What is the anatomy of the superior mesenteric artery (SMA) in an elderly patient with cardiovascular disease (CVD) and suspected acute mesenteric ischemia?

Anatomy of the Superior Mesenteric Artery

Origin and Topographic Location

The superior mesenteric artery (SMA) most commonly originates from the anterior abdominal aorta at the level of the lower third of the first lumbar vertebra (L1), approximately 1-2 cm below the celiac trunk. 1

• The SMA arises between the lower third of T12 and lower third of L1, with 77% originating at the lower third of L1 1
• The average distance from the celiac trunk to the SMA origin is 1.84 cm (range 0.60-1.50 cm in 85% of cases) 1
• The SMA is particularly vulnerable to embolic occlusion due to its relatively large diameter and low takeoff angle from the aorta 2

Course and Anatomical Relationships

• The SMA descends anterior to the uncinate process of the pancreas and third part of the duodenum 2
• The average distance from SMA origin to the inferior mesenteric artery is 6.67 cm 1
• The average distance from SMA origin to the aortic bifurcation is 10.39 cm 1

Branching Pattern and Main Branches

The SMA typically gives rise to three major colic branches: the middle colic artery (MCA), right colic artery (RCA), and ileocolic artery (ICA). 3

Standard Branching Pattern (Type A - Most Common)

• Independent origin of all three main branches occurs in 73-83% of cases 3
• This represents the classic anatomical description found in most textbooks 3

Common Anatomical Variations

Pattern IIb (common trunk between RCA and ICA) is the second most frequent variation, occurring in 15-32% of cases. 3

• Pattern IIa: Common trunk between RCA and MCA (4-20% of cases) 3
• Pattern IIc: Common trunk for all three branches (0.35% of cases) 3
• Pattern III: Absence of the right colic artery (2-8% of cases) 3
• The right colic artery is responsible for the majority of anatomical variations 3

Rare Anatomical Variants

• The SMA may originate from the right hepatic artery (6.13% of cases) 4
• Celiacomesenteric trunk (CMT): shared origin of celiac trunk and SMA, representing the least frequently reported vascular anomaly 5
• The inferior mesenteric artery may originate from the SMA in rare cases 4

Territory of Supply

The SMA supplies the entire midgut-derived structures, including the small intestine from the distal duodenum, the entire jejunum and ileum, and the large intestine from cecum through the proximal two-thirds of the transverse colon. 4, 3

• The ileocolic artery supplies the terminal ileum, cecum, appendix, and ascending colon 2, 6
• When the ileocolic artery is involved in thrombosis, necrosis of the proximal colon results 6
• The middle colic artery supplies the transverse colon 3

Collateral Circulation

Extensive collateral networks exist between the celiac trunk, SMA, and inferior mesenteric artery through the pancreaticoduodenal arcade and marginal artery of Drummond. 2, 7

• The pancreaticoduodenal arcade connects the celiac trunk (via gastroduodenal artery) to the SMA 7
• This collateral pathway can be used for retrograde transcollateral recanalization in cases of SMA occlusion 7
• The marginal artery of Drummond connects the SMA and IMA territories along the colon 3
• Incomplete marginal artery occurs in approximately 5-6% of cases 3

Clinical Relevance in Acute Mesenteric Ischemia

Embolic Occlusion Pattern

Emboli typically lodge 3-10 cm distal to the SMA origin, classically sparing the proximal jejunum and colon. 2

• This occurs at points of normal anatomic arterial narrowing 2
• The distal lodging pattern explains why proximal jejunum is often spared in embolic events 2
• More than 20% of SMA emboli are associated with concurrent emboli to other arterial beds (spleen, kidney) 2

Thrombotic Occlusion Pattern

Thrombotic occlusion typically occurs at the ostial or proximal SMA origin, often in patients with pre-existing atherosclerotic disease. 2

• Thrombosis usually develops at the origin of visceral arteries where atherosclerotic plaques accumulate 2
• Symptomatic SMA thrombosis most often accompanies celiac trunk occlusion due to the need for collateral flow 2
• When ileocolic artery involvement occurs, proximal colon necrosis results 6

Critical Anatomical Considerations

• Due to extensive collaterals in mesenteric circulation, isolated celiac trunk or inferior mesenteric artery occlusion infrequently leads to intestinal infarction 2
• The small intestine can compensate for 75% reduction in mesenteric blood flow for up to 12 hours 2
• In non-occlusive mesenteric ischemia (NOMI), SMA vasoconstriction affects both small bowel and proximal colon via the ileocolic artery 2, 8