Extrinsic Compression

Because the bowel is a flexible organ, anything that compresses it externally can decrease the intraluminal diameter and precipitate obstruction. For example, adhesions are fibrous bands of tissue that spontaneously arise within the peritoneum in reaction to insults, such as peritonitis or abdominal surgery, and can encircle and constrict loops of bowel. They are relatively common after surgery, with up to a 25% 5-year readmission rate caused by adhesions, depending on the type of operation.

Intraabdominal masses, such as Wilms’ tumor, can compress and obstruct bowel by their sheer size (“mass effect”). Annular pancreas is an anatomic variant in which a ring of pancreatic tissue encircles and can cause duodenal obstruction. Superior mesenteric artery (SMA) syndrome results from compression of the third duodenal segment by the artery against the aorta. It is sometimes seen in patients who have had precipitous weight loss with resultant loss of the mesenteric fat pad that protects the duodenum from compression; external body cast application; or after intestinal or orthopedic surgery that results in traction upon the SMA, pulling it down upon the duodenum. Another example is closed loop obstruction (internal hernia), in which a loop of bowel passes through a mesenteric defect or similar orifice (e.g., inguinal hernia) and becomes lodged within it, resulting in occlusion of both the proximal and distal ends of the loop.

Intrinsic Decrease in Lumen Caliber

Acquired Defects

Chronic bowel inflammation can, over time, lead to narrowing of the lumen. An important example is Crohn’s disease, in which there is discontinuous full-thickness bowel inflammation anywhere from the mouth to the anus (see Chapter 110). Over time, inflamed areas may stricture and obstruct. Another example of this is eosinophilic esophagitis, an entity that lies within the spectrum of food allergies and atopic disease. Because of persistent eosinophilic inflammation in the presence of the offending antigen, the esophagus can eventually stricture over time, leading to dysphagia and the need for esophageal dilatation to relieve the obstruction. Malignancy, although relatively uncommon in children compared with adults, is another important cause of decreased lumen caliber. For example, the GI tract is the most common extranodal site affected in non-Hodgkin’s lymphoma. Infiltration of the bowel wall by tumor leads to decreased effective luminal diameter and a higher risk of obstruction. Achalasia causes functional esophageal obstruction caused by aperistalsis and inadequate lower esophageal sphincter relaxation. An example of gastric outlet obstruction caused by decreased diameter is hypertrophic pyloric stenosis, a disease primarily affecting infants in the first few months of life in which smooth muscle hypertrophy leads to occlusion of the pylorus (Figure 109-3).

Figure 109-3 Hypertrophic, pyloric stenosis.

Internal Blockage

A wide variety of etiologies may be associated with internal blockage of the GI tract. Areas that are inherently narrow relative to the rest of the GI tract (upper esophageal sphincter, esophagus adjacent to the carina, lower esophageal sphincter, pylorus, ileocecal valve) are at more risk of becoming obstructed by large foreign bodies. In a special example of this, bezoars are accumulations of indigestible material (e.g., hair, fiber from vegetables) that become trapped in the GI tract (typically the stomach) and can cause obstruction when they grow large enough. Injury to the wall of the small intestine from blunt abdominal trauma, as can occur when bicycle handlebars injure a child’s duodenum, may instigate a hematoma, which can obstruct the small intestine because of its small caliber.

In neonates, meconium ileus is a cause of distal GI obstruction shortly after birth. Meconium is a conglomeration of sloughed intestinal mucosa, bile salts, and other debris that accumulates during fetal development. It is normally passed within the first day of life, but when it is exceedingly inspissated, as occurs in cystic fibrosis, patients may be unable to expel it, leading to obstructive symptoms. In young children, intussusception is an example in which the bowel obstructs itself because of telescoping of one portion of bowel into the adjacent region of bowel.

Defects in Bowel Arrangement

Malrotation is an important entity that clinicians must be aware of when evaluating intestinal obstruction of unknown etiology. Malrotation is a congenital defect typically involving both the small and large intestines that occurs when the bowel assumes an abnormal spatial arrangement within the abdomen. Intestinal development is a complex, continuous process spanning the fourth fetal week to birth that takes the primitive gut, which begins as a short, straight tube, and transforms it into a long, highly specialized organ. As part of this process, the intestinal tract, after rotating 270 degrees, becomes fixed in a specific arrangement such that the duodenum ends at the ligament of Treitz in the left upper quadrant and the cecum is found in the right lower quadrant. When this final arrangement fails to take place properly, the bowel is malrotated. Although this may not result in any clinical symptoms in mild cases, the most feared complication is volvulus, a surgical emergency in which the bowel, as a result of its inappropriate positioning and lack of usual support, twists upon its mesenteric stalk, causing not only intestinal obstruction but also ischemia (Figure 109-4). Ladd’s bands, intraperitoneal fibrous bands that can encircle and obstruct the duodenum, are also associated with malrotation.

Figure 109-4 Malrotation and volvulus.

Motility Failure

Even when there is not an occlusive obstruction of the intestine, failure of appropriate peristalsis can nonetheless create obstructive symptoms. A common example is postinfectious ileus, which is an interruption in normal intestinal motility and forward propulsion of contents without physical occlusion that most often occurs after viral gastroenteritis. It is usually mild and self-limited, but in rare instances, it can result in severe and prolonged gastroparesis, feeding intolerance, and the need for postpyloric nasogastric enteral feeding until it resolves. Intestinal surgery can cause postoperative ileus, a temporary disruption of normal motility, particularly when combined with narcotics, which have intrinsic antimotility effects.

Another important consideration is intestinal pseudoobstruction, which is a spectrum of congenital and acquired disorders involving the small or large bowel (or both) that ultimately results from failure of either the enteric nervous system or the intestinal smooth muscle to coordinate appropriate peristalsis. Examples of factors that can trigger pseudoobstruction are trauma, electrolyte abnormalities, sepsis, orthopedic and cardiovascular surgery, and pregnancy.

What Is the Age of the Patient?

Although many etiologies cross age lines, the age of a patient can raise the level of suspicion for particular problems. The neonatal period is when many congenital disorders, such as atresias and webs, become evident. Although malrotation can present at any point in life, most are diagnosed within the first year in patients with recurrent or bilious vomiting. Meconium ileus becomes evident within the first few days of life and prompts concern for cystic fibrosis. Hypertrophic pyloric stenosis presents within the first few months of life as nonbilious “projectile” vomiting. Intussusception most often presents within the first few years of life as recurrent bouts of vomiting associated with intense pain that both completely resolve between episodes.