(Radiographics. 1999;19:1219-1236.)
© RSNA, 1999
Congenital Anomalies of the Small Intestine, Colon, and Rectum1
Teresa Berrocal, MD,
Manuel Lamas, MD,
Julia Gutiérrez, MD,
Isabel Torres, MD,
Consuelo Prieto, MD and
María Luisa del Hoyo, MD
1 From the Servicio de Radiodiagnóstico, Hospital Infantil "La Paz," Paseo de la Castellana 261, 28046 Madrid, Spain. Presented as a scientific exhibit at the 1997 RSNA scientific assembly. Received June 23, 1998; revision requested July 27 and received September 15; accepted September 16. Address reprint requests to T.B.
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Abstract
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Congenital anomalies of the gastrointestinal tract are a significant cause of morbidity in children and, less frequently, in adults. These abnormalities include developmental obstructive defects of the small intestine, anomalies of the colon, anomalies of rotation and fixation, anorectal anomalies, and intestinal duplications. Neonates with complete high intestinal obstruction do not usually require further radiologic evaluation following radiography, whereas those with complete low obstruction should undergo a contrast material enema examination. An upper gastrointestinal series must be performed in all patients with incomplete intestinal obstruction because management is different in each case. In low intestinal obstruction, ultrasonography (US) may help differentiate between small bowel obstruction and colonic obstruction. In addition, US can help correctly identify meconium ileus and meconium peritonitis and is useful in the diagnosis of enteric duplication cysts. In malrotation and anorectal anomalies, computed tomography (CT) and magnetic resonance (MR) imaging can provide superb anatomic detail and added diagnostic specificity. Intestinal duplications manifest as an abdominal mass at radiography, contrast enema examination, or US. At CT, most duplications manifest as smoothly rounded, fluid-filled cysts or tubular structures with thin, slightly enhancing walls. At MR imaging, the intracystic fluid has heterogeneous signal intensity on T1-weighted images and homogeneous high signal intensity on T2-weighted images. Familiarity with these gastrointestinal abnormalities is essential for correct diagnosis and appropriate management.
Index Terms: Colon, abnormalities, 75.143, 75.145 • Gastrointestinal tract, abnormalities, 70.141, 70.143, 70.144, 70.145, 70.146 • Intestines, abnormalities, 74.141, 74.143, 74.144, 74.146 • Rectum, abnormalities, 757.143
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INTRODUCTION
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With few exceptions, congenital abnormalities involving the small bowel or colon are detected in neonates only when they are the direct cause of obstruction. Such abnormalities must be rectified surgically if the patient is to survive. Clinical signs and symptoms including abdominal distention, vomiting, and obstipation prompt the clinician to consult the radiologist, who must determine the presence, location, and cause of an obstruction.
In a healthy neonate, air can usually be identified in the stomach within minutes of birth, and within 3 hours the entire small bowel usually contains gas. After 8–9 hours, healthy neonates demonstrate sigmoid gas. The diagnosis of obstruction is based on some interruption in this dispersion of air. Delayed passage of gas through the neonatal gut may occur as a result of traumatic delivery, septicemia, hypoglycemia, or brain damage. Absence of gas in the bowel may be noted in neonates with severe respiratory distress who are undergoing mechanical ventilation and in cases of continuous nasogastric suction (1).
Radiography is the most valuable means of determining whether obstruction is present. This modality is often diagnostic; even if it is not, however, it may help determine the next most useful diagnostic procedure.
Congenital anomalies causing incomplete obstruction (eg, stenoses, webs, duplications, malrotations, peritoneal bands, aganglionosis) may not manifest until later in life, and other types of examinations (eg, barium enema studies, ultrasonography [US], computed tomography [CT], magnetic resonance [MR] imaging) are generally needed for diagnosis.
In this article, we discuss the importance of pediatric radiation protection and various means of ensuring adequate protection. We also discuss and illustrate a variety of congenital anomalies affecting the small bowel, colon, and rectum; evaluate the efficacy of various imaging modalities in the diagnosis and management of these conditions; and discuss the embryologic and pathologic basis of radiologic findings in appropriate cases as well as differential diagnoses and diagnostic pitfalls.
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RADIATION PROTECTION FOR CHILDREN
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Minimizing radiation exposure for children is extremely important because they are more sensitive to radiation and have a longer life expectancy than adults. However, protecting pediatric patients from overexposure is difficult because technical conditions are less optimal in pediatric radiology than in adult imaging (2,3). The smaller body size, age-dependent body composition, and lack of cooperation as well as several functional differences (eg, faster heart rate and respiration rate, inability to stop breathing on command, increased intestinal gas) make it impossible to achieve image quality in pediatric radiology that meets the standards for adult imaging. This does not imply that all quality criteria are inappropriate; they must, however, be adapted to pediatric imaging.
Effective immobilization of the patient is one of the most important steps in pediatric radiation protection. No diagnostic radiation exposure should be allowed unless there is a high probability that the exact patient position can be maintained because incorrect positioning is the most frequent cause of inadequate image quality in pediatric radiography (4). The field size must be limited exclusively to the area of interest with use of accurate collimation to avoid unnecessary irradiation of parts of the body outside this area. Recent developments in materials for cassettes, grids, tabletops, and front plates of film changers containing carbon fiber and new plastics have significantly reduced patient doses. Careful attention to radiographic exposure factors such as radiographic voltage, nominal focal spot value, filtration, film-focus distance, and so on is also necessary because these factors strongly affect patient doses. For all pediatric examinations, standard lead-rubber shielding must be available to help protect the body in the immediate proximity of the diagnostic field. Special shielding has to be added for certain examinations to protect against external scattered and extrafocal radiation. The gonads should be protected with properly adjusted capsules whenever this can be done without sacrificing necessary diagnostic information. In fluoroscopy, examination time should be reduced to a minimum. Pulsed fluoroscopy is the technique of choice because it reduces the time that the x-ray beam is on, thus reducing total radiation exposure. Finally, the imaging staff must have sufficient skill and experience and ample time must be available to ensure high-quality imaging in young children.
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DEVELOPMENTAL OBSTRUCTIVE DEFECTS
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Neonatal intestinal obstructions may be classified as high or low. Obstructions occurring proximal to the midileum are called high or upper intestinal obstructions, whereas those involving the distal ileum or colon are called low intestinal obstructions. The distinction is critical because children with high obstructions usually need little or no further radiologic evaluation after radiography, and the specific diagnosis is made at surgery. Neonates with low obstructions require a contrast enema examination, which frequently provides a specific diagnosis and may be therapeutic (5). However, the lower the obstructive lesion in the small bowel, the more severe the abdominal distention and the more difficult the accurate localization of the site of obstruction.
High Intestinal Obstruction
High intestinal obstruction is characterized by bilious vomiting (which frequently occurs after the first feeding) and abdominal distention at clinical examination. The generic diagnosis is usually straightforward at radiography, which demonstrates a few dilated bowel loops. Specific common causes of upper intestinal obstruction include atresia of the jejunum or proximal ileum and peritoneal bands. Partial obstruction can be caused by jejunal stenosis, peritoneal bands, duplication cyst, malrotation, and Meckel diverticulum (6–8).
Jejunal atresia is caused by an ischemic injury to the developing gut. Abdominal radiography shows two or three dilated bowel loops, which is more than would be seen in duodenal atresia and fewer than in ileal atresia or other causes of low bowel obstruction. There is no gas in the lower portion of the abdomen in jejunal atresia (Fig 1). The patient usually requires no further radiologic investigation, although barium enema examinations are commonly performed in attempts to exclude second and third areas of atresia lower in the bowel. In isolated proximal atresia of the jejunum, the colon is normal in size because the remaining small bowel distal to the atresia produces sufficient intestinal secretions to produce a normal-caliber colon. Although an upper gastrointestinal series is clearly not indicated, a small amount of air may be injected through a nasogastric tube to confirm complete or partial jejunal obstruction. A rare form of inherited jejunal atresia is the "apple peel" small bowel, which consists of proximal jejunal atresia with absence of the distal superior mesenteric artery, shortening of the small bowel distal to the atresia, and absence of the dorsal mesentery (Fig 2) (9,10). The distal small intestine spirals around its vascular supply and resembles an apple peel. The result is a very short intestine with a propensity toward necrotizing enterocolitis (10). This defect has historically been associated with high mortality, although recent reports suggest an improved prognosis (11).
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Figure 1a. Jejunal atresia. (a) Supine radiograph in a neonate with associated esophageal atresia shows three dilated loops of bowel. st = stomach. (b) Upright radiograph obtained in a different patient shows air-fluid levels in the stomach and the first part of the small bowel. No distal gas is seen.
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Figure 1b. Jejunal atresia. (a) Supine radiograph in a neonate with associated esophageal atresia shows three dilated loops of bowel. st = stomach. (b) Upright radiograph obtained in a different patient shows air-fluid levels in the stomach and the first part of the small bowel. No distal gas is seen.
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Figure 2. Gross specimen demonstrates apple peel small bowel. Note the distention of the proximal small bowel (white arrowheads), the shortening of the dorsal mesentery (arrow), and the distal spiraled segment of the small bowel (black arrowheads).
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Low Intestinal Obstruction
Low intestinal obstruction is defined as an obstruction that occurs in the distal ileum or colon. Signs include large bowel obstruction with vomiting, abdominal distention, and failure to pass meconium. In neonates, the differential diagnosis includes ileal and colonic atresia, meconium ileus or peritonitis, Hirschsprung disease, and functional immaturity of the colon. Anorectal malformations are also an important cause of low intestinal obstruction but are almost always evident at physical examination. The diagnosis of low obstruction is usually apparent at abdominal radiography because of the presence of many dilated intestinal loops, but the differentiation between ileal and colonic obstruction is difficult if not impossible. This distinction can readily be made with a barium enema study, which helps determine the presence of microcolon, indicates the position of the cecum with regard to possible malrotation, and shows the level of the obstruction in colonic atresia.
Ileal atresia is an important cause of low intestinal obstruction. It develops identically to similar lesions in other portions of the alimentary tract (eg, focal ischemia secondary to intrauterine vascular insufficiency). Radiography performed with the patient upright shows numerous dilated loops of bowel occupying the entire abdominal cavity and multiple air-fluid levels (12–14). When this degree of distention is reached, the mucosal pattern of the small bowel is effaced and it may be impossible to differentiate small bowel from colon. In such
a case, a barium enema study is mandatory to determine the presence of a colonic lesion. In ileal atresia, the colon has a normal location but a minute caliber (functional microcolon) (Fig 3) (15).
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Figure 3a. Ileal atresia. (a) Upright radiograph shows multiple air-fluid levels occupying the entire abdominal cavity. (b) Image from a barium enema study shows numerous dilated, air-filled loops of bowel and a small, unused colon (functional microcolon).
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Figure 3b. Ileal atresia. (a) Upright radiograph shows multiple air-fluid levels occupying the entire abdominal cavity. (b) Image from a barium enema study shows numerous dilated, air-filled loops of bowel and a small, unused colon (functional microcolon).
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Colonic atresia is less common than ileal atresia. It is often indistinguishable from obstruction of the distal ileum, especially when the atresia is located in the ascending colon. The colon proximal to the point of atresia is often massively dilated, and a mottled pattern of gas and feces may be identified. Barium enema examination usually reveals a distal microcolon with obstruction to the retrograde flow of barium at the site of the atresia (Fig 4) (16). The US features of colonic atresia are dilation of the distal small bowel and proximal colon. This area is often markedly echogenic secondary to retained meconium (17,18). US differentiation between small bowel obstruction and colonic obstruction in young children is possible only if distal portions of the colon are visualized and appear to be collapsed.
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Figure 4a. Colonic atresia. (a) Radiograph shows distended loops of bowel similar to those seen in low small bowel obstruction. (b) Image from a barium enema study demonstrates microcolon with complete obstruction to the retrograde flow of barium in the transverse portion of the colon.
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Figure 4b. Colonic atresia. (a) Radiograph shows distended loops of bowel similar to those seen in low small bowel obstruction. (b) Image from a barium enema study demonstrates microcolon with complete obstruction to the retrograde flow of barium in the transverse portion of the colon.
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Meconium ileus is the result of intraluminal obstruction of the colon and lower small bowel due to impaction of meconium and represents the earliest clinical manifestation of cystic fibrosis. Mechanical obstruction occurs when desiccated meconium pellets occlude the distal small bowel and the more proximal small bowel loops are distended with tenacious meconium paste. The abdomen is filled with gas-distended loops (Fig 5), and occasionally there is a relative absence of air-fluid levels due to abnormally thick intraluminal meconium (19). The admixture of gas with meconium may give rise to a soap bubble appearance similar to the fecal pattern in the colon in older patients. A similar fecal pattern may be seen in ileal atresia and aganglionosis of the terminal ileum. Contrast enema examination will show a functional microcolon. Meconium ileus may be complicated by volvulus of a distal intestinal loop, perforation, atresia, or peritonitis.
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Figure 5a. Meconium ileus. (a) Abdominal scout radiograph shows marked distention of the small bowel and a "soap bubble" appearance in the right side of the abdomen (arrows), a finding suggestive of mottled air and feces. (b) US image shows dilated, fluid-filled intestinal loops containing echogenic material (calcified meconium) (arrows). Associated ileal atresia was seen at surgery. (c) Gross specimen shows marked distention of the small bowel loops.
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Figure 5b. Meconium ileus. (a) Abdominal scout radiograph shows marked distention of the small bowel and a "soap bubble" appearance in the right side of the abdomen (arrows), a finding suggestive of mottled air and feces. (b) US image shows dilated, fluid-filled intestinal loops containing echogenic material (calcified meconium) (arrows). Associated ileal atresia was seen at surgery. (c) Gross specimen shows marked distention of the small bowel loops.
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Figure 5c. Meconium ileus. (a) Abdominal scout radiograph shows marked distention of the small bowel and a "soap bubble" appearance in the right side of the abdomen (arrows), a finding suggestive of mottled air and feces. (b) US image shows dilated, fluid-filled intestinal loops containing echogenic material (calcified meconium) (arrows). Associated ileal atresia was seen at surgery. (c) Gross specimen shows marked distention of the small bowel loops.
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Meconium peritonitis is a chemical peritonitis resulting from intrauterine bowel perforation. Common underlying disorders include small bowel atresia, meconium ileus, volvulus, and intussusception, although some cases are idiopathic. The extruded bowel contents provoke an intense peritoneal inflammatory reaction, leading to the formation of dense fibrotic tissue. This tissue often calcifies, resulting in the characteristic intraperitoneal calcifications identified prior to birth with US and after birth with abdominal radiography. The calcifications of meconium peritonitis may extend into the scrotum through a patent vaginal process to produce a calcified mass in the scrotum (Fig 6). US depicts these calcifications as highly echogenic linear or clumped foci in the abdomen or pelvis. Often, some of these foci exhibit posterior acoustic shadowing (20). Meconium peritonitis is typically either generalized or cystic in appearance at US (21). In the generalized condition, highly echogenic material spreads throughout the abdomen and around the bowel loops to produce a characteristic "snowstorm" appearance (22). Encysted collections of meconium range from homogeneous to heterogeneous in echogenicity and may be ill defined or well defined (23).
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Figure 6. Meconium peritonitis in a male neonate with cystic fibrosis. Abdominal radiograph shows linear and flocculent areas of calcification within the peritoneal cavity (arrows) and scattered areas of calcification in the scrotum (arrowheads).
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Hirschsprung disease is a form of low intestinal obstruction caused by the absence of normal myenteric ganglion cells in a segment of the colon. This condition accounts for approximately 15%–20% of cases of neonatal bowel obstruction. About 2% of patients with Hirschsprung disease also have Down syndrome. The aganglionosis varies in length but always extends proximally from the anal canal, and the rectosigmoid area is involved in 80% of cases. Ultrashort segment disease (in which aganglionosis is essentially limited to the region of the internal sphincter) is very rare, as is aganglionosis involving the entire alimentary tract (5,24). In children with Hirschsprung disease, the absence of ganglion cells results in the failure of the distal intestine to relax normally. Peristaltic waves do not pass through the aganglionic segment and there is no normal defecation, leading to functional obstruction. Abdominal distention, constipation, and bilious vomiting are the predominant signs and symptoms of obstruction and appear within a few days after birth (25–27). Radiography performed in children with Hirschsprung disease yields findings similar to those in other forms of low small bowel obstruction: variable gaseous distention of the colon and small bowel, often with air-fluid levels. The colon is usually difficult to identify accurately, and gas is usually absent in the rectum. Barium enema studies demonstrate patency of the colon, which is short but usually normal in caliber (25). A transition zone between the narrow and dilated portions of the colon in the shape of an inverted cone is the most characteristic radiologic finding (Fig 7). When this transition zone is observed, the examination should be discontinued because filling of the more proximal dilated bowel beyond the transition zone may lead to impaction. However, the distention of the bowel proximal to the segment of deficient innervation is gradual, and a transition zone is seen in only 50% of neonates during the first week of life. Abnormal contractions and irregular peristaltic activity of the aganglionic portion of the colon may be useful indicators of the disease (27,28), although they are nonspecific findings that are also seen in colitis. Twelve-hour-delayed postevacuation images are useful in dubious cases. The radiologic diagnosis of total colonic aganglionosis is difficult. Findings at barium enema examination may be normal or may include a short colon of normal caliber, microcolon, or a transition zone in the ileum (5,27–31) (Fig 8).
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Figure 7a. Hirschsprung disease in a 6-month-old infant with a history of chronic constipation. (a, b) Frontal (a) and lateral (b) images from a barium enema study show the proximal sigmoid colon and descending colon as greatly dilated compared with the distal colon and rectum.
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Figure 7b. Hirschsprung disease in a 6-month-old infant with a history of chronic constipation. (a, b) Frontal (a) and lateral (b) images from a barium enema study show the proximal sigmoid colon and descending colon as greatly dilated compared with the distal colon and rectum.
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Figure 8a. Total colonic aganglionosis. (a, b) Frontal (a) and lateral (b) images from a barium enema study show irregularity in the caliber of the colon with fewer redundant flexures than normal. (c) Gross specimen demonstrates total colonic aganglionosis.
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Figure 8b. Total colonic aganglionosis. (a, b) Frontal (a) and lateral (b) images from a barium enema study show irregularity in the caliber of the colon with fewer redundant flexures than normal. (c) Gross specimen demonstrates total colonic aganglionosis.
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Figure 8c. Total colonic aganglionosis. (a, b) Frontal (a) and lateral (b) images from a barium enema study show irregularity in the caliber of the colon with fewer redundant flexures than normal. (c) Gross specimen demonstrates total colonic aganglionosis.
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Functional immaturity of the colon is a common cause of neonatal obstruction, particularly in premature neonates and in those whose mothers were treated during labor with magnesium preparations or high doses of opiates or other sedatives. The condition has also been encountered in children with septicemia, hypothyroidism, or hypoglycemia and in those with diabetic mothers. Functional immaturity of the colon comprises several entities, most notably small left colon syndrome and meconium plug syndrome (32). Affected patients have abdominal distention, difficulty in initiating evacuation, and sometimes vomiting; typically, however, the bowel distention is less severe than with an organic obstruction. The condition is both diagnosed and treated with a contrast enema. In small left colon syndrome, barium enema examination demonstrates a distended right and transverse colon with a transition to a very small diameter descending and rectosigmoid colon near the splenic flexure. The rectum is usually quite distensible (5). In meconium plug syndrome, barium enema examination reveals a large meconium plug, which is then evacuated (Fig 9). No organic obstruction is seen. Typically, there is clinical improvement following the enema, and over the course of hours to days the radiographic and clinical signs of obstruction subside (33–35). In some cases, meconium ileus or Hirschsprung disease may be incorrectly diagnosed. If the difficulty is not resolved promptly and completely, a sweat test for cystic fibrosis and further observation for Hirschsprung disease are indicated (31).
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Figure 9a. Meconium plug syndrome. (a) Image from a barium enema study shows a normal-sized rectum and colon with inspissated meconium filling defects (arrows). (b) Gross specimen shows the colon (C) and the typical appearance of an evacuated plug (arrows).
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Figure 9b. Meconium plug syndrome. (a) Image from a barium enema study shows a normal-sized rectum and colon with inspissated meconium filling defects (arrows). (b) Gross specimen shows the colon (C) and the typical appearance of an evacuated plug (arrows).
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ANOMALIES OF ROTATION AND FIXATION
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The alimentary tract initially develops as a pouchlike extension of the yolk sac. At 6 weeks gestation, the intestinal tract is a continuous, tubular structure that is divided into the foregut (supplied by the celiac artery), midgut (supplied by the superior mesenteric artery), and hindgut (supplied mainly by the inferior mesenteric artery). During embryonic development, the bowel introduces itself into the abdomen and both the duodenojejunal and ileocolic segments of the primitive digestive tube rotate 270° counterclockwise about the omphalomesenteric vessels (ie, the future superior mesenteric artery and superior mesenteric vein) to reach their final normal positions (1,35). Before 6 weeks gestation, the duodenum initially rotates 90° counterclockwise so that it lies to the right of the superior mesenteric artery. Similarly, the cecum rotates 90° counterclockwise so that it lies to the left of the superior mesenteric artery. During the 6th week of gestation, the duodenum rotates another 90° counterclockwise so that it lies posterior to the superior mesenteric artery; the rest of the midgut is in the umbilical cord. By the 10th to 12th weeks of gestation, the intestine slides back into the peritoneal cavity, where the final 90° rotation of the duodenum and 180° rotation of the cecum occur. The right colon is the last portion of the gastrointestinal tract to rotate completely, allowing the cecum to descend into the right lower quadrant (Fig 10). This rotation is followed in the last stage by peritoneal fixation of the bowel. The small bowel mesentery is normally broad based, with its attachment extending from the ligament of Trietz to the ileocecal valve. This wide base prevents the small intestine from twisting around the superior mesenteric artery (1,5).
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Figure 10. Drawings illustrate the stages of intestinal rotation. In A, the duodenum has rotated 90° counterclockwise to lie to the right of the superior mesenteric artery. The distal large bowel also rotates 90° counterclockwise. In B, the duodenum has rotated another 90° counterclockwise. In C, the duodenum has rotated its final 90° counterclockwise with the duodenojejunal flexure lying to the left of the midline. The cecum continues to rotate. In D, the normally rotated bowel is depicted. (Reprinted, with permission, from reference 1.)
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The most important (and most radiologically significant) anomalies of rotation and fixation are nonrotation, malrotation, and reversed rotation. In nonrotation, the midgut returns to the peritoneal cavity after rotating only 180° instead of the normal 270°. The postarterial (colonic) limb reenters the abdomen first instead of last; this leaves the small intestine on the right side of the abdomen and the colon and cecum on the left side. The ileum crosses the midline from right to left to enter the cecum (Fig 11). Nonrotation is often an incidental finding in older children and adults during routine examination, but volvulus as a result of local clockwise rotation may accompany this anomaly (Fig 12). Malrotation implies that rotation occurs, but it is incomplete. In such cases, the prearterial segment, which returns to the abdomen first, is usually in a normal position and the degree of malrotation is indicated by the position of the cecum: The more complete the process, the more normal its position. The cecum may be on the left side, higher than normal on the right side, or in an intermediate position (Fig 13). Reversed rotation occurs when the postarterial segment of the midgut returns to the abdomen first. The cecum begins its migration and passes to the right behind the superior mesenteric artery. This reversed migration unwinds the normal counterclockwise rotation that occurred during the first stage and substitutes a final clockwise rotation of 90°, so that the transverse colon lies behind the duodenum and is separated from it by the superior mesenteric artery.
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Figure 11a. Nonrotation. Images from a contrast material-enhanced gastrointestinal examination show the small intestine on the right side of the abdomen and the colon and cecum on the left side. The ileum is seen crossing the midline from right to left (arrows in b).
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Figure 11b. Nonrotation. Images from a contrast material-enhanced gastrointestinal examination show the small intestine on the right side of the abdomen and the colon and cecum on the left side. The ileum is seen crossing the midline from right to left (arrows in b).
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Figure 12. Drawings illustrate midgut volvulus. Narrow mesenteric attachment of nonrotation (A) or incomplete rotation (B) may lead to midgut volvulus (C). (Reprinted, with permission, from reference 1.)
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Figure 13. Malrotation. On an image from a barium enema study, the intestine occupies an intermediate position between that of nonrotation and the normal postnatal position. The cecum and the terminal ileum are displaced upward and medially.
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Early diagnosis of these conditions is a major diagnostic challenge for radiologists and clinicians who strive to prevent the catastrophic complications of midgut volvulus and small bowel necrosis (36,37). In itself, abnormal positioning of the gut does not cause symptoms; these arise from the attendant abnormal mesenteric position and fixation that allow the gut to twist (Fig 12). The symptoms of rotation anomalies are identical to those of proximal bowel obstruction and may be accompanied by symptoms of vascular occlusion. The obstruction may be complete or partial and is the result of midgut volvulus or, less frequently, Ladd bands or internal hernia. In the neonate, complete obstruction is the rule and vomiting is the most common presenting symptom within the first month of life (77% of cases) (38). Vomiting often occurs within the first week of life (39% of cases) (5,38–40). In older children, the obstruction is usually partial and provokes recurrent attacks of vomiting and occasionally distention. Malrotation can even manifest in adults, and if volvulus develops, a delay in diagnosis can lead to necrotic bowel (35).
A neonate with bilious emesis on the first day of life in whom radiography shows complete duodenal obstruction does not require further evaluation. Any other neonate with bilious emesis or incomplete obstruction as indicated by the presence of gas in the distal portion of the intestinal tract does require evaluation. An upper gastrointestinal series is usually performed initially to demonstrate the level and nature of the obstruction. When a volvulus is present, the small intestine has a "corkscrew" appearance because it twists around the superior mesenteric artery (Fig 14). US, CT, and MR imaging may allow detection of malrotation by showing the superior mesenteric artery and superior mesenteric vein in abnormal relative positions (39–41). A superior mesenteric vein that is inverted relative to the superior mesenteric artery is highly indicative of malrotation (Fig 15). Pracros et al (42) described the "whirlpool" sign of actual midgut volvulus at color Doppler US, a sign that is characterized by clockwise wrapping of the superior mesenteric vein and mesentery around the superior mesenteric artery (43). Three-dimensional helical CT angiography can also exquisitely depict the twisting of the superior mesenteric vein about the superior mesenteric artery. A dilated, fluid-filled, obstructed stomach and proximal duodenum, thick-walled loops of ischemic right-sided small bowel, and free intraperitoneal fluid are associated findings that are readily recognizable at US or CT (43,44). However, the absence of an abnormal relationship between the superior mesenteric artery and superior mesenteric vein does not totally exclude the possibility of malrotation (45); moreover, some patients with abnormal relative positions of these vessels do not have malrotation. Thus, an upper gastrointestinal series remains the standard of reference in the diagnosis of malrotation (5).
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Figure 14. Midgut volvulus. Image from a contrast-enhanced upper gastrointestinal series clearly demonstrates the "corkscrew" appearance of the proximal small bowel (arrows) as it twists around the superior mesenteric artery.
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Figure 15. Malrotation. Abdominal CT scan shows the superior mesenteric vein (+) lying anterior to the superior mesenteric artery. The superior mesenteric vein normally lies on the right side of the superior mesenteric artery; in malrotation, it lies either in front or on the left side.
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INTESTINAL DUPLICATIONS
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Gastrointestinal tract duplications are uncommon congenital abnormalities that may occur anywhere along the alimentary tract from the tongue to the anus. By definition, they are located in or adjacent to the wall of part of the gastrointestinal tract, have smooth muscle in their walls, and are lined by mucosa similar to that of some other portion of the alimentary tract, most often gastric mucosa. The mucosal lining is not necessarily that of the adjacent segment of the digestive tube (46). The origin of duplications remains unknown, and several theories have been proposed to explain their pathogenesis. Incomplete recanalization of the digestive tract lumen adequately explains duplications in those portions of the gastrointestinal tract that go through the solid stage (eg, esophagus, small bowel, colon) (47). The intrauterine vascular accident theory suggests that duplications arise as the result of focal areas of vascular insufficiency secondary to fetal stress and anoxia (39,48) and may be a valid explanation for small bowel atresias and their associated duplications. These duplications are spheric cysts or, less frequently, tubular structures that tend to be located on the mesenteric aspect of the alimentary canal and that share a common muscular wall and blood supply with the canal but have a separate mucosal lining (46). They may be large or small, communicating or noncommunicating with the enteric lumen, and symptomatic or asymptomatic. The most common site is the ileum, followed by the esophagus and duodenum. The colon is less frequently involved, and there is a gradient of decreasing frequency from the cecum to the rectum.
The symptoms of small bowel duplication are due to encroachment of the duplication on the adjacent bowel, which causes partial or complete intestinal obstruction. The accumulation of secretions in the closed lumen causes distention, and the weight of the duplication can lead to small bowel volvulus. Bleeding usually indicates a long communicating duplication, with gastric mucosa causing ulceration of distal intestinal mucosa. Occasionally, a cyst located in the ileum at or near the ileocecal junction can manifest as an intussusception.
Abdominal radiography may show a soft-tissue mass within the abdomen. Barium enema examination usually shows a mass extrinsic to the bowel lumen. Contrast material may gain access to a duplication that communicates with the lumen. US reveals either a sonolucent mass with good through transmission due to clear fluid content or an echogenic mass due to hemorrhage and inspissated material within the duplication. If the typical inner echogenic mucosal and outer hypoechoic muscle layers are seen, the diagnosis of duplication can be established. An associated intussusception can be suspected on the basis of its US features and confirmed with a contrast enema study (Fig 16) (49,50). CT and MR imaging are alternative modalities that may be used when US findings are inconclusive. The majority of duplications appear on CT scans as smoothly rounded, fluid-filled cysts or tubular structures with thin, slightly enhancing walls located in or adjacent to the wall of part of the alimentary canal (51). The intracystic fluid has heterogeneous signal intensity on T1-weighted MR images and homogeneous high signal intensity on T2-weighted images. The differential diagnosis includes all cystic intraabdominal masses such as mesenteric and omental cysts, pancreatic pseudocysts, ovarian cysts, and so on.
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Figure 16a. Ileal duplication. (a) Image from a barium enema study shows extrinsic compression of the cecum by an extraluminal mass. (b) US image shows a cystic mass (C) that corresponds to a surgically proved duplication cyst. k = kidney.
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Figure 16b. Ileal duplication. (a) Image from a barium enema study shows extrinsic compression of the cecum by an extraluminal mass. (b) US image shows a cystic mass (C) that corresponds to a surgically proved duplication cyst. k = kidney.
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Duplications of the colon and rectum may also be cystic or tubular, the second type being seen more frequently in the rectum. About 40% of cystic colonic duplications involve the cecum and have imaging features similar to those of small bowel duplications. Constipation may be the only symptom, although acute obstruction has also been reported (52). When a cystic duplication communicates with the colonic lumen, air (Fig 17) or other matter from the intestine can enter the cyst. About 20% of rectal duplications communicate with the rectal lumen or extend to the exterior as a chronic perianal fistula. If the cyst contains ectopic gastric mucosa or pancreatic tissue and communicates with the rectum, rectal bleeding may occur (53).
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Figure 17. Cystic communicating duplication of the colon in a 54-year-old woman with abdominal pain. Abdominal radiograph shows a round collection of air near the ascending colon (arrows).
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ANORECTAL ANOMALIES
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Because of the complexity of the embryologic development of the anorectal segment, many anomalies are possible. Although various classification schemes and reviews of these conditions have been offered, the one by Gans (54)
is perhaps the simplest and most useful. In his classification scheme, rectal atresia is differentiated from imperforate or ectopic anus. In rectal atresia, the anus is open, but a variable segment of rectum superior to the anus is atretic and no fistula is present. Ectopic anus is the most common abnormality of the anorectal segment and occurs when the terminal bowel fails to descend normally, resulting in a lack of communication with the anus. As a result, the bowel opens via a fistula at an abnormal location (peritoneum, vestibule, vagina, urethra, bladder, or cloaca). In imperforate anus, the terminal bowel ends blindly and no fistula exists. Anal and rectal stenosis represent the last entity in this classification scheme and refer to cases of incomplete anal or rectal atresia (55). Embryologically, ectopic anus is currently thought to represent the failure of the hindgut to descend properly (54). Although the anal dimple and external sphincter are usually in the normal position, the distal colon, instead of descending and joining the anus, is arrested at a higher level and empties ectopically through a fistula. Depending on the exact site of arrest, the distal colon may have passed through the puborectal sling. In cases of high colon arrest, the colon ends at or above the puborectal sling, which is hypoplastic or even absent. If colon arrest is low, the colon will have passed through the puborectal sling, which will usually be well developed and functional. After an anorectal anomaly is identified, radiography must be performed and will usually show a low small bowel obstruction or colonic obstruction early in the neonatal period. Prone shoot-through radiography is useful in determining the level of the atresia and enables assessment of the sacrum. The "M" line (1,25,54) running horizontally through the junction of the lower third and upper two-thirds of the ischium accurately represents the level of the puborectal muscle (Fig 18). This "M" line is used to classify lesions as high, intermediate, or low. US is useful in delineating the distance from the distal pouch to the perineum (56–58). A distance of less than 10 mm indicates a low lesion, which can be safely treated with simple perineal anoplasty; a distance greater than 15 mm indicates a lesion that requires diversion with colostomy. Cystography is routinely used in the assessment of these patients and delineates associated fistulas between the terminal bowel and the urinary tract (Figs 19, 20). US is actually a useful method for identifying the distal pouch, although the differentiation between a low lesion and a high lesion in a relatively low position can be difficult if not impossible. CT and MR imaging are the modalities of choice because they can help determine the presence of the puborectal muscle and external sphincter as well as the rectal pouch prior to surgery (59–62). Associated anomalies, especially of the vertebra, kidney, esophagus, or trachea, are present in nearly 40% of cases. Anorectal anomalies may manifest as part of the VACTERL pattern of anomalies (ie, vertebral, anal, cardiac, tracheal, esophageal, renal, and limb anomalies). Renal tract anomalies are common; therefore, initial US of the urinary tract is essential in all affected patients.
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Figures 18-20. (18) Imperforate anus. Lateral radiograph shows an imperforate anus below the "M" line drawn through the junction of the upper two-thirds and lower one-third of the ischium (perineal surgical approach). (19) Ectopic anus. Voiding cystogram demonstrates a recto-urethral fistula (arrow). (20) Imperforate anus. Lateral voiding cystogram demonstrates an air-filled distal rectal pouch (arrows) ending blindly below the "M" line, a finding indicative of a low lesion. There is no fistula opening in the terminal bowel.
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Figures 18-20. (18) Imperforate anus. Lateral radiograph shows an imperforate anus below the "M" line drawn through the junction of the upper two-thirds and lower one-third of the ischium (perineal surgical approach). (19) Ectopic anus. Voiding cystogram demonstrates a recto-urethral fistula (arrow). (20) Imperforate anus. Lateral voiding cystogram demonstrates an air-filled distal rectal pouch (arrows) ending blindly below the "M" line, a finding indicative of a low lesion. There is no fistula opening in the terminal bowel.
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Figures 18-20. (18) Imperforate anus. Lateral radiograph shows an imperforate anus below the "M" line drawn through the junction of the upper two-thirds and lower one-third of the ischium (perineal surgical approach). (19) Ectopic anus. Voiding cystogram demonstrates a recto-urethral fistula (arrow). (20) Imperforate anus. Lateral voiding cystogram demonstrates an air-filled distal rectal pouch (arrows) ending blindly below the "M" line, a finding indicative of a low lesion. There is no fistula opening in the terminal bowel.
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SUMMARY
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Diagnosis of a gastrointestinal tract anomaly in the neonate or infant relies predominantly on clinical findings. The role of diagnostic imaging is to help determine as accurately as possible the exact nature of the abnormality. Intestinal obstructions in the neonate are classified as high or low and as complete or incomplete. These distinctions are critical and can almost always be made with abdominal radiography. Neonates with complete high intestinal obstruction do not usually require further radiologic evaluation following radiography. Neonates with complete low intestinal obstruction should undergo a contrast enema examination, which may be both diagnostic and therapeutic. An upper gastrointestinal series must be performed in patients with incomplete high or low intestinal obstruction regardless of age to try to determine the cause of the obstruction because management is different in each case.
US is often the next step because it is noninvasive and the equipment is portable. In low intestinal obstruction, US may help differentiate between small bowel obstruction and colonic obstruction. Meconium ileus and meconium peritonitis can be correctly identified at US both before and after birth. US is also very useful in the diagnosis of enteric duplication cysts. Demonstration of a cystic mass with a double-layered wall, which consists of an echogenic inner layer (mucosa and submucosa) and a thin, sonolucent outer layer (muscle) is virtually diagnostic for an enteric duplication cyst. In cases of malrotation, the relative positions of the superior mesenteric artery and superior mesenteric vein are correctly evaluated with US and the whirlpool sign of midgut volvulus can be demonstrated with color Doppler US. US can readily help identify the distal pouch in the evaluation of patients with anorectal anomalies.
CT and MR imaging are not suitable for general screening but provide superb anatomic detail and added diagnostic specificity. They are especially useful in the evaluation of malrotations and anorectal anomalies. Some of these anomalies remain asymptomatic and are diagnosed on the basis of incidental findings at routine examination for other conditions in adulthood.
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Footnotes
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CME FEATURE This article meets the criteria for 1.0 credit hour in category 1 of the AMA Physician's Recognition Award. To obtain credit, see the questionnaire on pp 1281-1288.
LEARNING OBJECTIVES After reading this article and taking the test, the reader will be able to:
• Summarize the normal and pathologic embryo-logic development of the small bowel, colon, and rectum.
• Recognize the clinical manifestations and imaging features of a variety of abnormalities and their differential diagnoses.
• Compare the utility of various imaging modali-ties in the management of these pathologic conditions.
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Abstract
INTRODUCTION
