Author(s): Michael Tadros 1 , Shounak Majumder 2 , John W Birk [*] 3
colitis; diarrhea; ischemia; rectal bleeding; vascular disease
Ischemic colitis is the most common form of intestinal ischemia, accounting for 75% of all cases, most commonly in patients over 60 years of age [1,2] . Recent studies calculated the incidence to be 15.6/100,000 person-years  . Ischemic colitis accounts for 1-3% of US hospital admissions  annually. However, our current incidence and prevalence data may be misleading as mild cases often go unnoticed or misdiagnosed.
Anatomic considerations & pathogenesis
Two main arteries supply the colon: i) the superior mesenteric artery which has three colonic branches (ileocolic, right colic and middle colic artery) and ii) the inferior mesenteric artery which also has three colonic branches (left colic, sigmoid and superior rectal artery). Two risk areas for ischemia are the terminal end of the right ileo-colic artery, which is the farthest from the origin of the superior mesenteric artery, and the marginal artery of Drummond which runs along the colon wall at the splenic flexure and allows for communication between the two main arteries. This splenic flexure area with its anastomosis is commonly called 'the watershed area' and is particularly prone to ischemia. The lower rectum receives an additional blood supply from the internal iliac artery and thus is least susceptible to ischemia  .
Ischemic colitis is caused by transient loss of blood supply to a segment of the colon. The majority of cases of ischemic colitis are caused by non-occlusive injury to the small blood vessels supplying the colon without a clear precipitating factor; this is in contrast to mesenteric ischemia which is frequently caused by an embolus or thrombus [6−10] . In fact many physicians get the two entities confused. In ischemic colitis the non-occlusive event causes a local hypoperfusion injury to the colon which is usually reversible.
As described above, ischemic colitis is most commonly caused by unidentified non-occlusive insult to the small vessels supplying the colon. However, specific causes should be considered  . First, the clinician is reminded that under special circumstances ischemic colitis can affect younger patients  . In younger patients a precipitating cause should be entertained such as drugs, long-distance running  or an underlying hypercoagulable state  . In some cases, a work-up to exclude underlying hypercoagulable state is warranted  . The hypercoagulable work-up may be necessary in special situations such as recurrent ischemic colitis, young adults and prior history of other thrombotic events. Factor V Leiden mutation is the most common hypercoagulable state that causes ischemic colitis  . Drugs should always be evaluated in the assessment of ischemic colitis [11,16,17] . Clinicians ought to review medication of all patients. (Table 1) includes a list of the medications commonly associated with ischemic colitis along with their presumed mechanism by which they cause colonic ischemia. Non-steroidal anti-inflammatory drugs, medications that slow colonic transit, vasoconstrictor drugs, diuretics, oral contraceptives and laxatives are some of the common drug classes that should be screened for in patients with ischemic colitis. Alosetron,...