Imaging of Congenital Anomalies of the Gastrointestinal Tract
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《美国医学杂志》
Department of Radiodiagnosis, All India Institute of Medical Sciences, New Delhi, India, India
Abstract
Abstract. The radiological imaging plays a vital role in the evaluatin of patients with congenital anomalies of the gastrointestinal tract. The evaluation of these patients, most of which present early after birth, frequently requires the use of various imaging modalities for making the correct diagnosis and planning surgical correction. This article reviews the common congenital anomalies of the gastrointestinal tract including obstructive lesions, anomalies of rotation and fixation, anorectal anomalies, and intestinal duplications. The plain radiograph is often diagnostic in neonates with complete gastric of upper intestinal obstruction and further radiologic evaluation may be unnecessary. An upper gastrointestinal series should be performed in all patients with incomplete intestinal obstruction. Sonography is useful in the evaluation of many congenital anomalies affecting pediatric gastrointestinal tract especially hypertrophic pyloric stenosis, enteric duplication cysts, midgut malrotation, meconium ileus and meconium peritonitis. Moreover, CT and MRI has assumed a greater importance as these provide excellent anatomic details which may be necessary for correct diagnosis as well as treatment planning. This is particularly true in evaluation of congenital anomalies such as esophageal/enteric duplications, vascular rings and anorectal anomalies. It is important to be familiar with the role nad usefulness of the various imaging modalities so that these can be used judiciously to avoid unnecessary radiation exposure while minimizing the patient discomfort. [Indian J Pediatr 2005; 72 (5) : -414]
Keywords: GIT; Congenital anomalies; Intestinal abnormalities
A wide spectrum of congenital anomalies may affect gastrointestinal tract, some of which manifest early after birth while others may not present till late childhood or adulthood. Imaging plays a very useful role in these developmental lesions, most of which present with obstruction. In most cases, plain radiography, gastrointestinal contrast study or Ultrasound (US) suffice. Computed tomography (CT) and Magnetic Resonance Imaging (MRI) are increasingly proving more useful to provide an accurate diagnosis specially in difficult cases.
Developmental lesions of the neonatal gastrointestinal tract can be grouped as follows:[ 1]
Structural
Attributed to embryologic maldevelopment
Esophageal atresia with or without fistula
Antro-pyloric atresia
Antral diaphragm
Duodenal atresia
Duodenal stenosis
Intrinsic: windsock duodenum
Extrinsic: annular pancreas
Midgut malrotation with peritoneal bands
Duplication or mesenteric cyst
Anorectal atresia
Attributed to in utero vascular (ischemic) complication
Jejuno-ileal atresia
Colonic atresia or stenosis
Complicated meconium ileus
Functional
Meconium plug syndrome and its variants
Megacystis-microcolon-intestinal hypoperistalsis
Structural and Functional Combined
Hypertrophic pyloric stenosis
Midgut vovulus (complicating midgut malrotation)
Uncomplicated meconium ileus
Colonic aganglionosis
In a normal neonate, swallowing begins almost immediately after birth and gas should be present in the stomach within few minutes.[3] Within 3 hours of birth, the entire small bowel usually contains gas while the sigmoid colon is seen only after 8-9 hours.[4] Disruption of this common pattern is seen in obstruction or presence of underlying illness such as brain damage, septicemia or hypoglycemia.
Plain radiograph is a useful, simple and most inexpensive tool in the evaluation of the neonate with gastrointestinal (GI) obstruction. Unlike adults and older children, in neonates the small and large bowel usually cannot be distinguished. The gas is distributed throughout the small and large bowel where a little fluid is present resulting in sharp bowel-air interfaces which appear as multiple closely apposed rounded or polyhedral structures on plain radiograph.[2] The precise level of obstruction may be evident in high GI obstruction but is difficult to determine in a low gut obstruction. High GI obstruction occurs most commonly at the level of duodenum and proximal jejunum and plain radiography alone is often diagnostic.
Bilious vomiting indicates obstruction distal to the ampulla of Vater. Although there is little role of contrast examintion in high gut obstruction,[5] GI contrast examination should be performed in obstructions presenting beyond first few days of life to rule out malrotation and midgut volvulus. The stomach is emptied through a nasogastric tube before upper GI contrast study is undertaken.
Barium suspension is not used in cases of suspected perforation or if there is a risk of barium inspissations. Aspiration into lungs should be avoided while using contrast agents because commonly used high osmolality ionic contrast may produce severe pulmonary edema. Hypertonic water-soluble ionic contrast media may be useful in relieving obstruction in meconium ileus by drawing water into the bowel lumen. However, this may cause fluid and electrolyte imbalance. Therefore, infant should be hydrated and serum electrolytes monitored before the procedure. Non-ionic low osmolality contrast media are preferred in most circumstances. Barium is preferred in cases of suspected Hirschsprung's disease or other conditions where delayed films have diagnostic value.
High Intestinal Obstruction: The usual presentation is vomiting which may be bile stained if obstruction is distal to ampulla of Vater. The plain radiograph is often diagnostic and further radiological evaluation may be unnecessary. However, an upper gastrointestinal series must be performed in all patients with incomplete intestinal obstruction. The causes of high obstruction include pyloric atresia, duodenal atresia, malrotation with midgut volvulus or Ladd's bands and proximal jejunal atresia.[2] Partial obstruction results from jejunal stenosis, peritoneal bands, duplication cyst, malrotation and Meckel's diverticulum.[4]
Low Intestinal Obstruction: Failure to pass meconium in the first 24-48 hours of life may be due to structural or functional reasons. The causes include ileal or colon atresia, Anorectal malformations, Hirschsprung's disease, meconium plug syndrome, and neonatal small left colon syndrome.
Ultrasonography (US) is often the first modality to be used in investigation of child with abdominal lump or suspected hypertrophic pyloric stenosis. US is highly accurate in the diagnosis of hypertrophic pyloric stenosis and extremely useful in the investigation of mass lesions such as enteric duplication cysts and mesenteric or omental cysts.[1]
US is the modality of choice for prenatal screening, but occasionally additional imaging information is needed.[6] MRI is being increasingly used for prenatal imaging of congenital anomalies. The use of fast sequences like single shot fast spin echo and echoplanar imaging has enabled successful prenatal MR imaging. Esophageal, duodenal, or small bowel atresia can be diagnosed on antenatal MRI. On MRI, the signal intensity differences of the dilated bowel may provide additional information complementary to the US in identifying the site of obstruction. The proximal small bowel (fluid content) appears hyperintense on single-shot fast spin echo (SSFSE) and hypointense on T1-weighted fast spin echo (FSE) imaging. In contrast, the distal small bowel and colon appear hypointense on SSFSE and hyperintense on T1-weighted FSE imaging due to presence of meconium. [7,8]
Esophagus
Esophageal Atresia (EA) and Tracheo-Esophageal fistula (TEF)
The presence of polyhydramnios, reduced intraluminal liquid in the fetal gut and inability to detect the fetal stomach on prenatal ultrasound may provide an early clue to the possibility of esophageal atresia.[9]
The chest radiographs (AP and Lateral) demonstrate proximal esophageal pouch filled with air. The abdomen is always included in the radiograph to look for air in the gastrointestinal tract. The absence of gas in stomach or intestinal tract is suggestive of pure esophageal atresia (EA) without fistula or EA with proximal tracheo-esophageal fistula. The position of aortic arch should be conveyed to the surgeon as it may help determine the surgical approach. When plain radiographs fail to indicate the side of the arch, and as a preoperative work-up, echocardiography should be performed before surgery to prognosticate and plan successful outcome.[10] Chest radiograph may show consolidation due to aspiration pneumonia especially in the dependant upper lobes in supine position.
Radiographically, there is inability to pass a red rubber catheter into the stomach. The proximal pouch may be outlined due to swallowed air (Fig 1a). Gentle injection of air via the tube may distend the proximal pouch. The distance between proximal and distal esophageal pouch can be assessed only after a gastrostomy in a staged procedure. Isolated long gap esophageal atresia is known to be associated with 13 pair of ribs.[12]
The diagnosis is frequently delayed in H-type fistula. The fistula is more precisely termed as N-type fistula as it courses obliquely from the esophagus upwards to the trachea (Fig 1b). Most H-fistulas are at thoracic inlet (T2-3). Tube esophagogram under fluoroscopic guidance with patient in the lateral or steep prone oblique position is the best way to demonstrate H-type fistula. The nasogastric tube placed at GE junction is gradually withdrawn while injecting contrast at various levels.[10] A small amount of isotonic non-ionic contrast medium or properly diluted barium should be used. The contrast should be aspirated back immediately after the study.
Esophageal stenosis and webs may be associated with tracheoesophageal fistula. [13,14] Up to one quarter of patients with EA & TEF have associated with GI anomalies including pyloric stenosis, duodenal/small bowel atresia, anorectal malformations. There should be high index of suspicion for associated anomalies. The pattern of air distribution on plain radiographs may provide clue to these anomalies. Air confined to the stomach and or duodenum suggests associated duodenal atresia. Follow up radiographs may be obtained if initial findings are suspicious.
Associated anomalies are frequently seen in such individuals including features of the VACTREL spectrum (vertebral anomalies, anorectal malformation, cardiovascular malformation, tracheo-esophageal fistula with esophageal atresia, renal anomalies and limb defects). The other recognized associations are trisomy 18, trisomy 21, CHARGE syndrome, Potter syndrome and presence of 13 pair of ribs. [3,15]
The dilatation of upper pouch may persist for months following repair of atresia. Patients with repaired esophageal atresia show a high incidence of abnormal peristalsis in the esophagus, which may cause dysphagia. [3,16] Anastomotic strictures and gastro-esophageal reflux occur commonly after EA and TEF repair. The incidence of stricture increases in patients with long gap EA with a delayed primary repair[17] and reflux.
Stomach
Gastric Atresia, Antro-pyloric Membranes: Congenital gastric obstruction is rare. Isolated pyloric atresia, pyloric/ prepyloric membrane, or antral webs are uncommon causes of gastric obstruction in the newborn. Pyloric atresia is classified into three types: (a) complete atresia with no connection between the stomach and duodenum, (b) complete atresia with a fibrous band connecting the stomach and duodenum, (c) and a gastric membrane or diaphragm. [18,19] The congenital antro-pyloric webs can be diagnosed on UGI contrast study where they are seen as linear defects resulting in ' pseudo double bulb' appearance, as the inflowing barium outlines the space between the antrum and pylorus first and then the duodenal bulb.[20] The membrane may be detected on sonography if the stomach is filled with clear fluid.
Congenital Hypertrophic Pyloric Stenosis (CHPS):
CHPS is characterized by hypertrophy of the circular muscles. Recent work suggests that impaired neuronal nitric oxide synthase synthesis in the myenteric plexus is an important contributing factor in the pathogenesis of infantile hypertrophic pyloric stenosis as well as of achalasia, diabetic gastroparesis, Hirschsprung's disease, and Chaga's disease.[21] The mainstay of diagnosis remains palpation of the hypertrophied pylorus in a quietly feeding baby. Ultrasonography is the primary imaging method, if required, and has replaced contrast evaluation. There is controversy over exact measurements of the gastric outlet in normal and hypertrophic pylorus. The thickness of the muscle is the most discriminating and accurate criterion for the diagnosis of hypertrophic pyloric stenosis.[22]
Ultrasonographically the antro-pyloric muscle of each patient is measured in the midlongitudinal plane (Fig 2a). The stomach is usually well distended allowing easy identification of the antro-pyloric region. The water or glucose solution may be given in cases of inadequate distention but should be removed after examination to prevent vomiting and aspiration. The antro-pyloric muscle less than 2 mm thick should be considered unequivocally normal.[23] The muscle thickness of 3 mm or more is abnormal (Fig 2b) and diagnostic for pyloric stenosis, whereas muscle thickness from 2mm to less than 3 mm is abnormal but not specifically diagnostic for pyloric stenosis.[24] The sonographic measurements in pylorospasm may overlap those accepted as positive for hypertrophic pyloric stenosis. In pylorospasm, there is considerable variability in measurement or image appearance with time during the study.[24] This is an important clue for diagnosing pylorospasm. The sonographic double-track sign can be seen in cases of pylorospasm as well as hypertrophic pyloric stenosis. It is not pathognomonic for hypertrophic pyloric stenosis.[25]
A barium study may be performed if ultrasound is inconclusive or if gastro-oesophageal reflux is suspected (Fig 2c). The hypertrophied muscle mass causes elongation and narrowing of pyloric canal ('string sign') as well as a bulge in the distal antrum with streak of barium pointing towards pyloric channel ('beak sign'). The barium may outline crowded mucosal folds as parallel lines ('double/triple track sign'). These findings, if transient, should be interpreted with caution. The importance of clinical examination and test feed is emphasized especially in cases with equivocal sonographic or barium findings. This will reduce the risk of false-positive diagnoses and negative laparotomies.[26]
Duodenum
Duodenal Atresia/Stenoses
Complete duodenal obstruction is much more common than congenital gastric obstruction. Vomiting is usually bilious as obstruction is often distal to the ampulla of Vater. The plain radiograph of the abdomen shows 'double bubble' sign corresponding to air in stomach and dilated proximal duodenum with absence of air in distal gastrointestinal tract. This condition may be diagnosed with antenatal ultrasound showing fluid filled double-bubble and polyhydramnios. Fetal karyotype should be obtained as up to 30% of patients have Down's syndrome.[9] There is a frequent association with other anomalies in up to 50% of cases. Partial duodenal obstruction is caused by annular pancreas, Ladd's bands, midgut volvulus, pre-duodenal portal vein and duplication cyst.
A pre-duodenal portal vein (persistent left vitelline vein) results from normal situs asymmetry, and is commonly found in patients with heterotaxy. The resultant portal vein courses anterior to the pancreas and duodenum. The diagnosis is made by identifying the prepancreatic course of the portal vein on sonography, CT or MR imaging. It is now believed that in most cases of duodenal obstruction associated with preduodenal portal vein, the obstruction is due to a primary, obstructing duodenal lesion such as intraluminal membrane or web and such a lesion should be suspected in these patients if duodenal obstruction is present. [2, 27]
Small Bowel
Jejunal/ileal atresia
Intestinal atresias account for about one third of all cases of neonatal intestinal obstruction. Prenatal ultrasonography is more reliable in detection of duodenal atresia than the more distal lesions. The survival rate has improved significantly in most of the series with the operative mortality being less than 1%. More distal the atresia better is the survival. An increased mortality is observed in multiple atresias, 'apple peel' syndrome, and when atresia is associated with meconium ileus, meconium peritonitis and gastroschisis.[28] Intestinal atresia occurs in approximately 10% to 20% of neonates with gastroschisis and may be missed at the initial closure if a thick 'peel' obscures the bowel.[29]
Jejuno-ileal atresia results from vascular occlusion and ischemic injury to the developing gut. Jejuno-ileal atresia is classified into four types. Type I is a simple intraluminal diaphragm composed of mucosal and submucosal elements. No interruption of the muscularis is present. Type II represents an atresia with solid fibrous cord (all the layers of intestinal wall interrupted) connecting the blind ends of the bowel. Type III is an atresia with complete separation of the blind ends and an associated mesenteric defect. The familial form of multiple atresias is considered as Type IV. [10, 30]
The disproportionate dilatation of the bowel immediately proximal to the atresia results in a bulbous contour suggestive of congenital small bowel obstruction.[31] Plain radiography is usually diagnostic and further radiological evaluation (upper GI series) is not necessary (Fig. 3). In cases of partial obstruction little amount of distal gas is usually present. A small amount of air may be injected through a nasogastric tube to confirm complete or partial jejunal obstruction. In isolated proximal atresia of the duodenum or jejunum, the colon is of normal size because the remaining bowel distal to the atresia produces sufficient intestinal secretions to produce a normal caliber colon.[4] In ileal atresia, the colon has a normal location but the caliber is reduced (functional microcolon) [32]. In low intestinal obstruction contrast enema should be done to exclude colonic lesion, meconium ileus or Hirschsprung's disease especially when dilated small bowel or colon cannot be differentiated on plain radiography.[4], [10] Sonography may be useful in differentiating meconium ileus and ileal atresia. In meconium ileus, the dilated bowel loops are filled with echogenic material, whereas in ileal atresia the bowel contents are echo-poor.[33]
The 'apple peel' syndrome is thought to follow intrauterine occlusion of the distal superior mesenteric artery (SMA) resulting in a proximal jejunal atresia with absence of mid-small bowel and the dorsal mesentery. The distal bowel derives its blood supply from the proximal SMA. The distal small bowel spirals around its single vascular supply and resembles an "apple peel." [34, 35]
Intestinal stenoses are much less common than intestinal atresias. Stenoses result from three major causes: external compression of the bowel lumen, intramural narrowing resulting from rests of heterotopic tissue, and incompletely perforated intraluninal webs.[10] Stenoses caused by extrinsic impression on the bowel lumen are almost entirely confined to the duodenum as a result of annular pancreas[36] or peritoneal bands.
Malrotation
In the fetal life, the primitive midgut herniates into the extraembryonic celom and later returns into the abdominal cavity between 6-10 weeks. During this process, the primitive midgut rotates anticlockwise through 270 degrees around the axis of the superior mesenteric artery. Both, duodeno-jejunal junction and caecal pole undergo 270 degree anti-clockwise rotation followed by fixation of duodeno-jejunal junction in the left upper quadrant and the ileo-caecal junction in the right lower quadrant. Interruption of this process at any point results in a narrow base of small bowel mesentery with predisposition to volvulus.[2] The term 'malrotation' or 'Nonrotation' is a misnomer as it results from lack of complete intestinal rotation The Ladd's bands or hyperfixation bands can form due to faulty peritoneal attachments as a result of malfixation and malrotation of the intestine. Rotational abnormalities are seen in approximately 70% of patients with situs ambiguous (heterotaxy).
A neonate with bilious vomiting on the first day of life and radiographic evidence of complete duodenal obstruction does not require further radiological evaluation. On the other hand a neonate with bilious vomiting who has been normal for first few days or with radiographic evidence of incomplete obstruction (presence of gas in distal part of gut) requires further evaluation.[4] An upper gastrointestinal series is usually performed initially to demonstrate the level and nature of obstruction. On a supine radiograph the normal C- loop crosses the midline and duodeno-jejunal junction lies to the left of the left vertebral pedicles at the level of the duodenal bulb. The normal duodeno-jejunal junction may be mobile and easily displaceable in infancy, especially in children less than 4 months of age.[37]
Any duodenal obstruction that occurs after immediate postnatal period should be assumed to be midgut volvulus until proved otherwise.[38] The upper gastrointestinal examination shows a typical corkscrew appearance of the duodenum and proximal jejunum (Fig 4). The distal jejunum lies to the right of midline. The Ladd's bands result in Z-shaped duodeno-jejunal configuration and usually obstruct at the third portion of the duodenum but can obstruct more distally.[39] A barium meal should be done for suspected malrotation since a normal barium enema does not exclude malrotation. The position of caecum may be normal in a significant number of patients with malrotation.[20] The UGI examination remains the gold standard for the diagnosis of malrotation
Superior mesenteric vein (SMV) normally lies to the right of superior mesenteric artery (SMA). Malrotation may be suggested on US if the SMV is to the left or anterior to the artery. However, this is neither a sensitive nor a specific sign. The sonographic "whirlpool sign" is objective and a definite sign as volvulus, is shown at color Doppler US as clockwise spiraling of the mesentery and SMV around the SMA. [40, 41] Recent literature is replete with many diagnostic criteria for various imaging modalities. The multiplicity of the findings is a testimony to the unsatisfactory nature of their yield.
Meconium peritonitis
Meconium peritonitis results from an in-utero bowel perforation proximal to a complete obstruction. Although commonly associated with bowel atresias, it may be present in asymptomatic neonates with in-utero sealed bowel perforation. Plain abdominal radiograph may show peritoneal calcifications due to calcified meconium
(Fig 5). The small bowel loops may be dilated in cases of associated bowel atreasia. These calcific densities may be seen in the scrotum also if the processus vaginalis is patent. On ultrasound, the calcifications are seen as echogenic densities outside the bowel wall.
Meconium ileus
Meconium ileus is a low intestinal obstruction caused by inspissated meconium usually in the terminal ileum and is almost always associated with cystic fibrosis. The plain abdominal radiograph usually shows low small bowel obstruction with dilated small bowel loops but absent or scant fluid levels. Although the absence of air-fluid levels strongly suggests meconium ileus, the presence of air fluid levels does not exclude it as some cases do demonstrate air-fluid levels, especially those with associated complications such as volvulus and stenosis or atresia. [18,42] Volvulus of a meconium laden heavy bowel loop is common and can lead to intestinal stenoses, atresias, gangrene and perforation. The "soap bubble" appearance caused by admixture of air and meconium is frequently seen (Fig 6). It can, however, also be seen with ileal atresia, colon atresia, Hirschsprung's disease and the meconium plug syndrome.[18] Calcifications or pseudocyst may be seen due to meconium peritonitis and localized ascitis
Enteric Duplication Cyst
Enteric duplication cysts can occur anywhere along the length of the gut where they lie along the mesenteric border and share a common muscle wall and blood supply. The duplications commonly occur in the distal ileum (Fig 7) and esophagus. Duplication cysts are usually spherical lesions and less often tubular. The communication with the adjacent bowel is uncommon in spherical lesions and is more likely to occur with tubular duplications.[43]
Ultrasound is the imaging modality of choice for the evaluation of an abdominal mass in the neonate. The most common clinical manifestation of intra-abdominal enteric duplication cysts is intestinal obstruction. Occasionally, haemorrhage or gastrointestinal bleeding may occur due to ectopic gastric mucosa which is seen in 10-20% of cases which makes it visible with technetium-99m pertechnetate scanning and may be of diagnostic value.
The cyst shows the echogenic inner rim of mucosa and hypoechoic outer rim of the muscle layer (double wall sign). This is most easily identified in the dependent portion of the cyst.[44] Peristalsis of the cyst wall when visible, is another useful sign.[45 ]The cyst is fluid filled, usually unilocular and anechoic. Occasionally, internal echoes and septations may be seen due to hemorrhage or inspissated mucoid material. Rarely, the cyst is completely echogenic and gives false impression of a solid lesion. Multiple duplication cysts may be present in 15-20% of cases.[46]
Esophageal duplication cyst: duplications of the esophagus are the second most common duplication of the gastrointestinal tract after ileal duplications. [43,47] An esophageal duplication cyst may present with pressure symptoms such as dysphagea, stridor, or breathing difficulty. On chest radiograph, duplication cyst is seen as posterior mediastinal mass. Ultrasound may demonstrate double-layered wall especially in large cysts abutting posterior chest wall. CT and MRI demonstrate well delineated outlines with non-enhancing attenuation values of water. Endoscopic US can accurately diagnose duplication cyst by demonstrating contiguity of the muscularis propria of the esophagus with the muscle layer of the cyst wall.[48]
Duodenal duplication cyst is a rare congenital anomaly and usually presents with symptoms of obstruction, but may cause biliary obstruction and pancreatitis. [49] The differential diagnosis of duodenal duplication includes a choledochocele, a pancreatic pseudocyst and intraluminal diverticulum.[50]
Mesenteric Cyst (Lymphangioma)
Mesenteric cyst is a congenital malformation arising due to sequestration of lymphatic vessels.[51] These are usually seen in the mesentery and less often in omentum and retroperitoneum. Children usually present in the first decade, with increasing abdominal girth or a palpable abdominal mass. Sonography reveals a thin-walled unilocular or multilocular cystic lesion, whereas CT and MRI demonstrate variable characteristics of the cyst contents (usually water-to fat) depending upon whether fluid is chylous, infected or haemorrhagic. Ultrasound is particularly useful to demonstrate the thin septations which may not be well seen on CT. Rarely, a mesenteric lymphangioma may contain calcification mimicking a mesenteric teratoma. [52]
Large Bowel
Colonic Obstruction
Colonic atresia is quite rare and often indistinguishable from obstruction of the distal ileum. The right colon is most commonly affected.[53] Plain radiograph demonstrates features of a low intestinal obstruction with air-fluid levels or mottled pattern due to retained meconium. Contrast enema shows a distal microcolon and complete obstruction at the level of atresia. On US, dilatation of the distal small bowel and proximal colon with echogenic contents due to retained meconium may be seen.[4]
Hirschsprung's Disease
Approximately one-fifth of neonatal intestinal obstructions are due to Hirschsprung's disease.[2] In about three quarters of these cases, the area of aganglionosis is limited to the rectum and sigmoid (short segment disease).[54] Long segment disease involves a variable portion of the colon proximal to the sigmoid and in total aganglionosis coli the entire colon and a part of the terminal ileum[20] is involved. Total colonic aganglionosis and 'ultra short segment' disease is rare.
The majority of patients present within the first 6 weeks in life. Neonates present with failure to pass meconium, abdominal distention, vomiting or enterocolitis. Enterocolitis is the leading cause of death in Hirschsprung's disease.[55] Enterocolitis and perforation is more common in the long segment disease. The perforation occurs most commonly in the ascending colon or in the appendix. [56, 57]
Imaging Features : Plain radiograph shows features of low bowel obstruction with colonic dilatation out of proportion to the small bowel.[58] There may be absence of rectal gas or small amount of gas may be seen in rectum (especially on prone films) on plain radiograph. However, the absence of rectal gas is not specific for Hirschsprung's disease, being more commonly seen in infants with sepsis and necrotizing enterocolitis.[59]
Digital examination of the rectum or enemas,in patients scheduled for barium studies should not be done as it may distort the findings.[10] For the same reason, the study (enema) is never performed with an inflated Foley catheter in the rectum.[10 ]Barium enema is performed carefully on an unprepared patient by inserting a straight-tipped catheter to a point just beyond the anal sphincter. The patient should be placed in lateral position and barium infused slowly as rapid infusion can distend and mask the transition zone. The contrast agent should be prepared with normal saline to avoid possibility of water absorption from the large surface area of dilated colon.[3] The diagnostic feature in short segment disease is funnel shaped transition zone and reversal of the recto-sigmoid ratio (Fig 7). Normally, the rectum is the most distensible portion of the bowel and has a diameter greater than that of the sigmoid colon (recto: sigmoid ratio >1).[20] The radiological transition zone is commonly found distal to the pathological transition zone.[60] There may be irregular, uncoordinated contractions in the aganglionic segment (20% of cases) which may be mistaken for mucosal ulcerations. Delayed films after 24 hrs may demonstrate prolonged retention of barium in the sigmoid colon (not rectum which normally holds barium for 24 hours). This is a strong indicator of Hirschsprung's disease, even when the enema findings have been inconclusive. Rectal biopsy is always necessary for confirmation.
The diagnosis of total colonic aganglionosis is much more difficult. The radiological findings viz. foreshortening, rounding of the colonic flexures and small caliber colon[2 ]are relatively nonspecific. The prolonged retention of barium on delayed films should alert one to suspect the diagnosis.[61 ]Presence of skip lesions in Hirschsprung's disease is extremely rare (controversial ). Awareness of this variant aids in the interpretation of the barium enema in children with signs and symptoms of aganglionosis, especially when the rectal biopsy is normal.[62 ]The associations of Hirschsprung's disease include Down's syndrome, ileal and colon atresia and neuro-cristopathies.[63-65]
Anorectal Anomalies
The anorectal malformations are divided into high and low types depending upon whether the termination of hindgut is above or below the puborectalis sling. Anorectal anomalies are often associated with fistulous tracts opening into the genitourinary system. Although the distinction between high and low lesions can be made clinically, the radiologic study is required to evaluate the internal anatomy. This is usually accomplished using contrast studies. In both male and female infants with low lesions, there is usually a visible perineal opening and the communication with the genitourinary tract is absent [10,20 ] However, there are cases with imperforate anus with external signs of a low lesion with demonstrable fistula into the anterior urethra either at surgery or by radiographic studies.[66] These cases should be considered an intermediate form of imperforate anus with external signs of a low lesion but with fistula characteristically to the membranous or bulbous urethra.[65 ]High lesions are treated with initial diverting colostomy and definitive repair at a later age. A pressure augmented distal colostogram is recommended prior to definitive repair, both to confirm the level of rectal atresia and to define any associated fistulous communication.[67] Water- soluble contrast medium is injected under mild pressure through a Foley's catheter with its balloon inflated to occlude the stoma. The fistulous tract not identified on the conventional contrast colostogram (Fig 8) may be delineated when an augmented-pressure modification of the technique is utilized.[66 ]Micturating cystourethrography (MCU) and/or retrograde urethrography (RGU) are also useful to demonstrate fistula and presence of vesicoureteric reflux.
The easily available plain radiograghy is simple and is often the preliminary investigation to be used in evaluation of these patients. Prone cross table lateral view with babies kept in genu-pectoral position is useful in determining the level of atresia.[68 ]The use of invertogram is to be discouraged as it causes unnecessary stress. During invertogram, the baby usually keeps crying causing contraction of puborectalis sling and obliteration of the lower rectum. Also, the rectum may be pulled in the cephalic direction resulting in error of classification. In those anomalies, which are associated with fistulae with the urinary or genital tract, the fistula becomes the highest point of rectum in an invertogram and gas may escape resulting in less distension of the rectum. In prone position, the fistula is lowest and therefore rectum is better distended causing adequate delineation of the rectal gas [68]. The pubococcygeal line and M line are imaginary lines used to represent the level of puborectalis sling on lateral radiographs. The "M" line is a better anatomic correlate of puborectalis sling. The "M" line passes horizontally through the junction of the lower third and upper two thirds of the ischium.
Ultrasonography can assess the distance of the rectal pouch in relation to the perineal surface. A pouch perineal distance of < 1.5 cm is indicative of low anomalies, while a pouch that terminates above the base of the bladder (superior extent of urogenital diaphragm) is indicative of a high lesion.[69] However, the distinction between high and intermediate anomaly cannot be made out with this technique. Recently, anal endosonography has been used as an accurate alternative to MRI in the assessment of anorectal malformations after repair.[70]
CT and MRI are the modalities of choice to delineate pelvic anatomy including puborectalis muscle and external sphincter. Congenital anorectal malformations are found in many forms, and are frequently associated with other anomalies, especially of the spine, spinal cord, and urogenital system.[71 ] Magnetic resonance imaging has proven to be the single stop modality to answer all the crucial questions such as level and type of malformation, type of fistula, developmental state of the sphincter muscle complex, and the presence of associated anomalies.[71-73] MRI has contributed to a better insight in the morphology and pathogenesis of such complex congenital malformations.[71].
Spinal radiographs must be examined carefully for abnormalities, because spinal pathology has profound effect on the outcome. Normal radiographic and sonographic appearance of spinal anatomy in children with anorectal malformation makes MRI superfluous, but if radiographs or ultrasound are uninformative/ abnormal, MRI should be used to accurately depict possible intraspinal pathology.[74] Sonography of the urinary tract for associated renal anomalies is essential in all patients with anorectal malformation.
Megacystis-Microcolon-Intestinal Hypoperistalsis Syndrome (Berdon Syndrome)[75
]This syndrome is a rare congenital cause of intestinal obstruction. The abdominal distention is due to dilated bladder and dilated short bowel in absence of organic obstruction. There is hypomotility of small bowel with retrograde peristalsis on upper GI contrast study while an enema shows a microcolon. Ultrasound will show a dilated bladder and bilateral hydrourteronephrosis. The anterior abdominal wall musculature is thin, similar to prune belly syndrome.
Meconium Plug Syndrome
Immature left colon is a form of functional obstruction associated with prematurity, maternal drug ingestion (magnesium preparations or high doses of sedatives),and in babies of diabetic mothers. [4,76] The affected infants present with distal bowel obstruction and delayed meconium passage. However, the bowel distention is less severe than with a mechanical obstruction. The contrast enema typically shows a mildly dilated right and transverse colon and a small left colon with an abrupt transition at the level of splenic flexure. The meconium plugs are seen as multiple elongated filling defects in dilated colon. The contrast enema using water-soluble contrast media is both diagnostic as well as therapeutic. If clinical improvement does not follow over next few hours or days, suction rectal biopsy is recommended to rule out Hirschsprung's disease.
Conclusion
0The goal of imaging is to provide an accurate diagnosis and other relevant information to the clinician while minimizing patient discomfort and radiation exposure. The judicious choice of various imaging modalities requires a concerted effort of the clinician and the radiologist aimed at providing best information and help to the treating clinician to plan out the most suitable treatment efficaciously
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Abstract
Abstract. The radiological imaging plays a vital role in the evaluatin of patients with congenital anomalies of the gastrointestinal tract. The evaluation of these patients, most of which present early after birth, frequently requires the use of various imaging modalities for making the correct diagnosis and planning surgical correction. This article reviews the common congenital anomalies of the gastrointestinal tract including obstructive lesions, anomalies of rotation and fixation, anorectal anomalies, and intestinal duplications. The plain radiograph is often diagnostic in neonates with complete gastric of upper intestinal obstruction and further radiologic evaluation may be unnecessary. An upper gastrointestinal series should be performed in all patients with incomplete intestinal obstruction. Sonography is useful in the evaluation of many congenital anomalies affecting pediatric gastrointestinal tract especially hypertrophic pyloric stenosis, enteric duplication cysts, midgut malrotation, meconium ileus and meconium peritonitis. Moreover, CT and MRI has assumed a greater importance as these provide excellent anatomic details which may be necessary for correct diagnosis as well as treatment planning. This is particularly true in evaluation of congenital anomalies such as esophageal/enteric duplications, vascular rings and anorectal anomalies. It is important to be familiar with the role nad usefulness of the various imaging modalities so that these can be used judiciously to avoid unnecessary radiation exposure while minimizing the patient discomfort. [Indian J Pediatr 2005; 72 (5) : -414]
Keywords: GIT; Congenital anomalies; Intestinal abnormalities
A wide spectrum of congenital anomalies may affect gastrointestinal tract, some of which manifest early after birth while others may not present till late childhood or adulthood. Imaging plays a very useful role in these developmental lesions, most of which present with obstruction. In most cases, plain radiography, gastrointestinal contrast study or Ultrasound (US) suffice. Computed tomography (CT) and Magnetic Resonance Imaging (MRI) are increasingly proving more useful to provide an accurate diagnosis specially in difficult cases.
Developmental lesions of the neonatal gastrointestinal tract can be grouped as follows:[ 1]
Structural
Attributed to embryologic maldevelopment
Esophageal atresia with or without fistula
Antro-pyloric atresia
Antral diaphragm
Duodenal atresia
Duodenal stenosis
Intrinsic: windsock duodenum
Extrinsic: annular pancreas
Midgut malrotation with peritoneal bands
Duplication or mesenteric cyst
Anorectal atresia
Attributed to in utero vascular (ischemic) complication
Jejuno-ileal atresia
Colonic atresia or stenosis
Complicated meconium ileus
Functional
Meconium plug syndrome and its variants
Megacystis-microcolon-intestinal hypoperistalsis
Structural and Functional Combined
Hypertrophic pyloric stenosis
Midgut vovulus (complicating midgut malrotation)
Uncomplicated meconium ileus
Colonic aganglionosis
In a normal neonate, swallowing begins almost immediately after birth and gas should be present in the stomach within few minutes.[3] Within 3 hours of birth, the entire small bowel usually contains gas while the sigmoid colon is seen only after 8-9 hours.[4] Disruption of this common pattern is seen in obstruction or presence of underlying illness such as brain damage, septicemia or hypoglycemia.
Plain radiograph is a useful, simple and most inexpensive tool in the evaluation of the neonate with gastrointestinal (GI) obstruction. Unlike adults and older children, in neonates the small and large bowel usually cannot be distinguished. The gas is distributed throughout the small and large bowel where a little fluid is present resulting in sharp bowel-air interfaces which appear as multiple closely apposed rounded or polyhedral structures on plain radiograph.[2] The precise level of obstruction may be evident in high GI obstruction but is difficult to determine in a low gut obstruction. High GI obstruction occurs most commonly at the level of duodenum and proximal jejunum and plain radiography alone is often diagnostic.
Bilious vomiting indicates obstruction distal to the ampulla of Vater. Although there is little role of contrast examintion in high gut obstruction,[5] GI contrast examination should be performed in obstructions presenting beyond first few days of life to rule out malrotation and midgut volvulus. The stomach is emptied through a nasogastric tube before upper GI contrast study is undertaken.
Barium suspension is not used in cases of suspected perforation or if there is a risk of barium inspissations. Aspiration into lungs should be avoided while using contrast agents because commonly used high osmolality ionic contrast may produce severe pulmonary edema. Hypertonic water-soluble ionic contrast media may be useful in relieving obstruction in meconium ileus by drawing water into the bowel lumen. However, this may cause fluid and electrolyte imbalance. Therefore, infant should be hydrated and serum electrolytes monitored before the procedure. Non-ionic low osmolality contrast media are preferred in most circumstances. Barium is preferred in cases of suspected Hirschsprung's disease or other conditions where delayed films have diagnostic value.
High Intestinal Obstruction: The usual presentation is vomiting which may be bile stained if obstruction is distal to ampulla of Vater. The plain radiograph is often diagnostic and further radiological evaluation may be unnecessary. However, an upper gastrointestinal series must be performed in all patients with incomplete intestinal obstruction. The causes of high obstruction include pyloric atresia, duodenal atresia, malrotation with midgut volvulus or Ladd's bands and proximal jejunal atresia.[2] Partial obstruction results from jejunal stenosis, peritoneal bands, duplication cyst, malrotation and Meckel's diverticulum.[4]
Low Intestinal Obstruction: Failure to pass meconium in the first 24-48 hours of life may be due to structural or functional reasons. The causes include ileal or colon atresia, Anorectal malformations, Hirschsprung's disease, meconium plug syndrome, and neonatal small left colon syndrome.
Ultrasonography (US) is often the first modality to be used in investigation of child with abdominal lump or suspected hypertrophic pyloric stenosis. US is highly accurate in the diagnosis of hypertrophic pyloric stenosis and extremely useful in the investigation of mass lesions such as enteric duplication cysts and mesenteric or omental cysts.[1]
US is the modality of choice for prenatal screening, but occasionally additional imaging information is needed.[6] MRI is being increasingly used for prenatal imaging of congenital anomalies. The use of fast sequences like single shot fast spin echo and echoplanar imaging has enabled successful prenatal MR imaging. Esophageal, duodenal, or small bowel atresia can be diagnosed on antenatal MRI. On MRI, the signal intensity differences of the dilated bowel may provide additional information complementary to the US in identifying the site of obstruction. The proximal small bowel (fluid content) appears hyperintense on single-shot fast spin echo (SSFSE) and hypointense on T1-weighted fast spin echo (FSE) imaging. In contrast, the distal small bowel and colon appear hypointense on SSFSE and hyperintense on T1-weighted FSE imaging due to presence of meconium. [7,8]
Esophagus
Esophageal Atresia (EA) and Tracheo-Esophageal fistula (TEF)
The presence of polyhydramnios, reduced intraluminal liquid in the fetal gut and inability to detect the fetal stomach on prenatal ultrasound may provide an early clue to the possibility of esophageal atresia.[9]
The chest radiographs (AP and Lateral) demonstrate proximal esophageal pouch filled with air. The abdomen is always included in the radiograph to look for air in the gastrointestinal tract. The absence of gas in stomach or intestinal tract is suggestive of pure esophageal atresia (EA) without fistula or EA with proximal tracheo-esophageal fistula. The position of aortic arch should be conveyed to the surgeon as it may help determine the surgical approach. When plain radiographs fail to indicate the side of the arch, and as a preoperative work-up, echocardiography should be performed before surgery to prognosticate and plan successful outcome.[10] Chest radiograph may show consolidation due to aspiration pneumonia especially in the dependant upper lobes in supine position.
Radiographically, there is inability to pass a red rubber catheter into the stomach. The proximal pouch may be outlined due to swallowed air (Fig 1a). Gentle injection of air via the tube may distend the proximal pouch. The distance between proximal and distal esophageal pouch can be assessed only after a gastrostomy in a staged procedure. Isolated long gap esophageal atresia is known to be associated with 13 pair of ribs.[12]
The diagnosis is frequently delayed in H-type fistula. The fistula is more precisely termed as N-type fistula as it courses obliquely from the esophagus upwards to the trachea (Fig 1b). Most H-fistulas are at thoracic inlet (T2-3). Tube esophagogram under fluoroscopic guidance with patient in the lateral or steep prone oblique position is the best way to demonstrate H-type fistula. The nasogastric tube placed at GE junction is gradually withdrawn while injecting contrast at various levels.[10] A small amount of isotonic non-ionic contrast medium or properly diluted barium should be used. The contrast should be aspirated back immediately after the study.
Esophageal stenosis and webs may be associated with tracheoesophageal fistula. [13,14] Up to one quarter of patients with EA & TEF have associated with GI anomalies including pyloric stenosis, duodenal/small bowel atresia, anorectal malformations. There should be high index of suspicion for associated anomalies. The pattern of air distribution on plain radiographs may provide clue to these anomalies. Air confined to the stomach and or duodenum suggests associated duodenal atresia. Follow up radiographs may be obtained if initial findings are suspicious.
Associated anomalies are frequently seen in such individuals including features of the VACTREL spectrum (vertebral anomalies, anorectal malformation, cardiovascular malformation, tracheo-esophageal fistula with esophageal atresia, renal anomalies and limb defects). The other recognized associations are trisomy 18, trisomy 21, CHARGE syndrome, Potter syndrome and presence of 13 pair of ribs. [3,15]
The dilatation of upper pouch may persist for months following repair of atresia. Patients with repaired esophageal atresia show a high incidence of abnormal peristalsis in the esophagus, which may cause dysphagia. [3,16] Anastomotic strictures and gastro-esophageal reflux occur commonly after EA and TEF repair. The incidence of stricture increases in patients with long gap EA with a delayed primary repair[17] and reflux.
Stomach
Gastric Atresia, Antro-pyloric Membranes: Congenital gastric obstruction is rare. Isolated pyloric atresia, pyloric/ prepyloric membrane, or antral webs are uncommon causes of gastric obstruction in the newborn. Pyloric atresia is classified into three types: (a) complete atresia with no connection between the stomach and duodenum, (b) complete atresia with a fibrous band connecting the stomach and duodenum, (c) and a gastric membrane or diaphragm. [18,19] The congenital antro-pyloric webs can be diagnosed on UGI contrast study where they are seen as linear defects resulting in ' pseudo double bulb' appearance, as the inflowing barium outlines the space between the antrum and pylorus first and then the duodenal bulb.[20] The membrane may be detected on sonography if the stomach is filled with clear fluid.
Congenital Hypertrophic Pyloric Stenosis (CHPS):
CHPS is characterized by hypertrophy of the circular muscles. Recent work suggests that impaired neuronal nitric oxide synthase synthesis in the myenteric plexus is an important contributing factor in the pathogenesis of infantile hypertrophic pyloric stenosis as well as of achalasia, diabetic gastroparesis, Hirschsprung's disease, and Chaga's disease.[21] The mainstay of diagnosis remains palpation of the hypertrophied pylorus in a quietly feeding baby. Ultrasonography is the primary imaging method, if required, and has replaced contrast evaluation. There is controversy over exact measurements of the gastric outlet in normal and hypertrophic pylorus. The thickness of the muscle is the most discriminating and accurate criterion for the diagnosis of hypertrophic pyloric stenosis.[22]
Ultrasonographically the antro-pyloric muscle of each patient is measured in the midlongitudinal plane (Fig 2a). The stomach is usually well distended allowing easy identification of the antro-pyloric region. The water or glucose solution may be given in cases of inadequate distention but should be removed after examination to prevent vomiting and aspiration. The antro-pyloric muscle less than 2 mm thick should be considered unequivocally normal.[23] The muscle thickness of 3 mm or more is abnormal (Fig 2b) and diagnostic for pyloric stenosis, whereas muscle thickness from 2mm to less than 3 mm is abnormal but not specifically diagnostic for pyloric stenosis.[24] The sonographic measurements in pylorospasm may overlap those accepted as positive for hypertrophic pyloric stenosis. In pylorospasm, there is considerable variability in measurement or image appearance with time during the study.[24] This is an important clue for diagnosing pylorospasm. The sonographic double-track sign can be seen in cases of pylorospasm as well as hypertrophic pyloric stenosis. It is not pathognomonic for hypertrophic pyloric stenosis.[25]
A barium study may be performed if ultrasound is inconclusive or if gastro-oesophageal reflux is suspected (Fig 2c). The hypertrophied muscle mass causes elongation and narrowing of pyloric canal ('string sign') as well as a bulge in the distal antrum with streak of barium pointing towards pyloric channel ('beak sign'). The barium may outline crowded mucosal folds as parallel lines ('double/triple track sign'). These findings, if transient, should be interpreted with caution. The importance of clinical examination and test feed is emphasized especially in cases with equivocal sonographic or barium findings. This will reduce the risk of false-positive diagnoses and negative laparotomies.[26]
Duodenum
Duodenal Atresia/Stenoses
Complete duodenal obstruction is much more common than congenital gastric obstruction. Vomiting is usually bilious as obstruction is often distal to the ampulla of Vater. The plain radiograph of the abdomen shows 'double bubble' sign corresponding to air in stomach and dilated proximal duodenum with absence of air in distal gastrointestinal tract. This condition may be diagnosed with antenatal ultrasound showing fluid filled double-bubble and polyhydramnios. Fetal karyotype should be obtained as up to 30% of patients have Down's syndrome.[9] There is a frequent association with other anomalies in up to 50% of cases. Partial duodenal obstruction is caused by annular pancreas, Ladd's bands, midgut volvulus, pre-duodenal portal vein and duplication cyst.
A pre-duodenal portal vein (persistent left vitelline vein) results from normal situs asymmetry, and is commonly found in patients with heterotaxy. The resultant portal vein courses anterior to the pancreas and duodenum. The diagnosis is made by identifying the prepancreatic course of the portal vein on sonography, CT or MR imaging. It is now believed that in most cases of duodenal obstruction associated with preduodenal portal vein, the obstruction is due to a primary, obstructing duodenal lesion such as intraluminal membrane or web and such a lesion should be suspected in these patients if duodenal obstruction is present. [2, 27]
Small Bowel
Jejunal/ileal atresia
Intestinal atresias account for about one third of all cases of neonatal intestinal obstruction. Prenatal ultrasonography is more reliable in detection of duodenal atresia than the more distal lesions. The survival rate has improved significantly in most of the series with the operative mortality being less than 1%. More distal the atresia better is the survival. An increased mortality is observed in multiple atresias, 'apple peel' syndrome, and when atresia is associated with meconium ileus, meconium peritonitis and gastroschisis.[28] Intestinal atresia occurs in approximately 10% to 20% of neonates with gastroschisis and may be missed at the initial closure if a thick 'peel' obscures the bowel.[29]
Jejuno-ileal atresia results from vascular occlusion and ischemic injury to the developing gut. Jejuno-ileal atresia is classified into four types. Type I is a simple intraluminal diaphragm composed of mucosal and submucosal elements. No interruption of the muscularis is present. Type II represents an atresia with solid fibrous cord (all the layers of intestinal wall interrupted) connecting the blind ends of the bowel. Type III is an atresia with complete separation of the blind ends and an associated mesenteric defect. The familial form of multiple atresias is considered as Type IV. [10, 30]
The disproportionate dilatation of the bowel immediately proximal to the atresia results in a bulbous contour suggestive of congenital small bowel obstruction.[31] Plain radiography is usually diagnostic and further radiological evaluation (upper GI series) is not necessary (Fig. 3). In cases of partial obstruction little amount of distal gas is usually present. A small amount of air may be injected through a nasogastric tube to confirm complete or partial jejunal obstruction. In isolated proximal atresia of the duodenum or jejunum, the colon is of normal size because the remaining bowel distal to the atresia produces sufficient intestinal secretions to produce a normal caliber colon.[4] In ileal atresia, the colon has a normal location but the caliber is reduced (functional microcolon) [32]. In low intestinal obstruction contrast enema should be done to exclude colonic lesion, meconium ileus or Hirschsprung's disease especially when dilated small bowel or colon cannot be differentiated on plain radiography.[4], [10] Sonography may be useful in differentiating meconium ileus and ileal atresia. In meconium ileus, the dilated bowel loops are filled with echogenic material, whereas in ileal atresia the bowel contents are echo-poor.[33]
The 'apple peel' syndrome is thought to follow intrauterine occlusion of the distal superior mesenteric artery (SMA) resulting in a proximal jejunal atresia with absence of mid-small bowel and the dorsal mesentery. The distal bowel derives its blood supply from the proximal SMA. The distal small bowel spirals around its single vascular supply and resembles an "apple peel." [34, 35]
Intestinal stenoses are much less common than intestinal atresias. Stenoses result from three major causes: external compression of the bowel lumen, intramural narrowing resulting from rests of heterotopic tissue, and incompletely perforated intraluninal webs.[10] Stenoses caused by extrinsic impression on the bowel lumen are almost entirely confined to the duodenum as a result of annular pancreas[36] or peritoneal bands.
Malrotation
In the fetal life, the primitive midgut herniates into the extraembryonic celom and later returns into the abdominal cavity between 6-10 weeks. During this process, the primitive midgut rotates anticlockwise through 270 degrees around the axis of the superior mesenteric artery. Both, duodeno-jejunal junction and caecal pole undergo 270 degree anti-clockwise rotation followed by fixation of duodeno-jejunal junction in the left upper quadrant and the ileo-caecal junction in the right lower quadrant. Interruption of this process at any point results in a narrow base of small bowel mesentery with predisposition to volvulus.[2] The term 'malrotation' or 'Nonrotation' is a misnomer as it results from lack of complete intestinal rotation The Ladd's bands or hyperfixation bands can form due to faulty peritoneal attachments as a result of malfixation and malrotation of the intestine. Rotational abnormalities are seen in approximately 70% of patients with situs ambiguous (heterotaxy).
A neonate with bilious vomiting on the first day of life and radiographic evidence of complete duodenal obstruction does not require further radiological evaluation. On the other hand a neonate with bilious vomiting who has been normal for first few days or with radiographic evidence of incomplete obstruction (presence of gas in distal part of gut) requires further evaluation.[4] An upper gastrointestinal series is usually performed initially to demonstrate the level and nature of obstruction. On a supine radiograph the normal C- loop crosses the midline and duodeno-jejunal junction lies to the left of the left vertebral pedicles at the level of the duodenal bulb. The normal duodeno-jejunal junction may be mobile and easily displaceable in infancy, especially in children less than 4 months of age.[37]
Any duodenal obstruction that occurs after immediate postnatal period should be assumed to be midgut volvulus until proved otherwise.[38] The upper gastrointestinal examination shows a typical corkscrew appearance of the duodenum and proximal jejunum (Fig 4). The distal jejunum lies to the right of midline. The Ladd's bands result in Z-shaped duodeno-jejunal configuration and usually obstruct at the third portion of the duodenum but can obstruct more distally.[39] A barium meal should be done for suspected malrotation since a normal barium enema does not exclude malrotation. The position of caecum may be normal in a significant number of patients with malrotation.[20] The UGI examination remains the gold standard for the diagnosis of malrotation
Superior mesenteric vein (SMV) normally lies to the right of superior mesenteric artery (SMA). Malrotation may be suggested on US if the SMV is to the left or anterior to the artery. However, this is neither a sensitive nor a specific sign. The sonographic "whirlpool sign" is objective and a definite sign as volvulus, is shown at color Doppler US as clockwise spiraling of the mesentery and SMV around the SMA. [40, 41] Recent literature is replete with many diagnostic criteria for various imaging modalities. The multiplicity of the findings is a testimony to the unsatisfactory nature of their yield.
Meconium peritonitis
Meconium peritonitis results from an in-utero bowel perforation proximal to a complete obstruction. Although commonly associated with bowel atresias, it may be present in asymptomatic neonates with in-utero sealed bowel perforation. Plain abdominal radiograph may show peritoneal calcifications due to calcified meconium
(Fig 5). The small bowel loops may be dilated in cases of associated bowel atreasia. These calcific densities may be seen in the scrotum also if the processus vaginalis is patent. On ultrasound, the calcifications are seen as echogenic densities outside the bowel wall.
Meconium ileus
Meconium ileus is a low intestinal obstruction caused by inspissated meconium usually in the terminal ileum and is almost always associated with cystic fibrosis. The plain abdominal radiograph usually shows low small bowel obstruction with dilated small bowel loops but absent or scant fluid levels. Although the absence of air-fluid levels strongly suggests meconium ileus, the presence of air fluid levels does not exclude it as some cases do demonstrate air-fluid levels, especially those with associated complications such as volvulus and stenosis or atresia. [18,42] Volvulus of a meconium laden heavy bowel loop is common and can lead to intestinal stenoses, atresias, gangrene and perforation. The "soap bubble" appearance caused by admixture of air and meconium is frequently seen (Fig 6). It can, however, also be seen with ileal atresia, colon atresia, Hirschsprung's disease and the meconium plug syndrome.[18] Calcifications or pseudocyst may be seen due to meconium peritonitis and localized ascitis
Enteric Duplication Cyst
Enteric duplication cysts can occur anywhere along the length of the gut where they lie along the mesenteric border and share a common muscle wall and blood supply. The duplications commonly occur in the distal ileum (Fig 7) and esophagus. Duplication cysts are usually spherical lesions and less often tubular. The communication with the adjacent bowel is uncommon in spherical lesions and is more likely to occur with tubular duplications.[43]
Ultrasound is the imaging modality of choice for the evaluation of an abdominal mass in the neonate. The most common clinical manifestation of intra-abdominal enteric duplication cysts is intestinal obstruction. Occasionally, haemorrhage or gastrointestinal bleeding may occur due to ectopic gastric mucosa which is seen in 10-20% of cases which makes it visible with technetium-99m pertechnetate scanning and may be of diagnostic value.
The cyst shows the echogenic inner rim of mucosa and hypoechoic outer rim of the muscle layer (double wall sign). This is most easily identified in the dependent portion of the cyst.[44] Peristalsis of the cyst wall when visible, is another useful sign.[45 ]The cyst is fluid filled, usually unilocular and anechoic. Occasionally, internal echoes and septations may be seen due to hemorrhage or inspissated mucoid material. Rarely, the cyst is completely echogenic and gives false impression of a solid lesion. Multiple duplication cysts may be present in 15-20% of cases.[46]
Esophageal duplication cyst: duplications of the esophagus are the second most common duplication of the gastrointestinal tract after ileal duplications. [43,47] An esophageal duplication cyst may present with pressure symptoms such as dysphagea, stridor, or breathing difficulty. On chest radiograph, duplication cyst is seen as posterior mediastinal mass. Ultrasound may demonstrate double-layered wall especially in large cysts abutting posterior chest wall. CT and MRI demonstrate well delineated outlines with non-enhancing attenuation values of water. Endoscopic US can accurately diagnose duplication cyst by demonstrating contiguity of the muscularis propria of the esophagus with the muscle layer of the cyst wall.[48]
Duodenal duplication cyst is a rare congenital anomaly and usually presents with symptoms of obstruction, but may cause biliary obstruction and pancreatitis. [49] The differential diagnosis of duodenal duplication includes a choledochocele, a pancreatic pseudocyst and intraluminal diverticulum.[50]
Mesenteric Cyst (Lymphangioma)
Mesenteric cyst is a congenital malformation arising due to sequestration of lymphatic vessels.[51] These are usually seen in the mesentery and less often in omentum and retroperitoneum. Children usually present in the first decade, with increasing abdominal girth or a palpable abdominal mass. Sonography reveals a thin-walled unilocular or multilocular cystic lesion, whereas CT and MRI demonstrate variable characteristics of the cyst contents (usually water-to fat) depending upon whether fluid is chylous, infected or haemorrhagic. Ultrasound is particularly useful to demonstrate the thin septations which may not be well seen on CT. Rarely, a mesenteric lymphangioma may contain calcification mimicking a mesenteric teratoma. [52]
Large Bowel
Colonic Obstruction
Colonic atresia is quite rare and often indistinguishable from obstruction of the distal ileum. The right colon is most commonly affected.[53] Plain radiograph demonstrates features of a low intestinal obstruction with air-fluid levels or mottled pattern due to retained meconium. Contrast enema shows a distal microcolon and complete obstruction at the level of atresia. On US, dilatation of the distal small bowel and proximal colon with echogenic contents due to retained meconium may be seen.[4]
Hirschsprung's Disease
Approximately one-fifth of neonatal intestinal obstructions are due to Hirschsprung's disease.[2] In about three quarters of these cases, the area of aganglionosis is limited to the rectum and sigmoid (short segment disease).[54] Long segment disease involves a variable portion of the colon proximal to the sigmoid and in total aganglionosis coli the entire colon and a part of the terminal ileum[20] is involved. Total colonic aganglionosis and 'ultra short segment' disease is rare.
The majority of patients present within the first 6 weeks in life. Neonates present with failure to pass meconium, abdominal distention, vomiting or enterocolitis. Enterocolitis is the leading cause of death in Hirschsprung's disease.[55] Enterocolitis and perforation is more common in the long segment disease. The perforation occurs most commonly in the ascending colon or in the appendix. [56, 57]
Imaging Features : Plain radiograph shows features of low bowel obstruction with colonic dilatation out of proportion to the small bowel.[58] There may be absence of rectal gas or small amount of gas may be seen in rectum (especially on prone films) on plain radiograph. However, the absence of rectal gas is not specific for Hirschsprung's disease, being more commonly seen in infants with sepsis and necrotizing enterocolitis.[59]
Digital examination of the rectum or enemas,in patients scheduled for barium studies should not be done as it may distort the findings.[10] For the same reason, the study (enema) is never performed with an inflated Foley catheter in the rectum.[10 ]Barium enema is performed carefully on an unprepared patient by inserting a straight-tipped catheter to a point just beyond the anal sphincter. The patient should be placed in lateral position and barium infused slowly as rapid infusion can distend and mask the transition zone. The contrast agent should be prepared with normal saline to avoid possibility of water absorption from the large surface area of dilated colon.[3] The diagnostic feature in short segment disease is funnel shaped transition zone and reversal of the recto-sigmoid ratio (Fig 7). Normally, the rectum is the most distensible portion of the bowel and has a diameter greater than that of the sigmoid colon (recto: sigmoid ratio >1).[20] The radiological transition zone is commonly found distal to the pathological transition zone.[60] There may be irregular, uncoordinated contractions in the aganglionic segment (20% of cases) which may be mistaken for mucosal ulcerations. Delayed films after 24 hrs may demonstrate prolonged retention of barium in the sigmoid colon (not rectum which normally holds barium for 24 hours). This is a strong indicator of Hirschsprung's disease, even when the enema findings have been inconclusive. Rectal biopsy is always necessary for confirmation.
The diagnosis of total colonic aganglionosis is much more difficult. The radiological findings viz. foreshortening, rounding of the colonic flexures and small caliber colon[2 ]are relatively nonspecific. The prolonged retention of barium on delayed films should alert one to suspect the diagnosis.[61 ]Presence of skip lesions in Hirschsprung's disease is extremely rare (controversial ). Awareness of this variant aids in the interpretation of the barium enema in children with signs and symptoms of aganglionosis, especially when the rectal biopsy is normal.[62 ]The associations of Hirschsprung's disease include Down's syndrome, ileal and colon atresia and neuro-cristopathies.[63-65]
Anorectal Anomalies
The anorectal malformations are divided into high and low types depending upon whether the termination of hindgut is above or below the puborectalis sling. Anorectal anomalies are often associated with fistulous tracts opening into the genitourinary system. Although the distinction between high and low lesions can be made clinically, the radiologic study is required to evaluate the internal anatomy. This is usually accomplished using contrast studies. In both male and female infants with low lesions, there is usually a visible perineal opening and the communication with the genitourinary tract is absent [10,20 ] However, there are cases with imperforate anus with external signs of a low lesion with demonstrable fistula into the anterior urethra either at surgery or by radiographic studies.[66] These cases should be considered an intermediate form of imperforate anus with external signs of a low lesion but with fistula characteristically to the membranous or bulbous urethra.[65 ]High lesions are treated with initial diverting colostomy and definitive repair at a later age. A pressure augmented distal colostogram is recommended prior to definitive repair, both to confirm the level of rectal atresia and to define any associated fistulous communication.[67] Water- soluble contrast medium is injected under mild pressure through a Foley's catheter with its balloon inflated to occlude the stoma. The fistulous tract not identified on the conventional contrast colostogram (Fig 8) may be delineated when an augmented-pressure modification of the technique is utilized.[66 ]Micturating cystourethrography (MCU) and/or retrograde urethrography (RGU) are also useful to demonstrate fistula and presence of vesicoureteric reflux.
The easily available plain radiograghy is simple and is often the preliminary investigation to be used in evaluation of these patients. Prone cross table lateral view with babies kept in genu-pectoral position is useful in determining the level of atresia.[68 ]The use of invertogram is to be discouraged as it causes unnecessary stress. During invertogram, the baby usually keeps crying causing contraction of puborectalis sling and obliteration of the lower rectum. Also, the rectum may be pulled in the cephalic direction resulting in error of classification. In those anomalies, which are associated with fistulae with the urinary or genital tract, the fistula becomes the highest point of rectum in an invertogram and gas may escape resulting in less distension of the rectum. In prone position, the fistula is lowest and therefore rectum is better distended causing adequate delineation of the rectal gas [68]. The pubococcygeal line and M line are imaginary lines used to represent the level of puborectalis sling on lateral radiographs. The "M" line is a better anatomic correlate of puborectalis sling. The "M" line passes horizontally through the junction of the lower third and upper two thirds of the ischium.
Ultrasonography can assess the distance of the rectal pouch in relation to the perineal surface. A pouch perineal distance of < 1.5 cm is indicative of low anomalies, while a pouch that terminates above the base of the bladder (superior extent of urogenital diaphragm) is indicative of a high lesion.[69] However, the distinction between high and intermediate anomaly cannot be made out with this technique. Recently, anal endosonography has been used as an accurate alternative to MRI in the assessment of anorectal malformations after repair.[70]
CT and MRI are the modalities of choice to delineate pelvic anatomy including puborectalis muscle and external sphincter. Congenital anorectal malformations are found in many forms, and are frequently associated with other anomalies, especially of the spine, spinal cord, and urogenital system.[71 ] Magnetic resonance imaging has proven to be the single stop modality to answer all the crucial questions such as level and type of malformation, type of fistula, developmental state of the sphincter muscle complex, and the presence of associated anomalies.[71-73] MRI has contributed to a better insight in the morphology and pathogenesis of such complex congenital malformations.[71].
Spinal radiographs must be examined carefully for abnormalities, because spinal pathology has profound effect on the outcome. Normal radiographic and sonographic appearance of spinal anatomy in children with anorectal malformation makes MRI superfluous, but if radiographs or ultrasound are uninformative/ abnormal, MRI should be used to accurately depict possible intraspinal pathology.[74] Sonography of the urinary tract for associated renal anomalies is essential in all patients with anorectal malformation.
Megacystis-Microcolon-Intestinal Hypoperistalsis Syndrome (Berdon Syndrome)[75
]This syndrome is a rare congenital cause of intestinal obstruction. The abdominal distention is due to dilated bladder and dilated short bowel in absence of organic obstruction. There is hypomotility of small bowel with retrograde peristalsis on upper GI contrast study while an enema shows a microcolon. Ultrasound will show a dilated bladder and bilateral hydrourteronephrosis. The anterior abdominal wall musculature is thin, similar to prune belly syndrome.
Meconium Plug Syndrome
Immature left colon is a form of functional obstruction associated with prematurity, maternal drug ingestion (magnesium preparations or high doses of sedatives),and in babies of diabetic mothers. [4,76] The affected infants present with distal bowel obstruction and delayed meconium passage. However, the bowel distention is less severe than with a mechanical obstruction. The contrast enema typically shows a mildly dilated right and transverse colon and a small left colon with an abrupt transition at the level of splenic flexure. The meconium plugs are seen as multiple elongated filling defects in dilated colon. The contrast enema using water-soluble contrast media is both diagnostic as well as therapeutic. If clinical improvement does not follow over next few hours or days, suction rectal biopsy is recommended to rule out Hirschsprung's disease.
Conclusion
0The goal of imaging is to provide an accurate diagnosis and other relevant information to the clinician while minimizing patient discomfort and radiation exposure. The judicious choice of various imaging modalities requires a concerted effort of the clinician and the radiologist aimed at providing best information and help to the treating clinician to plan out the most suitable treatment efficaciously
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