A 72-year-old woman with a history of hypertension and atrial fibrillation (on warfarin) presents with acute onset severe abdominal pain, nausea, and loose stools for 6 hours. She denies prior abdominal surgery. On examination, she is in severe distress, blood pressure is 110/70 mmHg, heart rate 110 bpm and irregular. The abdomen is tender but not rigid; bowel sounds are initially present but become absent. Plain abdominal X-ray shows dilated small bowel loops with air-fluid levels, but no free air. CT angiography with IV contrast shows segmental narrowing of the superior mesenteric artery with reduced enhancement of the small bowel wall. What is the most likely diagnosis?

Explanation

## Diagnosis: Acute Mesenteric Ischemia (AMI) — SMA Thrombosis ### Critical Clinical Clues **Key Point:** The combination of **sudden severe pain out of proportion to physical findings**, **atrial fibrillation** (source of thromboembolism), **reduced small bowel wall enhancement on CT**, and **segmental SMA narrowing** is diagnostic of acute mesenteric ischemia. ### Pathophysiology of SMA Thrombosis 1. **Atrial fibrillation** → cardiac thrombus formation → embolism to SMA 2. **Alternative mechanism:** In-situ thrombosis of atherosclerotic SMA (less common acutely) 3. **Result:** Sudden interruption of mesenteric blood flow → mucosal ischemia → full-thickness necrosis if untreated 4. **Timeline:** Progression from ischemia to infarction occurs over hours [cite:Harrison 21e Ch 297] ### Why the Imaging Findings Are Diagnostic **High-Yield:** CT angiography is the gold standard for diagnosing acute mesenteric ischemia: - **Segmental SMA narrowing** = arterial occlusion - **Reduced bowel wall enhancement** = hypoperfusion and ischemia - **Dilated small bowel** = secondary to ischemic dysfunction, not mechanical obstruction - **Absence of free air** = full-thickness perforation has not yet occurred (but is imminent) ### Classic Presentation Triad **Mnemonic: PAD** — **P**ain out of proportion, **A**bdominal exam benign, **D**iarrhea/loose stools (from mucosal sloughing). ### Clinical Pearl **Clinical Pearl:** The **absence of rigidity or guarding** despite severe pain is the hallmark of early mesenteric ischemia. Once peritonitis develops (free perforation), the exam becomes rigid — a late and ominous sign. ### Progression of Findings Over Time ```mermaid flowchart TD A[SMA thrombosis]:::outcome --> B[Mucosal ischemia]:::outcome B --> C[Severe pain, diarrhea]:::outcome C --> D{Reperfusion?}:::decision D -->|No| E[Transmural necrosis]:::urgent D -->|Yes| F[Recovery possible]:::action E --> G[Perforation, peritonitis]:::urgent G --> H[Septic shock, death]:::urgent F --> I[Healing, stricture risk]:::outcome ``` ### Why Bowel Sounds Change **Key Point:** Initially, ischemic bowel may show hyperactive sounds (as it attempts to move ischemic contents). As necrosis progresses, sounds become absent (paralysis). This dynamic change is a critical clinical sign. ### Immediate Management 1. **IV access, aggressive fluid resuscitation** (ischemic bowel is third-spaced) 2. **Anticoagulation** (heparin) to prevent propagation of thrombus 3. **Urgent surgical exploration** or **percutaneous catheter-directed thrombolysis** if diagnosis made early 4. **Broad-spectrum antibiotics** (translocation risk) 5. **Avoid vasoconstrictors** (worsen ischemia) ### Mortality and Prognosis **High-Yield:** Mortality of acute mesenteric ischemia is 60–80% if diagnosis is delayed beyond 24 hours. Early recognition and intervention are life-saving.