Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

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Table 4.7. Conditions associated with mechanical small bowel obstruction

Intraluminal

Bezoar (foreign body)

Gallstone

Intussusception

Parasites

Polyp (neoplastic or nonneoplastic)

Meconium

Intramural

Infiltrating neoplasm

Hematoma and hemorrhage

Inflammation

Crohn’s disease

Infection

Ischemia

Radiation enteritis

Congenital stricture

Extrinsic Adhesions

Adjacent inflammation/abscess involving bowel Hernia (internal or external)

Volvulus

Extrinsic neoplasm, cyst, etc. compressing bowel Congenital bands

Endometriosis

sion tube early in their course, even before a definitive diagnosis is established, followed by watchful observation, a poorly defined term having a different meanings for different physicians. Gastrointestinal radiologists prefer that these patients undergo imaging studies without undue delay. What should be the sequence of imaging studies?

Because of ready availability, conventional radiographs should be obtained before bowel decompression. Likewise, CT is best performed before bowel decompression. If conventional radiographs do not reveal small bowel dilation, the decompression catheter, preferably a sumptype catheter rather than a simple nasogastric tube, is advanced distal to the pylorus. Barium instilled through the tube provides a fluoroscopic small bowel study, and is used for enteroclysis or as an aid to identify the small bowel during a subsequent CT study.

Occasional but recurrent publications advocate the use of a water-soluble contrast agent to

study small bowel obstruction. Although in infants use of a nonionic agent is advantageous, in the study of adult small bowel obstruction the water-soluble contrast agents have little to recommend. Erratic amounts of additional fluid are drawn intraluminally, the already distended bowel becomes even more distended,the patient becomes more uncomfortable, and often the only conclusion possible from such a study is identifying whether the bowel is grossly dilated or not. In fact, some of the studies claiming a superiority for CT over a small bowel contrast study used water-soluble contrast agents rather than barium for the comparison study, leading to preordained results. In most patients, if bowel wall integrity is not compromised, a small bowel contrast study should be performed with barium.

Some investigators use solid, 4-mm radiopaque markers to identify a partial small bowel obstruction (93); the markers coalesced close to the obstruction. This technique is currently little used and appears inferior to more conventional imaging tests.

Small bowel dilation per se, without colonic dilation, is not pathognomonic of obstruction; for example, patients with celiac sprue tend to have dilated, fluid-filled loops of small bowel, with an overall appearance similar to distal small bowel obstruction. On the other hand, regardless of the imaging study performed, small bowel obstruction is implied if proximal small bowel is dilated while more distal loops are collapsed. These dilated proximal loops tend to be filled mostly with liquid intestinal content. Numerous bowel caliber dimensions have been published in an attempt to differentiate between normal and dilated caliber bowel, but these measurements have little use in clinical practice; the borderland between normal caliber and dilated bowel is sufficiently vague and varies considerably depending on site, degree, and age of an obstruction, resultant peristaltic activity, and presence of underlying disease.

The presence of massively dilated small bowel loops suggests a chronic, long-standing obstruction.

Conventional Radiography

A number of investigators advocate CT rather than conventional radiographs when bowel obstruction is suspected clinically. Generally

three questions are raised in this setting: Is obstruction indeed present? Where is the obstruction? What is the etiology of an obstruction? From a surgical perspective the first question is most important and generally both the first and often also the second question can be answered by conventional radiography. The underlying etiology is of less interest to the surgeon, except to exclude ischemia and incarceration, etiologies notoriously difficult to detect early by any imaging modality. Bowel obstruction (or the lack of it) can be approached with either an oral barium study or CT, although many radiology departments have evolved to the point where CT is more readily available and is encouraged.

Conventional radiographs aid in differentiating complete or high-grade partial small bowel obstruction from low grade or no obstruction; findings suggesting a high-grade small bowel obstruction include the presence of gas-fluid levels at a differential height in the same loop and presence of a mean gas-fluid level width ≥25mm on upright abdominal radiographs (94).

Barium Study

A small bowel barium study is a viable option in the patient with a suspected small bowel obstruction. Contraindications to barium are few: an acute abdomen, suspected bowel perforation, or distal colonic obstruction. Some surgeons still argue for a water-soluble agent to avoid possible barium spillage into the peritoneal cavity during subsequent surgery, an argument of dubious validity; the surgeon should spill neither barium nor the usually infected intestinal contents proximal to a small bowel obstruction.

A number of studies have documented the advantages of enteroclysis over a conventional small bowel study, yet in most institutions enteroclysis is rarely performed in this clinical setting.

Prior to performing a barium study, be it a conventional small bowel study, enteroclysis, or a CT-enteroclysis study, thought must be given to whether another diagnostic study will be needed that is obviated by the intraluminal retention of barium. Even endoscopists complain if their field of view is obscured by barium.

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Small bowel obstruction is not a contraindication to an antegrade barium study. Barium proximal to a small bowel obstruction remains in suspension. It does not become more viscous, as happens in the colon, and in the small bowel barium does not influence the degree of obstruction. Contrary to an occasional surgical report (95),barium does not make a small bowel obstruction worse. Likewise, the occasional surgeon’s request to use “thin” barium in a setting of small bowel obstruction is meaningless; marked fluid retention proximal to an obstruction invariably leads to barium dilution.

A barium enema with reflux into the small bowel is a viable study in a patient with suspected distal small bowel or colon obstruction. This study differentiates between obstructive ileus and adynamic ileus in most patients and often provides an etiology for an obstruction.

Computed Tomography

Reported CT sensitivities in detecting small bowel obstruction have ranged up to 100%, with examiner enthusiasm and interest in the technique probably influencing results. Nevertheless, a number of studies over the past decade have shown that CT not only detects a small bowel obstruction but also often identifies a cause. CT can correctly distinguished between small bowel obstruction and adynamic ileus in almost all and established a cause of obstruction in most. In general, CT is more accurate in a setting of a high-grade rather than low-grade obstruction.

Comparing conventional radiography and CT in patients with suspected small bowel obstruction, the positive predictive value of conventional radiography was 80% and of CT 95% (96); of interest is that the false-negative rate was 8% with conventional radiography but only 1.6% with CT. Nevertheless, the authors recommended that conventional radiography still be the initial study of choice. The authors did not evaluate and compare their CT results to barium studies.

A blinded retrospective analysis comparing conventional radiography and CT in patients suspected of having a small-bowel obstruction achieved a similar overall accuracy with both examinations (Table 4.8); CT, however, revealed

a cause of obstruction in 95%, a finding not usually possible with conventional radiography. The authors concluded that conventional radiography should remain the initial imaging study in patients with suspected small bowel obstruction. Comparing enteroclysis and CT in clinically equivocal small bowel obstruction, CT correctly identified 79% of 43 proven intestinal obstructions (98); CT was most accurate with a complex or long narrowed segment and least accurate with short stenotic segments. It was falsely positive in two patients with mesenteric infarction; enteroclysis had no false positives or false negatives. The author concluded that enteroclysis is more accurate in detecting and localizing an obstruction, but CT is superior in determining the cause of the obstruction and in detecting any underlying strangulation.

A retrospective CT study in children achieved a sensitivity of 87% and specificity of 86% for detecting small bowel obstruction (99); the etiology of the obstruction was correctly identified in 47% of scans.

The conclusions of these studies apply to the patient with a suspected small bowel obstruction where an examination is performed to confirm that an obstruction is indeed present and to suggest an etiology. Another large group of patients consists of those hospitalized with an obstruction but due to delays, often erroneous but well-intentioned, the obstruction has now resolved clinically and the radiologist is asked to suggest an etiology for the previous obstruction. The analogy is similar to a chest

radiograph being obtained after pneumonia has cleared. In such a situation enteroclysis appears to be the examination of choice in detecting a subtle underlying tumor, residual adhesion, or similar abnormality; CT is of limited value in such a setting.Also, CT appears to have a limited role in evaluating a partial small bowel obstruction. In such a setting enteroclysis appears superior.

Typical CT criteria for small bowel obstruction consist of a discrepancy between more proximal dilated and more distal nondilated small bowel loops (Fig. 4.23). Distal small bowel obstruction consists of generalized small bowel dilation and a narrowed colon lumen. A CT finding of gas and particulate material in dilated segments of small bowel is considered abnormal, and most of these patients have a more distal small bowel obstruction, although an occasional patient with slow small bowel transit, regardless of etiology, has similar findings. It is, however, an uncommon finding in obstruction.

Thickened bowel wall containing a thin hyperdense component, called a target sign or halo sign, represents fluid within the bowel wall and is seen with bowel ischemia. A somewhat similar appearance is found with small bowel intussusception and with an inflamed loop, such as in Crohn’s disease. This sign is also seen in some normal bowel where it represents fat infiltration and appears related to obesity (100).

Rather than relying on axial images, coronal and sagittal images are often more helpful in

Figure 4.23. An incisional hernia causing small bowel obstruction after laparoscopic surgery. CT identifies both the hernia (arrow) and marked proximal small bowel dilation. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

identifying a point of obstruction and suggesting a possible etiology.

Computed tomography enteroclysis achieves a sensitivity similar to that of conventional enteroclysis in detecting a site of partial small bowel obstruction, assesses the degree of obstruction, and often also identifies a cause of the obstruction. This study is superior to conventional CT, especially in a setting of partial small bowel obstruction. Still, CT enteroclysis should be approached with caution; instillation of large amounts of contrast into a small bowel already distended by an obstruction simply distends it further and risks inducing vomiting and aspiration.

Ultrasonography

Ultrasonography studies of patients with acute abdominal pain have achieved sensitivities of 75% to 95% in detecting an obstruction, yet closer perusal makes it difficult to place these studies in a proper perspective. Ultrasonography is more often employed to study the bowel in Europe rather than the United States or Canada, where it has been supplanted by CT and, to a lesser extent, by MRI.

Conventional US readily identifies extraluminal fluid; such fluid is present in about two thirds of patients with small bowel obstruction.

Duplex Doppler US has been used to differentiate between obstruction and paralytic ileus, with results similar to those obtained during a

physical examination, namely, in the early stages of mechanical obstruction. Doppler US reveals hyperperistalsis proximal to the obstruction; in long-standing obstructions, as expected, a decrease in intensity and duration of peristalsis predominates. Dilated and atonic segments are identified in adynamic ileus, although some bowel motility usually is still present. Fluoroscopic observation of contrast-filled loops of bowel identifies similar peristaltic activity, although most fluoroscopists tend to rely little on these observations.

Magnetic Resonance Imaging

Magnetic resonance imaging is not often employed in suspected small bowel obstruction; rather, obstruction is occasionally detected when a study is performed for other reasons. Preliminary evidence suggests, however, that a MR study dedicated to the small bowel is rather accurate. A prospective study of patients with suspected inflammatory bowel disease or small bowel obstruction found that MR enteroclysis findings were similar to those obtained with conventional enteroclysis or surgery (6).

Adhesions

The most common cause of small bowel obstruction is adhesions, usually secondary to prior surgery. The surgery does not necessarily have to involve small bowel. In one hospital, small bowel obstruction due to adhesions was highest after appendicectomy and colonic resections and lowest after gallbladder and pancreatic surgery (101). Even some urologic procedures lead to small bowel obstruction. Adhesions range from single to multiple. Some adhesions fixate or kink a loop of bowel; others, extrinsic to the bowel, act as a nidus for an internal hernia.

Computed tomography reveals an adhesive obstruction as a sharp bowel angulation at the site of obstruction. A barium study identifies an adhesion as a smooth, extrinsic, linear impression at the point of change in bowel caliber.

Cancer

Not all obstructions in patients with a known cancer are secondary to tumor; about one third

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have a benign etiology. At times CT can suggest an etiology for intestinal obstruction. Patients with known intraabdominal malignancy and small bowel obstruction often have other sites of obstruction not detectable by CT (102); additional information altering subsequent treatment is often provided by a contrast enema. In fact, the entire gastrointestinal tract should be studied in cancer patients because multiple sites of obstruction are relatively common.

Surgery relieves most small bowel obstructions. In patients with a malignant obstruction, however, surgical relief of an obstruction usually does not influence survival.

Closed Loop Obstruction/Volvulus

A closed loop obstruction consists of a loop of bowel obstructed at two points. The closed loop refers to bowel lumen and not blood vessels. In time, increasing intraluminal pressure leads to venous stasis, ischemia, and strangulation. Although many closed loop obstructions eventually evolve into strangulation, from an imaging viewpoint the two entities can be distinguished: a closed loop obstruction implies lumen obstruction; strangulation signifies bowel ischemia. Obstruction in a hernia is a type of closed loop obstruction.

A volvulus is due to intestinal twisting and resultant lumen obstruction.A volvulus is a type of closed loop obstruction if the obstruction involves both inflow and outflow to the twisted loop.

Because etiologies of volvulus differ between infants and adults, these populations are best approached separately.

Infants

Small bowel volvulus in infants is almost always associated with midgut malrotation or nonrotation. Malrotation predisposes to a small bowel twist around its mesentery, which contains the superior mesenteric artery and vein; resultant vascular compromise leads to small bowel necrosis unless emergent surgical intervention ensues. The proximal end of malrotation is in the descending duodenum, distal to the papilla of Vater, and resultant luminal obstruction at this level accounts for the common clinical pres-

entation of bilious vomiting, often within first days of life.

Conventional radiographs often reveal partial rather than complete duodenal obstruction. Complete duodenal obstruction and a gasless distal bowel is more common with duodenal atresia. Ischemia developing in a setting of neglected partial volvulus also results in a gasless abdomen. A conventional radiographic finding compatible with duodenal obstruction is seen only in about half of the infants requiring surgical correction. When present, however, a need for further imaging prior to surgery is debatable.

The duodenal corkscrew appearance on a contrast study is familiar to most radiologists. Barium is used by most, with a minority preferring water-soluble low osmolarity contrast agents. Some advocate US using water as a contrast agent, but such an approach is very operator dependent and not widely practiced.

The usual anatomic relationship of superior mesenteric artery and vein is reversed in malrotation. Ultrasonography reveals a whirlpool, with the superior mesenteric vein and mesentery twisted around the superior mesenteric artery. Although this finding is useful in detecting malrotation both with CT and US, falsepositive and false-negative findings do occur. In one study, however, color Doppler US identified a whirlpool sign that rotated clockwise (with caudal movement of the transducer) in 12 of 13 pediatric patients with surgically confirmed midgut volvulus and counterclockwise in three patients without midgut volvulus (103); the sensitivity of this clockwise US whirlpool sign in detecting midgut volvulus was 92% and the specificity 100%, and the authors suggest that color Doppler US should be an initial imaging study in children with suspected midgut volvulus.

Adults

In adults a closed loop small bowel obstruction is most often secondary to adhesions. Small bowel volvulus without a predisposing cause is not common in Western Europe or North America, but in parts of Turkey it represents 13% of small bowel obstruction (104). In some parts of the world volvulus is associated with ingestion of large quantities of milky foods. Ascariasis infestation predisposes to small