A 72-year-old man with chronic kidney disease (eGFR 28 mL/min/1.73m²) and hypertension presents with sudden-onset severe lower abdominal pain and bloody diarrhea. He has been taking aspirin 100 mg daily for secondary stroke prevention and atorvastatin. On examination, he is afebrile, BP 142/88 mmHg, and has mild left lower quadrant tenderness without peritoneal signs. Laboratory studies show Hb 11.2 g/dL, WBC 8500/μL, and creatinine 2.8 mg/dL (baseline 2.2 mg/dL). CT angiography of the abdomen shows segmental colonic wall thickening in the left colon with preserved enhancement, no arterial occlusion, and no pneumatosis. What is the most likely diagnosis?

Explanation

## Ischemic Colitis — Clinical Recognition and Diagnosis **Key Point:** Ischemic colitis is the most common form of mesenteric ischemia and typically presents with sudden-onset abdominal pain and bloody diarrhea in elderly patients with cardiovascular risk factors and low-flow states. ### Clinical Presentation and Risk Factors This patient has multiple risk factors for ischemic colitis: | Risk Factor | Present in Case | |---|---| | **Age > 60 years** | Yes (72 years) | | **Cardiovascular disease** | Yes (prior stroke, on aspirin) | | **Chronic kidney disease** | Yes (eGFR 28, baseline Cr 2.2) | | **Hypertension** | Yes (BP 142/88) | | **Acute renal function deterioration** | Yes (Cr 2.8, ↑ from 2.2) | | **Low-flow state** | Implied (dehydration, renal disease) | **Clinical Pearl:** Ischemic colitis typically affects **watershed areas** of the colon (splenic flexure, rectosigmoid junction) due to tenuous blood supply. Left colon involvement (as seen here) is classic for low-flow ischemia. ### CT Imaging Findings — The Diagnostic Clue The CT findings are **pathognomonic for ischemic colitis**: 1. **Segmental colonic wall thickening** — edema from ischemic injury 2. **Preserved enhancement** — rules out transmural necrosis (which shows poor/absent enhancement) 3. **No arterial occlusion** — excludes acute mesenteric ischemia from thrombosis or embolism 4. **No pneumatosis** — indicates non-transmural disease; good prognosis **High-Yield:** Preserved mucosal enhancement on CT is the hallmark of **non-occlusive mesenteric ischemia (NOMI)** or low-flow ischemic colitis. Absent/poor enhancement suggests transmural necrosis and poor prognosis [cite:Harrison 21e Ch 297]. ### Pathophysiology of Low-Flow Ischemic Colitis ```mermaid flowchart TD A["Chronic Kidney Disease<br/>+ Hypertension"]:::outcome --> B["Endothelial dysfunction<br/>Reduced vasodilatory capacity"]:::action C["Acute Renal Function Decline<br/>Dehydration/Hypotension"]:::outcome --> D["Splanchnic vasoconstriction<br/>Compensatory response"]:::action B --> E["Reduced colonic perfusion<br/>Low-flow state"]:::urgent D --> E E --> F["Ischemic injury to mucosa<br/>& submucosa"]:::action F --> G["Bloody diarrhea<br/>Abdominal pain"]:::outcome H["Preserved enhancement on CT"]:::decision -.->|Indicates| I["Non-transmural disease<br/>Good prognosis"]:::outcome ``` ### Differential Diagnosis — Why Others Are Wrong | Diagnosis | Key Distinguishing Feature | Why Excluded Here | |---|---|---| | **Ischemic colitis (low-flow)** | Preserved enhancement, no arterial occlusion, watershed distribution | **BEST FIT** — all findings match | | **Infectious colitis (C. difficile)** | Recent antibiotic exposure, pseudomembranes on colonoscopy | No antibiotic history; CT shows segmental thickening, not pseudomembranes | | **Ulcerative colitis** | Continuous involvement from rectum, younger age, no acute renal decline | Age 72, acute presentation, left colon only, renal dysfunction trigger | | **Acute mesenteric ischemia (arterial)** | Absent/poor enhancement, arterial occlusion on imaging, severe pain out of proportion | CT shows **preserved enhancement** and **no arterial occlusion** — rules this out | **Mnemonic:** **NOMI** = Non-Occlusive Mesenteric Ischemia; caused by low-flow states (sepsis, heart failure, renal failure, dehydration) rather than arterial thrombosis. ### Management Approach 1. **Supportive care** — IV fluids, NPO, electrolyte correction 2. **Avoid vasoconstrictors** — can worsen splanchnic ischemia 3. **Discontinue aspirin temporarily** (bleeding risk) 4. **Treat underlying cause** — optimize renal function, manage hypertension 5. **Colonoscopy** — if diagnosis uncertain or to exclude other pathology (reserved for stable patients) 6. **Monitor for transmural disease** — repeat imaging if clinical deterioration **Clinical Pearl:** Most cases of non-occlusive ischemic colitis resolve with conservative management; mortality is < 5% if transmural necrosis is absent (preserved enhancement). **High-Yield:** The **absence of arterial occlusion on imaging** combined with **preserved colonic enhancement** is diagnostic of low-flow ischemic colitis, not acute arterial mesenteric ischemia.