Why are patients with IBD more prone to develop an inflammatory arthritis

Why are patients with IBD more prone to develop an inflammatory arthritis? 

The pathogenesis of gut-joint iteropathy is unknown. However, inflammation of the gut and joints appear to be tightly linked. When ileocolonoscopies are done on spondyloarthropathy (ankylosing spondylitis, reactive arthritis) patients without GI symptoms, up to 25% have macroscopic lesions and up to 60% have microscopic evidence of asymptomatic CD. Over time, 6% to 10% of these patients develop overt symptomatic CD. Alternatively, up to 10% of IBD patients without evidence of a spondyloarthropathy at onset of their GI symptoms will develop overt arthritis on followup. 

Environmental antigens capable of inciting rheumatic disorders enter the body’s circulation by traversing the respiratory mucosa, skin, or GI mucosa. The human GI tract has an estimated surface area of 400 m (200 times the body’s skin surface area) and functions not only to absorb nutrients but also to exclude potentially harmful antigens. The gut-associated lymphoid tissue, which includes Peyer patches, the lamina propria, and intraepithelial T cells, constitutes 25% of the GI mucosa and helps to exclude entry of bacteria and other foreign antigens. Whereas the upper GI tract is normally exposed to 10 mucosa-adhering bacteria, the lower GI tract is constantly in contact with millions of bacteria (up to 10 12 /g of feces). The total number of bacterial cells called the human microbiota that we are exposed to is 10 times that of the number of cells of the body. 

Inflammation, whether from idiopathic IBD or from infection with pathogenic microorganisms, can disrupt the normal integrity and function of the bowel, leading to increased gut permeability. This increased permeability may allow nonviable bacterial antigens in the gut lumen to enter the circulation more easily. These microbial antigens could either deposit directly in the joint synovia, leading to a local inflammatory reaction, or cause a systemic immune response, resulting in immune complexes that then deposit in joints and other tissues. Genetic susceptibility is required to develop the immunologic response in the gut and joint, which results in persistent inflammation and tissue injury.

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