Embryology

Critical embryologic events occur during normal intestinal fixation to the developing body wall. Normal intestinal fixation requires sequential growth, elongation, and rotation of the intestine beginning as early as the fifth week of gestation as illustrated in Figure 56-1 . Three distinct events must occur for normal intestinal fixation. The first stage involves herniation of the primary intestinal midgut loop into the base of the umbilical cord, where it remains until the 10th week of gestation. The axis of the midgut loop is the vascular pedicle of the superior mesenteric artery (SMA). The SMA axis divides the midgut into prearterial and postarterial segments, with the omphalomesenteric duct located at the apex of the midgut loop. This loop rotates 180° counterclockwise so that the proximal prearterial half of the loop passes posterior to the SMA. The prearterial segment gives rise to the proximal duodenum, which normally lies to the right of midline. The more distal prearterial segment passes posterior and to the left of the SMA, becoming the third and fourth portions of the duodenum. The distal duodenum is normally fixed to the left of the aorta at the ligament of Treitz, having rotated 270° counterclockwise from its original position. The jejunoileal segment undergoes substantial elongation to form the remainder of the small intestine. The embryonic postarterial segment, which gives rise to the cecum and right colon, also undergoes growth and elongation with 270° counterclockwise rotation. Therefore, although the cecum is initially positioned to the left, the normal rotational pattern places the cecum anterior and ultimately to the right of the SMA before attaching to the posterior wall of the right iliac fossa.

Normal intestinal rotation of the duodenum, small intestine, and cecum from the fifth gestational week through completion by the 12th gestational week. (A) The fifth gestational week herniation of the midgut loop along the axis of the superior mesenteric artery (SMA). (B) The prearterial limb rotates counterclockwise 180°, passing posterior to the SMA. (C) The distal prearterial segment passes posterior and left, rotating 270° counterclockwise to become the third and fourth portions of the duodenum. (D-E) The postarterial limb, which gives rise to the cecum and right colon, rotates counterclockwise 270°, placing the cecum in its final position in the right lower quadrant.

The second important state of midgut rotation and fixation is reduction of the extracelomic gut, occurring between weeks 10 and 12 of gestation. At this point in fetal development, the duodenojejunal junction has passed posterior to the SMA and the midgut has rotated 180° counterclockwise. The small intestine initially remains to the right side of midline, and the cecum and ascending colon are still anterior to the SMA on return of the intestine to the abdominal cavity. Many common abnormalities of intestinal fixation occur as a result of arrested development during this 2-week period.

The final stage of midgut development is fixation of the intestine to the posterior body wall, occurring after the 12th week of gestation. Normal points of fixation include the cecum in the right iliac fossa and the duodenojejunal junction at the ligament of Treitz ( Figure 56-2 ). The normal small bowel mesentery is therefore fixed to the posterior body wall, with a broad base extending from the ligament of Treitz to the cecum. This broad-based mesenteric attachment normally prevents torsion of the intestinal mesentery around its vascular supply. In contrast, abnormalities of intestinal rotation may cause the base of the mesentery to lack broad-based attachment or poor fixation to the posterior body wall. The lack of intestinal fixation, along with a narrow vascular pedicle, produces an anatomic predisposition that may lead to volvulus.

Normal fixation of the midgut mesentery to the posterior body wall with the duodenojejunal segment at the ligament of Treitz and the cecum in the right lower quadrant. The shaded portions of the colon are extraperitoneal.

Anatomy

The normal embryologic sequence for intestinal rotation and fixation can be interrupted at any developmental stage, producing a diverse spectrum of rotational and fixation abnormalities. There are a number of distinct congenital anomalies strongly associated with abnormalities of intestinal rotation and fixation secondary to persistent midgut herniation from the abdominal cavity during fetal development. These anomalies include congenital diaphragmatic hernia and congenital abdominal wall defects such as omphalocele and gastroschisis. Therefore, an associated anomaly in patients diagnosed with intestinal malrotation is quite common. Several conditions associated with an increased rate of intestinal malrotation are listed in Box 56-1 .

Although abnormalities of intestinal malrotation and fixation are often described under the general term malrotation , specific anatomic conditions are further characterized in the following subsections.

Nonrotation

This is a relatively common anomaly characterized by incomplete counterclockwise rotation of the midgut around the SMA. Instead of the normal 270° arc, rotation is either absent or arrested before exceeding 90° ( Figure 56-3 ). The small intestine resides on the right side of midline, the colon resides on the left, and the cecum is displaced anteriorly and midline. The duodenojejunal junction is to the right of midline and is more caudal and anterior in position. Nonrotation carries a significant clinical risk of midgut volvulus because the small bowel mesenteric vascular pedicle is very narrow at its base. Duodenal obstruction may also occur as a result of peritoneal attachments fixing the cecum to the posterior body wall. These peritoneal attachments, also known as Ladd’s bands, pass anterior and lateral to the distal duodenum. Duodenal obstruction secondary to Ladd’s bands is treated with division of the abnormal cecal attachments crossing the duodenum.

Nonrotation. The small intestine, including the duodenojejunal junction, resides in the right side of the abdomen with the cecum and colon on the left.

Incomplete (Mixed) Rotation

This abnormality of intestinal rotation is characterized by arrest of normal rotation at or near 180° rather than the normal 270° ( Figure 56-4 ). Instead of rotating posterior and left of the SMA, incomplete rotation of the prearterial segment leaves the duodenojejunal junction to the right of midline. The cecum does not complete counterclockwise passage anterior to the SMA and, therefore, the incompletely rotated cecum usually resides in the upper abdomen just to the left of the SMA. Similar to nonrotation, abnormal fixation of the cecum to the right posterolateral body wall by Ladd’s bands places the duodenum at risk for compression or obstruction. In addition, the mesenteric vascular pedicle is narrow and places the entire midgut at risk for volvulus.

Incomplete rotation. The small intestine, including the duodenojejunal junction, resides in the right side of the abdomen, whereas the cecum has partially rotated and lies anterior to the duodenum. Ladd’s bands from the posterior abdominal wall may compress and obstruct the duodenum.

Clinical Presentation

Abnormalities of intestinal rotation are historically estimated to be present in approximately 1% to 2% of the total population. However, the true incidence of intestinal malrotation is difficult to define given the spectrum of anatomic variation and the propensity for the majority of these abnormalities to remain asymptomatic. Data from a population-based birth defect registry in Hawaii demonstrated an identified case rate of 2.86 cases of malrotation per 10,000 live births and fetal deaths.

Many children with asymptomatic malrotation will be incidentally diagnosed by upper gastrointestinal contrast studies performed for other clinical reasons. Symptomatic malrotation is usually encountered in the clinical setting of duodenal obstruction or midgut volvulus. Duodenal obstruction occurs as a result of Ladd’s bands causing extrinsic compression and obstruction of the distal duodenum as they pass anterior to the duodenum. Older infants and children with symptomatic duodenal obstruction may present with bilious emesis associated with gastric and proximal duodenal distention. Newborn infants may present with bilious emesis without abdominal distension secondary to partial duodenal obstruction. A paucity of small bowel gas may be seen on plain abdominal films.

Midgut volvulus should be considered in the differential diagnosis of any infant or child presenting with bilious emesis. A prospective audit of 63 consecutive neonates with bilious emesis demonstrated a surgical cause of intestinal obstruction in 24 (38%), with four of the infants having intestinal malrotation. The clinical outcome of midgut volvulus is time dependent, and this is the fundamental reason that signs and symptoms of intestinal obstruction in an infant or child must be pursued on an aggressive, emergent basis until definitive diagnosis is made. Delay in diagnosis or definitive treatment will lead to intestinal vascular ischemia and subsequent strangulation of the entire small intestine. Initial symptoms may be subtle and limited to irritability and feeding intolerance with progressive bilious emesis. Guaiac-positive stool associated with bilious emesis should be considered indicative of mechanical small bowel obstruction until proven otherwise. Late clinical findings include progressive abdominal distension and the development of peritonitis. Systemic symptoms and signs of metabolic acidosis, coagulopathy, hypotension, and shock reflect the progression of untreated bowel ische­mia and infarction. Systemic features are generally considered to represent poor prognostic factors for small bowel salvage and overall survival.

More than 50% of symptomatic intestinal rotational abnormalities are discovered within the first month of life, and approximately 90% present in children younger than 1 year of age. Symptomatic infants and children require emergent diagnosis, operative exploration, and definitive correction. Older children and adults may also present with acute symptomatic volvulus, but they may also have a history of vague, recurrent, and chronic abdominal pain associated with episodic symptoms of intestinal obstruction. Although volvulus from malrotation occurs more commonly in the neonate and infant, older children and adults with malrotation may present with volvulus in an unpredictable manner. Symptomatic intestinal malrotation presenting outside of the neonatal period may be characterized by intermittent abdominal pain, recurrent episodic emesis, and malnutrition associated with failure to thrive. Chronic diarrhea or malabsorptive symptoms may be found. Regardless of age at diagnosis, operative treatment of malrotation can be lifesaving in the setting of volvulus and may prevent a life-threatening situation in the asymptomatic individual.

Diagnosis

Diagnostic imaging evaluation generally begins with plain abdominal radiography. Classic radiographic findings of malrotation with associated duodenal obstruction include gastric and proximal duodenal distension with a paucity or absence of small bowel gas due to partial duodenal obstruction. The plain film alone may not differentiate malrotation from duodenal atresia or stenosis in a neonate; this is an important clinical distinction given the emergent need for intervention in the setting of volvulus versus the relatively elective repair of duodenal atresia. In most instances of suspected duodenal obstruction with concern of malrotation, an upper gastrointestinal series is a definitive imaging study ( Figure 56-5 ). Malrotation with volvulus typically produces incomplete duodenal obstruction with a “corkscrew” or “coiled spring” appearance of contrast passing into the distal duodenum and proximal jejunum. Duodenal atresia and stenosis may occur anywhere within the duodenum, but is more commonly proximal than the obstruction observed with malrotation with volvulus. Complete absence of small bowel gas and a “double-bubble” appearance on plain abdominal film is typical of duodenal atresia, whereas diminished but discernible distal gas is characteristic of duodenal stenosis or malrotation with incomplete obstruction.

(A) Upper gastrointestinal contrast study (anteroposterior [AP] projection) demonstrating intestinal malrotation (nonrotation) without volvulus. (B) Contrast study (lateral projection) in a neonate demonstrating intestinal malrotation with volvulus. Note the “corkscrew” appearance of contrast in the proximal jejunum.

(Courtesy of Jack R. Sty, MD, Thomas T. Sato, MD. Children’s Hospital of Wisconsin.)

Other more subtle radiographic findings consistent with a diagnosis of malrotation include duodenal redundancy and incorrect position of the duodenojejunal junction, particularly in a location to the right of midline and inferior to the pylorus. A classic finding of malrotation observed with contrast enema is cecal malposition to the midline or left of the vertebral column. However, the reported sensitivity and specificity of gastrointestinal contrast studies in the diagnosis of malrotation vary, with false-positive rates as high as 15%.

Recent interest in ultrasonographic diagnosis of malrotation has led to emerging case series reporting sensitivity and accuracy in detecting malrotation. Ultrasonography relies, in part, on the relationship of the SMA to the superior mesenteric vein; normally, the superior mesenteric vein is to the right of the SMA on transverse sonograms. Abnormal position of the superior mesenteric vein either ventral or to the left of the SMA is associated with malrotation. In addition to inversion of the mesenteric vessels, the ultrasonographic presence of duodenal dilation with tapering has been reported to have a sensitivity of 89% and a specificity of 92% in the detection of volvulus. However, a normal relationship of the SMA and the superior mesenteric vein (SMV) does not exclude the presence of asymptomatic malrotation. Finally, helical CT scan imaging in the setting of malrotation with volvulus has been described as demonstrating a “whirlpool” sign from the twisted duodenojejunal junction. Regardless of the modality, several important points in the diagnostic work-up of malrotation include the following.

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  Symptomatic malrotation with or without volvulus is more common in the neonate and infant, but can occur at any age.

  
  
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  If malrotation with volvulus is suspected clinically, aggressive resuscitation along with emergent radiologic studies and surgical consultation are indicated.

  
  
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  In a symptomatic infant or child, radiographic evidence of malrotation alone is enough to warrant emergent exploration.

  
  
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  It may be difficult to differentiate the presence or absence of volvulus radiologically, and the absence of definitive radiographic evidence should not defer operative intervention if clinical suspicion is high.