Residency · Residency · Diagnostic Radiology

Acute Mesenteric Ischemia

Overview

Definition and Significance

Acute mesenteric ischemia is defined as insufficient blood flow to the mesentery to meet metabolic demands, resulting in bowel ischemia and necrosis. It is a time-critical emergency with an overall mortality of 50-80% that drops to 20-30% with early diagnosis and intervention. CT angiography is the imaging modality of choice, with sensitivity exceeding 90% and specificity exceeding 95%.

Etiologies

The etiologies are divided into three major categories. Arterial occlusive disease accounts for 50-70% of cases and includes SMA embolism (the most common overall cause) and SMA thrombosis. Venous thrombosis accounts for 5-15% of cases and involves SMV or portal vein thrombosis. Non-occlusive mesenteric ischemia (NOMI) represents 20-30% of cases and results from splanchnic vasoconstriction without vascular occlusion, typically occurring in critically ill patients on vasopressors or with low cardiac output.

Arterial Occlusive Mesenteric Ischemia

SMA Embolism

SMA embolism is the most common cause of acute mesenteric ischemia. The source is typically cardiac, arising from atrial fibrillation, left ventricular thrombus, or valvular disease. The embolus characteristically lodges 3-8 cm distal to the SMA origin, beyond the origin of the middle colic artery. The jejunum and ileum are most affected, though the proximal jejunum may be spared because it is fed by proximal branches that arise above the level of the embolus. On CTA the embolus appears as a filling defect within the SMA lumen with abrupt vessel cutoff.

SMA Thrombosis

SMA thrombosis usually occurs at the SMA origin on a background of atherosclerosis, producing more extensive ischemia (from the duodenum to the transverse colon) because the occlusion is proximal. On CTA there is occlusion at or near the SMA origin with calcified atherosclerotic plaque, and collateral vessels from prior chronic stenosis may be visible.

Celiac Axis Involvement

Isolated celiac occlusion rarely causes ischemia due to collateral pathways through the pancreaticoduodenal arcades and the arc of Buhler. However, combined celiac and SMA occlusion is catastrophic.

Mesenteric Venous Thrombosis

Risk Factors

Risk factors for mesenteric venous thrombosis include hypercoagulable states (protein C and S deficiency, Factor V Leiden, antiphospholipid syndrome), portal hypertension and cirrhosis, intra-abdominal infection or inflammation (pancreatitis, diverticulitis), malignancy (particularly pancreatic cancer), and oral contraceptives.

CTA Findings

The hallmark finding is a filling defect within the SMV and/or portal vein representing non-enhancing intraluminal thrombus. Associated findings include mesenteric edema and congestion (haziness of the mesenteric fat), bowel wall thickening with hyper- or hypoenhancement, and ascites. The presentation is often more subacute than arterial ischemia.

Non-Occlusive Mesenteric Ischemia (NOMI)

Clinical Context

NOMI occurs in critically ill patients with shock, heart failure, sepsis, or following cardiac surgery. Vasopressor use, especially norepinephrine and vasopressin, is a significant contributing factor. Importantly, there is no vascular occlusion on CTA.

CTA Findings

CTA shows patent mesenteric arteries but diffuse narrowing or spasm of the SMA and its branches, along with irregular narrowing of the mesenteric arterial arcades. Bowel wall changes may be segmental or diffuse. Diagnosis often requires clinical correlation and exclusion of occlusive disease.

CT Findings of Bowel Ischemia

Bowel Wall Changes

Hypoenhancement refers to decreased or absent enhancement of the bowel wall and indicates transmural ischemia from arterial insufficiency. Hyperenhancement, or increased mucosal enhancement, reflects reperfusion injury or venous congestion and may be seen in early ischemia or venous thrombosis. Bowel wall thickening results from edema and hemorrhage within the wall and is more common in venous ischemia. A paper-thin bowel wall is a late finding in arterial ischemia indicating full-thickness necrosis.

Mesenteric Findings

Mesenteric fat stranding reflects inflammatory changes and edema. Mesenteric vessel engorgement is particularly prominent in venous thrombosis. Mesenteric hemorrhage appears as high-attenuation stranding in the mesentery.

Signs of Transmural Necrosis and Perforation

Pneumatosis intestinalis is gas within the bowel wall, appearing as linear or bubbly lucencies, and indicates mucosal disruption and bacterial translocation. Portomesenteric venous gas (PMVG), or gas in the portal vein or mesenteric veins, correlates with poor prognosis when associated with ischemia. However, it is important to note that pneumatosis and PMVG are not always ischemic in origin, as benign causes include instrumentation, COPD, and immunosuppression. Free intraperitoneal air indicates perforation, and high-attenuation ascites represents hemorrhagic free fluid.

Distribution Patterns

The SMA territory supplies the jejunum, ileum, cecum, ascending colon, and transverse colon up to the splenic flexure. The IMA territory supplies the descending colon, sigmoid, and upper rectum. The watershed zones, specifically the splenic flexure (Griffiths point) and the rectosigmoid junction (Sudeck point), are most vulnerable to ischemia in low-flow states.

Chronic Mesenteric Ischemia

Clinical Features

Chronic mesenteric ischemia presents with postprandial abdominal pain ("intestinal angina"), food avoidance, and weight loss. It usually requires significant stenosis or occlusion of at least 2 of 3 mesenteric vessels (celiac, SMA, IMA) to become symptomatic.

CTA Findings

CTA shows calcified atherosclerotic stenosis or occlusion at the origins of the celiac axis and SMA. Prominent collateral pathways, including the arc of Riolan, marginal artery of Drummond, and pancreaticoduodenal arcades, may be visible. The bowel wall appearance is typically normal between episodes.

<image>An axial CTA diagram showing acute SMA embolism. The SMA is depicted in cross-section and longitudinal view. A filling defect (thrombus) is shown within the SMA lumen approximately 3-5 cm distal to its origin, beyond the takeoff of the middle colic artery. The proximal SMA is patent. Distally, several jejunal and ileal loops show decreased wall enhancement (hypoenhancing walls) indicating ischemia. Adjacent mesenteric fat stranding is visible. A small inset shows a magnified view of the filling defect with the embolus labeled. Arrows indicate the patent proximal SMA, the embolus, and the ischemic bowel segments.</image>

<image>A CT image panel showing the progression of bowel ischemia findings. Four sequential images arranged from left to right showing: (1) Early ischemia with hyperenhancing bowel wall and mesenteric edema. (2) Progressing ischemia with hypoenhancing bowel wall segments and more prominent mesenteric stranding. (3) Transmural necrosis with paper-thin bowel wall, pneumatosis intestinalis (linear gas in the bowel wall highlighted with arrows), and high-attenuation free fluid. (4) Portal venous gas shown as branching low-attenuation structures in the liver periphery on an axial CT through the liver. A severity arrow runs along the bottom from "early/reversible" to "late/irreversible."</image>

Clinical Pearls

Acute mesenteric ischemia has a narrow diagnostic window, and CT angiography should be performed urgently when clinical suspicion arises, particularly in patients presenting with acute abdominal pain out of proportion to the physical examination, atrial fibrillation, or recent myocardial infarction. SMA embolism typically lodges distal to the middle colic artery origin, sparing the proximal jejunum, while SMA thrombosis occurs at the vessel origin and causes more extensive ischemia. Pneumatosis intestinalis and portomesenteric venous gas are not always ischemic in origin, and clinical context is essential; however, in the setting of acute abdominal pain, these findings are ominous. The watershed zones (splenic flexure and rectosigmoid junction) are most vulnerable to ischemia in low-flow and non-occlusive states. In venous mesenteric ischemia, the pattern of SMV or portal vein thrombus, bowel wall thickening with hyperenhancement, mesenteric edema, and ascites differs distinctly from arterial ischemia. The status of all three mesenteric vessels (celiac, SMA, IMA) should always be reported on CTA along with collateral pathways.

References

• Bala M, et al. "Acute Mesenteric Ischemia: Guidelines of the World Society of Emergency Surgery." World Journal of Emergency Surgery, 2017
• ACR Appropriateness Criteria: Acute Mesenteric Ischemia, 2018

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