Abdominal wall defects are relatively
common fetal defects occurring in approximately 1 in 2,000 live births.
Since it is not possible for all major abdominal wall defects
to be identified early in pregnancy, views of the abdominal wall and cord insertion are recommended in all second
and third trimester ultrasound exams. When an abdominal wall
defect is detected it is important to identify the type of defect,
as this may affect the prognosis. Below are multiple abdominal or gastrointestinal defects.
As in all structural birth defects,
prenatal detection and management through and beyond delivery
requires a collaborative team approach involving experienced
sonography, genetic counseling, obstetrics, neonatal and surgical
experts. With years of involvement in this perinatal field,
nothing is more clear than that no one individual can provide
a comprehensive approach.
The two most common abdominal defects are
gastroschisis and omphalocele. Gastroschisis is a small defect
normally located to the right of the insertion of the umbilical cord and is not covered by a membrane. Though this opening a portion of the bowel protrudes. Usually only the small and
large bowel are involved though in some cases the stomach, liver, and
genitourinary system may be involved. The
bowel may become thickened and dilated due to the exposure to the amniotic fluid.
Bowel complications (such as malrotation and segmental atresia)
are present in approximately 15% of cases of gastroschisis.
However, other anomalies are not usually associated with this
defect. Gastroschisis is considered to be sporadic in occurrence
and is not typically associated with chromosome abnormalities.
However, some familial cases have been reported.
An omphalocele is a midline defect
of the abdominal wall that results in herniation of the bowel
and intrabdominal contents into the umbilical cord. The defect
may be categorized by the presence or absence of the liver in
the omphalocele sac. Unlike gastroschisis, the bowel contents
are covered by a membrane in an omphalocele. Often excess fluid will develop within the omphalocele sac.
Approximately
30% of fetuses with an omphalocele have a chromosome abnormality and therfore amniocentesis might be a benetifical tool.
The most common chromosome abnormalities are trisomy 18,
13 and 21 (Down syndrome),
Turner syndrome (45,X), and triploidy.
Another syndrome that may be associated with an omphalocele
is Beckwith-Wiedemann
syndrome. Other abnormalities, such as heart defects, are identified in approximately 67
to 88% of fetuses with an omphalocele. The prognosis of the
fetus often depends on the presence of associated abnormalities.
Esophageal atresia is a typically a sporadic defect of the gastrointestinal system that
results from incomplete division of the foregut into the trachea
and the esophagus. Esophageal atresia is often difficult to identify prior to 24 weeks gestation. The ultrasound findings of esophageal atresia include polyhydramnios,
non-visualization of the fetal stomach, and visualization of
regurgitation after fetal swallowing. Approximately 50 to 70%
of fetuses with esophageal atresia have another associated defect, such as a heart defect.
The prognosis of these fetuses depends on the type and severity
of the associated defects. Some cases of esophageal atresia are due to an underlying chromosome abnormality in the fetus (typically trisomy 21-Down syndrome or trisomy 18) and therefore amniocentesis may be benefiical.
Duodenal atresia is a condition in which the first part of the small bowel (the duodenum) has not developed properly resulting in a blockage between the stomach and the bowel. This defect is identified by sonography due to
the presence of polyhydramnios and the "double-bubble"
(fluid-filled stomach and duodenum). This defect is present
in approximately 1 in 5,000 pregnancies, and approximately 30%
of these fetuses have trisomy 21. Therefore amniocentesis might be beneficial. Also, other anomalies are present in
approximately 50% of these cases. The prognosis of these fetuses
depends on the presence of other anomalies.
REFERENCES
Nyberg, D.A., Mahony, B.S., and Pretorius, D.H. Abdominal Wall
Defects in Diagnostic Ultrasound of Fetal Anomalies. Mosby-Year
Book, Inc: St Louis, 1990. Pp 395-432.
Nyberg, D.A., Mahony, B.S., and
Pretorius, D.H. Intra-Abdominal Abnormalities in Diagnostic
Ultrasound of Fetal Anomalies. Mosby-Year Book, Inc: St Louis,
1990. Pp 342-394.
Romero, R., Pilu, G., Jeanty, P.,
Ghidini, A., and Hobbins, J.C. The Abdominal Wall in Prenatal
Diagnosis of Congenital Abnormalities. Appleton & Lange:
Connecticut, 1988. Pp 209-232.
Romero, R., Pilu, G., Jeanty, P.,
Ghidini, A., and Hobbins, J.C. The Gastrointestinal Tract and
Intraabdominal Organs in Prenatal Diagnosis of Congenital Abnormalities.
Appleton & Lange: Connecticut, 1988. Pp 233-254.
