Development and congenital anomalies of the gastrointestinal tract and its adnexae

Atresia of the intestine

Atresia of the gastrointestinal tract is known to occur at all levels, from the oesophagus to the rectum. In most instances, atresia may be understood from the perspective of developmental anatomy as a failure of normal developmental processes. Recent studies have identified a number of genes responsible for growth factors and their receptors in the gut during development. The interaction between the endodermal epithelium and underlying mesenchyme is particularly important for establishing normal gut structure, and disruption of these processes is thought to be an important contributing cause of atresia formation.

Atresias may occur as an isolated anomaly, in the context of multiple anomalies, and within a syndrome. They present with a range of severity ( Fig. 66.1 ). Many patterns of anomaly association have been recognized. One example of a broad expression of anomalies is seen in the VACTERL association. The full expression of this clinical entity may include atresia of the oesophagus and rectum; duodenal atresia may also be a part of this spectrum. It is well established that Down's syndrome may include atresia of the duodenum. Atresia involving the jejunum and ileum, and the much less common atresia of the colon, appear to be the result of an insult that acts through the common pathway of ischaemia at the site of the atresia : this has been shown to occur after the intestine has developed. A well-known example of this phenomenon may be seen in the neonate who has suffered an antenatal volvulus, and then at birth is found to have deletion of a significant portion of the intestine. In the majority of atresias, a short segment of intestine is affected and often the remainder of the intestine is healthy, although multiple atresias are known to occur in about 10% of such patients. The appearance of an atresia at the time of surgical correction quickly reminds surgeons that the intestine does function in utero , and the atresia has resulted in an intestinal obstruct­ion prior to the fetus reaching term. A variable expression of atresia is encountered in relation to the timing of an ischaemic insult. There may be evidence of the mechanism, such as an antenatal intussusception or segmental volvulus, if the event occurs close to term, and no such evidence if it has occurred much earlier in gestation. A secondary phenomenon that is observed in infants with intestinal atresia is related to the effect it may have on the peritoneal cavity. Intestine that has undergone infarction in utero will resorb, leaving behind remnants that suggest the timing of the event. When there has been spillage of intestinal contents with perforation occurring in utero , fetal peritonitis will result, producing ascites and adhesions that increase the complexity of surgical repair ( Fig. 66.2 ). In every instance of atresia, a significant discrepancy is found in the luminal size of the proximal dilated obstructed bowel and the distal collapsed defunctioned bowel. The bowel distal to the obstructing mechanism is described as microbowel, and the appearance at the time of a contrast enema very quickly brings the microcalibre of an unused colon to the surgeon's attention. This discrepancy in size, with a large dilated proximal bowel and a small collapsed distal bowel, adds to the challenge of restoring intestinal continuity: the luminal discrepancy ratio may be as much as 10 to 1. At the time of surgery there is a need to resect a portion of proximal bowel in order to reduce the luminal discrepancy, and to facilitate the re-establishment of intestinal continuity in order to optimize recovery of intestinal function. There is evidence that the proximal dilated bowel at the level of obstruction may have abnormal motility that is believed to be the result of the ischaemic insult and chronic dilation.

There are a number of less common and more severe anatomical patterns of atresia. These include a pattern of atresia following an ischaemic insult involving a major proximal branch of the superior mesenteric artery (type IIIb). This produces a long-segment deletion of bowel and an unusual retrograde pedunculated vascular supply to the remaining ileum, giving it a configuration that has been likened to an ‘apple peel’. This pattern of atresia poses significant challenges because the absence of a true mesentery supplying the remaining small bowel leaves it vulnerable to poor perfusion as well as potential volvulus. A second uncommon pattern of atresia, which may be diagnosed at the time of operation, is referred to as a ‘string of pearls’, where a string of atretic segments may extend along most, or all, of the small intestine, leaving the patient with very little functional small bowel. These patients may also have atresia involving the stomach, duodenum and colon. This pattern of atresia has been shown to be familial. Infants with either of these two patterns of atresia need to remain long-term in hospital, receiving supplemental parenteral nutrition and awaiting recovery of bowel function that will allow for enteral feeding. Fortunately, most intestinal atresias involve short segments; recovery following re-establishment of intestinal continuity is the rule, with preservation of adequate intestinal length.

Intestinal atresia at the level of the duodenum is not uncommon and is most often diagnosed antenatally. These fetuses will have developed a markedly enlarged duodenum; a large fluid-filled stomach and proximal duodenum, analogous to the classic double bubble seen on an abdominal X-ray, will be seen on fetal assessment ( Fig. 66.3 ). A clue to the presence of this anomaly is polyhydramnios, which is usually present and may be severe. At birth, these infants are found to have a markedly increased gastric capacity and have bilious emesis in the early hours of life. Prompt diagnosis, followed by nasogastric decompression of the stomach, will alleviate concerns regarding emesis and aspiration. The anomaly in this instance is usually ascribed to a failure of recanalization of the lumen of the proximal duodenum after a period of endodermal hyperplasia during the first trimester. The majority of these lesions will occur in the region of the second portion of the duodenum at the junction between foregut and midgut. The anomaly is in close proximity to the point of entry of the bile duct and pancreatic duct, and therefore is in an anatomical region where corrective surgery must avoid interference with drainage at the level of the ampulla. The pattern of atresia in the duodenum may be relatively simple, in that a diaphragm may account for the atresia, and in some instances there may even be continuity through a small opening that defines the lesion as a congenital stenosis. A diaphragmatic atresia lends itself to a repair that maintains duodenal continuity, and must be carried out with careful attention paid to the anatomical location of the ampulla. When there is a gap between the proximal and distal duodenum, the site of obstruction coincides with the region of the head of the pancreas. Pancreas is interposed between the two duodenal ends, making it seem as though pancreatic development interfered with duodenal development. It is believed that the atresia is primary, and that the prominence of the pancreas is secondary. When the anomaly is in close proximity to the pancreas, a carefully constructed duodenal anastomosis may be created, or alternatively it is possible to anastomose the proximal jejunum to the duodenum and avoid the risks of surgical manipulation in the region of the second part of the duodenum. Functional continuity of the bowel remains the objective. However, in the presence of a proximal megaduodenum, one can anticipate motor function to be somewhat delayed or to remain abnormal in the long term. The anatomical appearance of the duodenum even later in life may remain abnormal, with the proximal duodenum remaining large compared with the more distal duodenum.

Atresia of the small bowel distal to the duodenum may occur at any level of the jejunum or ileum but tends to be more common in the distal small bowel ( Fig. 66.4 ). It is estimated that as many as 1 in 10 may be multiple and surgeons are therefore obliged to look carefully for a second level of atresia that can be hidden by a higher-level atresia because of the decompression of the distal bowel. Multiple anastomoses may be carried out in order to preserve a healthy segment of bowel interposed between two sites of atresia. This is done in order to adhere to the principle of preservation of maximum functional intestine. As with atresia at all levels of the intestine, the success in re-establishing continuity of bowel is dependent on achieving both anatomical and functional continuity. The bowel adjacent to a zone of atresia may have abnormal peristaltic activity and may also have a relatively poor blood supply, which may dictate the need to resect or taper the proximal blind-ending bowel. If bowel adjacent to the level of an anastomosis has poor perfusion, this may lead to stricture formation. Intestinal atresia involving the colon is uncommon ( Fig. 66.5 ). It occurs more frequently in the right-side colon; the relative competence of the ileocaecal valve means that an atresia of the ascending colon can result in a large dilated proximal bowel that has the appearance of an enteric cyst.

Atresia rarely involves the stomach, but when recognized, it often occurs in the region of the pylorus. It tends to arise in association with the dermal disease epidermolysis bullosa. Surgical restoration of continuity between stomach and duodenum may be accomplished, but these infants have abnormal connective tissue and their ability to heal is often impaired.

Hirschsprung's disease should be considered in the differential diagnosis of a neonate or infant who presents with a low intestinal obstruction. It results from failure of enteric nerves to colonize the gut, leading to a portion of the gut having no enteric nervous system and being unable to generate peristaltic action. There is sustained contraction of the aganglionic portion of gut and distension of the proximal portion (megacolon). Affected neonates show delayed passage of meconium, constipation, vomiting and abdominal distension.