What is the most common form of colitis in an outpatient older than 50 years?
The most common form of colitis in an outpatient older than 50 years is ischemic colitis. The most common causes of ischemic colitis are low-flow states, such as congestive heart failure, shock, or dysrhythmia. Occlusion of large vessels owing to atherosclerosis or embolism infrequently causes colonic ischemia because large collateral arterial arcs (arc of Riolan, marginal artery of Drummond) can compensate for central vascular occlusion.
Ischemia may occur in any part of the colon; however, it commonly involves the distal transverse colon and descending colon because these regions are the sites of transition between the blood supply from the superior mesenteric and inferior mesenteric arteries. The splenic flexure region is known as the “watershed” area of the colonic arterial supply.