Colon: Non Neoplastic Disease: CT Evaluation of Mesenteric Ischemia
Karen M. Horton, MD and Elliot K. Fishman, MD
Introduction
Mesenteric ischemia is a complex disorder that encompasses a wide spectrum of conditions which result in decreased blood flow to the small intestine. Mesenteric ischemia has traditionally posed a diagnostic challenge to both clinicians and radiologists, because patients often present with nonspecific symptoms such as abdominal pain. In the past, angiography was the only radiological test which could identify these patients early. Non-invasive modalities such as computed tomography (CT) were not sensitive enough to identify early, reversible disease. However, recent advancements in CT technology such as the introduction of multidetector-row CT (MDCT) and three-dimensional CT imaging have renewed interest in utilizing CT for the evaluation of suspected mesenteric ischemia. This article will review the current role of CT in the evaluation of mesenteric ischemia.
Clinical Background
Mesenteric ischemia can result from a variety of conditions, all of which result in compromised blood flow to the gut. The condition can be organized into acute mesenteric ischemia and chronic mesenteric ischemia.
Acute Mesenteric Ischemia
Acute mesenteric ischemia results from decreased blood flow to the intestines. This can be a result of (1) arterial embolism or thrombosis, (2 ) venous occlusion or (3) low flow states compromising the splanchnic circulation.. Patients with acute mesenteric ischemia usually present with severe abdominal pain. The exact clinical presentation will depend on the etiology.
The most common etiology of acute mesenteric ischemia is an embolism to the SMA. Patients with mesenteric ischemia as a result of an SMA embolism will present with sudden onset of pain, typically in the periumbilical region or right lower quadrant. Classically the pain is out of proportion to the physical findings. The patient may also report nausea, vomiting or diarrhea. Laboratory studies may reveal en elevated white blood cell count and in cases of intestinal infarction, there may also be acidosis, hypovolemia and hemoconcentration. Most emboli originate in the heart. Therapy may involve systemic anticoagulation or angiography with intra-arterial thrombolysis. In severe cases, surgery may be necessary to reestablish blood flow and to resect infarcted bowel.
Patients with thrombosis of the superior mesenteric artery usually present with a more gradual onset of abdominal pain. Often these patient will have a history consistent with chronic mesenteric ischemia and will have underlying atherosclerotic disease effecting the mesenteric arteries. A thrombosis then forms on a pre-existing plaque, resulting in acute symptoms. This is estimated to account for up to 25% of cases of acute mesenteric ischemia.. Again the patient’s complaints may be significant, although the physical exam findings are not impressive, i.e. the abdomen is usually soft and non tender. Laboratory findings may be aid in the diagnosis, but are usually not significantly elevated until infarction has occurred. These patients may require emergent systemic anticoagulation or intra-arterial thrombolysis. Once they are stabilized, surgery will often be performed in order to bypass the vessels compromised by atherosclerotic plaque.
Non-occlusive intestinal ischemia accounts for up to a third of cases of acute mesenteric ischemia and occurs in patients with systemic shock, severe dehydration, hypovolemia from trauma, decreased cardiac output, or drug-induced vasospasm. Patients will present with vague abdominal pain, which is often severe. The clinical history is most helpful in identifying these patients. Treatment usually consists of selective arterial administration of vasodilators and fluid resuscitation in an effort to restore adequate blood flow to the intestines. Patients with peritoneal signs will require surgery.
The least common cause of acute mesenteric ischemia is venous thrombosis. Thrombus can form in the superior mesenteric vein in patients with hypercoagulable states, post-operative patients, patients with trauma or patients with portal hypertension, pancreatitis, or diverticulitis. Treatment usually consists of anticoagulation and treatment of the underlying disorder.
Chronic Mesenteric Ischemia
Chronic mesenteric ischemia typically has a more insidious course with patients complaining of repeated episodes of abdominal pain, frequently occurring after a large meal. As a result patients may develop sitophobia (fear of eating) and involuntary weight loss. Most patients are over the age of 50 and have widespread atherosclerotic disease. Almost 95% of cases of chronic mesenteric ischemia result from stenosis of the superior mesenteric artery by atherosclerotic plaque. The severity and frequency of symptoms depend on the availability of adequate collaterals. On physical exam, the patients are usually malnourished and when suffering from an acute attack, complain of sever abdominal pain. Typically there will be no rebound or guarding on physical examination. An abdominal bruit is occasionally present. Treatment may include long acting nitrates which result in mesenteric vasodilatation. However, surgery is usually required. Surgical treatment consists of arterial reconstruction or aortomesenteric bypass. Recently percutaneous transluminal angioplasty and endovascular stent have been attempted in this patients population.
CT Technique
Over the past 2 decades there have been significant advancements in CT technology which have allowed CT to take a more dominant role in evaluating patients with suspected mesenteric ischemia. These improvements include faster gantry rotation speeds, thinner collimation, the introduction of spiral CT in the early 1990’s, the development of mechanical power injectors and the widespread use of safe non-ionic contrast material. In addition, the recent introduction and increasing availability of multidetector-row scanners promises to once again revolutionize CT scanning. There are advantages of MDCT for the evaluation and diagnosis of patients with intestinal ischemia.
First, MDCT allows thinner collimation. Depending on the manufacturer, 0.5-1.25 mm slices can be obtained. The thinner slices are especially important when visualizing the mid and distal branches of the mesenteric vessels and visualization of the bowel wall. Second, MDCT are faster. The gantry rotation speed is 0.5 second and because of the multiple detector rows, four slices can be obtained per rotation. This means that the scanner is 8 times faster than a traditional 1 sec single detector spiral scanner. The faster scanners decrease respiratory motion artifact and allow more accurate timing of the bolus in the arterial and venous phases. Third, because MDCT allows thinner collimation and faster scanner and bolus timing, the 3D volume set is improved. 3D visualization of the data sets is especially important in this patient population because the mesenteric vasculature is complex and the branches are very difficult to identify on routine axial scans.
When evaluating a patient with suspected intestinal ischemia, careful attention to technique is necessary. The examination with focus on 2 areas: imaging the mesenteric arteries and veins, and imaging the bowel. For accurate mesenteric vessel evaluation, a rapid IV contrast bolus is required. We routinely administer 120 cc of non-ionic contrast through a peripheral catheter at a rate of 3 cc/sec. Since both the arteries and veins can be involved, dual phase imaging is obtained. We acquire images at 25 second (arterial) and 50 seconds (venous) after the start of the injection. Using our MDCT scanner (Siemens, Volume Zoom), we utilized the 4 X 1mm collimator setting to obtain 1.25mm slices. We routinely perform 3 D imaging on these patients, since it greatly improves visualization and identification of branches of the mesenteric arteries and veins . In addition to evaluation of the mesenteric vasculature, it is important to examine the intestines and bowel wall. The use of water as oral contrast will allow excellent visualization of the bowel wall and will not interfere with 3D imaging of the vasculature . We typically ask patients to drink 750 cc of water 20 minutes prior to the study and an additional 250 cc of water immediately prior to the study, in order to distend the stomach and duodenum.
CT Findings
Acute Mesenteric Ischemia
When imaging a patient for acute possible mesenteric ischemia, it is important to evaluate the mesenteric vasculature, both the mesenteric arteries (superior mesenteric artery, inferior mesenteric artery) and mesenteric veins (superior mesenteric vein, inferior mesenteric vein). Patients with acute ischemia may demonstrate thrombus within the mesenteric arteries or veins. This will typically appear as low density clot within the proximal portion of the vessels, although distal thrombi can also occur, especially if related to emboli from the heart. These distal thrombi may not be directly visualized since they are within distal branches, but segmental areas of wall thickening and /or pneumatosis may be seen. The thrombus can be occlusive or non-occlusive. Tumors with mesenteric vessel encasement such as pancreatic cancer and carcinoid tumors or conditions such as sclerosing mesenteritis can also result in acute mesenteric ischemia, depending on the availability and adequacy of collaterals . In these cases the mesenteric vessels will appear narrowed or even occluded due to local tumor invasion. Acute mesenteric ischemia can also be caused by low-flow states. This can be s result of hypovolemia after trauma or from decreased cardiac output or vasospasm , often related to drugs. On CT, the mesenteric arteries may appear small in caliber with decreased visualization of distal branches. 3D reconstruction of the data is very helpful in this setting.
In addition to abnormalities in the mesenteric vasculature, patients with acute mesenteric ischemia can demonstrate changes in the affected bowel loops. If the proximal SMA or SMV is involved, the entire small bowel, right colon and transverse colon may be involved. Involvement of smaller branches of the mesenteric vessel as occurs is patients with embolic branches of the arteries, will only affect the supplied small bowel segments. Segmental ischemia can also occur due to mechanical obstruction, such as a hernia or small bowel volvulus.
The most common findings in patients with acute mesenteric ischemia, is nonspecific small bowel thickening. The affected loops will appear thickened and there is often associated mesenteric stranding and fluid. The bowel wall may appear low in density compatible with submucosal edema. Ischemia can also result in submucosal hemorrhage. This can be easily detected if non-contrast scans are obtained. The affected loops may also be dilated, most likely due to disruption of its normal peristaltic activity, similar to ileus.
In addition, changes in small bowel enhancement have also been described and include, decreased enhancement, delayed enhancement or rarely lack of enhancement. Pneumatosis (air within the bowel wall) is usually a late finding and indicates irreversible disease, i.e. infarction. At this point, surgical resection will be necessary. In addition to the presence of pneumatosis in cases of advanced ischemia and infraction, air can also be occasionally detected in the mesenteric veins or portal vein. This is an ominous finding and is associated with a high mortality rate. Chronic Mesenteric Ischemia
Patients with chronic mesenteric ischemia will typically have atherosclerotic plaque in the proximal portion of the celiac axis or superior mesenteric artery. The inferior mesenteric artery can occasionally also be involved. Unlike acute mesenteric ischemia, the mesenteric veins are typically not involved in patients with chronic mesenteric ischemia, unless there are superimposed acute symptoms. Atherosclerotic plaque is often calcified and can therefore be easily be identified on CT as focal calcification near the origin of the mesenteric arteries . The amount of plaque will determine the degree of vessel narrowing /stenosis and it is important not only to detect the presence of plaque but also to determine the degree of vessel compromise. It is also important to realize that atherosclerotic disease of the mesenteric vessels is a common finding in older individuals, most of whom will be asymptomatic. Therefore the presence of atherosclerotic plaque involving the mesenteric arteries needs to be correlated with the patients history and symptoms, and does not in itself indicate the presence of ischemia.
Since atherosclerotic disease occurs gradually overtime, collateral vessels will form to maintain adequate blood flow to the gut . These collaterals can also be visualized on CT and are important to identify prior to surgical planning. Surgeons will need an accurate road map of the mesenteric vessels and collaterals before attempting arterial reconstruction of aorto-mesenteric bypass.
Unlike acute mesenteric ischemia, patients with chronic mesenteric ischemia will not usually demonstrates changes in the bowel wall thickness or enhancement, unless there is superimposed acute thrombus or emboli.
Conclusion
Mesenteric ischemia is a complicated condition which is difficult to diagnose both clinically and radiologically. However, recent advances in CT technology including the development of multidetector row CT scanners and 3D imaging software, have greatly improved CT evaluation of the bowel wall and mesenteric vasculature. This has resulted in renewed interest in utilizing CT for detection of mesenteric ischemia which, in turn can help guide both medical and surgical treatment. This article reviewed the current status and potential of MDCT for this clinical application.