Miscellaneous Abnormalities of the Small Bowel

Chapter Outline
Abnormalities of Small Bowel Development in Adults
Endometriosis
Intestinal Edema
Enteroliths and Bezoars

Abnormalities of Small Bowel Development in Adults

Meckel’s Diverticulum

The yolk sac provides nutrition to the fetus before the placenta develops. The yolk sac is connected to the midgut by the omphalomesenteric duct (vitellointestinal duct). This duct is obliterated during the seventh to eighth weeks of embryogenesis as the placenta assumes the nutritional feeding of the fetus. Persistence of various portions of the omphalomesenteric duct leads to a variety of anomalies. Failure of the entire omphalomesenteric duct to atrophy leads to an enteroumbilical fistula. Failure of one portion of the duct to atrophy may result in a fusiform area of dilation, termed an omphalomesenteric cyst. Persistence of the vitellointestinal duct as a fibrous cord can lead to volvulus or compressive obstruction.

Meckel’s diverticulum results from persistence of the omphalomesenteric duct at its attachment to the ileum. It is the most common congenital abnormality of the gastrointestinal (GI) tract; the prevalence of Meckel’s diverticulum at autopsy is 1% to 4%. However, most people with this congenital anomaly never develop symptoms.

Meckel’s diverticulum arises from the antimesenteric border of the ileum, usually within 100 cm of the ileocecal valve. It may be connected to the umbilicus by a fibrous band or to other intestinal loops by congenital bands or adhesions. The diverticulum usually varies from 2 to 15 cm in length and is about 2 cm in width. Meckel’s diverticulum contains all layers of the intestinal wall. The diverticulum is lined by small bowel epithelium and often contains heterotopic gastric or pancreatic tissue or Brunner’s glands.

Infants (<2 years) with Meckel’s diverticulum may present with GI bleeding caused by secretion of acid by ectopic gastric mucosa and subsequent ulceration. Adults may present with GI bleeding, obstruction, or perforation. Diverticulitis results from ulceration and focal perforation of the diverticulum caused by ectopic gastric mucosa or an enterolith or foreign body impaction. Obstruction results from a variety of mechanisms, including intussusception, volvulus around a persistent fibrous or adhesive band, and ileal narrowing related to ulceration. The diverticulum may become incarcerated in an inguinal, femoral, or umbilical hernia, also known as a Littré hernia. A variety of tumors may arise in Meckel’s diverticulum, including carcinoid tumors, adenocarcinomas, and benign or malignant mesenchymal tumors.

Technetium pertechnetate scintigraphy can detect ectopic gastric mucosa in more than 85% of infants, children, and adults with Meckel’s diverticulum who have acute or chronic GI bleeding. However, most adults are asymptomatic or develop symptoms related to obstruction. All imaging modalities (except enteroclysis) have poor sensitivity for the detection of Meckel’s diverticulum. In a small percentage of cases, plain abdominal radiographs may reveal radiopaque enteroliths in a Meckel diverticulum or a dilated, gas-filled diverticulum in the right lower quadrant. Meckel’s diverticulum is rarely diagnosed on small bowel follow-throughs, except in patients with excessive mesenteric fat related to obesity or Crohn’s disease. Computed tomography (CT) or ultrasound may demonstrate a cystic or tubular structure attached to bowel in the right lower quadrant.

In adults without GI bleeding, enteroclysis is the best radiologic test for Meckel’s diverticulum. A Meckel’s diverticulum is detected in 2% to 3% of all patients undergoing enteroclysis, approaching the incidence at autopsy. A blind-ending tubular or cystic sac ( Fig. 49-1 ) is usually seen to communicate with the antimesenteric border of the distal ileum. One clue to the presence of the diverticulum is a triradiate fold pattern at the junction of the sac with the intestinal lumen. Folds in the diverticulum will be perpendicular to folds in the adjacent ileum. The surface of the diverticulum may be abnormal, containing a granular mucosa, focal ulceration, or focal polypoid mound of ectopic gastric mucosa or tumor. In other patients, an inverted Meckel diverticulum may appear as a polypoid intraluminal lesion ( Fig. 49-2 ), which sometimes acts as lead point for a small bowel intussusception.

Midgut Duplications

Small bowel duplication is a congenital incomplete or complete doubling of a variably long segment of bowel. The duplication may be served by an independent mesentery or by the same mesentery as the adjacent bowel. Midgut duplication cysts contain all layers of the bowel wall, including a mucosa, submucosa, and inner circular muscle layer, with its associated myenteric plexus. Some duplications have a full longitudinal muscle layer, and others have no longitudinal muscle. The mucosal lining of these cysts is usually intestinal. Duplication cysts may also contain gastric mucosa, pancreatic tissue, thyroid stroma, ciliated bronchial epithelium, lung, and cartilage.

Most midgut duplications involve the ileum, particularly the region of the ileocecal valve, and the duodenum. These are usually elongated lesions attached to the muscular layer of the adjacent small bowel. If secretions accumulate, the duplication may become a cystic mass protruding into the small bowel mesentery. Multiple duplications are found in about 5% of patients. Approximately 20% of these duplications communicate with the bowel lumen at the proximal or distal end of the cyst or at both ends.

Ectopic gastric epithelium lining a duplication cyst may cause peptic ulceration, with subsequent GI bleeding or perforation. Obstruction may result from volvulus, intussusception, or compression of the adjacent bowel by the cyst. Rarely, duplication cysts are complicated by tumor.

Duplication cysts may be manifested on barium studies by an extrinsic mass indenting and compressing the mesenteric border of the adjacent bowel. Barium enters the cyst in only a small percentage of cases ( Fig. 49-3 ). CT or ultrasound may show a cystic mass embedded in the small bowel wall. If there is concern about GI ulceration or bleeding, a ^99mTc-pertechnetate scan usually reveals heterotopic gastric mucosa in the cyst.

Heterotopic Tissue

Ectopic gastric mucosa is present in a wide variety of locations in the GI tract, including the esophagus, duodenum, and mesenteric small intestine, as well as in congenital abnormalities such as duplication cysts and Meckel’s diverticulum. Congenital ectopic mucosa contains an orderly arrangement of superficial foveolar epithelium and underlying fundic glands partially lined by parietal and chief cells. Ectopic mucosa should be distinguished from the more common foveolar metaplasia found in the duodenal bulb in patients with peptic duodenitis. However, foveolar metaplasia in peptic duodenitis lacks organized gastric pits and glands.

Ectopic pancreas is most frequently encountered in the duodenum (28%), stomach (26%), or jejunum (16%). The ectopic tissue may arise in various levels of the bowel wall, including the mucosa, submucosa, and serosa. Ectopic pancreatic tissue is composed of varying numbers of acini, ducts, and islet cells. Ectopic pancreas has also been reported in jejunal and ileal diverticula and Meckel’s diverticulum, as well as in the gallbladder, bile ducts, umbilicus, and fallopian tubes.

Ectopic pancreas in the mesenteric small bowel is usually discovered incidentally as a nodule or mass of lobulated solid or cystic tissue abutting the bowel in patients operated on for other reasons. Although microscopic pancreatitis is not uncommon, clinical pancreatitis is very unusual. One case of ectopic pancreas in the jejunum has been reported in which the patient developed pancreatitis and pseudocyst formation. In this patient, a cystic lesion abutted a jejunal loop, mimicking jejunal diverticulitis or a small perforated tumor ( Fig. 49-4 ).

Segmental Dilation

The small intestine may have a focally dilated, aperistaltic segment, termed segmental dilation. In most cases, the isolated atonic loop is in the ileum, giving rise to the term ileal dysgenesis. The cause of this condition is uncertain, but it has been postulated that it results from congenital neuromuscular dysfunction. Ganglion cells are present. In children, ileal dysgenesis is associated with Meckel’s diverticulum and omphaloceles. Ectopic mucosa (especially gastric mucosa) may be found in the dilated segment and may cause ulceration.

Segmental dilation may be manifested on barium studies by a focally dilated spherical or tubular segment of ileum ( Fig. 49-5 ) in direct contiguity with the adjacent inflow and outflow loops of ileum. The aperistaltic segment functions as a barrier to intestinal flow, resulting in partial small bowel obstruction. Ulcerated mucosa may be present in some patients. This condition can be distinguished from Meckel’s diverticulum by its direct continuity with adjacent ileal loops. Ileal dysgenesis can also be distinguished from primary small bowel lymphoma with aneurysmal dilation by the normal mucosal surface of the dilated ileal loop.

Intestinal Malrotation

Symptomatic patients with intestinal malrotation are usually infants and children with high-grade obstruction caused by midgut volvulus or Ladd’s bands. The embryologic, clinical, and radiographic aspects of intestinal malrotation in infants and children are described in detail in Chapters 113 and 116 . Adults with intestinal malrotation are usually asymptomatic or have vague abdominal complaints.

The midgut is that portion of bowel related to the superior mesentery artery and vein axis; it is composed of the third and fourth portions of the duodenum, jejunum, ileum, cecum, appendix, ascending colon and transverse colon. Intestinal malrotation encompasses a number of variations based on the degree of rotation of the midgut in the umbilical cord and degree of rotation when and after it returns to the coelomic cavity. The variations include nonrotation, malrotation, hyperrotation, and reversed rotation.

During the eighth fetal week, the midgut loop rotates 90 degrees counterclockwise while it is within the umbilical cord, so that the cecum lies on the left side of the fetus and the jejunum on the right. If the midgut returns to the abdomen without further rotation, it maintains this orientation. Although this variation represents a 90-degree rotation, it has been confusingly termed nonrotation because the small bowel is not rotating within the coelomic cavity. Nonrotation actually represents small bowel rotation that stopped at 90 degrees of counterclockwise rotation within the umbilical cord. The third and fourth portions of the duodenum and duodenojejunal flexure are absent. Instead, the jejunum is in direct continuation with the second portion of the duodenum ( Fig. 49-6 ). The jejunum and ileum lie in the right side of the abdomen, whereas the colon lies on the left side. The cecum is in the midline, with the terminal ileum entering the cecum from its right side. The appendix originates from a midline position, often low in the midline of the abdomen.

During the 10th fetal week, the midgut normally rotates another 90 degrees counterclockwise within the umbilical cord, so that the cecum lies superiorly on the right and the jejunum inferiorly on the left.

The jejunum is then the first midgut section to reenter the coelomic cavity, passing behind the superior mesenteric artery. During this part of rotation in the coelomic cavity, the jejunum comes to lie between the posteriorly located duodenum and anteriorly positioned colon. During the 11th week, the cecum rotates still another 90 degrees into the right lower quadrant, completing a 270-degree counterclockwise rotation.

Intestinal malrotation occurs when the midgut fails to complete its 180-degree counterclockwise rotation after the initial 90-degree rotation in the umbilical cord. The duodenojejunal junction is not fixed in its normal location to the left of the spine but instead lies inferiorly and to the right of the spine. The small bowel lies predominantly in the right or midabdomen. Mesenteric bands from the liver and posterior abdominal wall cross the second portion of the duodenum and extend to the cecum (Ladd’s bands).

Intestinal hyperrotation occurs when the small intestine has rotated more than the usual 180 degrees, resulting in a long ascending colon, with the cecum located in the left upper quadrant. In a reversed rotation, the bowel enters the abdomen with a clockwise rotation, so the transverse colon lies posterior to the duodenum in the right upper quadrant.

Rather than being confused by the series of somewhat misnamed rotational terms, the radiologist can focus on the following: (1) how much duodenum is present; (2) the location of the duodenal-jejunal junction; (3) the anterior-posterior relationship of the jejunum and ileum to the transverse colon and duodenum; (4) the location of the cecum and the relationship of the appendix and ileocecal valve to the cecum; (5) the location and length of the ascending colon; and (6) the relationship of the superior mesenteric artery (SMA) to the superior mesenteric vein (SMV).

The location of the duodenojejunal junction should be ascertained in any adult with vomiting or abdominal pain. The normal duodenojejunal junction lies to the left of the spine, at about the level of the duodenal bulb. When the ligament of Treitz is in its normal position, the first loops of the jejunum may cross to the right of the spine as a normal variant—as long as the jejunum is not entering a posteriorly located right paraduodenal hernia. On the other hand, the duodenojejunal junction not infrequently has an abnormal location. An abnormally positioned duodenojejunal junction is important because it may cause twisting and obstruction of the proximal small bowel, manifested by duodenal dilation and slow transit of contrast material on barium studies. However, the typical corkscrew sign of intestinal malrotation and midgut volvulus in infants (see Chapter 113 ) is rarely found in adults. Intestinal malrotation in adults is often recognized on CT, margentic resonance imaging (MRI), or ultrasound by an abnormal relationship between the SMA and SMV, so that the vein lies anterior and to the left of the artery ( Fig. 49-7 ).