone marrow oedema can be present in 48% of patients. Although 'aunt minnie' appearance on MRI is pathognomonic, specialised imaging like SPECT/PET CT can be considered in difficult cases to confirm the diagnosis. We reviewed case notes of patients with incidental diagnosis of OCI on imaging and followed their clinical progression to see if the diagnosis was modified later. Methods Retrospective radiological database analysis was performed to find imaging reported as OCI or osteitis from 2015-2020. Electronic patient records were analyzed and data was compiled through Microsoft Excel. Results Data from 24 patients was reviewed. Most patients were females (91.7%), with a median age of 43 years (range 19-79 years). CT and MRI were the chief modalities of imaging (46% and 38% respectively) while 20% had plain x-rays of the pelvis. The main indications for imaging were pelvic or hip girdle pain (58.3%), cancer surveillance (25%), abdominal pain (6%), and infection (6%). 20% had an underlying rheumatological diagnosis of which 8% had spondyloarthritis. 12.5% of patients had seronegative features of spondyloarthritis and 8.3% were HLA B27 positive. Approximately 40% of patients were seen by the Rheumatology team for further assessment after imaging and 12.5% of patients were diagnosed with axial spondyloarthritis and remained under follow-up. Conclusion OCI can mimic sacroiliitis on imaging. Although only a minority of patients with OCI reported on initial imaging were subsequently diagnosed with axial spondyloarthritis, thorough clinical assessment and multidisciplinary discussion can reduce the chance of a missed axial spondyloarthritis diagnosis. Further large-scale studies are required to find the exact incidence of spondyloarthritis in patients with imaging features of OCI. Disclosure S. Saeed: None. M. Jamal: None. D. Ostry: None. G. Robinson: None. R. Sengupta: None.

Clinical and imaging characteristics of osteitis condensans ilii as compared with axial spondyloarthritis

Objectives Osteitis condensans ilii (OCI) has become an important differential diagnosis for axial spondyloarthritis (axSpA). The objective of this matched case–control study was to investigate demographic, clinical, laboratory and MRI characteristics of OCI as compared with axial spondyloarthritis (axSpA). Methods A total of 60 patients diagnosed with OCI were included in the final analysis. From 27 of these patients, MRIs of the sacroiliac joints were available. OCI patients were matched with a 1:1 ratio by back pain duration to patients with definite axSpA in order to compare clinical, laboratory and MRI characteristics. Results The OCI patients were nearly all females (96.7 vs 46.7%), had a significantly lower prevalence of inflammatory back pain (39.5 vs 88.9%), a significantly lower percentage of HLA-B27 positives (35.2 vs 80.0%) and a lower prevalence of the majority of other SpA features as compared with axSpA patients. Interestingly, there was no difference in the prevalence of osteitis in the sacroiliac joints (92.6 vs 85.2% in OCI and axSpA, respectively, P = 0.44), but there was a difference in the prevalence of erosions (7.4 vs 66.7%, respectively, P = 0.0001). In addition, in OCI nearly all lesions were localized in the anterior part of the sacroiliac joints while in axSpA lesions were localized predominantly in the middle part of the joint (for osteitis: 96 vs 4% in OCI and 28.6 vs 71.4% in axSpA; P = 0.0002 for the inter-group difference). Conclusion Clinical and imaging features of OCI compared with axSpA are described that should help in differential diagnosis.