Ischaemic Colitis

The vascular supply to the bowel is plentiful and there are extensive connections along the lumen at the edge of the mesentery, as well as the radial supplies from the branches of mesenteric vessels. Those areas where there is a "watershed" of supply between major arteries seem to be the most liable to the consequences of poor perfusion. In the colon the commonest site for ischaemia is at the splenic flexure, between superior and inferior mesenteric supply.

Interference with bowel perfusion can be one or more of three processes:

Pathologies that narrow or obstruct the lumen of major or minor vessels; from vasculitis through to collagen disease, embolus and thrombosis.

Mechanical factors that compress or distort supplying vessels, including; Intussusception, volvulus, herniae or trauma.

Processes that lower effective perfusion pressure as in cardiac failure. A temporary ileus may be seen after myocardial infarct or cardiac surgery.

Inflammatory bowel pathology is often seen as an unusually large length of undilated gas-filled bowel, which implies a local lack of peristaltic activity. Experienced Radiologists will always point out that without an erect view, intraluminal fluid may obscure the true diameter of small-bowel in the supine view (see more detail in Ileus document). The colon is less likely to accumulate fluid; it is nearer the point of excretion and one of its normal functions is to resorb fluid from the luminal contents. Poor bowel perfusion will affect protective mechanisms against proteases in the bowel lumen or against bacterial invasion. The earliest sign is an ileus, perhaps with thickening of the bowel wall from oedema and haemorrage. Gas in the bowel wall from the lumen or gas generation in the bowel wall are seen later as the pathology progresses.

Antibiotics, that alter the bowel flora, may contribute to this process as in pseudomembranous colitis.

Bowel dilatation can increase to a point where the tension and stretching of the wall may impair its blood supply. Even if the pressure inside remains the same, the geometry means that an increase in radius 'r' will mean an increase in the tension 't' in the wall, distorting the small intra-mural vessels, to potentially reduce perfusion. The point at which normal colon may perforate is about 10 cm. diameter. Any pathology that weakens the bowel wall will increase the risk of perforation by lowering the threshold diameter at which perforation may occur. To restate, at any constant pressure, wall tension increases with the bowel diameter. The crucial point is that abnormal bowel with reduced perfusion may perforate earlier.