Discussion

Fetal bowel dilatation is characterized by fluid-filled intestinal loops which measure at least 7mm in diameter at any time during pregnancy [1]. Dilated bowel on ultrasound is suggestive of intestinal malformation or obstruction. It is most commonly caused by intestinal atresia and meconium peritonitis, but can also be a result of malrotation with volvulus, anorectal malformation, or Hirschsprung’s disease [1-3]. In a review of 40 cases with prenatal bowel dilation, half were found to have intestinal malformations and these patients were more likely to have associated polyhydramnios, fetal ascites, enlarged stomach, and echogenic bowel on prenatal ultrasound [1].

This case is unique in that the dilated bowel identified prenatally was found at autopsy to be caused by an internal hernia through the mesentery of the small bowel. Congenital internal hernias are rare. In a study reviewing 1827 patients who underwent surgery for small and large bowel obstructions, 10 (0.5%) were found to be due to congenital internal hernia [4]. Internal hernias are classified based on location: paraduodenal, pericecal, foramen of Winslow, transmesenteric, transmesocolic, intersigmoid and retroanastomotic [5]. Among internal hernias, the incidence of congenital transmesenteric hernias ranges from 60% in some studies [4], to 5-10% in others [6,7]. The etiology of mesenteric defects is poorly understood. Hypotheses include regression of the dorsal mesentery, developmental enlargement of a hypovascular area, rapid lengthening of a segment of mesentery and compression of the mesentery by the colon during fetal midgut herniation [5,8,9].

We reviewed the literature for all cases of congenital transmesenteric internal hernias in the past 15 years. In neonates <30 days of age, 17 cases have been reported (Table 1) [5,8-20]. These infants often present with bilious vomiting and abdominal distension. The defect is most commonly located in the terminal ileal mesentery and measures 1.5 – 4cm in size. Nearly 60% had small bowel necrosis or ischemia and approximately 12% had perforation. Four neonates had associated conditions including cystic fibrosis, cardiac anomalies, and microcolon, and approximately 40% of neonates were born preterm. Survival rate was 82%, and of the infants who died, two were born preterm and one had a multiple anomalies.

Prenatal diagnosis of small bowel obstruction due to internal hernia has not been reported. Imaging signs of internal hernia are non-specific and thus pre-operative diagnosis is rare [15]. In the fetal period, ascites [5] or non-reassuring fetal status [15] may be identified, prompting delivery and additional post-natal evaluations.

References

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