Histologic evaluation of disease activity in inflammatory bowel disease

Abbreviations

Introduction

Histologic assessment of biopsy and resection specimens of the small and large intestine has always been an integral part of the diagnosis and management of idiopathic inflammatory bowel disease (IBD). Most IBD cases can be classified into two major subtypes: ulcerative colitis (UC) and Crohn’s disease (CD). UC is a chronic inflammatory disease of the colon, while CD is an inflammatory disease with a propensity to involve the gastrointestinal (GI) tract . From a histopathologic standpoint, despite many shared characteristics, each subtype has unique macroscopic (gross) and microscopic (histologic) features that can help distinguish one from another.

Macroscopic features of UC and CD and guidelines of standard endoscopy sampling with adequate specimens for histopathology evaluation will be discussed first in this chapter. This is followed by discussions of histologic features of each entity and histologic grading and scoring systems that have been used in clinical trials of UC and CD. The remainder of the chapter is focused on histologic mucosal healing in IBD.

Macroscopic features of ulcerative colitis and Crohn’s disease

UC classically starts in the rectum and can extend proximally to involve other segments or even the entire colon. The extent of involvement is determined by the severity of the disease. Small intestine or upper GI tract is usually spared in UC. Inflammation in UC is contiguous and homogenous in the affected segment of the colon without any uninflamed areas. Further, inflammation in UC is superficial and predominantly confined to the mucosa muscularis mucosae, or superficial submucosa. Ulcers in UC are shallow and typically do not involve the muscle wall. The deep layers of the bowel wall and mesentery are intact in UC ( Fig. 15.1A and Table 15.1 ).

In contrast, CD can affect any part of the GI tract, though the small intestine and proximal colon being the most often-affected segments. Inflammation in CD is segmental and discontinuous. Inflamed areas are intermixed with uninflamed areas, resulting in an appearance of “skip lesions.” In addition, inflammation in CD is transmural and, therefore, can cause injuries to the muscle wall, serosa, or mesentery. Deep-penetrating ulcers, fissures, fistula tracts, strictures, and fat wrapping along the antimesenteric border are hallmark features of CD ( Fig. 15.1B and Table 15.1 ).