ischemic bowel disease

**Anatomy/Pathophysiology** - Three arteries supply the gut: celiac (foregut), SMA (midgut: distal duodenum to proximal 2/3 of transverse colon), and IMA (hindgut: distal 1/3 of transverse colon to upper rectum). - Collateral circulation: the marginal artery of Drummond and the arc of Riolan connect the SMA and IMA. The middle and inferior rectal arteries (internal iliac) supply the rectum. - Watershed areas are the most vulnerable in hypoperfusion: the splenic flexure (Griffiths point, SMA/IMA junction) and the rectosigmoid junction (Sudeck point, IMA/superior rectal junction). - Mucosa is the most metabolically active layer and is injured first. Reperfusion injury from reactive oxygen species adds to the damage. Transmural infarction leads to necrosis, perforation, peritonitis, and sepsis.

**Types** 1. *Acute mesenteric ischemia (AMI)* - **Arterial embolism (about 50%):** Source is the heart (atrial fibrillation, mural thrombus after MI, endocarditis). It lodges in the SMA distal to the origin of the middle colic artery, which spares the proximal jejunum and transverse colon. - **Arterial thrombosis (about 25%):** Occurs at the SMA origin on top of atherosclerosis. Patients often have prior postprandial pain and weight loss. - **Nonocclusive mesenteric ischemia (NOMI, about 20%):** Splanchnic vasoconstriction in low-flow states (shock, heart failure, sepsis) or with vasopressors, digoxin, or cocaine. - **Mesenteric venous thrombosis (about 10%):** Associated with hypercoagulable states (factor V Leiden, malignancy, OCPs, cirrhosis/portal hypertension, pancreatitis). Presentation is more insidious. 2. *Chronic mesenteric ischemia:* Atherosclerotic stenosis of at least 2 of the 3 mesenteric vessels. Features are postprandial abdominal pain 15–60 minutes after eating, "food fear" (sitopheobia), and weight loss. Patients are often female, older, and smokers. 3. *Ischemic colitis:* The most common form of GI ischemia, usually caused by low flow rather than occlusion. It affects the watershed areas (splenic flexure, rectosigmoid). Rectal sparing is common. Presentation is crampy left-sided abdominal pain with hematochezia, often after hypotension or aortic surgery, such as AAA repair with IMA ligation.

**Clinical Features** - Acute: sudden severe periumbilical pain **out of proportion to exam** early on. Nausea, vomiting, and diarrhea or bloody stool may occur. Later signs are peritonitis, fever, and shock. - Labs: leukocytosis, **elevated lactate** (a late and nonspecific finding), metabolic acidosis, and elevated amylase.

**Diagnosis** - **CT angiography** is the first-line test for suspected AMI. It shows arterial or venous filling defects, bowel wall thickening, pneumatosis intestinalis, and portal venous gas. - Conventional angiography is the gold standard. It is also therapeutic, allowing vasodilator infusion (papaverine) in NOMI. - Ischemic colitis: colonoscopy shows a segmental pale or cyanotic mucosa with a line of demarcation. CT may show "thumbprinting." Barium enema was historically described as showing thumbprinting. - Chronic: duplex ultrasound, CTA, or MRA. - AXR is usually nonspecific. Avoid delaying imaging or surgery in unstable patients.

**Management** - Resuscitate, give broad-spectrum antibiotics, and start IV heparin for embolic, thrombotic, and venous causes. - Embolic or thrombotic AMI: urgent revascularization (embolectomy, bypass, or endovascular stenting) and resection of necrotic bowel. Perform a second-look laparotomy as needed. - NOMI: treat the underlying cause, stop vasoconstrictors, and give intra-arterial vasodilators. - Venous thrombosis: anticoagulation (surgery only if there is infarction). - Chronic: angioplasty with stenting or surgical bypass. - Ischemic colitis: usually supportive (bowel rest, IV fluids, antibiotics). Surgery is reserved for gangrene, perforation, or peritonitis.

**Complications:** bowel necrosis, perforation, sepsis, short bowel syndrome, and stricture formation after ischemic colitis.

**High-yield Associations** - Older patient with atrial fibrillation and sudden severe abdominal pain → SMA embolus. - Postprandial pain with weight loss → chronic mesenteric ischemia. - Cirrhosis or hypercoagulability with abdominal pain → mesenteric venous thrombosis. - Pathology: coagulative necrosis with a hemorrhagic infarct. Histology of ischemic colitis shows "ghost" crypts and a hyalinized lamina propria. - Differential: diverticulitis, IBD, appendicitis, SBO, and pancreatitis.