Nutrition in Short Bowel Syndrome

KEY POINTS

• 1.

  Intestinal failure (IF) is defined as the loss of functional gut mass to levels below those needed for digestion and absorption of fluid and nutrients required to support adequate nutrition and growth.

• 2.

  Short bowel syndrome (SBS) is a subtype of IF that occurs due to the anatomic loss of a part of the intestines.

• 3.

  Gastroschisis and necrotizing enterocolitis (NEC) are the most frequently seen congenital and acquired causes of IF in neonates and young infants.

• 4.

  Loss of specific parts of the intestine may predispose an infant to various nutritional deficiencies.

• 5.

  SBS from surgical resection of the intestine may show phasic adaptive recovery: an initial acute period of intestinal dysfunction, subacute recovery over several weeks, and then a chronic phase of consolidation over months to years depending on the extent of the loss.

• 6.

  Many infants with IF require parenteral nutrition for long periods and are consequently at risk of cholestasis and liver failure.

• 7.

  In many patients, specific nutritional supplements such as dietary fiber, lipid preparations such as fish oil, probiotics; and motility-altering agents may be helpful.

Introduction

Intestinal failure (IF) is defined as the reduction of functional gut mass to levels below those needed for digestion and absorption of fluid and nutrients at levels adequate for nutrition and growth, resulting in prolonged dependence on parenteral nutrition. ^, It can result from either anatomic or functional loss of the intestines ( Table 21.1 ). The term “short bowel syndrome” (SBS) refers to a specific subtype of IF that results from the anatomic loss of absorptive and digestive surface of the intestines. IF can also result from mucosal dysfunction due to various congenital and acquired enteropathies and from gastrointestinal dysmotility states.

Congenital IF is associated most frequently with gastroschisis, which accounts for about 15% of all cases of IF and SBS in infants (Pediatric Intestinal Failure Consortium study). In the same study, necrotizing enterocolitis (NEC) was noted to be the most frequent postnatal cause, accounting for 26% of IF in infants ( Fig. 21.1 ). These data are not surprising, because NEC still has a fairly high incidence; in a study from 820 centers in the United States, NEC occurred in 7.6% of all very low birth weight (VLBW) infants. In another study conducted by the Neonatal Research Network of the National Institute for Child Health and Human Development (NICHD), NEC caused 96% of all IF and SBS in VLBW infants. The increasing prevalence of SBS is likely related to improved survival of extremely premature infants with severe NEC and other gastrointestinal conditions. ^, The NICHD network has reported the incidence of SBS to be 7 per 1000 VLBW infants and 11 per 1000 extremely low birth weight infants. SBS is also an important cause of infant morbidity and mortality in other countries; the Canadian Collaborative Study Group reported an overall incidence of 0.25 cases of SBS per all 1000 live births, increasing to 3.5 per 1000 in preterm infants.

Development of the Gastrointestinal System and Relevance to Short Bowel Syndrome

The human gut develops from an infolding of the endodermal layer of the embryo, beginning at about 2 to 3 weeks after conception. All three germ layers are involved: the endoderm, the mesoderm, and the ectoderm. Many endodermal cells differentiate into gut epithelium and associated glands; mesoderm contributes to the lamina propria, muscularis mucosa, muscularis externa, and the blood vessels; and the ectoderm gives rise to the gut neuronal network in the submucosal and myenteric plexuses. The longitudinal growth of the small intestines can be described in three phases: an initial linear phase, a second and more accelerated phase between 20 and 40 weeks of gestation, and then a second phase of linear growth during infancy. The small intestines grow from about 2 to 20 cm between the 7th and 14th weeks of gestation and then double in length from 20 to 40 weeks. The mean length of the small intestines at 20 weeks is 125 cm, and this increases to 275 cm at full term. The cylindrical growth, both linear and circumferential, occurs secondary to binary fission or duplication of intestinal crypts and is most prominent in the submucosa. The surface area of the intestinal mucosa increases with the formation of mucosal folds of Kerckring, deepening of the crypts, and the development of microvilli in the brush border. This mucosal and submucosal growth increases the absorptive area of the intestines by more than 600 times. Finger-shaped villi with apical microvilli can be seen as early as the 14 weeks’ gestation, and this development continues during early infancy. ^, This “reserve” ability of the small intestines to grow in structure and function in the second half of gestation provides some adaptability to the loss of some length secondary to disease and/or surgical resection.

Nutritional Challenges in Short Bowel Syndrome

The signs and symptoms of SBS are defined by the loss of function of specific parts of the gastrointestinal tract. The direct impact of SBS is attributed to the loss of the digestive and absorptive surface of the mucosal ultrastructure and/or shortened length of the intestine. The transit of the food may also be reduced, and the resulting feeding intolerance (emesis) may reduce its contact period with the mucosa, thus impairing digestion and absorption. The loss of specific parts of the gastrointestinal tract, which have evolved to absorb different nutrients, may also predispose the infant to various nutritional deficiencies. Fig. 21.2 summarizes these data.

The stomach is resected less frequently, but the disturbances to gastric physiology can lead to several signs and symptoms seen in SBS. The stomach primarily receives the ingested food, initiates digestion, and transfers the food in a series of well-coordinated movements between the fundus-body and the pylorus-antrum. Gastric hypersecretion is a state of increased gastric secretions often noted with resection of the terminal ileum. The terminal ileum secretes a hormone called peptide YY, which slows down the motility and secretion of the stomach and the duodenum. ^, After resection of the ileum, this negative feedback look is lost, leading to states of hypergastrinemia and hypermotility. Incoordination between different parts of the stomach can lead to gastroparesis, impairing the ability to transfer gastric content to the duodenum, leading to increased gastrointestinal output.

Pancreatic enzymes are detected as early as 15 weeks’ gestation, but the ontogeny of pancreatic exocrine function is slower compared with the anatomic development of the gut. The pancreatic exocrine function is fairly mature beyond 20 to 25 weeks but then plateaus to continue to develop through early infancy. In SBS, the loss of pancreatic function leads to exocrine pancreas insufficiency and subsequent malabsorption of nutrients. Pancreatic amylase in the duodenum resumes the digestion of complex carbohydrates initiated by salivary amylase in the mouth. Thereafter, brush border hydrolases found on the enterocytes, such as lactase, maltase, and sucrase, continue the disintegration of disaccharides into monosaccharides and facilitate their absorption into the bloodstream via hexose transporters such as SGLUT-1 and GLUT5. Intestinal lactase activity is low between 14 and 20 weeks’ gestation before reaching a high level at term. Although human milk is high in lactose concentration, VLBW infants tolerate human milk very well. This is probably aided by the colonic salvage pathways that convert unabsorbed lactose into short-chain fatty acids, which are then absorbed and used for energy production. In fact, early feeding promotes increased intestinal lactase in preterm infants. Many other effects of SBS may not be directly related to the loss of digestion and absorption of nutrients. Examples include intestinal failure–associated liver disease (IFALD), cholelithiasis, bloodstream infections, metabolic bone disease, oral aversion, and long-term growth failure with neurodevelopmental delays.

Intestinal Adaptation/Rehabilitation

The events and recovery of intestinal functions after the sentinel event resulting in IF can be broadly categorized into three stages ( Fig. 21.3 ):

• 1.

  Acute phase, seen immediately after a surgical event, lasting between a few days to weeks. The acute phase is characterized by postoperative ileus and consequent high outputs either through the gastric tube or the gastrostomy. Consequent to high gastrointestinal outputs, this is accompanied by fluid and electrolyte losses and acid–base imbalance.

• 2.

  Subacute phase, spanning several weeks. The intestinal function gradually returns; the previously high gastrointestinal output decreases with less severe fluid and electrolyte disturbances. During this period, enteral nutrition is initiated and advanced on a foundation of a stable parenteral nutritional regimen. This phase is significant for linear and circumferential growth and the adaptation of previously retained sections of the intestines. This process facilitates feed tolerance, enabling the advancement of enteral nutrition and weaning of parenteral nutrition.

• 3.

  Chronic consolidation phase, spanning several months to years depending on the extent of the loss.

All stages show progressive intestinal adaptation, marked by a series of anatomic and physiologic changes that begin as early as 48 hours after surgery and continue throughout rehabilitation. ^, During this process, compensatory changes are observed in the mucosa and muscular layers of the intestines. Changes in the mucosal cytoarchitecture involve lengthening of villi, deepening of crypts, and enterocyte proliferation resulting in expansion of the subluminal mucosal surface, increasing the surface area by many times. This process results in increased enterocyte per villus and is achieved by increasing mucosal DNA and RNA content. This process of intestinal adaptation is mediated by gastrointestinal hormones such as growth hormone, insulin-like growth factor, glucagon-like peptides, peptide YY, and neurotensin. This process of intestinal adaptation is further enhanced by exposure of the intestinal mucosa to enteral feeds, which prompts the release of hormonal mediators mentioned above, facilitating a trophic effect. The extent of intestinal adaptation varies with the anatomic site of the gastrointestinal tract; studies suggest that the capacity for anatomic and function adaptation is highest in the ileum and limited in the jejunum. The process of intestinal adaptation provides the host with the anatomic and physiologic machinery required to improve the function of the residual bowel. As the intestinal adaptation progresses, it results in the gradual return of the function of the residual bowel, restoring the capacity to digest and absorb fluids, electrolytes, and nutrients.

Goals of Intestinal Rehabilitation

The goals of intestinal rehabilitation are listed sequentially in the order of appearance in Table 21.2 . Enteral autonomy is often mentioned as the final goal in the process of intestinal rehabilitation. Enteral autonomy is defined as a clinical state in which the demands for adequate nutrition are met entirely by enteral feeds with complete independence from parenteral nutrition. However, the ultimate goal should be to promote and achieve appropriate developmental milestones with the best-tolerated combination of enteral and parenteral nutrition while striving toward enteral autonomy. There is wide variability in the definition of enteral autonomy, with different durations of independence from parenteral nutrition used as criteria. Short-term definitions have included achievement of feeds of 130 mL/kg/day with freedom from parenteral nutrition for a period as short as 48 hours. ^, However, long-term definitions in which independence from parenteral nutrition for over 3 to 12 months with appropriate growth parameters offer a more meaningful definition of enteral autonomy. The latter is a better definition because it stands the test of a more significant duration of time and incorporates goals of growth and development. Although the parenteral nutrition–free period of 12 months is more stringent, it is reported that a period of 3 months free from parenteral nutrition captures most instances of enteral autonomy without any relapses.

Nutritional Strategies in Short Bowel Syndrome