GYNAECOLOGY AND OBSTETRICS
Year : 2005 | Volume
: 15 | Issue : 3 | Page : 361--372
Prenatal diagnosis of anterior abdominal wall defects: Pictorial essay
From the Meera Hospital, Shiv Marg, Bani Park, Jaipur, India
S-9, Bhawani Singh Road, C-Scheme, Jaipur-302005
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Agarwal R. Prenatal diagnosis of anterior abdominal wall defects: Pictorial essay.Indian J Radiol Imaging 2005;15:361-372
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Agarwal R. Prenatal diagnosis of anterior abdominal wall defects: Pictorial essay. Indian J Radiol Imaging [serial online] 2005 [cited 2019 Jun 24 ];15:361-372
Available from: http://www.ijri.org/text.asp?2005/15/3/361/29157
Congenital anterior abdominal wall defects include omphalocele, gastroschisis, body stalk anomaly and prune-belly syndrome. Omphalocele is a midline anterior abdominal wall defect with herniation of the abdominal viscera into the base of the umbilical cord.. Gastroschisis is a defect lateral to midline with evisceration of abdominal contents directly into the amniotic cavity. Body stalk anomaly is an extensive abnormality of the anterior wall with adhesion of eviscerated viscera to the placenta. Prune-belly syndrome is an anomaly in which intestinal pattern is evident through the thin, lax, protruding abdominal wall in the infants.
Omphalocele and gastroschisis are most frequently encountered congenital ventral wall defects. Majority cases of omphalocele are associated with other serious structural defects and chromosomal abnormalities. Whereas, gastroschisis is usually an isolated lesion and is not associated with other structural defects and abnormal karyotype. Body stalk anomaly is also an isolated anomaly with rare association with chromosomal anomalies, but is lethal. Prune-belly syndrome is mostly associated with obstructive uropathy and severe maldevelopment of urinary tract. The final outcome of these defects is significantly affected by the presence of additional structural and / or chromosomal abnormalities. So, accurate detection and appropriate classification of associated fetal anomaly is of great importance for the further course of pregnancy ,. For this reason, finding of anterior abdominal wall defect requires further assessment of the affected pregnancy by targeted ultrasonography, echocardiography and karyotyping. In cases associated with lethal or multiple severe anomalies, parents may opt for elective termination of the pregnancy. However, in absence of associated life threatening anomalies, infants can have an uncomplicated course with a normal long-term quality of life  but decision to continue the pregnancy should be made by a multidisciplinary team including experienced sonologist, perinatologist, geneticist and cardiologists. Fetuses with an isolated ventral wall defect should be delivered in a unit with easy access to pediatric surgical facilities. As far as the mode of delivery is concerned, these fetuses may safely be delivered vaginally, and cesarean delivery should be performed for obstetric indication only .
Antenatal sonography is the key imaging modality available at present time. The widespread use of the fetal ultrasonography in routine antenatal care now allows majority of ventral wall defects to be identified before the age of viability. Anterior abdominal wall defects cause elevation in the MSAFP. Therefore, examination of the ventral wall is a prerequisite part of the sonographic evaluation in all pregnancies complicated by raised MSAFP. Although the etiologies of anterior abdominal wall defects are likely to be widely discrepant, the pathophysiology of each defect leads to key characteristics that make it possible to differentiate one entity from another. Among these features are the location of the defect in relation to cord insertion [Figure 1], the size and contents of the defect, and associated anomalies. These basic features of simple abdominal wall defects such as omphalocele and gastroschisis are used as the initial points of assessment . Using these basic features, diagnosis of omphalocele and gastroschisis can be made as early as 10 weeks and 12 weeks of gestation respectively. But before the early diagnosis of these defects is considered one must be familiar with the physiologic midgut herniation [Figure 2], which subsides at 12 weeks of gestation.
Accurate prenatal diagnosis of ventral wall defects, using ultrasonography, is important because it affects patient management and prognosis. However, detection rate of omphalocele and gastroschisis
was found to be 66.7 % during the second and third trimester . In another study ultrasound examination between 16 and 22 weeks gestation detected 60% of defects with a false positive rate of 5.3 % and fetuses with gastroschisis were incorrectly assigned as exomphalos in 14.7 % . Failure in correctly diagnosing abdominal wall defects occurred mostly in cases with small defects, ruptured omphalocele, multiple fetal anomalies, intrauterine fetal death, twin pregnancies or cases referred in late gestation. A significant regional variation in the ultrasonographic detection of fetal abdominal wall defects was also found in Europe . The variation reflects differences in screening policies, equipment and operator experience. Misdiagnosis of exomphalos as gastroschisis has serious implications because exomphalos is often associated with chromosomal anomalies and karyotyping may not be performed because gastroschisis is rarely associated with chromosomal anomalies. On the other hand, gastroschisis may be misdiagnosed as an omphalocele, which may result in unnecessary amniocentesis exposing the patient to the risks involved in the procedure. Therefore, some of the problems of diagnostic accuracy need to be considered when counseling couples with a ventral wall defect.
Although the specific factors leading to omphalocele and gastroschisis have not been elucidated, the focus has rested on environmental and nutritional factors. Maternal illness, infections, frequent medication during pregnancy, smoking, and genetic abnormalities may be associated with birth of babies with anterior abdominal wall defects. Folic acid deficiency, hypoxia and salicylates have caused laboratory rats to develop abdominal wall defects. One recent study has shown that periconceptional multivitamin use is associated with a 60% reduction in the risk of nonsyndromic omphalocele . However, another investigation found no association between maternal folic acid use and abdominal wall defects . Abnormal levels of carotene, glutathione, and high nitrosoamnies may be related with ventral wall defects . Gastroschisis is seen more frequently in mothers who use vasoactive substances such as nicotine and cocaine . There are several reports describing a higher rate of smoking in women whose fetuses are found to have gastroschisis . However, according to one study maternal smoking has not been associated with omphalocele . Studies from the California birth defects monitoring program have proposed that a low prepregnancy body mass may represent a risk factor for offspring with gastroschisis . One study reported an increased rate of abdominal wall defects among infants born to women who were obese but not diabetic . Another study reported an increased risk of these defects with socioeconomic deprivation . Prevalence of exomphalos increase with maternal age and decrease with gestational age  whereas, gastroschisis tend to occur in younger mothers which may hypothetically be related to lifestyle factors ,,. However, other investigations reported no clear association between omphalocele risk and maternal age .
Normal development of the anterior abdominal wall depends on the fusion of four ectomesodermic folds; cephalic, caudal and two lateral folds. Failure of lateral body folds to migrate centrally results in omphalocele. If the anomaly of the ventral wall is more extensive and, in addition to exomphalos involves cephalic embryonic fold then it results in pentalogy of Cantrell. Similarly, if the lateral fold defect is associated with caudal fold failure, it results in exstrophy of bladder or cloaca.
Exomphalos is a sporadic abnormality with a birth prevalence of about 1 in 4000. Prenatal diagnosis of an omphalocele by ultrasound is based on the demonstration of the midline abdominal wall defect, the herniated sac with its visceral contents and the umbilical insertion at the apex of the sac [Figure 3][Figure 4][Figure 5]a,b,c,[Figure 6]a,b. The sac is composed of peritoneum, amnion and Whartons jelly. Visualization of the sac confirms the diagnosis of omphalocele and virtually excludes gastroschisis. However, the amnio-peritoneal sac is not always visible [Figure 3][Figure 4]. Rarely, the sac may rupture in utero and omphalocele masquerades as gastroschisis . Visceral contents in the sac may include loops of intestine, liver, [Figure 5]a,b,c, [Figure 6]a,b, [Figure 7] and stomach. Ascites is common in the herniated sac [Figure 5]a,b,c,[Figure 6]a,b,[Figure 7]. Size of abdominal opening in an omphalocele may range from a simple hernia of the cord containing a few loops of bowel to giant omphaloceles in which large part of liver protrudes. The size of omphalocele does not alter the prognosis but surgical reduction and repair correlates with size of the abdominal wall defect. Sometimes, complete exteriorization of the liver is seen [Figure 8]a,b; in such cases abdominal and thoracic cavity may be small and under developed. Associated pulmonary hypoplasia, restrictive lung disease and oligohydramnios complicate the out come. For pregnancies in which isolated omphalocele is detected at early ultrasound, follow-up scan is advised especially at 20-24 weeks gestation for the detection of late-manifesting fetal anomalies. Follow-up ultrasound also helps in detection of complete disappearance of a small defect, which may occur later in the pregnancy . Additional follow-up ultrasound examinations are also required until delivery.
A higher proportion of omphaloceles is associated with concurrent malformations [Figure 9], syndromes and chromosomal anomalies . Cardiac anomalies ,,,,, gastrointestinal, genitourinary , neural tube ,, and musculoskeletal defects are frequently found in association with exomphalos. Omphalocele is involved in many polymalformative syndromes such as Beckwith-Widemann , pentalogy of Cantrell , Meckel-Gruber syndrome , and lethal cleft palate-omphalocele syndrome . The most common syndrome associated with omphalocele is Beckwith-Widemann syndrome, which is characterized by omphalocele, organomegaly, gigantism, hemihypertrophy, and polyhydramnios . Associated chromosomal anomalies include trisomies 18, 13, and 21, Turner, Klinefelter, and triploidy syndromes ,. Karyotypic abnormalities are more common in association with omphaloceles that contains only bowel compared with those that contains only liver or bowel and liver both ,. Nonsyndromal omphalocele may be familial . Prevalence of chromosomal defects increase with maternal age and decrease with gestational age . Associated polyhydramnios or oligohydramnios also suggests increased risk of chromosomal anomalies.
Isolated omphalocele diagnosed during the early stages of gestation typically has a good prognosis . Perinatal mortality rate is low in such case .
Differential diagnosis of an omphalocele include; physiologic bowel herniation, umbilical hernia, gastroschisis, amniotic band syndrome, exstrophy of urinary bladder and cloaca, pentalogy of Cantrell, body stalk anomaly, cavernous hemangiomas, pseudo-omphalocele, blood clots and acardiac monster.
At 8-10 weeks of gestation, all fetuses demonstrate physiologic umbilical herniation of the midgut [Figure 2] that is visualized as a hyperechogenic mass in the base of umbilical cord; retraction into the abdominal cavity occurs at 10-12 weeks and is completed by 11weeks and 5 days ,,. A physiologic midgut herniation seldom exceeds 7 mm in diameter and is invariably smaller compared with diameter of the abdomen. Because of physiologic herniation of bowel, diagnosis of an omphalocele may be difficult before 12 weeks gestation. However, there are some reports in the literature describing detection of omphalocele as early as 10 weeks of gestation ,. Van Zalen- Sprock et al also reported 14 cases of exomphalos diagnosed at 11-14 weeks of gestation . Early detection is especially possible when liver is identified as an eviscerated organ. Extracorporeal liver has typical echogenic property within the herniated sac and it never 'migrates' physiologically outside the permanent place below the diaphragm.
Amniotic band syndrome is a common cause of abdominal wall disruption defects. An atypical location of the abdominal wall defect along with extremity deformity with adherent band suggests amniotic band syndrome ,. Multiple cavernous hemangiomas are often found over the lower body and present as multiple surface masses that causes limb hypertrophy. Compression of the lateral thoracic wall due to transducer pressure or oligohydramnios may change the shape of the fetal abdomen, which may be confused with an exomphalos. Prenatal diagnosis of 'hernia' of the fetal abdominal wall has been reported. Sonography showed a large extra-abdominal mass on the right of the normal umbilical cord insertion and was not definable either as an omphalocele or as gastroschisis [47,48]. Blood clots around the umbilicus [Figure 10] a,b secondary to placental abruption may mimic an omphalocele or gastroschisis. Rarely, acardiac monster [Figure 11]a,b, lying near the anterior abdominal wall of the normal twin fetus, because of its extremely bizarre appearance, may appear as an omphalocele or gastroschisis.
Pentalogy of Cantrell
This syndrome was first described by Cantrell and his colleagues in 1958 . Anomalies observed in this disorder are  a midline, supraumbilical abdominal wall defect  a defect of the lower sternum  a deficiency of the anterior diaphragm  a defect in the diaphragmatic pericardium  congenital intracardiac defects ,. The most common intracardiac defects are atrial septal defect, ventricular septal defect, and teratology of Fallot . Diagnosis of the complete syndrome requires the above five criteria described by Cantrell but incomplete variant forms exhibiting three or four of the features have been described . In sonography, ectopia cordis associated with an omphalocele should suggest the diagnosis of pentalogy of Cantrell [Figure 12]a,b. Earliest prenatal diagnosis of the syndrome has been reported at 9 weeks and 5 days. The syndrome may be associated with other anomalies such as agenesis of the gallbladder, and polysplenia , cystic hygroma, renal dysplasia , exencephaly and amniotic band syndrome . Differential diagnosis includes isolated ectopia cordis, ectopia cordis associated with amniotic band syndrome, omphalocele and body stalk anomaly. In isolated defects, primary repair in the neonatal period is the best type of management for this rare condition . However, the out come depends on the severity of congenital cardiac anomaly .
Bladder and cloacal exstrophy.
Both bladder exstrophy and cloacal exstrophy are sporadic abnormalities. Bladder exstrophy is found in 1 per 30000 births and cloacal exstrophy is found in about 1 in per 200000 births. The severity ranges from a small vesicocutaneous fistula in the abdominal wall or simple epispadias to complete exstrophy of the cloaca involving exposure of the entire hindgut and the bladder. Sonographically, bladder extrophy may appear as a well-defined, solid or complex anterior abdominal mass below the umbilical cord insertion, immediately superior to the fetal genitalia. Prolonged and repeated scans fail to reveal the fetal urinary bladder in presence of normal renal collecting system and ureters and amniotic fluid ,. In addition, a small penis with anteriorly displaced scrotum and abnormal widening of the iliac crests may be found . Umbilical cord insertion may be abnormal. The protruding anterior abdominal mass does not contain any large cystic area as it does not contain the urine that is excreted directly from the ureters into the amniotic fluid. Since there is no obstruction to urinary flow, upper urinary tract and amniotic fluid index is found normal . In cloacal exstrophy, both urinary and gastrointestinal tracts are involved. Cloacal exstrophy (also referred to as OEIS complex) is the association of an omphalocele, exstrophy of the bladder, imperforate anus, and spinal defects such as meningomyelocele ,. Associated anomalies are common including cardiovascular, central nervous system, vertebral, small bowel atresia, single umbilical artery, club foot and ambiguous genitalia. Ambiguous genitalia is an important finding and visualization of normal external genitalia will probably exclude the diagnosis of bladder and cloacal exstrophy. Cloacal exstrophy is commonly associated with chromosomal abnormalities. Associated abnormalities are rare in bladder exstrophy.
The prognosis depends on the presence of associated anomalies. In isolated defects, with aggressive reconstructive surgery, postoperative survival is more than 80%. However, if the diagnosis is made before viability then termination of pregnancy is an option.
Gastroschisis is sporadic anomaly with a birth prevalence of 1 per 4000. This deformation abnormality is probably caused by disruption of the right omphalomesenteric artery and a resultant full-thickness defect in the abdominal wall located just lateral and usually to the right of an intact umbilical cord. As a result, evisceration of small bowel and, on occasion, even large bowel occurs into the amniotic space . Prenatal diagnosis by ultrasound is based on the demonstration of the normally situated umbilical cord insertion and the herniated free-floating loops of intestine without any membranous covering or a sac [Figure 13] a,b,c. Since these free-floating bowel loops lie uncovered in the amniotic fluid, they may become thick, edematous and matted and appear as an echogenic cauliflower-shaped mass protruding through the fetal abdomen  [Figure 13]a,b,c or an echogenic mass with ragged edge. In addition to bowel, occasionally, herniation of liver, pancreas, stomach, spleen, bladder, uterus, ovaries, and fallopian tubes may also occur. The anomaly can be diagnosed as early as 12 weeks of gestation ,[64 ] but there is a sparsity of reports on first-trimester diagnosis. Gastroschisis is usually an isolated anomaly but sometimes it may be associated with congenital heart abnormalities , ectopia cordis , neural tube  and diaphragmatic defects . Karyotype abnormalities are exceedingly rare . However, some familial cases have been reported . An autosomal recessive model of inheritance was found to be the most parsimonious explanation for the families of infants with isolated omphalocele and gastroschisis .
Affected patients have malrotated bowel. Vascular compromise may occur from a volvulus. Serial ultrasound follow-up is important because later in pregnancy bowel obstruction, peritonitis, bowel perforation, and fetal growth restriction may occur . A bowel diameter greater than 17 mm represents significant bowel dilation due to obstruction [Figure 13]b. Bowel diameter more than 11 mm is usually associated with a greater number of postnatal bowel complications. Sonographic findings of bowel abnormalities are associated with difficult abdominal wall repair and increased incidence of complications. However, sonographic evaluation of bowel dilation for the purpose of preventing bowel injury by early delivery is not generally helpful . Overall prognosis is usually favorable. Postoperative survival is about 95% and is largely the result of lack of other severe anomalies associated with this defect . However, the postoperative hospital stay is often lengthy and complications related to the gastrointestinal tract are very common . Mortality is usually the consequence of short gut syndrome.
Differential diagnosis includes physiologic bowel herniation, omphalocele especially with ruptured sac, umbilical hernia, amniotic band syndrome, bladder and cloacal exstrophy, body stalk anomaly, cavernous hemangioma, pentalogy of Cantrell, blood clots due to placental abruption [Figure 10] a,b, and sometimes acardiac monster [Figure 11]a.
Body stalk anomaly
Body stalk anomaly is a sporadic, lethal abnormality, found in about 1 per 10,000 pregnancies. The pathogenesis is uncertain but possible causes include abnormal folding of the trilaminal embryo during the first 4 weeks of development, early amnion rupture with amniotic band syndrome, and early generalized compromise of embryonic blood flow. The ultrasonographic features are, a major abdominal wall defect, severe kyphoscoliosis, a short or absent or rudimentary umbilical cord, and limb abnormalities. Absence of the umbilicus and umbilical cord causes adherence of the placenta to the herniated viscera such as liver and intestines rendering the fetus immobile , [Figure 14]a,b,c. The typical features of body stalk anomaly can be detected by ultrasound by the end of the first trimester. The anomaly is usually associated with abnormal nuchal thickness measurements. Sometimes, in the first trimester, it is possible to demonstrate the upper part of the fetal body in the amniotic cavity and the lower part in the celomic cavity. The finding suggests the early amnion rupture before obliteration of the celomic cavity is a possible cause of the syndrome . The anomaly is usually not associated with karyotypic abnormalities . Smrcek JM et al reported a case in which mosaic trisomy 2 was found .
Prune- belly syndrome
This syndrome, also called triad syndrome or Eagle-Barrett syndrome, occurs in approximately 1 in 40,000 births; 95% of affected individuals are male. Its etiology and pathogenesis is uncertain and may result from primary obstructive urinary anomalies or defective mesodermal development. It is characterized by a triad of distinctive features including deficient abdominal muscles, undescended testes, and urinary tract abnormalities probably due to severe urethral obstruction in fetal life . Urinary tract abnormalities include massive dilatation of the ureters and upper tracts and a very large bladder. Anterior and posterior urethra may be dilated, resulting in megalourethra. The kidneys usually show various degrees of dysplasia. In females, anomalies of the urethra, uterus, and vagina are usually present. The diagnosis should be suspected in fetuses with very large abdominal masses. These masses are most typically a result of bladder obstructions due urethral valves [Figure 15]a or urethral agenesis [Figure 15]b, but other large abdominal masses such as ovarian cyst, hydrometrocolpos, massively enlarged kidneys [Figure 15]c, and bowel (especially due to Hirschsprung's disease) can also be the cause. The syndrome may be associated with cardiovascular malformation , gastrointestinal anomalies , musculoskeletal defects including limb abnormalities  and scoliosis. Differential diagnosis includes megacystis megaureter, urethral obstruction, primary vesicourethral reflux , neurogenic bladder, and megacystis microcolon intestinal hypoperistalsis syndrome.
Termination of pregnancy can be offered before viability. The prognosis depends on the degree of renal function compromise. Early urinary obstruction leads to renal failure, pulmonary hypoplasia and death in neonatal period.
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