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1
Question:

A 21-year-old woman at 18 weeks gestation comes to the office for a routine prenatal visit.  A quadruple screen reveals an elevated maternal serum alpha-fetoprotein.  Sonographic imaging of the fetus shows an anterior abdominal wall defect with herniation of bowel contents adjacent to the umbilicus.  No covering membrane is identified.  Color Doppler ultrasound imaging shows a normal 3-vessel cord inserting medial to this defect.  The remainder of the anatomical survey is unremarkable.  Which of the following is the most likely cause of these fetal findings?

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Pediatric abdominal wall defects

Diagnosis

Pathophysiology

Clinical features

Congenital umbilical hernia

  • Incomplete closure of umbilical ring (fascial opening)
  • Benign, reducible protrusion
  • Defect covered by skin
  • Midline (umbilical cord at apex)

Omphalocele

  • Abnormal gut rotation without physiologic reduction back into abdominal cavity
  • Sac containing bowel ± other abdominal organs
  • Defect covered by peritoneum (no skin, fascia, muscle)
  • Midline (umbilical cord at apex)

Gastroschisis

  • Full-thickness abdominal wall defect
  • Eviscerated bowel
  • Not covered by membrane
  • Located to the right of midline/umbilicus

This fetus has an anterior abdominal wall defect with herniation of bowel contents.  These findings suggest one of 2 possible diagnoses that differ based on location and covering membrane:

  • Omphaloceles occur due to abnormal gut rotation and failed physiologic return of bowel to the abdominal cavity during embryonic development.  Omphaloceles have herniated bowel contents confined to the umbilical cord sac, which results in a midline defect with an associated peritoneal covering (Choice E).

  • Gastroschisis is a congenital anomaly caused by disrupted or defective abdominal wall formation.  Because the underlying etiology is a full-thickness abdominal wall defect, the fetal bowel eviscerates into the amniotic cavity without a covering membrane.  The herniation is adjacent to the umbilicus (ie, paraumbilical), and the umbilical cord insertion site is usually normal and separate from the defect.

Because fetal bowel produces alpha-fetoprotein (AFP), fetuses with gastroschisis (no covering membrane) have direct AFP leakage into the amniotic fluid.  The high concentration of amniotic fluid AFP is absorbed into the maternal circulation, resulting in abnormally elevated maternal serum AFP levels.  Omphaloceles, by comparison, do not elevate amniotic fluid or maternal serum AFP levels as highly because the fetal bowel remains covered by peritoneum.

(Choice A)  In contrast to gastroschisis, congenital umbilical hernias (caused by failed closure of the umbilical fascial ring after birth) are midline defects covered by subcutaneous tissue and skin.  They are typically benign and easily reducible.

(Choice C)  Hirschsprung disease occurs due to failure of neural crest cell migration.  Patients have an aganglionic sigmoid colon/rectum that cannot relax (eg, bowel obstruction, megacolon).  It does not present with eviscerated bowel in utero.

(Choice D)  Meckel diverticulum results from failure of obliteration of the vitelline (omphalomesenteric) duct; the diverticulum can contain ectopic, acid-secreting gastric mucosa that ulcerates adjacent small bowel.  Therefore, the most common presentation is painless lower gastrointestinal bleeding in early childhood.  There is no associated herniation of bowel contents.

Educational objective:
Gastroschisis is a congenital abdominal wall defect characterized by herniation of uncovered bowel (ie, no covering membrane) located adjacent to the umbilicus.  It can be diagnosed on fetal ultrasound, and the condition is associated with abnormally elevated maternal serum alpha-fetoprotein levels.