Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

bowel volvulus. On a rare occasion a small bowel mesenteric tumor, even a lipoma or lymphangioma, acts as a nidus for a twist, leading to volvulus. A rare cause was a vena cava perforation by a Greenfield filter resulting in smallbowel volvulus (105).

Some adults with chronic small bowel volvulus have few clinical signs and symptoms and some even do not require surgical correction.

The classic conventional radiography description of a closed loop small bowel obstruction is a coffee bean sign, often a late finding caused by the mostly fluid-filled dilated loops. The diagnosis is established with barium studies, CT, MRI, or even angiography. Computed tomography shows radial convergence of stretched mesenteric vessels toward a point of obstruction. The involved small bowel loops taper toward the point of obstruction, and two converging bowel loops, representing the afferent and efferent loops, are identified at the point of obstruction. At times such converging loops are identified even before clinical obstruction is evident. Small bowel loops involved in the closed loop are dilated and have a C or U shape. Veins draining the closed loop are engorged. Eventually bowel and mesenteric folds become edematous and radiate toward the site of torsion.

Computed tomography and US of a volvulus reveal a similar appearance to a closed loop obstruction, but a volvulus also contains a twist or whirl due to rotation of the involved loop of bowel and its mesentery; the radiating folds mimic a spoke wheel. Mesenteric edema and ascites are relatively common findings. In a volvulus involving most of the small bowel, the superior mesenteric artery and vein are reversed in their relative positions.A mesenteric twist is not pathognomonic of volvulus but is also seen with adhesions and prior bowel resection.

Ileosigmoid Knot

A rare cause of both small bowel and sigmoid colon obstruction, the ileosigmoid knot consists of ileal loops wrapping around the base of a redundant sigmoid. Both an elongated small bowel mesentery and a redundant sigmoid colon predispose to this entity. The twisted ileal loops readily become ischemic and gangrenous; thus there is a high prevalence of strangulation.

ADVANCED IMAGING OF THE ABDOMEN

Occasionally conventional radiographs suggest the diagnosis by identifying a small bowel obstruction,a dilated sigmoid colon,and medial displacement of the cecum and distal descending colon. A barium small bowel study simply shows small bowel obstruction, although this study tends to be unsatisfactory due to the often associated peritonitis and superimposed slow bowel transit time. Computed tomography is suggestive by showing a twist, or whirl, of the involved bowel.

Strangulation

In a strangulated obstruction, blood flow to the obstructed loop is compromised, leading initially to ischemia and eventually to bowel necrosis. The diagnostic studies must thus be performed with dispatch.

The most common cause of bowel strangulation is a loop trapped in a hernia, either internal or external. An acute volvulus often also compromises blood supply to the twisted loop and leads to strangulation. Strangulating obstructions are difficult to diagnose both clinically and with conventional radiologic techniques. Enteroclysis should detect a closed loop obstruction, but cannot detect strangulation unless bowel edema or other signs of ischemia are evident. A target sign, mesenteric edema or infiltration by blood, and pneumatosis intestinalis are evidence of ischemia. Strangulation progressing to pneumatosis intestinalis, regardless of how diagnosed, signifies bowel wall necrosis.

Computed tomography of a strangulating loop shows a serrated beak at the site of the bowel obstruction. Intramural edema and hemorrhage develop due to ischemia. Contrastenhanced CT reveals delayed enhancement of diseased bowel loops. Intramesenteric hemorrhage is identified in some patients. Computed tomography findings pointing to a strangulated obstructions rather than a simple small bowel obstruction are poor or no bowel wall enhancement, findings of low sensitivity but high specificity. Less often found are an unusual course for mesenteric vessels and mesenteric vessel engorgement.Ascites develops eventually. Overall,CT identifies about 85% of patients with a strangulated obstruction, keeping in mind that higher sensitivities can be achieved at the expense of lower specificity.

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Abnormal US findings are detected in about 90% of patients with bowel strangulation; these consist of small bowel distention, aperistalsis, bowel wall thickening, and intraperitoneal fluid, findings also seen with prolonged simple small bowel obstruction, but with strangulation these findings develop earlier in the course. An US finding of fluid–fluid levels due to intestinal content sedimentation throughout the small bowel is found with small bowel obstruction, regardless of cause, once adynamic ileus ensues. On the other hand, US detection of intraluminal fluid–fluid levels in a segment of adynamic bowel together with peristalsis in other loops suggests a strangulating obstruction (106).

Figure 4.24. Transient jejunojejunal intussusception in a patient with terminal ileal Crohn’s disease, a rare association.

Intussusception

Ileocolic intussusception is discussed in the Chapter 5.

Clinical

The final end point of an intussusception is invagination of one loop of bowel (intussusceptum) into another (intussuscipiens). Almost any segment of the small bowel and colon can be involved. Most intussusceptions are antegrade, with only an occasional one being retrograde. Detected on barium studies or CT, most are transient and cannot be duplicated. Experienced radiologists detect random, transient nonobstructing intussusceptions sufficiently often that these are often considered to be a normal variant (Fig. 4.24). Indeed, whether such transient intussusceptions are of clinical significance in the absence of other findings is debatable. Nevertheless, transient intussusceptions are more common than normal in certain disorders, such as celiac sprue.

It is the longer, more persistent intussusceptions, with the intussusceptum consisting not only of bowel but also its associated mesentery, that cause trouble both in adults and children. Among adults with one or more intussusceptions detected with CT or MR, 30% had a neoplastic lead point (107). Both jejunal and ileal polyps, benign and malignant, act as lead points. A Meckel’s diverticulum or enlarged lymph nodes occasionally serve as a lead point both in adults and children. Hypertrophied Peyer’s patches, a duplication, or even ectopic

pancreas are encountered as a lead point mostly in children. Enteric intussusceptions due to metastases are rare. Rare also is an intussusception due to Crohn’s disease. Small bowel intussusception is a complication encountered after prior abdominal surgery. At times intussusception is obscured by postoperative or chemotherapeutic findings.

Jejunal intussusceptions in children are not common, and although some are idiopathic, more often a polyp, such as a hamartoma, is a lead point. More distal intussusceptions in young children tend to be idiopathic.

In distinction to infants, the signs and symptoms in most adults with intussusception are nonspecific. An obstruction is not even initially suspected in many patients.

Imaging

Jejunogastric intussusception is one of the complications encountered after a hemigastrectomy and gastrojejunostomy (Billroth II operation). A barium study reveals coiled-spring–appearing jejunal loops within the gastric remnant, thus confirming the diagnosis. Computed tomography or US should also establish the diagnosis; a coiled-spring appearance to the jejunal loops (intussusceptum) within the stomach should be evident.

In children with an ileoileocolic intussusception, an air enema tends to identify the intussusceptum as two or more intraluminal polyps

once it has been reduced to the ileocecal valve; this appearance differs from the smooth or somewhat lobulated appearance found with most simple ileocolic intussusceptions to allow differentiation of ileoileocolic from ileocolic intussusceptions.

Computed tomography of an enteroenteric intussusception reveals a characteristic target lesion containing an intraluminal soft tissue tumor (intussusceptum), asymmetric mesentery, and, at times, a dilated intussuscipiens (Fig. 4.25). When imaged along its long axis, an intussusception has more of a sausage-shaped appearance. Occasionally CT also defines an underlying lead point.

The US appearance of an uncomplicated enteric intussusception is that of a target lesion.

Fluid within bowel provides intrinsic contrast for T2-weighted MRI. A bowel-within-bowel or coiled spring appearance is seen on both axial and coronal images.

Gallstone Ileus

Most gallstones impact in the distal ileum (not in the terminal ileum). The next most common site is in the duodenum (discussed in Chapter 3), and the least common is colonic obstruction, where it occurs proximal to a stricture.

A typical clinical scenario is the elderly woman with no previous abdominal surgery presenting with small bowel obstruction. At the other extreme, gallstone ileus has been detected even in a teenager.

ADVANCED IMAGING OF THE ABDOMEN

Gallstone ileus is often suspected from a conventional radiographic examination. The classic Rigler triad consists of small bowel obstruction, biliary tract gas, and a distal small bowel calcification, although all three findings are uncommon in any one patient. Similar findings are also evident with CT, and preliminary data suggest that CT allows a more confident diagnosis.

Abdominal US detects small bowel obstruction and does locate an ectopic gallstone in some patients.

Mortality with gallstone ileus remains high, partly due to an often late diagnosis, aged patient population, and frequently coexisting other medical problems.

Foreign Body

Gastrointestinal foreign bodies are encountered in both children and adults (Fig. 4.26). A majority pass spontaneously without complications.

Small bowel bezoars are less common than gastric ones. Risk for bezoars increases after vagotomy and partial gastric resection. An unusual small bowel obstruction developed during enzymatic treatment for a gastric persimmon bezoar (108). Conventional radiographs simply suggest bowel obstruction without identifying a bezoar. Some bezoars contain sufficient gas to suggest pneumatosis intestinalis. A barium study identifies the intraluminal bezoar. Computed tomography findings consist of an intraluminal heterogeneous soft tissue tumor, often containing focal gas.

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Figure 4.26. A swallowed tube-like foreign body contains contrast (arrow). (Courtesy of Algidas Basevicius, M.D., Kaunas Medical University, Kaunas, Lithuania.)

A trichobezoar is an uncommon cause of small bowel obstruction. Most are suggested by conventional radiography or a barium study and further imaging is not warranted.

Retained surgical gauze eventually becomes encased in a fibrotic mass that can lead to small bowel obstruction or a fistula and mimic a neoplasm. In a region of the world endemic for tuberculosis, a retained surgical sponge simulated intestinal tuberculosis (109). Retained surgical staples are generally of little significance, although when incorporated in an adhesion they appear next to an obstruction. Occasionally a severed gastrostomy tube obstructs the small bowel.

The current popularity of a high-fiber diet is also not without risk. Thus in one patient recurrent small bowel ileus was caused by ingestion of high-fiber canned asparagus (110).

Other Obstructions

Acute sigmoid diverticulitis is a not uncommon cause of small bowel obstructions; a loop of small bowel becomes trapped by focal inflammation or simply becomes adynamic.

Any intramural infiltration sufficient to narrow the lumen eventually leads to lumen obstruction. Similar to the duodenum, intramural hemorrhage or a hematoma can obstruct the jejunal or ileal lumen; these obstructions tend to be transient, and they clear as a hematoma resolves.

Small bowel obstruction is a rare but major complication during pregnancy. As in the general population, those women who have had previous abdominal surgery are at increased risk of developing a small bowel obstruction.

Eosinophilic gastroenteritis–associated bowel stenosis is a rare cause of small bowel obstruction.

Therapy

Once appropriate imaging studies are obtained and an obstruction diagnosed, an indwelling catheter is useful in decompressing the small bowel; this should make the patient more comfortable and aid the return of bowel function.

Some surgeons claim a purported therapeutic effect by hyperosmotic water-soluble contrast in relieving small bowel obstructions, but radiologists generally scoff at such claims. A prospective, randomized study of patients with suspected postoperative small bowel obstruction comparing the effect of instilling 100mL of a hyperosmotic water-soluble contrast agent via a nasogastric tube found no difference in length of hospital stay or in the rate of complications, whether a contrast agent was used or not (111).

Traditionally, patients with an unresolving small bowel obstruction underwent a laparotomy. Laparoscopic therapy of small bowel obstruction is also performed, especially in a setting of postoperative adhesions.

Adynamic Ileus

Acute

Peritoneal inflammation, either primary or secondary, leads to bowel paralysis. The resultant adynamic ileus ranges from focal (sentinel loop) to generalized and involves both the small and large bowel. Some surgeons attempt to distinguish between a normal postoperative ileus and adynamic ileus. Most radiologists consider these to be the same entity.

Ingestion of certain raw foods induces adynamic ileus. In Japan, adynamic ileus developed in some patients after eating raw squid (112); the squid contained a specific type of

larva.Acute small bowel angioneurotic edema is also a cause of adynamic ileus.

Acute adynamic ileus can generally be suspected from conventional radiographs; both the small bowel and colon are dilated. Superficially, distal colonic obstruction and an incompetent ileocecal valve mimic these findings.

Chronic (Pseudo-Obstruction)

Chronic adynamic ileus, also called intestinal pseudo-obstruction, is not a single entity but an intestinal condition associated with a number of neuropathies and myopathies. Familial visceral neuropathy results in chronic myenteric plexus destruction leading to poor motility and bowel distention on a chronic basis. Myotonic muscular dystrophy and familial visceral myopathy involve intestinal smooth muscle and lead to small bowel dysmotility and distention (Fig. 4.27). Endoscopic biopsy is generally not helpful in these patients because abnormalities are in the external muscle layers. Barium contrast studies are often necessary to exclude mechanical obstruction.

Figure 4.27. Familial oculo-intestinal myopathy in a 21–year- old patient. Enteroclysis reveals marked bowel hypotonia. The bowel wall is of normal thickness. (Courtesy of Arunas Gasparaitis, M.D., University of Chicago.)

ADVANCED IMAGING OF THE ABDOMEN

At times intestinal pseudo-obstruction is associated with an underlying neoplasms, such as lung carcinoma or carcinoid, with the intestinal dilation presumably secondary to a paraneoplastic process. In some patients intestinal dysmotility resolves after tumor resection. Intestinal pseudo-obstruction and a secretory diarrhea were the initial presentation in a man found to have Crohn’s disease (113).

Infants with fetal alcohol syndrome develop intestinal pseudo-obstruction, probably due to an enteric neuropathy.

Various radiopaque markers have been used to measure small bowel transit time, but they have achieved limited clinical application. Gastric emptying, small bowel transit and colonic transit can be combined into a single scintigraphic whole-gut transit time study. The clinical relevance of such a test, however, is not clear.

Therapy with erythromycin and cisapride are often effective therapies in this condition.

Systemic Sclerosis (Scleroderma)

Systemic sclerosis is readily differentiated from celiac disease. In the latter entity the duodenum tends not to be dilated, close to normal small bowel motility is evident, dilation involves primarily the distal small bowel, and more intraluminal bowel content is present—findings differing from those seen in systemic sclerosis.

Some patients with systemic sclerosis develop pneumatosis cystoides intestinalis and even idiopathic pneumoperitoneum. Although a de novo pneumoperitoneum usually suggests a surgical abdomen, some of these patients are managed medically. Nevertheless, an autopsy study of patients with systemic sclerosis found bowel perforation to be more common than expected (114); these perforations ranged from the esophagus to colon, with some being initially silent. The authors suggest that the bowel wall in these patients is inherently weak, and cautioned physicians performing invasive procedures to keep this in mind.

Other Disorders

Collagenous sprue mimics celiac disease both clinically and radiologically. Duodenal and

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jejunal biopsies reveal villous atrophy in both entities, but in collagenous sprue collagen is deposited in small bowel lamina propria.

The small bowel is not a major target organ in Chagas disease. An occasional patient develops marked jejunal dilation due to stasis, bacterial overgrowth, and resultant malabsorption.

The small bowel appearance in dermatomyositis is similar to that seen in sprue, although findings in sprue tend to be more striking.

Ehlers-Danlos syndrome is an inherited connective tissue disorder resulting in skin hyperextensibility, articular hypermobility, and tissue fragility. The bowel is dilated. These patients are prone to developing intestinal hemorrhage and bowel perforation.

Generally a carbon-14 D-xylose breath test is used in patients with suspected small bowel bacterial overgrowth. One limitation of this test is that in individuals with gastrointestinal motor dysfunction, delayed gastric emptying tends to prevent carbon-14 D-xylose from reaching the small bowel loops involved by bacterial overgrowth and thus results in a negative test. A liquid phase gastric emptying test performed at the same time as D-xylose test corrects for gastric stasis.

Perforation/Fistula

Bowel perforation and resultant peritonitis is discussed in Chapter 14.

Migration of a biliary stent into the small bowel is generally of little consequence; rarely, it has led to small bowel perforation.An uncommon cause of bowel perforation is a suction biopsy. A swallowed toothpick or sharp bone can lead to small bowel perforation. Some perforations seal over spontaneously and have few, if any, sequelae; others develop into peritonitis or an abscess.

Pneumoperitoneum is the classic sign of bowel perforation. As is well known, not all perforations result in a pneumoperitoneum; intraperitoneal fluid is more common. Computed tomography visualization of bowel wall discontinuity at a perforation site is rare. Oral contrast extravasation likewise is rarely identified, although renal excretion after oral administration of a water-soluble contrast agent

should raise suspicion of bowel perforation. A caveat: in some patients orally ingested watersoluble contrast is absorbed from normal bowel. Nonabsorbable barium contrast agents are not used in a clinical setting of suspected acute bowel perforation.

A fistula involving the small bowel can develop to any adjacent structure. In adults most fistulas are secondary to either prior surgery or Crohn’s disease. Less often a small bowel malignancy or disorders in adjacent structures lead to a fistula. A small bowel fistula to bone results in osteomyelitis.

Subtle small bowel fistulas and sinus tracts are best studied with enteroclysis.

Most enterocutaneous fistulas are initially managed medically. Fluoroscopically guided fistulous tract and associated bowel catheterization should be considered for those not healing; eventual fistula closure can be achieved in most. Several chronic enterocutaneous fistulas were successfully treated by injecting biologic glue (N-butyl-2-cyanoacrylate-histoacryl) into the fistulous tract (115).

Diverticula

Meckel’s diverticula were discussed earlier (see Congenital Abnormalities).

Jejunal and ileal diverticula are not rare. Most are innocuous and considered to be incidental findings (Fig. 4.28). An occasional diverticulum will bleed (at times massively), perforate, or even obstruct the small bowel. A rare diverticulum evolves into diverticulitis or an intraabdominal abscess, involves extraperitoneum structures, or develops into an abdominal wall abscess. Clinically, ileal diverticulitis mimics appendicitis.

Tumors, including adenocarcinomas and sarcomas, do develop in diverticula; both barium studies and CT detect them, if they are large enough.

Both small bowel and colonic diverticula are more prevalent in patients with connective tissue disorders such as Marfan’s syndrome and Ehlers-Danlos syndrome. These patients also develop diverticula-related complications.

Occasionally detected is extensive diverticulosis of the entire small bowel. Stasis, bacterial