Vascular compromise of the gut is responsible for approximately 0.1% of all hospital admissions and 1.0% of admission for an acute abdomen.
Despite advances made in the diagnostic and therapeutic field,
acute intestinal ischemia remains a highly lethal condition. Higher prevalence in the elderly population and nonspecific clinical presentation leading to delayed diagnosis contribute to the high mortality.
Prompt diagnostic and intervention are essential to improve patient survival in this potentially lethal disorder.
Multidetector CT (MDCT) has become the gold standard in evaluating patients with suspected mesenteric ischemia.
The modality provides rapid,
allows to exclude the other acute abdominal pathologies and affords exquisite visualization of the bowel wall,
mesentery and omenta,
and the abdominal viscus.
To accurately diagnose mesenteric ischaemic disease,
the radiologist must be acquainted with both the mesenteric arterial and venous anatomies of the bowel.
The three major arteries that supply the small and large bowel are the coeliac trunk,
superior mesenteric artery (SMA) and inferior mesenteric artery (IMA)(figure 1).
The coeliac trunk generally provides the blood supply from the distal oesophagus to the second portion of the duodenum.
The superior mesenteric artery supplies the third and fourth portions of the duodenum,
ileum and colon to the level of the splenic flexure.
The IMA supplies the distal colon from the level of the distal transverse portion to the upper rectum.
Middle and inferior rectal arteries,
branches of the internal iliac arteries,
supply the distal rectum.
There are numerous important mesenteric collateral pathways that provide a rich vascular safety net for the mesenteric blood supply.
Collateral pathways (figure 2)
•Celiac axis (CA) / Superior mesenteric artery (SMA):
Gastroduodenal arteries (GD)
•SMA / Inferior mesenteric artery (IMA) collaterals:
The marginal artery of Drummond:
between right and middle colic artery (mesenteric border)
The arc of Riolan :
communication of the left colic artery with the middle colic artery
•IMA / internal iliac artery:
Communication between superior hemorrhoidal artery ( IMA ) and the middle and inferior hemorrhoidal arteries (iliac arteries)
The venous system returns essentially parallel to the arterial supply.
The superior and inferior mesenteric veins run parallel to the arteries and drain the respective part of the bowel.
The inferior mesenteric vein (IMV) joins the splenic vein,
and the splenic vein joins the superior mesenteric vein (SMV) to form the main portal vein.
Numerous collateral venous pathways can form between the mesenteric and systemic veins including the gastric and oesophageal,
lumbar and pelvic veins.
Intestinal ischaemia refers to acute or chronic insufficient blood flow within the mesenteric circulation to meet the metabolic demands in the bowel.
Mesenteric ischaemia can be of acute (90%) or chronic type (10%).
In acute type the causes are arterial embolism,
nonocclusive form or venous occlusion.
Acute occlusions of the superior mesenteric artery due to thrombosis or embolisation are responsible for approximately 60%–70% of cases of acute bowel ischaemia,
whereas nonocclusive conditions account for approximately 20%–30% of cases and mesenteric venous thromboses account for 5%–10% of the total.
Acute mesenteric arterial embolism:
Mesenteric emboli can originate from the left atrium,
associated with cardiac dysrhythmias such as atrial fibrillation,
left ventricle with global myocardial dysfunction associated with poor ejection fraction,
or cardiac valves due to endocarditis.
Occasionally emboli generated from an atherosclerotic aorta.
Emboli typically lodge at points of normal anatomic narrowing,
and the SMA is particularly vulnerable because of its relatively large diameter and low takeoff angle from the aorta.
More than 20% of emboli to the SMA are associated with concurrent emboli to another arterial bed including the spleen,
findings of changes in these organs on CTA suggest a proximal embolic source.
Acute mesenteric arterial thrombosis:
Thrombosis of the SMA is usually associated with pre-existing chronic atherosclerotic disease leading to stenosis.
Many of these patients have a history consistent with chronic mesenteric ischemia (CMI) with symptoms of mesenteric angina including postprandial pain,
or “food fear”,
and thus a systematic history is important when evaluating a patient suspected to have AMI.
Thrombosis usually occurs at the origin of visceral arteries,
an underlying plaque in the SMA usually progresses to a critical stenosis over years resulting in collateral beds.
SMA thrombosis may also occur due to vasculitis,
or a mycotic aneurysm.
Acute non-occlusive mesenteric ischemia(NOMI):
NOMI is usually a consequence of SMA vasoconstriction associated with low splanchnic blood flow.
Patients with NOMI typically suffer from severe coexisting illness,
commonly cardiac failure which may be precipitated by sepsis.
Hypovolemia and the use of vasoconstrictive agents may precipitate NOMI.
Venous acute mesenteric ischaemia (VAMI):
Mesenteric venous thrombosis may be caused by infiltrative,
neoplastic or inflammatory/infectious conditions.
Although occasionally idiopathic,
up to 50% of patients have an identifiable risk factor,
such as previous deep venous thrombosis or pulmonary embolism.
Other risk factors include a hypercoagulability state such as Leiden factor V mutation,
oral contraceptive use,
cirrhosis and advanced malignancy.