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Fetal abdominal abnormalities evaluated on prenatal imaging can be grouped into one of several general categories: obstructions, ventral wall defects, echogenic bowel and peritoneal abnormalities, and finally masses and solid organ abnormalities.
In circumstances when ultrasound (US) cannot accurately determine the extent or nature of an abnormality, fetal magnetic resonance imaging (MRI) can be useful. Sequence selection includes standard single-shot fast spin echo (SSFSE) T2-weighted images at 3- to 5-mm slice thickness and steady-state free-precession (SSFP) imaging at overlapping intervals. Echo-time lengths range from 80 to 250 ms; longer lengths are advantageous for delineation of cystic abnormalities. T1-weighted fast gradient echo (GRE) sequences may be useful in the fetal abdomen, specifically for identification of the liver, which is mildly hyperintense, and very bright signal meconium. Imaging modalities using ionizing radiation have no present role in the evaluation of a fetus with a gastrointestinal (GI) tract abnormality.
Interruption of the fetal GI tract can occur at any point from the esophagus to the anus, and the etiology for the loss of continuity is not the same at each site. The cause of esophageal atresia (EA) with or without a fistula to the trachea is not known but is believed to be a result of failure of the “system of folds” in the cranial and caudal ends of the tracheoesophageal space (see Chapter 96 ). EA occurs in approximately 1 in 4,000 live births and can be associated with a number of additional abnormalities in the fetus. Most commonly, these abnormalities include those comprising the VACTERL complex ( V ertebral, A nal, C ardiac, T racheo E sophageal, R enal, and L imb anomalies) and those seen in Down syndrome, such as common atrioventricular canal, absent or hypoplastic nasal bone, and nuchal thickening. EA occurs most commonly with a distal fistula (84%).
Gastric atresia is the rarest atresia and is almost always pyloric with a distended proximal stomach. It likely results from vascular impairment during fetal development and is associated with epidermolysis bullosa, as well as multiple hereditary atresia and other conditions.
In contrast to pyloric atresia, duodenal atresia (DA) is relatively common and, along with EA, is associated with Down syndrome. The etiology of DA has been thought to be related to failure of recanalization of the lumen of this segment of the proximal small bowel; more recent work implicates potential disruptions in fibroblast growth factor receptor 2 (FGFR2) and retinoic acid signaling pathways (see Chapter 102 ).
Atresias distal to the duodenum are thought to be the result of ischemia or other focal insult to the bowel and may be multiple. Small bowel atresias are the most common cause of fetal and neonatal bowel obstructions, with colonic segments affected much more rarely (in 10% of all atresias). Although most atresias are sporadic, some populations are prone to multiple atresia syndromes. Meconium ileus in persons with cystic fibrosis also can have small bowel atresia, and colonic atresia can be seen in persons with Hirschsprung disease. Anorectal atresia/malformation deserves special consideration because it often is seen in combination with other malformations, such as VACTERL, caudal regression, cloacal malformation, or the Currarino association (see Chapter 105 ).
Despite the likelihood of a distal fistula that might allow for flow of fluid into the gut, the most consistent clues to the diagnosis of EA on US are a persistently small-volume stomach and polyhydramnios. Distension of a proximal esophageal pouch typically is intermittent and therefore unreliably witnessed either on US or MRI, although in the rare cases where it is vital to make a prenatal diagnosis, serial midline sagittal MRI with cine T2-weighted sequences may prove useful ( Fig. 85.1 ).
A lack of visualization of the stomach is most likely EA, with or without a distal fistula. Other considerations in the differential diagnosis of a small fetal stomach include congenital microgastria; oligohydramnios/anhydramnios; diminished swallowing function (e.g., in arthrogryposis, with little to no fetal movement); or obstruction to normal swallowing, including from increased intrathoracic pressure ( Fig. 85.2 ). A markedly distended stomach, on the other hand, might raise concern for outlet obstruction, as with pyloric atresia. Gastric volvulus rarely has been reported in utero and is more of a risk when the stomach is not in its normal location. A right-sided stomach in situs ambiguous or inversus should be evident as long as one is careful to establish the orientation of the fetus to the mother during the examination.
DA appears on fetal imaging as the classic fluid-filled double bubble of dilated stomach and duodenal bulb ( Fig. 85.3 ). Duodenal stenosis sometimes can be distinguished by hyperperistalsis of the dilated proximal duodenum as seen on US whereas, in DA, no peristalsis may exist at all. DA can be associated with malrotation of the bowel. Other causes for proximal obstruction include annular pancreas or an adhesive Ladd band in a patient with primary midgut malrotation. As with other obstructions to the GI tract, the more proximal the atresia, the earlier and the more frequent presentation with polyhydramnios; therefore polyhydramnios is typical in duodenal and proximal jejunal atresias.
On T2-weighted sequences, high signal in liquid small bowel content should be evident on both sides of the abdomen; location of the small bowel primarily on the right may be an indication of a rotational abnormality ( Fig. 85.4 ). The small bowel is considered to be dilated if it is larger than 7 mm; the colon is usually up to 15 mm in the third trimester, while the normal rectum, which should be the largest segment, can be larger ( Fig. 85.5 ). In proximal jejunal atresia, more loops will be dilated than just the double bubble seen with DA (“triple bubble” in very proximal jejunal atresia) ( Fig. 85.6 ); distal jejunal atresia and ileal atresia, on the other hand, may be more difficult to distinguish prospectively because multiple dilated, fluid-filled loops are seen in both conditions, and typically a normal amniotic fluid volume is present.
The differential diagnosis for multiple dilated loops signifying a distal obstruction parallels that in newborn imaging and, in addition to atresia, includes meconium ileus, midgut volvulus, volvulus around a residual of the omphalomesenteric duct, and total colonic aganglionosis (Hirschsprung disease). One clue to the diagnosis of meconium ileus might be the presence of hyperechoic debris filling distended loops near the point of obstruction in the distal small bowel on US; MRI shows that the internal contents are hyperintense on T1-weighted images and intermediate on T2 imaging, consistent with meconium. However, practically speaking, the MR pattern of dilated bowel containing meconium-typical signal characteristics is nonspecific, and both small bowel and colonic obstructions can show a pattern similar to that described for meconium ileus ( Fig. 85.7 ). Vigorous hyperperistalsis of the bowel is indicative of obstruction ( Fig. 85.8 and ). Cases of in utero volvulus, although uncommon, have been prospectively identified either with a classic “whirlpool” twisting of the bowel or when the dilated ischemic segments are thick-walled, nonperistaltic, and have internal echogenicity from hemorrhage. High-grade mechanical and functional obstructions of the distal small bowel are associated with congenitally small colons (“microcolon”), typically detectable on MR beginning in the late second trimester.
Colonic atresias are difficult to diagnose specifically unless the obstructed segment can be identified with certainty in the expected location of the colon or interhaustral notches can be separated from valvulae conniventes on US. Meconium typically is seen in the rectum by 20 weeks' gestation, in the left colon by 24 weeks' gestation, and in the right colon by 31 weeks' gestation, but this pattern may be altered by the pathology that is present. On FSE T2-weighted sequences, meconium has a very low signal, whereas it shows a high signal on T1 weighting.
In general, when dilated loops are discovered, US is sufficient to diagnose the level of obstruction as proximal versus distal and to generate a focused differential diagnosis. Fetal MRI is reserved for complex cases or for those in which other abnormalities are suspected and might be confirmed by additional imaging. The advantage of MRI is in its larger field of view and reproducible multiplanar capability, showing the anatomy relative to other landmarks. This advantage usually is demonstrated well with single-shot T2-weighted sequences in multiple planes, whereas the addition of GRE T1-weighted sequences to show the distribution of high-signal meconium can provide useful additional information ( Fig. 85.9 ). It is crucial to understand that meconium is not exclusively restricted to the colon, can normally be seen in the distal ileum, and meconium signal may be present in dilated loops proximal to distal obstructions. Box 85.1 lists causes of absent or abnormal meconium signal in the colon.
Distal bowel obstruction
Bowel perforation (meconium peritonitis/pseudocyst)
Megacystis–microcolon–hypoperistalsis syndrome
Cloacal exstrophy
Cloacal malformation
Congenital diarrhea syndromes
Although one might anticipate that bowel loops would be dilated in cases of anorectal atresia, such dilation is uncommon, and the diagnosis often is made on the basis of a high level of suspicion because of associated features. On US, the diagnosis may be suspected if the characteristic hypoechoic configuration of the anal musculature with a central focus of echogenic mucosa ( Fig. 85.10 ) is absent. Also, a cord or ridge of abnormal fibrous tissue at the perineum may be imaged by US or MRI ( e-Fig. 85.11 ). If a fistula between the colon and the urinary tract allows mixing of meconium and urine, intraluminal precipitant calcifications may be detected ( Fig. 85.12 ).
Hirschsprung aganglionosis only very rarely is identified by bowel dilatation antenatally in the third trimester, although it is among the most common causes of neonatal intestinal obstruction, and therefore this diagnosis is more often made postnatally. Intestinal neuronal dysplasia type B, a malformation of the parasympathetic plexus, has been identified retrospectively in a fetus as a cause of bowel obstruction but is extraordinarily unusual. Another rare form of functional obstruction that can be seen in the neonate, congenital segmental dilation of the bowel, has been described with both short segment or more extensive antenatal bowel enlargement, the latter indistinguishable from a pattern of mechanical obstruction.
Congenital diarrhea syndromes have been reported as showing dilation of the bowel to the rectum but without meconium signal on T1-weighted imaging, reflecting the loss of normal meconium. Therefore lack of colonic meconium is not exclusive to mechanical small bowel obstructions. Differential considerations include secretory sodium or chloride diarrheas, megacystis-microcolon-hypoperistalsis syndrome, pseudoobstruction, and total colonic aganglionosis (Hirschsprung disease).
At this time, no criteria have been established for intervention in the fetus based on bowel obstruction unrelated to ventral wall defects, aside from amnioreduction for polyhydramnios. Selected patients may have amniocentesis to evaluate for the possibility of chromosomal abnormalities or cystic fibrosis. Follow-up is performed with ultrasonography, with a plan to have the newborn evaluated at birth by the appropriate pediatric specialists.
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