Definitions and Epidemiology
Short bowel syndrome (SBS) is a disorder of malabsorption resulting from significant small bowel loss secondary to congenital disease or surgical resection. The incidence of SBS is estimated at 1200 per 100,000 live births.1 SBS is the most common cause of intestinal failure. This is defined as a significant reduction in functional small bowel mass, leading to inadequate digestion and absorption, with subsequent growth failure. Other less common causes of intestinal failure include structural enterocyte defects and severe disorders of intestinal motility.2
At birth, the estimated small bowel length is approximately 250 cm for term or near-term infants and approximately 100–120 cm for premature infants <30 weeks gestation. Small bowel length is thought to double in the last trimester of pregnancy. Different systems have been used to describe SBS including those based on etiology, age, and anatomical considerations. In the context of surgical resection, the remaining small bowel length is often used to categorize disease severity. In a short resection, >100–150 cm of small bowel remains, compared to 40–100 cm with a large resection and <40 cm remaining after a massive resection.2
The small bowel has considerable adaptive capacity to compensate for intestinal loss. Intestinal adaptation is defined as a growth process of the remaining small bowel, through morphological and functional changes, leading to improved absorption. This process starts shortly after bowel loss but often continues for months to years. Depending on the segment and length of the lost small bowel, patients with SBS are frequently dependent on parenteral nutrition for prolonged periods of time, sometimes indefinitely. The use of parenteral nutrition has significantly improved the life expectancy of children with SBS. On the other hand, long-term parenteral nutrition can be associated with a variety of complications, some of which can have grave consequences. Patients with SBS who fail medical therapy or develop complications may require surgical interventions, including transplantation. Because of the relatively high mortality, SBS is considered among the most lethal disorders in young children.3
The small bowel is a vital organ involved in digestion, absorption, and fluid balance. The small bowel is divided into three anatomical segments (Figure 20–1). The first is the duodenum, which extends from the pylorus to the duodenojejunal junction, defined by the ligament of Treitz. The proximal half of the remaining small intestine is composed of jejunum and the distal portion is ileum. Some absorption takes place in the duodenum, including that of iron. The main function of the duodenum is to neutralize acidic gastric contents and mix them with intestinal, pancreatic, and hepatic digestive secretions. Absorption mostly takes place in the jejunum and ileum. The jejunum has an abundant surface area enhanced by folds and numerous tall villi. The luminal surface of enterocytes, or intestinal epithelial cells, is in turn covered with finger-like projections termed microvilli, which are collectively referred to as the brush border. Villi become shorter and less abundant in the ileum, which therefore has less absorptive surface area than the jejunum. The center of each villus is occupied by a capillary network that absorbs nutrients which are eventually transported to the liver by the portal venous system. The terminal ileum has a high concentration of lymphoid tissue that assists in immune regulation. The terminal ileum also has site-specific receptors for absorption of bile acids and vitamin B12. The ileocecal valve, separating the ileum from the proximal colon, slows the movement of fluid into the cecum and limits bacterial migration from the colon into the small bowel.4
During embryonal development, the small intestine arises from the midgut, which extends from the mid-duodenum to the distal transverse colon. The duodenum has an extensive blood supply derived from the celiac axis and the superior mesenteric artery. This dual blood supply helps avoid ischemia if the supply from one of the major vessels becomes compromised. The remainder of the midgut, extending from the jejunum to the proximal two-thirds of the transverse colon, depends on blood supply from branches of the superior mesenteric artery. Therefore, extensive bowel loss can result from compromise of the superior mesenteric arterial blood flow. The midgut drains through the superior mesenteric vein, which joins the splenic vein in forming the portal vein.4 About 70–80% of blood entering the liver is venous blood from the portal vein, and the remainder is arterial blood supplied by the hepatic artery. This is important as small bowel disease can have a significant negative impact on the liver with possible bacterial translocation and liver dysfunction.
Major physiologic disturbances can result from loss of large sections of the small bowel absorptive surface area. Consequences especially include malabsorption of fluid, electrolytes, macronutrients (proteins, carbohydrates, and fats), and micronutrients (vitamins and trace elements). In general, jejunal resections are better tolerated than is loss of ileal segments. When a significant portion of jejunum is lost, intestinal adaptation of the ileum can compensate for many of the jejunal functions. Certain jejunal functions, however, cannot be replaced, including loss of enteric hormone production (such as cholecystokinin and motilin) that affects intestinal motility and digestion. Loss of jejunum is often accompanied by decreasing biliary and pancreatic secretions and increased gastrin levels, leading to gastric hypersecretion. Major ileal resections can reduce absorption of fluid, electrolytes, bile acids, and vitamin B12. These functions cannot be taken over by the remaining jejunum. Bile acids, excreted into the duodenum by the liver, are required for the absorption of long-chain fatty acids and fat-soluble vitamins in the ileum. Loss of bile acids may therefore contribute to diarrhea by worsening fat malabsorption. Vitamin B12 binds to intrinsic factor, which is produced in the stomach, and the complex is subsequently absorbed in the terminal ileum. Bile acid and vitamin B12 absorption cannot be replaced by the jejunum. The ileocecal valve plays an important role in slowing intestinal transit to allow more time for absorption. The absence of the ileocecal valve, in addition to reducing transit time, can also lead to small bowel bacterial overgrowth (SBBO) from colonization by colonic bacteria. SBBO can result in mucosal injury and worsening diarrhea.5 The colon, if present, can play an important role in absorbing water, sodium, and short-chain fatty acids in SBS patients. A summary of possible consequences of loss of specific intestinal segments is presented in Table 20–1.
|• Loss of duodenum
|• Iron malabsorption
|• Loss of jejunum
|• Calcium malabsorption
|• Folate malabsorption
|• Fat-soluble vitamin malabsorption
|• Gastric hypersecretion
|• Loss of ileum
|• Bile acid malabsorption
|• Vitamin B12 malabsorption
|• Loss of the ileocecal valve
|• Small bowel bacterial overgrowth
|• Increased fluid and electrolyte losses
In children, the causes of SBS are variable and can be classified by the age at the time of intestinal loss (Table 20–2).3,5,6 The outcome will depend on the length and functionality of the remaining small bowel. Other contributing factors include the presence or absence of the ileocecal valve and colon, and associated complications, especially liver disease.
|• Prenatal/neonatal period
|• Intestinal atresia
|• Midgut or segmental volvulus
|• Necrotizing enterocolitis
|• Extensive Hirschsprung’s following resection
|• Postnatal period
|• Midgut volvulus
|• Mesenteric infarction
|• Crohn’s disease requiring resection
|• Radiation enteritis
|• Intestinal tumor
A thorough review of the past medical and surgical history is essential. Children with SBS often have an obvious event or events that lead to bowel loss. Frequently noted causes include a major congenital anomaly such as gastroschisis or intestinal atresia, or the occurrence of an abdominal catastrophe such as midgut volvulus or extensive bowel resection after necrotizing enterocolitis (Table 20–2). Occasionally, patients may undergo large or multiple intestinal resections from Crohn’s disease or extensive Hirschsprung’s disease. Inquiring about the underlying etiology of SBS is important, because it can shed light on the portion of small bowel affected and the functionality of the remaining bowel. Other important historical aspects include the remaining length and segment of small bowel, the presence of an ileocecal valve and colon, and the continuity of the remaining bowel (versus presence of ostomies). These can be based on a thorough review of the surgical records and direct discussions with the surgical service.
It is critical to perform a thorough nutritional assessment. History of previous and current nutritional intake and tolerance should be obtained. Patients with SBS are typically on total or near-total parenteral nutrition through a central venous line. Most are receiving little to no enteral nutrition shortly after the event that leads to bowel loss. For parenteral nutrition, the amount of intravenous dextrose, amino acids, and lipids as well as the electrolyte, multivitamin, and trace element solutions should be determined. For enteral nutrition, the type (breast milk versus formula), caloric density, total volume, and mode (continuous versus bolus, oral versus tube feeding) of feedings should be known. Caloric intake should be assessed separately for parenteral and enteral nutrition. The aim of therapy is to gradually increase the caloric intake from enteral nutrition and decrease that from the parenteral route while maintaining appropriate growth and development.
Previous and current central line access should be reviewed. Any previous history of central line thrombosis or infection should be obtained, including causative organisms and related clinical complications. Frequent line infections requiring line changes can raise concern about line care quality, intestinal bacterial translocation, worsening-associated liver disease, and limited long-term vascular access. This complication can lead to adjustments in central line care and the feeding regimen, as well as earlier consideration for other surgical interventions, including transplantation.
The clinical symptoms associated with SBS can vary but often include diarrhea, weight loss or poor growth, fatigue, and lethargy. These symptoms arise from underlying dehydration, electrolyte abnormalities, and calorie and nutrient deficiencies. Stool output will be higher in children with SBS because of limited absorptive surface even with only partial enteral feeding. Worsening diarrhea can imply reaching or going beyond the maximum absorptive capacity of the remaining bowel. Diarrhea can also be seen with SBBO, which is common among SBS patients. A history of recurrent abdominal distention, foul-smelling stools, and flatulence should alert the managing physician to possible SBBO, especially when the ileocecal valve has been resected, allowing retrograde contamination of the small intestine with colonic contents. Weight loss or poor growth suggests inadequate caloric intake, and should lead to adjustments in the nutritional management. Certain vitamin and other micronutrient deficiencies can lead to specific symptoms that should be recognized and treated promptly (see Table 8–6).
Liver disease can affect 40–60% of infants receiving prolonged parenteral nutrition. The presence of liver disease, especially cholestasis, significantly worsens the outcome of children with SBS.7 The history should therefore include an assessment for possible liver disease and related complications including portal hypertension, history of gastrointestinal bleeding, and signs of fat malabsorption.
Accurate weight and height measurements are needed. The presence of temporal wasting, loss of muscle mass and subcutaneous fat, poor dentition, and peripheral edema suggest severe protein and energy malnutrition. Other general features of malnutrition include dry skin, prominent nail ridges, and blunted lingual papillae.
Patients can have physical findings and symptoms related to specific micronutrient deficiencies. Essential fatty acid deficiency (linoleic acid and linolenic acid) may occur, manifesting as growth retardation, dermatitis, and alopecia. Patients with SBS and cholestasis should be closely monitored clinically and biochemically for fat-soluble vitamin deficiency (A, D, E, and K). Vitamin A deficiency is associated with significant ocular impairment. Vitamin A absorption and metabolism can also be impaired by underlying zinc deficiency. Signs of zinc deficiency include poor growth and wound healing, diarrhea, alopecia, and angular stomatitis. Manifestations of vitamin E deficiency can include ocular palsy, wide-based gait, and decreased deep tendon reflexes. The presence of ecchymoses, purpura, or bleeding diatheses should raise concern about vitamin K deficiency. Vitamin D deficiency is associated with rickets and osteomalacia. Biochemically, vitamin D-deficient patients can have hypocalcemia, hypophosphatemia, and elevated alkaline phosphatase levels.
Patients with significant resections of the distal ileum are at high risk of developing vitamin B12 deficiency. Deficiency in vitamin B12 (cobalamin) can result in megaloblastic anemia and demyelination. Folic acid deficiency can lead to megaloblastic anemia, neutropenia, and impaired growth. Iron deficiency can present with pallor, fatigue, spooned nails, and glossitis. In general, micronutrient deficiencies are uncommon occurrences with parenteral nutrition and are more likely to develop once parenteral nutrition is discontinued, as intestinal absorption may be suboptimal.
Physical signs of associated liver disease can include jaundice, scleral icterus, excoriations secondary to pruritis, splenomegaly, ascites, and poor growth. Abdominal distention and tympany may suggest SBBO from colonic bacterial translocation into the small bowel or from intestinal strictures following surgery.
Because children with SBS usually have a specific event leading to bowel loss or resection, the diagnosis is rarely confused with other conditions. However, some patients may have a long history of multiple enterectomies leading eventually to SBS, such as patients with small bowel Crohn’s disease. Common symptoms in SBS patients, such as diarrhea and lack of growth, can be exacerbated by other factors, including gastrointestinal and extraintestinal infections, SBBO, intestinal strictures, loss of bowel function (despite reasonable remaining bowel length), dysmotility, and Munchausen by proxy. These possibilities should be investigated and addressed appropriately.