MSCT illuminates intestinal ischemia

Early diagnosis of bowel ischemia remains an extremely difficult clinical and radiological problem. This condition is associated with a mortality rate of between 70% and 90%, most often because of diagnostic delay.

Acute mesenteric ischemia may be caused by arterial or venous occlusion or by mesenteric hypoperfusion. Patients present with a variety of clinical and radiological manifestations. Causative pathology can range from localized transient ischemia to intestinal necrosis. The high mortality rate is due in part to difficulties interpreting nonspecific clinical signs and symptoms. The likelihood of normal or nondiagnostic radiographs during the condition's early stages also plays a role.

Multislice CT is being used increasingly to evaluate patients presenting with an acute abdomen, for which acute bowel ischemia is one of several possible diagnoses. CT can visualize the cause of ischemia directly. It can also demonstrate changes resulting from an ischemic bowel and show important coexistent findings or complications. Secondary signs such as portal venous gas, bowel wall thickening, and pneumatosis can also help confirm the diagnosis. Given the often subtle nature of radiological signs of acute small bowel ischemia, MSCT can make a significant contribution to diagnosis of this complex condition.

The small bowel extends from the duodenojejunal flexure (ligament of Treitz) to the ileocecal valve. It is approximately five meters long. The proximal two-fifths are known as the jejunum, and the distal three-fifths as the ileum. The circular valvulae conniventes are about 1 to 2 mm thick and extend through the entire length of the small bowel. Blood is supplied from the branches of the superior mesenteric artery, which arises from the abdominal aorta. The small bowel is attached to the posterior abdominal wall by its mesentery, allowing it to be relatively mobile within the peritoneal cavity. The jejunum lies in the left upper abdomen, and the ileum mainly in the right lower abdomen.

The ischemic process causes a wide spectrum of bowel injuries ranging from increased capillary permeability of the microvasculature and mucosal epithelium to transmural necrosis in 24 to 48 hours. Changes due to bowel necrosis can be seen on CT within just a few hours of onset.

Underlying causes of acute occlusive ischemia include thromboembolism (50% of cases), bowel obstruction, neoplasms, vasculitis, inflammatory conditions, trauma, chemotherapy, radiation, and corrosive injury. Nonocclusive ischemia can be secondary to systemic conditions that alter cardiac output, such as heart failure, hypovolumic shock, sepsis, and medication reactions. Acute occlusive ischemia is usually caused by embolic occlusion of the superior mesenteric artery. Emboli may arise secondary to atrial fibrillation or following left ventricular thrombi in a post-myocardial infarction patient. Superior mesenteric vein occlusion is an uncommon etiology of occlusive ischemia. This is associated with a number of conditions: portal hypertension, abdominal surgery, trauma, hypercoagulable states, and inflammation to the abdomen or pelvis.

Clinical signs and symptoms of acute ischemia are nonspecific, leading to delayed diagnosis and management. Patients usually present with sudden, ill-defined abdominal pain. Leukocytosis, acidosis, and pyrexia may also occur, but these are not specific signs.

RADIOLOGY'S ROLE

Plain radiographs are generally performed in patients presenting with acute abdominal pain. These remain an important tool in patients suspected of mesenteric infarction, and they can provide information complementary to CT.¹ Plain-film findings of bowel wall thickening, mucosal irregularity, and a generalized ileus tend to occur during later stages of acute bowel ischemia. More specific signs, such as intramural air and portal gas, can be seen in cases of intestinal infarction. Observation of portal venous gas on a plain radiograph generally indicates a poor prognosis.

Ultrasound has a limited role in the diagnosis of acute intestinal ischemia, though it can provide clues to the diagnosis by demonstrating bowel wall thickening and ascites. Dynamic assessment of the bowel, including peristaltic activity, can also be helpful.

At Southmead Hospital, we perform CT on a four-slice scanner (MX8000, Philips Medical Systems), using a standard protocol of 120 kV, 200 mA (effective 150 mAs), pitch 1, and 4 x 5-mm slice width. If ischemia is suggested as a clinical possibility, we prefer not to administer oral contrast or to administer 500 to 700 mL of water as oral contrast material. This is important, because in patients with suspected mesenteric ischemia, low-attenuation contrast material allows better visualization of the enhancing bowel wall, and multiplanar and 3D reformatting of the mesenteric vessels is made easier if high-attenuation oral contrast is not present. We inject a 100-mL bolus of intravenous contrast at 3 mL/sec. Images acquired in the arterial and portal venous phase are reconstructed and then reviewed on a workstation.

Bowel wall thickening, dilated bowel loops, and ascites can all be seen on CT during the early stages of ischemia, but these signs are rather nonspecific (Figure 1). The same signs can be associated with a number of infections, inflammatory bowel disease, and primary and secondary neoplasms, including lymphomas.^2,3 Bowel wall thickening is the most common sign of acute gastrointestinal ischemia, though its nonspecific nature complicates diagnosis.⁴ The so-called halo or target sign refers to alternating rings or layers of high and low attenuation in the thickened intestinal wall. This can be caused by submucosal edema or intramural hemorrhage (Figure 1).

The most specific ischemic changes seen on CT are gas in the splanchnic and portal vasculature and intramural gas. Portal vein gas by itself is not pathognomonic of bowel infarction in adults. It may also be seen in intra-abdominal abscesses and ulcerative colitis.⁵ Intramural gas in association with portal venous gas is an equally specific but more sensitive finding (Figures 2 and 3). A study combining plain abdominal radiographs with contrast-enhanced MSCT showed specific findings of abdominal infarction in 65% of proven cases.¹

Contrast-enhanced CT can identify thromboembolism as the causative etiology of ischemia (Figure 4). Occlusion from both emboli and thrombi, however, can lead to reflex vasoconstriction of distal mesenteric blood vessels that mimics nonocclusive mesenteric ischemia. A low-attenuation clot may be identified in the superior mesenteric vein, surrounded by a rim of contrast, in cases of venous infarction (Figure 5).⁶ Proximal extension of the thrombus into the portal vein may also occur.

Dynamic bowel obstruction can be secondary to a number of conditions, such as adhesions, hernias, and volvulus (Figure 6). It may not be possible to identify the exact cause of bowel obstruction, especially in cases of adhesions. MSCT may provide supportive findings for an early diagnosis of mesenteric hypoperfusion or ischemia, and it can also rule out a number of other causes of acute abdominal pain. The performance of CT in diagnosing ischemia and complete obstruction in cases of small bowel obstruction has been assessed in a comprehensive literature review.⁷

While neoplasms are an unusual and uncommon cause of small bowel ischemia, radiologists should remain aware of their potential role (Figure 7).

FUTURE TRENDS

Selective mesenteric angiography has long been regarded as the gold standard in diagnosing acute mesenteric ischemia. MSCT is now emerging as a viable alternative. It provides high-quality images of the vascular tree and the target organ, and it may offer an alternative diagnosis. Endovascular treatment does have a role on its own or in combination with surgery, but treatment choice may also depend on local availability of emergency catheter angiographic services. Early diagnosis before bowel infarction can improve survival rates.⁸ Although fibrinolytic agents have been trialed in acute ischemia secondary to the superior mesenteric artery embolus, surgery remains the main treatment of choice.⁹

Diagnosis of acute small bowel ischemia is facilitated by radiological examinations such as MSCT. It is often the combination of clinical, laboratory, and radiological signs, however, that leads to a correct diagnosis. CT signs of early small bowel ischemia include bowel wall thickening, vascular engorgement, the target or halo sign, and mesenteric edema. Bowel infarction is likely if portal venous or intramural gas is seen, and the bowel wall shows poor or no enhancement.

DR. BURNEY and DR. PRABHU are specialist registrars in clinical radiology at Southmead Hospital in Bristol, U.K., and DR. LYBURN and DR. HOPKINS are consultant radiologists at Cheltenham General Hospital in the U.K.

REFERENCES

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