The term inflammatory bowel disease (IBD) refers to Crohn’s disease and ulcerative colitis. Both these diseases are important to pathologists, since they can only be diagnosed by macro- and microscopic examination of the bowel. The main aim of this post is to compare the basic pathological mechanisms of these diseases, in order to explain their distinct appearances.
Crohn’s can affect any part of the GI tract from mouth to anus, but most commonly affects the small or large bowel. The most common site of involvement is the terminal ileum, a fact which is significant for a couple of reasons. Firstly, it means that Crohn’s can initially present with pain in the right iliac fossa, in a manner that can be confused with appendicitis. Secondly, the terminal ileum is the only site in the body where vitamin B12 is absorbed – this means that Crohn’s patients who have resections in this area need to have B12 replacement, or they’ll eventually develop symptoms of B12 deficiency (megaloblastic anaemia and subacute combined degeneration of the spinal cord). Crohn’s affects the bowel in a segmental rather than continuous manner, meaning that instead of spreading predictably round the bowel, it affects intermittent areas of bowel. This means that unlike UC, large surgical resections aren’t always curative: the disease can just pop up somewhere else.
Microscopically, the hallmark of Crohn’s (and the key phrase to learn for exams) is transmural granulomatous inflammation. Inflammation in the intestinal crypts spreads deep into the bowel wall, which becomes very friable and develops deep cracks in the wall known as fissures. When looking at a surgical resection for severe Crohn’s, the deep fissures between islands of normal mucosa give a “cobblestone” appearance to the bowel. As the fissures extend through the bowel serosa, they can form fistulae (i.e. abnormal connections) between the bowel and other structures – skin, other bowel loops, the bladder or the vagina for example. Chronic inflammation of the bowel wall also leads to fibrosis, and stricture formation, which leads to permanent narrowing of the lumen and bowel obstruction.
Unlike Crohn’s, UC spreads in a very predictable way: it starts by involving the rectum, and spreads proximally through the large bowel in a continuous fashion before stopping at the terminal ileum. Occasionally a bit of inflammation may spread beyond this into the terminal ileum (a phenomenon called ‘backwash ileitis’).
Microscopically, UC doesn’t involve the full thickness of the bowel wall. The mucosa becomes very inflamed and friable, and ulcerated; when irritated by the passing stream of faeces, this explains why PR bleeding is more common in UC than Crohn’s. Inflammation in the mucosal crypts of the bowel results in the formation of prominent crypt abscesses, which are collections of neutrophils, and ongoing mucosal repair results in islands of regenerating mucosa known as pseudopolyps. Erosion of the bowel mucosa exposes the underlying nervous plexuses to faecal irritation, where impairment neuromuscular function can result in the complication of toxic megacolon.
Here’s a summary of everything so far:
Why is it important to get the diagnosis right?
Both of these diseases are usually diagnosed from biopsy specimens taken at colonoscopy. Whilst the features described above make the distinction sound clear-cut, in real life it’s actually not always easy to tell which is which. The pathologist can encounter the following problems:
- Tissue from biopsies is often suboptimal – only mucosa might be present, meaning it’s impossible to assess for full-thickness inflammation
- Granulomas, a key feature of Crohn’s, are only visible on 10% of Crohn’s biopsies (but 90% of bowel resection specimens)
- Prior medical treatment can cause the microscopic changes in UC to be patchy, mimicking Crohn’s
We can usually tell IBD from infective colitis (which has a lack of chronic changes on the biopsy), and the treatment for both types of IBD is broadly similar – so are we just splitting hairs by trying to call it UC or Crohn’s? The rationale comes with the surgical management of IBD. Patients with severe disease who need the whole of their colon surgically removed have two options: an end ileostomy or an ileo-anal pouch. A pouch creates an artificial rectum, meaning the patient doesn’t have to have a stoma. The pouch has a high failure rate in Crohn’s, but is much more successful in UC: making the correct diagnosis allows the most appropriate surgery to be planned for the individual patient.
Sample examination questions:
1. Features more common in Crohn’s disease than ulcerative colitis include:
A. Segmental small and large bowel inflammation
B. Rectal involvement
C. Transmural granulomatous inflammation
E. Pseudopolyp formation
2. Complications of ulcerative colitis include:
A. Increased risk of colorectal carcinoma
B. Sclerosing cholangitis
C. Primary biliary cirrhosis
E. Toxic megacolon
(Answers: 1 = A, C, D; 2 = A only)