My initial index of suspicion was Axial SpA, with first work up being X-ray to establish any extent of Sacro-illitis alongside bloods to facilitate a decision and refer into our Rheumatology team. Bloods came back with raised ESR, CRP and a negative HLBA-27.
OCI presents with a variety of signs and symptoms of low back and hip symptoms making the diagnosis difficult and has been reported as being easily confused for Inflammatory Sacroiliitis and Metastatic Bone Lesions as well as mimicing symptoms and image findings of SpA (Mitra, 2010). OCI is a benign cause of recurrent axial low back pain and is an incidental finding on plain X-ray, characterised by sclerosis of predominantly the iliac bone adjacent to an otherwise normal Sacro-Iliac Joint (SIJ). It has an average frequency of 0.9–2.5 %, and particularly affects women during the fertile decades with normal inflammatory parameters, negative HLA-B27 and no bone destruction or erosion. The radiological and laboratory criteria in addition to the history and other clinical findings can differentiate it from similar conditions affecting the SIJ, such as Ankylosing Spondylitis, Seronegative Spondyloarthropathies, Sacroilitis, Primary Hyperparathyroidism, Paget’s disease, Renal Osteodystrophy, Lymphoma and Bone Secondaries. Treatments for the condition are primarily conservative with surgical resection being reserved for refractory cases (Mitra 2010, Jenks, 2012, Cidem et al, 2012 ; Shipp & Haggart, 1950;Thompson et al, 1957; Rodríguez-Pla et al, 2004).
The pathophysiology of OCI has been described as unknown and the research is vague. It has been postulated that the gravid uterus may compress the abdominal aorta and cause ischemia in the inferior portion of the ilium. Other studies have suggested that the mechanical stress of pregnancy itself may overload the sacroiliac articulation resulting in OCI (Hare & Haggart, 1945; Thompson,1954). Gillespie and Lloyd-Roberts (1953) describe a series of cases, all occurring in females who had borne children and they report the probable pathogenesis as an obliterative endarteritis that follows parturition. Obliterating endarteritis is severe proliferating endarteritis or inflammation of the intima that results in an occlusion of the lumen of the artery. It can occur due to a variety of medical conditions such as a complication of radiation poisoning, TB meningitis or a syphilis infection (Song et al, 2005, Ghosh & Amit K, 2008; Gillespie & Lloyd-Roberts, 1953).
The AP plain radiograph shows dense triangular sclerosis on the iliac side of both sacro-iliac joints. Another thought in the ￼vascular hypothesis is the involvement of the nutrient artery supplying the affected region of the ilium, which has been suggested to lead to ischaemic changes in the bone. The constant position of a nutrient artery in the inferior juxta-articular region of the ilium could account for the characteristic situation of the lesion (Rendich and Shapiro, 1936). There have also been reports of cases in which there was evidence of a chronic urinary infection or a history of Pyelitis. Consideration that the renal and ureteric infection could spread to the nutrient foramina in the iliac bone leading to ischaemia has been a factor of the work of Szabados (1947) and according to their findings, ischaemic iliac bone necrosis is the main contributing hypothesis for OCI, in a way similar to that of early hip avascular necrosis.
In summary, it was an incidental finding with a lack of consensus on pathophysiology and limited research, however we do come across it so worth knowing a bit more so we can qualify our thoughts and index of suspicion. This particular patient is awaiting full spinal MRI as part of the work up in relation to any underlying pathology.
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