Idiopathic chronic inflammatory bowel diseases show primary inflammation of the intestinal wall

The idiopathic inflammatory bowel diseases, of which there are two main members, have no known cause.
In Crohn's disease there is a granulomatous inflammatory pattern of disease that affects the full thickness of the bowel wall.
It is most common in the terminal ileum, but may affect any part of the gastrointestinal tract in a discontinuous pattern.
With ulcerative colitis, a chronic inflammatory disease of the rectal mucosa, inflammation may extend to involve the whole of the colon in continuity.
Importantly, both types of inflammatory bowel disease are associated with systemic manifestations outside of the intestine.

In diagnosing idiopathic inflammatory bowel disease, infective causes of inflammation have first to be excluded.
Investigation is by imaging and biopsy.

Crohn's disease

Crohn's disease is more common in women than in men, patients usually being 20-60 years old.
It particularly affects the terminal ileum (synonym: regional ileitis), but can occur anywhere in the gut, especially in the mouth, colon, and anus.

The macroscopic appearance of the bowel in Crohn's disease varies according to the stage of the disease.

in early disease affected bowel shows marked swelling of submucosa and mucosa, mainly due to severe submucosal oedema.
This leads to loss of the pattern of normal transverse folds, and small superficial areas of haemorrhagic ulceration arise which,
over time, develop into fissures.

In established chronic disease the bowel mucosa shows a cobblestone pattern due to a combination of submucosal oedema and interconnecting deep fissured ulcers.
The bowel wall is thickened by oedema and fibrosis and, commonly, there is stricture formation. Regional lymph nodes usually become enlarged.
Disease is not continuous and areas of normal bowel may be present between the diseased segments (skip lesions).The normal bowel proximal to a segment of Crohn's disease is often dilated due to partial obstruction.

Direct complications of Crohn's disease

Crohn's disease shows transmural inflammation with deep fissured ulcers Crohn's disease is histologically characterized by inflammation of all layers, submucosal oedema, ulcers that extend deepinto the bowel wall and form fissures, and fibrous scarring.
Non-caseating granulomas may be present.

Inflammation in Crohn's disease is transmural, and serosal involvement leads to inflammatory adhesion to other loops of bowel, to the parietal peritoneum of the anterior abdominal wall, or to the bladder.
Deep penetration of fissured ulcers, which may extend through the full thickness of the bowel wall into the adherent viscus, causes fistulae (tracks between two cavities) and sinuses (a track from a viscus to an outside surface).
This is particularly seen in the perianal region.

The inflammation in the wall is composed of lymphoid cells, macrophages and plasma cells.
Small non-caseating granulomas, which occur in about 70% of cases, may be seen in any layer of the bowel, e.g. mucosa.

Ulcerative colitis

Ulcerative colitis affects the rectum and variable amounts of colon

Ulcerative colitis starts at the rectum (proctitis) and may extend for a variable distance around the colon.

There are three clinical patterns of disease:

1 In active acute disease the mucosa in the rectum and affected colon shows areas of shallow ulceration; inflammation is limited to the mucosa and lamina propria.

2 In chronic quiescen or treated disease ulceration is not prominent and the mucosa appears red, granular and thinned.

3 In fulminant active disease the colon shows extensive confluent mucosal ulceration.The colon progressively dilates (toxic dilatation - 'acute toxic megacolon').

The direct local complications of ulcerative colitis include blood and fluid loss from extensive ulceration. Acute disease may progress
rapidly to toxic dilatation and perforation and, in long-standing disease, dysplasia and neoplastic change may occur.

In the most extensive disease the whole colonic mucosa is affected.
Patients typically develop diarrhoea, the faeces being mixed with blood, mucus and pus. The natural history of ulcerative colitis  can be divided into three main patterns:

 • 10% of patients develop severe disease requiring early surgery.

 • 10% of patients have persistent active disease despite treatment.

 • 80% of patients have chronic quiescent colitis with infrequent episodes of relapse.