Normal Herniated Bowel
Findings:A small (<5mm) heterogeneous soft-tissue mass (arrow) is seen adjacent to the anterior abdominal wall. This was not seen on later sonograms and is secondary to normal herniation of the bowel into the base of the umbilical cord. The sonogram on the right demonstrates a large homogeneous mass anterior to the fetal abdomen. This proved to be herniated liver into an omphalocele sac.
Discussion:Normally, at eight weeks gestational age the intestine elongates and moves outside of the embryonic abdomen herniating into the base of the umbilical cord. This is due to a number of factors including rapid growth of the cranial end of the midgut (which will form the small intestine), and the large size of the developing liver and kidneys. While the bowel is within the umbilical cord, the midgut rotates 90 degrees counterclockwise. At approximately 10-11 weeks the abdomen enlarges and allows the intestines to return within the abdominal cavity. The midgut rotates an additional 180 degrees counterclockwise fixing to the posterior retroperitioneum. The herniation can be visulalized sonographically as an echogenic mass at the base of the umbilical cord measuring approximately 5-10 mm. The maximum reported diameter of herniated bowel is 10 mm. A prospective study by Bowerman in 1993 found the upper limit of measured herniated bowel to be 7 mm. The herniated bowel has a heterogeneous appearance and has a somewhat conical shape. Two reports have documented complete return of the herniated contents by 11 gestational weeks. An additional study by Green, however, found that herniated bowel was still visible in 20% of the patients at 12 weeks. From a different perspective, Bowerman was unable to identify normal herniated bowel in an embryo with a crown-rump length greater than 44 mm. An abdominal wall defect may result from either failure of the herniated bowel to return into the abdomen or from failure of fusion of the four abdominal folds that normally join at the base of the umbilical cord. If the bowel does not enter the abdomen normally, the small bowel mesentery may fail to undergo normal fixation and the small bowel may twist around the small stalk containing the superior mesenteric artery. This may result in volvulus, bowel obstruction and possibly gangrene.
Differential Diagnosis:The major differential diagnostic entity to consider when a soft tissue "mass" is seen adjacent to the anterior abdominal wall in the first trimester embryo is an omphalocele. Based upon the above observations, an omphalocele should be considered when the following observations are made: 1) the soft-tissue mass is larger than 7mm prior to 10 weeks and larger than 10 mm between 10 and 12 weeks gestation and is homogeneous in texture (due to the liver) 2) the "mass" is round and is as large, if not larger, than the embryonic abdomen. 3) the mass persists beyond 12 weeks gestation, or 4) other malformations are identified. Even with these criteria there will be those cases where it may be difficult to differentiate a small omphalocele from a large umbilical cord hernia. In these cases a repeat examination after 14 weeks gestation is advised. Likewise, one should not assume that any soft-tissue seen anterior to abdominal wall will always be herniated bowel.
References:Curtis JA, Watson L. Sonographic diagnosis of omphalocele in the first trimester of fetal gestation. J Ultrasound Med 7:97-100, 1988
Pagliano M, Mossetti M, Ragno P. Echographic dignosis of omphalocele in the first trimester of pregnancy. J Clin Ultrasound 18:658-660, 1990
Brown DL, Emerson DS, Shulman LP, Carson SA. Sonographic diagnosis of omphalocele during the 10th week of gestation. Amer J Roengenol 153:825-826, 1989
Bowerman R, Avila N, Ginsberg H. High resolution songraphic identification of fetal midgut herniation into the umbilical cord: differentiation from fetal anterior abdominal wall defects. J Ultrasound Med 7:109 (suppl), 1988
Cyr DR, Mack LA, Schoenecker SA, Patten RM, Shepard TH et al. Bowel migration in the normal fetus: US detection. Radiology 161:119-121, 1986
Green JJ, Hobbns JC. Abdominal ultraasound examination of the first-trimester fetus. Am J Obstet Gynecol 159:165-175, 1988