Pathology of inflammatory bowel disease: a critical appraisal in diagnosis and management

The anatomic pathology of inflammatory bowel disease (IBD) is necessarily studied and continually evaluated for better understanding of its scope and meaningful classification. The reasoning behind classification is to enable grouping of ailments for reliable treatment and prognostication. The two prominent IBD are traditionally subclassified as ulcerative colitis (UC) and Crohn's disease (CD). They are separated from most other IBD by two features: (1) their idiopathic nature and (2) their chronicity. Superb descriptions of their anatomic manifestations abound in literature and their salient features can be found in selected articles and specialty texts [1-12]. Despite the apparent redundancy, some of this chapter will be devoted to the traditional descriptions. During much of the past century many inflammatory diseases of the alimentary tract have been subclassified and to a considerable degree great insight into pathogenesis has been gained [13-18]. This is particularly true for infectious gastroenterocolitides, food intolerance, and to some extent iatrogenic injury (such as radiation or drug effects). What remains are a number of idiopathic inflammatory diseases, the dominant ones being UC and CD. It is the responsibility of our generation and future ones to continue the progression and place our new understanding of IBD in the context of modern understanding of biology and medicine. With this in mind, even the traditional practice of gross examination and microscopic study can contribute to 21st-century understanding of IBD. The anatomic pathologic findings in IBD can be separated into those estabUshed traditional findings, and those that are more modern based on relatively recent descriptions. Interestingly, some of the modern anatomic pathological interpretations may have their root in our modern treatment and diagnostic techniques which are unmasking previously under-recognized elements of IBD. Of particular interest in this chapter are more recent anatomic descriptions regarding (1) patchy rather than diffuse changes in UC, (2) upper gastrointestinal CD, and (3) epithelial neoplasia in IBD. The general aspects of the anatomic pathology of UC and CD are reviewed here, and the histopathologic assessment is covered in detail in Chapter 37. Classification of disease is always a challenge in medicine, and IBD are no different. For this chapter the strategy for classification separates inflammatory diseases with a known etiology from idiopathic IBD. It also nearly excludes diseases in which the bowel is secondarily involved by systemic diseases (such as Behcet syndrome). However, even within IBD one can subclassify the diseases based on fulminant course, extent of colitis, and indeterminate clinicopathologic disease. Thus, it is important to preserve some large diagnostic groups for the purpose of rapid understanding and communication, and for this reason we stress separating IBD into the two major groups: UC and CD. Smaller concerns addressing ulcerative proctitis or indeterminate colitis are less strongly treated. There are ample anatomic pathologic features to separate UC and CD, but also some unifying principles. Fortunately, a good foundation of pathologic principles helps to understand the medically relevant aspects of IBD. All of IBD has, at its pathophysiologic root, chronic inflammation with activity. Long-standing inflammation has the ability to injure the bystander organ, often beyond the ability for repair and normal function. Like any chronic inflammatory process, the longterm consequences are parenchymal damage, atrophy, fibrosis, and loss of function to the injured anatomic compartment. Like so many other chronic inflammatory processes in the gastrointestinal (GI) tract there is an increased risk of malignancy. Thus, the pathophysiologic sequelae of UC and CD are in parallel with other organs of the GI system which suffer chronic inflammatory injury; the examples