Ultrasound Diagnosis of Abdominal Wall Defects
by Beth A. Pletcher, MD, March 2000

Abdominal wall defects occur in about 1 in 2000 live births and are being diagnosed prenatally with increasing frequency because of widespread acceptance of maternal serum AFP screening and use of prenatal ultrasound in otherwise uncomplicated pregnancies. The two most common defects identified are gastroschisis and omphalocele, which each carry with them very different risks for the co-occurrence of a genetic disorder as well as long term prognosis.

During the 5th to 6th week of gestation, the lateral and ventral body walls come together to form the umbilical cord, but it is not until about the 10th week of gestation that the bowel is completely encased in the abdominal cavity. Disruption of the natural intestinal migration process may lead to the appearance of an omphalocele whereas failure of closure of the lateral and ventral walls results in a gastroschisis defect. These two developmental anomalies must be differentiated from other midline defects such as a severe body-stalk anomaly or cloacal extrophy. Careful sonographic studies can differentiate between these clinical entities and provide valuable information to the couple facing this diagnosis.

Ultrasound can be used to answer several questions relating to this diagnosis and will assist in providing appropriate prognostic information and directing additional diagnostic studies. First, the presence or absence of a membrane covering the defect can help to differentiate between an omphalocele and gastroschisis. The former is virtually always covered by a peritoneal membrane whereas the latter is not. Second, evaluation of the umbilical cord location in relation to the defect is also quite valuable since an omphalocele is located at the insertion of the cord, a gastroschisis next to or beside the insertion of the cord and a cloacal extrophy below the cord insertion. A body stalk anomaly (also called limb-body wall complex), as with an omphalocele, is directly involved with the umbilical cord insertion, but is associated with additional severe fetal malformations. If the abdominal wall defect is found to be located above the umbilical cord insertion, the diagnosis of ectopia cordis must be considered. Third, identification of specific organs extruded beyond the abdominal wall can help to differentiate between these clinically distinct conditions and may also provide additional prognostic data. Clearly, evisceration of the heart would define ectopia cordis and bladder, cloacal extrophy. However, the presence of bowel alone can be seen with either gastroschisis or a small omphalocele. Identification of liver in the defect is seen most often with omphalocele and body-stalk defects. The presence of liver in an omphalocele defect might suggest that this is a large defect and more difficult to repair surgically, but interestingly, the smaller omphaloceles without extrusion of the liver, are more frequently associated with chromosomal disorders. If this was indeed the case, it would carry a much graver prognosis. Fourth, sonographic appearance of the bowel involved in the defect may be helpful prognostically (especially with gastroschisis) since this defect results in chronic exposure of the intestine to amniotic fluid and may lead to bowel wall thickening, atresia and/or infarction. For this reason, when gastroschisis is suspected, serial ultrasounds may assist in making decisions about timing of delivery, especially if bowel dilatation or wall nodularity are noted late in gestation. Last, the presence or absence of additional malformations is most helpful in providing prognostic information for the parents. Since the greatest risk for associated defects and/or chromosomal abnormalities is with the finding of an omphalocele, chromosome studies should be considered in any fetus with this finding. Over half of all fetuses with an omphalocele will have at least one additional malformation and one third will have an abnormal karyotype. The occurrence of severe scoliosis with an omphalocele is practically diagnostic of a body-stalk anomaly. Since gastroschisis is sometimes associated with amniotic band syndrome, a sonographic survey is also useful in this clinical setting to evaluate the fetus for possible additional related defects such as limb amputations, orofacial clefts or cranial defects.

The prognosis for these abdominal wall defects is quite variable with isolated gastroschisis perhaps carrying the best overall prognosis. Improvements in diagnosis and postnatal surgical care in recent years have seen a steady decrease in mortality in newborns with this condition; over 90% of such infants now survive. Survival of infants with omphaloceles depends primarily on the presence or absence of additional defects and whether there is an underlying cytogenetic abnormality. Associated abnormalities include: cardiac defects, CNS malformations as well as GI and GU anomalies. Trisomy 18 and trisomy 13 are the most common chromosomal problems seen with fetal omphaloceles, but is also associated with trisomy 21, triploidy and Turner syndrome. Mortality for fetuses with omphaloceles and additional defects is rather high at 80% and with chromosomal anomalies approaches 100%. However, with isolated omphalocele, mortality is much lower and recent studies suggest a mortality rate of only about 20%. Both ectopia cordis and limb-body wall complex carry extremely poor prognoses and are generally fatal. Cloacal extrophy is associated with a variety of anomalies involving the GU tract, GI tract and occasionally the heart or CNS. Mortality rates are intermediate with about 50% survival, but long term management is frequently more complex, requiring multiple reconstructive surgeries and in a small subset of males, sex reassignment.

Probably the best known syndrome associated with omphaloceles is Beckwith-Wiedemann syndrome (BWS), and should be considered in the differential diagnosis when a fetus is found to have this anomaly. For this reason, targeted ultrasound may be employed to assess the fetus with an omphalocele for macrosomia, enlarged or lobulated kidneys, macroglossia or polyhydramnios which may or may not be present. With advances in molecular diagnosis for BWS, it may also be possible in some fetuses to confirm this diagnosis, but neither ultrasound nor DNA-based testing are good enough at this time to establish this diagnosis with certainty in most cases. However, if BWS is suspected prenatally, it could provide the parents with some useful and encouraging information, since the prognosis for this condition is generally quite good. Making this diagnosis may also assist the pediatrician or neonatologist to direct care in the newborn period and beyond.