Abdominal Wall Defects
Omphalocele and gastroschisis are the major anomalies encountered in the neonate. an omphalocelea defect in abdominal wall musculature and skin with protrusion of abdominal viscera
contained within a membranous sac.
gastroschisisthe defect in the abdominal wall was displaced to the right of the
umbilicus and eviscerated bowel was not covered by a membrane. Survival of an
infant born with an abdominal wall defect, especially gastroschisis, was unusual
before the advent of modern antibiotics, nutritional support, and neonatal intensive
care .
An accurate estimate of the incidence of gastroschisis and omphalocele is difficult
to obtain due to the under reporting of stillbirths, confusion in defining rupturedomphaloceles as gastroschisis defects, and the voluntary terminations of
pregnancies with sonographic evidence of abdominal wall defects. Reports from the
United States suggest a combined incidence of 1 in 2000 live births, . In the
1960s and 1970s, omphaloceles outnumbered gastroschisis 3:1, but over the last 20
years gastroschisis defects have predominated 2-3:1. This increase in incidence may
represent increased selective termination of pregnancies of omphaloceles, more accurate
classification of defects, or an actual increase in the gastroschisis birth rate.
Males and females are equally affected by gastroschisis, and there may be a slight
male predominance in omphalocele. No predilections based on race, maternal age,
parity or geography have been substantiated.
Etiology
No specific etiologies for abdominal wall defects have been identified in humans.
Omphalocele
At approximately the fourth week of gestation, the fetal abdominal wall is separatedonly by somatopleure, a thin membrane of ectoderm and mesoderm that is
later replaced by simultaneous ingrowth of four mesodermal tissue folds in cranial, caudad, and lateral to medial directions. Cranial downgrowth forms the thoracic
and epigastric wall; caudad ingrowth forms the hypogastrium, bladder, and the hindgut.
By the sixth week, further ingrowth occurs from lateral paravertebral myotomes.
These ingrowths flatten medially to form the rectus abdominus muscles and fuse in
the midline to complete enclosure of the abdominal cavity. It is theorized that the
failure of ingrowth of lateral mesoderm gives rise to an arrested somatopleure defect
characteristic of a standard omphalocele, and failure of ingrowth of cranial and caudal elements accounts for the spectrum of defects which accompany pentalogy of Cantrell, exstrophy of the urinary bladder, cloacal exstrophy, and imperforate anus.
These fusion defects are thought to occur within the first 4-7 weeks of gestationand can be associated with other first trimester developmental anomalies and a 10-30% incidence of chromosomal anomalies (i.e., autosomal trisomies of 12, 18, and 21).
Gastroschisis
Gastroschisis appears to arise from a specific weakness in the abdominal wallwith secondary rupture and herniation of abdominal viscera. More rapid dissolution
of the right umbilical vein occurs outside the standard period of organogenesis,
leaving an area of relative weakness in the mesenchyme through which bowel or
abdominal viscera can herniate and eventually rupture. This theory explains the
95% incidence of the defect being to the right of the umbilicus. Gastroschisis is
usually an isolated mechanical defect and typically not associated with an increased
incidence of other developmental anomalies.
Clinical Presentation
Omphalocele
At birth, omphalocele is recognized as a central defect of the abdominal
wall beneath the umbilical ring. It is greater than 4 cm (defects less than 4 cm are
generally called hernias of the cord) and is covered by a membranous sac or amnion.
The umbilical cord inserts directly into the sac in an apical or occasionally lateral
position. The sac may rupture in utero in 10-18% or from the delivery process in
4%. If the sac is intact, it contains normal appearing abdominal viscera including
the liver in 48%. Giant omphaloceles have sacs that replace most of the abdominal
wall, contain most of the intra-abdominal viscera, and have associated underdeveloped peritoneal cavity and pulmonary hypoplasia.
The incidence of associated major congenital anomalies has previously been
estimated at 35%, but with improvement in neonatal survival, data collection, andimaging techniques, more recent series report associated anomalies in up to 81%.
Cardiovascular defects are present in 20% of the patients and include tetralogy of
Fallot, ASD, and VSD most commonly. Defects associated with cranial fold failure
include congenital heart disease, diaphragmatic hernia, ectopia cordis, sternal cleft,
and when all of these elements are present represent the pentalogy of Cantrell. Defects
of caudad fold ingrowth may include imperforate anus, genitourinary malformations,
bladder or cloacal exstrophy, colon atresia, sacral and vertebral anomalies, and
meningomyelocele. Syndromes associated with omphalocele include autosomal trisomies 13, 18,
and 21 in up to 30%. Also associated is the Beckwith-Weideman syndrome consisting
of macroglossia, hypoglycemia, and visceromegaly.
Gastroschisis
At birth, gastroschisis is recognized by an isolated opening in the
abdominal wall to the right (95%) of the umbilicus with free evisceration of abdominal contents.
In utero exposure of bowel to amniotic fluid eventually causes thickening,
shortening, and the development of a thick fibrous peel which seems accentuated
with prolonged exposure and term deliveries. Associated defects are uncommon and
relate mechanically to the abdominal wall defect. Evisceration of the bowel leads to
malrotation. Constriction of the base of the herniated intestines may cause intestinal
stenosis, atresia, and volvulus (usually ileal in location). Undescended testicles
are also more common with this defect. Upon repair, intestinal edema, thickening,
and the fibrous peel eventually resolve coinciding with resolution of a prolonged
ileus that may last up to 4 weeks. Late appearance of necrotizing enterocolitis (NEC)
has been reported in up to 17% of patients following standard repairs. Patients with
gastroschisis are more likely to be born preterm or small for gestational age (SGA)
compared to omphalocele patients who are usually term, large babies.
Treatment
Prenatal diagnosis of an abdominal wall defect is not in itself an indication for acesarean delivery. Prenatal management calls for and evaluation for associated anomalies including a complete fetal ultrasound and possible amniocentesis. For gastroschisis and omphalocelepatients, serial ultrasound examinations showing progressive dilatation and thickening of bowel has been correlated with poor intestinal function and is used in some centers as an indication for induction of labor.
Neonatal management begins with the preservation of eviscerated bowel or intact
amnion with sterile moistened saline gauze dressings, transparent film wrap, or bowel
bags. Intravenous fluids at 1.25-1.5 times maintenance and antibiotics are administered.
Great care is taken to conserve body heat. A thorough examination for associated
anomalies is performed including:
1. plain radiographs of the chest, spine and pelvis,
2. echocardiogram,
3. renal ultrasonography, and
4. chromosomal analysis.
Omphalocele
Primary closure of the small to medium sized omphalocele is preferred. This
includes excision of the sac, possible correction of the associated malrotation, and
general inspection of the abdominal contents. Intraoperative alternatives to primary
closure include prosthetic patch closure, simple closure of mobilized skin flaps, or
placement of a silo for sequential tightening and staged closure. Giant omphaloceles
or patients who are not suitable candidates for anesthesia may be treated with topical
application of Betadine ointment or silver sulfadiazine to the intact sac. This
allows secondary eschar formation and eventual epidermal ingrowth. Residual
abdominal wall hernias are then repaired at one year of ageAbdominal
Gastroschisis
Primary closure of gastroschisis usually begins with enlarging the abdominal wall
defect to allow for the reduction of intestinal contents. The abdominal wall is gently
stretched to enlarge the peritoneal cavity, and preoperative enemas may be helpful
to decompress the colon. Severe matting of the bowel or peel formation may preclude
an immediate repair of associated atresia or stenosis. These can be reduced and repaired in 6-8 weeks when the bowel injury/peel has resolved. In cases of volvulus and necrosis, nonviable bowel is resected. Bowel continuity is restored primarily or more rarely proximal enterostomies are performed as needed.
Complete reduction of bowel contents under minimal pressure is preferable, but a prosthetic patch, skin flaps, or staged closure may be necessary. Parenteral nutrition is begun postoperatively and continued until adequate oral nutrition is attained. Careful postoperative fluid management, postoperative antibiotics, and adequate ventilatory support contribute to successful outcomes.
Outcomes
Morbidity and mortality rates with omphalocele closure are closely tied to prematurity, large-sized defects, and major associated anomalies.Most modern series report survival rates form 71-93%. Children with gastroschisis often have a prolonged ileus, but once bowel function returns, these children thrive and ultimately do well since they have few associated anomalies. At present, survival rates are generally 90-95%.