Clinical meaning
Ischemic colitis is the most common form of gastrointestinal ischemia, occurring when blood flow to the colon is transiently reduced to a level insufficient to maintain cellular metabolic function, resulting in mucosal injury that ranges from reversible mucosal ischemia to transmural necrosis and gangrene. The colon receives its blood supply from branches of the superior mesenteric artery (SMA), which supplies the right colon and proximal two-thirds of the transverse colon, and the inferior mesenteric artery (IMA), which supplies the distal transverse colon, descending colon, sigmoid colon, and upper rectum. The watershed areas -- regions located at the junction of two arterial territories where collateral flow is weakest -- are the most vulnerable to ischemic injury. The two most clinically significant watershed areas are the splenic flexure (Griffiths point, at the junction of the SMA and IMA territories) and the rectosigmoid junction (Sudeck point, at the junction of the IMA and hypogastric artery territories). The rectum is typically spared because it has dual blood supply from the IMA and the internal iliac (hypogastric) arteries. The pathophysiology involves a reduction in mesenteric blood flow that can be occlusive (caused by thrombosis, embolism, or surgical ligation of mesenteric vessels) or, more commonly, non-occlusive (caused by systemic hypotension, low cardiac output states, vasoconstriction from medications, or atherosclerotic narrowing that reduces flow below the critical threshold during episodes of physiological stress). When oxygen delivery falls below cellular demand, the mucosal layer is injured first because it is the most metabolically active layer with the highest oxygen requirement. The mucosal barrier breaks down, allowing bacterial translocation from the bowel lumen into the damaged tissue, triggering an inflammatory cascade that worsens tissue injury. In mild cases, only the mucosa and submucosa are involved (non-gangrenous ischemic colitis), and the injury is often reversible with supportive care; the mucosa can regenerate within days to weeks. In severe cases, full-thickness (transmural) ischemia leads to gangrene, perforation, and peritonitis, requiring emergent surgical intervention with colonic resection. The typical clinical presentation is an elderly patient with atherosclerotic cardiovascular disease who develops sudden onset of mild-to-moderate crampy left-sided abdominal pain followed by an urgent desire to defecate and passage of bright red or maroon blood mixed with stool within 24 hours. The bleeding is usually mild and self-limited, in contrast to the massive hemorrhage seen with diverticular bleeding or angiodysplasia. Diagnosis is confirmed by colonoscopy performed within 48 hours, which reveals edematous, friable mucosa with segmental hemorrhagic lesions, often with a sharp demarcation between affected and normal segments at the watershed boundary. CT scan with IV contrast may show bowel wall thickening, thumbprinting (submucosal edema and hemorrhage), and stranding of pericolic fat. The majority of cases (approximately 80 percent) are non-gangrenous and resolve with conservative management: bowel rest (NPO), IV fluid resuscitation, broad-spectrum antibiotics to prevent secondary infection of damaged mucosa, and careful monitoring for signs of deterioration. The practical nurse monitors for complications including stricture formation (which occurs in 10-15 percent of cases weeks to months later), progression to gangrene (evidenced by worsening pain, peritoneal signs, sepsis, and lactic acidosis), and recurrence.