Extraintestinal considerations in inflammatory bowel disease.
If one reviews the literature with zeal, it is increasingly apparent that few organs escape recruitment when IBD is chronic or progressive. Insights into mucosal pathophysiology have helped with understanding the more frequent extraintestinal manifestations, but the mechanisms attendant to the development of less common events (e.g. acute pancreatitis, concurrent gluten sensitive enteropathy, or active pulmonary disease) remain either poorly studied or obscure. It is particularly interesting, however, to read reports of abnormal pulmonary function, generally of the obstructive type, correlated to measurements of abnormal intestinal permeability in patients with either active pulmonary sarcoid or pulmonary involvement in Crohn's disease. It has been further speculated that similarities in the mucosal immune system of the lung and intestine are responsible for evidence of bronchial hyperreactivity in patients with active IBD. Finally, it is important to recognize that extensions of the inflammatory process are not restricted to the development of organ-based events but may be responsible for some of the most frequent systemic abnormalities detected in IBD patients. It is now also well confirmed that the cytokine environment in IBD can support activated coagulation and, in some clinical situations, overt vascular thrombosis. The cerebrovascular complications of IBD are well recognized and range from peripheral venous thrombosis to central stroke syndromes and pseudotumor cerebri. Reports of focal white matter lesions in the brains of patients with IBD or an increased incidence of polyneuropathy may be other clinical examples of regional microvascular clotting. Microvascular injury appears to be more ubiquitously present, with reports ranging from a speculated primary causative role (e.g., granulomatous vasculitis in the mesenteric circulation) to the utility of nailbed vasospasm, in Crohn's disease, as a clinical marker for disease activity. It is also reported that IL-6 suppression of erythropoietin production is a major feature of the chronic anemia seen in active IBD. Moreover, the capacity of peripheral monocytes from active IBD patients to secrete TNF and IL-8 is reported predictive for the degree of therapeutic response from recombinant erythropoietin. These collected observations constitute another excellent example of the symmetry between basic science and clinical utility. It is from the context of applied basic science that many future therapies will arise. Empiricism will lose much of its appeal as clinical observations will be increasingly translated into cellular language. Already in animal models, elemental diets diminish IL-6-related acute inflammatory injury, and reductions in dietary lipid alter the antigenicity of bacteria. Provocatively, in humans, unconfirmed reports have even associated diet therapy with the resolution of uveitis and pyoderma gangrenosum. It is likely that efforts will also be made to induce oral tolerance if specific triggering proteins are discovered or to alter bowel flora if such an arcane area of investigation becomes resurgent.