A primer on IBD: Phenotypes, diagnosis, treatment, and clinical challenges

Inflammatory bowel disease (IBD) is a chronic inflammatory condition of the gastrointestinal tract, most commonly divided into ulcerative colitis (UC) and Crohn's disease. We have seen an increase in incidence in IBD over the last few decades worldwide. UC and Crohn's disease have strong genetic underpinnings which have been steadily elucidated over the past 20 years. Additionally, there are number of environmental factors that have been recognized as triggers of disease, including dietary/microbiome and smoking. In this chapter, we lay out the primary phenotypes observed in Crohn's and UC. We next discuss the approach to diagnosis, which is generally multifactorial, including blood and stool testing, abdominal imaging, and colonoscopy with biopsy. Next, we summarize the treatment algorithms for both disease, including the pre- and post-biologic era. Greater concentration is given to the discussion of the anti-tumor necrosis factor (TNF) alpha, which to date has been the greatest game changer in IBD management. We also discuss the newer pharmacologic mechanisms of targeting the disease including lymphocyte adhesion blockers, anti-IL 12/23 inhibitors, and drugs that target the JAK/STAT pathway. Some of the newer agents in the pipeline are also briefly discussed. In the final section, we explore clinical correlates of the genetic findings to date. We delve into some of the key findings including the discovery of the NOD2 risk gene, as well as the most up-to-date genome wide association studies (GWAS) findings. Through this we explore how these genetic findings correlate with specific disease phenotypes, and how the findings have helped choose targets for pharmacologic therapy.