# Acute Mesenteric Ischemia

## Overview

Acute mesenteric ischemia (AMI) is a vascular emergency with mortality rates of 60 to 80% despite advances in diagnosis and treatment. The key to reducing mortality is a high index of suspicion, early diagnosis with CT angiography, and prompt restoration of blood flow through revascularization or resection of non-viable bowel. The classic teaching -- "pain out of proportion to physical findings" -- remains the most important clinical clue to this devastating condition.

## Classification and Etiology

| Type | Frequency | Onset | Key Features | Treatment |
|------|-----------|-------|-------------|-----------|
| SMA Embolism | 40–50% | Sudden | A-fib/MI source; lodges 3–8 cm from SMA origin; spares proximal jejunum | Embolectomy or endovascular thrombectomy |
| SMA Thrombosis | 20–30% | Acute-on-chronic | Prior chronic mesenteric ischemia; occlusion at SMA origin; extensive involvement | Bypass or endarterectomy |
| NOMI | 20–30% | Insidious | Low cardiac output, vasopressors; patchy ischemia; ICU patients | Optimize cardiac output + papaverine |
| Mesenteric Venous Thrombosis | 5–15% | Subacute (days–weeks) | Hypercoagulable states; segmental edema/hemorrhagic infarction | Anticoagulation (heparin → long-term) |

### Superior Mesenteric Artery Embolism (40-50%)

SMA embolism is the most common cause of AMI. The embolic source is typically the left atrium (in atrial fibrillation), the left ventricle (post-MI mural thrombus), valvular disease, or aortic atherosclerosis. Emboli characteristically lodge 3 to 8 cm from the SMA origin, distal to the takeoff of the middle colic artery. This means the proximal jejunum is typically spared, while the mid and distal small bowel and the right colon are affected. The onset of severe abdominal pain is sudden and dramatic.

### SMA Thrombosis (20-30%)

SMA thrombosis occurs at the vessel origin, superimposed on pre-existing atherosclerotic stenosis. Patients often have a preceding history of chronic mesenteric ischemia with postprandial pain, food fear, and weight loss. Because the occlusion is proximal, bowel involvement is more extensive, potentially spanning from the duodenum to the transverse colon. Pre-existing collateral circulation may partially protect some segments.

### Non-Occlusive Mesenteric Ischemia (NOMI) (20-30%)

NOMI results from splanchnic vasoconstriction without vessel occlusion and is associated with low cardiac output states, shock, vasopressors (especially norepinephrine and vasopressin), digitalis use, and hemodialysis. The ischemia is patchy and segmental. NOMI typically affects ICU patients, and the diagnosis is frequently delayed because these patients often cannot communicate their symptoms effectively.

### Mesenteric Venous Thrombosis (5-15%)

Superior mesenteric vein thrombosis has a more insidious onset, with symptoms developing over days to weeks. Risk factors include hypercoagulable states (Factor V Leiden, protein C/S deficiency, antithrombin III deficiency), portal hypertension, cirrhosis, malignancy, intra-abdominal inflammation, and oral contraceptive use. The resulting bowel injury consists of segmental wall edema and hemorrhagic infarction.

## Clinical Presentation

### Early Findings

The hallmark of early mesenteric ischemia is **severe abdominal pain out of proportion to physical examination findings**. The abdomen may be soft and non-tender despite the patient's severe distress. Early in the course, rapid and forceful bowel evacuation (gut emptying) is common, along with nausea, vomiting, diarrhea, and tachycardia.

### Late Findings

Late findings indicate established bowel necrosis and include peritoneal signs (guarding, rigidity, rebound tenderness), abdominal distension with absent bowel sounds, bloody stool (from mucosal sloughing), fever, sepsis, hemodynamic instability, and metabolic acidosis with elevated lactate.

### Risk Factor Profile

The clinical context helps identify the etiology. Atrial fibrillation or recent MI suggests embolism. Known peripheral vascular disease and claudication point toward thrombosis. Critical illness with low cardiac output and vasopressor dependence raises concern for NOMI. A hypercoagulable state or DVT history suggests mesenteric venous thrombosis.

## Diagnostic Workup

### Laboratory Studies

**Lactate** is elevated in advanced ischemia, but a normal lactate does not exclude early AMI -- this is a critical point that cannot be overemphasized. Base deficit correlates with ischemia severity. D-dimer is elevated but non-specific. Leukocytosis is often marked, frequently exceeding 15,000 to 20,000. Amylase and lipase may be mildly elevated. LDH and CPK are elevated in advanced bowel necrosis. No single laboratory test is sufficiently sensitive or specific to diagnose or exclude AMI alone.

### Imaging

**CT angiography (CTA)** is the study of choice, with sensitivity of 93 to 100% and specificity of 95 to 100% for AMI. On the arterial phase, it demonstrates SMA embolus or thrombus as a filling defect, SMA origin occlusion in thrombosis, and reduced bowel wall enhancement. On the venous phase, it shows SMV thrombosis, bowel wall thickening, and edema from venous congestion. Signs of bowel ischemia or necrosis include pneumatosis intestinalis (intramural gas), portal or mesenteric venous gas, lack of bowel wall enhancement, mesenteric haziness, ascites, and bowel dilation. In NOMI, the CTA shows patent but narrowed SMA and branches with segmental poor enhancement.

**Conventional angiography** has been largely replaced by CTA for diagnosis but remains valuable for therapeutic intervention, including catheter-directed thrombolysis, mechanical thrombectomy, and papaverine infusion for NOMI. **Plain radiographs** are often normal early in the disease and are not adequate to diagnose or exclude AMI; late findings include ileus, "thumbprinting" (bowel wall edema), pneumatosis, and portal venous gas.

## Management

### General Principles

Initial management includes aggressive fluid resuscitation, broad-spectrum antibiotics covering gram-negatives and anaerobes, correction of the underlying cause (rate and rhythm control for atrial fibrillation, optimization of cardiac output), and anticoagulation with heparin unless contraindicated. Vasoconstrictors should be avoided if possible because they worsen mesenteric ischemia. Nasogastric decompression, serial abdominal examinations, and lactate monitoring are essential.

### SMA Embolism

When peritonitis or bowel necrosis is present, emergent laparotomy is required. The procedure involves SMA embolectomy (via transverse arteriotomy with a Fogarty catheter), resection of non-viable bowel, and a planned second-look laparotomy at 24 to 48 hours. When peritonitis is absent, an endovascular approach may be considered, including catheter-directed thrombolysis with tPA, mechanical or aspiration thrombectomy, and angioplasty with or without stenting if an underlying stenosis is found. Systemic anticoagulation with heparin is started in all cases.

### SMA Thrombosis

With peritonitis, emergent laparotomy is performed with aortomesenteric bypass (antegrade or retrograde) or SMA thromboendarterectomy, bowel resection as needed, and a second-look laparotomy. Without peritonitis, endovascular stenting and thrombolysis may be considered. Long-term anticoagulation is required.

### Non-Occlusive Mesenteric Ischemia

The primary treatment of NOMI is medical: optimizing cardiac output, discontinuing vasoconstrictors, and correcting hypovolemia. Selective mesenteric angiography with papaverine infusion directly into the SMA (30 to 60 mg per hour) provides targeted vasodilation. Surgery is reserved for peritonitis or bowel necrosis. Mortality remains very high at 70 to 90% due to the severity of the underlying critical illness.

### Mesenteric Venous Thrombosis

Systemic anticoagulation with heparin is the primary treatment, followed by transition to warfarin or a DOAC for a minimum of 6 to 12 months, and often lifelong if a hypercoagulable state is identified. Surgery is indicated only for peritonitis or bowel necrosis. Endovascular catheter-directed thrombolysis may be used for extensive thrombosis. A hypercoagulable workup is mandatory. The prognosis is better than arterial AMI, with mortality of 20 to 40%.

### Second-Look Laparotomy

A planned re-exploration at 24 to 48 hours to reassess bowel viability is critical after embolectomy or bypass when marginal bowel was left in situ. This decision should be made at the initial operation, not deferred until clinical deterioration occurs. The second-look allows definitive resection of non-viable segments and creation of anastomoses.

### Assessment of Bowel Viability

Clinical assessment evaluates color (pink indicates viability; dusky or black indicates necrosis), peristalsis, mesenteric pulsation, and bleeding from a cut edge. Adjuncts include IV fluorescein with a Wood lamp, handheld Doppler, and ICG fluorescence angiography, which is increasingly available. When viability is uncertain, the questionable bowel should be left and a second-look planned.

## Chronic Mesenteric Ischemia

Chronic mesenteric ischemia results from progressive atherosclerotic stenosis of the mesenteric vessels (celiac, SMA, IMA). The classic triad consists of postprandial abdominal pain ("intestinal angina"), food fear (sitophobia), and weight loss. Clinically significant ischemia usually requires stenosis of at least two of the three major mesenteric vessels. Diagnosis is made by CTA or MRA showing mesenteric stenosis, or by duplex ultrasound demonstrating an SMA peak systolic velocity exceeding 275 cm/s. Treatment options include endovascular stenting (the preferred first-line approach) or surgical bypass (antegrade aortomesenteric or retrograde iliac-to-SMA). Untreated chronic mesenteric ischemia can progress to acute thrombosis, making elective revascularization a preventive intervention.

<image>CT angiography findings in acute mesenteric ischemia showing four panels: (1) SMA embolism with a filling defect in the mid-SMA distal to the middle colic artery origin, (2) SMA thrombosis with occlusion at the SMA origin, (3) mesenteric venous thrombosis with SMV filling defect and bowel wall thickening, and (4) advanced ischemia with pneumatosis intestinalis and portal venous gas. Label each finding with arrows and annotations.</image>

<image>Anatomical illustration of the mesenteric vascular anatomy showing the celiac trunk, superior mesenteric artery with its branches (inferior pancreaticoduodenal, jejunal, ileal, ileocolic, right colic, middle colic), and inferior mesenteric artery with its branches (left colic, sigmoid, superior rectal). Highlight the collateral pathways: marginal artery of Drummond, arc of Riolan (meandering mesenteric artery), and pancreaticoduodenal arcades. Show common sites of embolic lodgment and thrombotic occlusion.</image>

<image>Surgical illustration of SMA embolectomy technique showing exposure of the SMA at the root of the mesentery, transverse arteriotomy, passage of a Fogarty balloon catheter for embolus extraction, and primary arteriotomy closure. Include a companion panel showing assessment of bowel viability with areas of viable (pink), questionable (dusky), and non-viable (necrotic/black) bowel segments after revascularization.</image>

## Clinical Pearls

"Pain out of proportion to physical exam" is the hallmark of early mesenteric ischemia -- a soft abdomen in a patient with severe pain should trigger immediate CTA. A normal lactate does not exclude mesenteric ischemia, especially early in the course, as lactate elevation is a late finding indicating established necrosis. CT angiography with arterial and venous phases is the diagnostic study of choice, with sensitivity and specificity both exceeding 95%. SMA embolism typically spares the proximal jejunum because emboli lodge distal to the middle colic artery origin. SMA thrombosis produces more extensive ischemia spanning from the duodenum to the transverse colon because the occlusion is at the vessel origin. A second-look laparotomy at 24 to 48 hours should always be planned at the initial operation when bowel viability is questionable, rather than deferred until clinical deterioration occurs. NOMI occurs in critically ill patients on vasopressors, and treatment is primarily medical -- optimizing cardiac output and administering papaverine infusion -- with surgery reserved for necrosis. Patients with chronic mesenteric ischemia presenting with postprandial pain, weight loss, and food fear are at risk for acute-on-chronic thrombosis, and elective revascularization can prevent this catastrophe.

## References

- Bala M, Kashuk J, Moore EE, et al. Acute mesenteric ischemia: guidelines of the World Society of Emergency Surgery. *World J Emerg Surg*. 2017;12:38.
- Clair DG, Beach JM. Mesenteric ischemia. *N Engl J Med*. 2016;374(10):959-968.
- Oldenburg WA, Lau LL, Rodenberg TJ, et al. Acute mesenteric ischemia: a clinical review. *Arch Intern Med*. 2004;164(10):1054-1062.
- Defined SR, Defined SMA. ACR Appropriateness Criteria: Acute mesenteric ischemia. *J Am Coll Radiol*. 2020;17(5S):S198-S206.