Mesenteric Ischemia

Mesenteric ischemia is a relatively rare disorder seen in the emergency department (ED); however, it is an important diagnosis to make because of its high mortality rate. Vague and nonspecific clinical findings and limitations of diagnostic studies make the diagnosis a significant challenge. Moreover, delays in diagnosis lead to increased mortality rates. Despite recent advances in diagnosis and treatment, mortality rates continue to remain high.

Pathophysiology
Mesenteric ischemia is caused by decreased intestinal blood flow that can be caused by a number of mechanisms. Decreased intestinal blood flow results in ischemia and subsequent reperfusion damage at the cellular level that may progress to the development of mucosal injury, tissue necrosis, and metabolic acidosis.

The blood supply to the intestine is derived predominantly from 3 major gastrointestinal arteries that arise from the abdominal aorta: the celiac axis, the superior mesenteric artery (SMA), and the inferior mesenteric artery (IMA). The intestine has significant collateral circulation at all levels that allows for some protection from ischemia and is able to compensate for approximately a 75% acute reduction in mesenteric blood flow for up to 12 hours, without substantial injury.

The pathophysiology of intestinal ischemia can be divided into arterial and venous etiologies and acute and chronic ischemia. The vast majority of cases are secondary to arterial causes. All diseases and conditions that affect arteries, including atherosclerosis, arteritis, aneurysms, arterial infections, dissections, arterial emboli, and thrombosis, are reported to occur in the intestinal arteries.

Acute mesenteric ischemia (AMI) can be further divided into embolic, thrombotic, or nonocclusive causes.


Arterial embolism
Arterial embolism accounts for approximately one third of acute cases of AMI.
Emboli to the mesenteric arteries are usually from a dislodged cardiac thrombus.
The SMA is most commonly affected with the IMA rarely affected due to its small caliber.
Arterial thrombosis
Arterial thrombosis accounts for approximately one third of acute cases of AMI.
It is usually due to acute worsening of ischemia in patients who have preexisting atherosclerosis of the mesenteric arteries.
Thrombosis often involves at least 2 of the major splanchnic vessels.
Nonocclusive etiology
Nonocclusive etiology accounts for approximately one third of acute cases of AMI.
The primary mechanism is severe and prolonged intestinal vasoconstriction.
The most common setting is severe systemic illness with systemic shock usually secondary to reduced cardiac output.
Intestinal vasospasm has also been seen to occur in cocaine ingestion, ergot poisoning, digoxin use, and with alpha-adrenergic agonists.
A small proportion of cases are from venous thrombosis, seen mostly in patients with hypercoagulable states.
Venous thrombosis of the visceral vessels may precipitate an acute ischemic event as compromised venous return leads to interstitial swelling of the bowel wall, with subsequent impedance of arterial flow and eventual tissue necrosis.

Chronic mesenteric ischemia (CMI) usually results from long-standing atherosclerotic disease of 2 or more mesenteric vessels. Other nonatheromatous causes of CMI include the vasculitides such as Takayasu arteritis. Symptoms are caused by the gradual reduction in blood flow to the intestine that occurs during eating since total blood flow to the intestine can increase by 15% during meals.

Frequency
United States
AMI is involved in up to 0.1% of all hospital admissions, although this number is likely to rise as the population ages.

Mortality/Morbidity

Mortality rates are high and range from 60-100% depending on the source of obstruction. Early and aggressive diagnosis and treatment has been shown to significantly decrease the mortality rate if the diagnosis is made prior to the development of peritonitis.
One report of 21 patients with SMA embolus, intestinal viability was achieved in 100% of patients before diagnosis if the duration of symptoms was less than 12 hours, in 56% if it was between 12 and 24 hours, and in only 18% if symptoms were more than 24 hours in duration.
Another study found that even at hospital centers with angiography available 24 hours, mortality rates still were approximately 70%.

Sex
No sex predilection exists.

Age
Mesenteric ischemia is generally a disease of the older population, with the typical age of onset being older than 60 years; however, with risk factors and other predisposing factors, it may be seen in younger patients.


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