Etiology

SBS occurs due to the loss of functioning surface area of bowel, leading to the decreased absorption of electrolytes, nutrients and water.[1] The severity and effects depend on the location and extent of intestinal loss. The mildest cases result from limited bowel resection without significant involvement of the terminal ileum. The most severe cases have had a total colectomy and extensive small bowel resection with resultant proximal jejunostomy.

In adults, a typical case of SBS might include a patient with Crohn disease with isolated terminal ileal resection, or a patient with massive small bowel resection secondary to ischemia or traumatic injury. Ischemic injury may be caused by mesenteric arterial thrombosis or embolism, mesenteric venous thrombosis, volvulus, or intussusception. Crohn disease is becoming a less frequent cause for SBS due to more restrictive surgeries and the use of strictureplasty. However, patients with abdominal or pelvic radiation injury who have had large bowel resections due to strictures and enterotomies are increasingly common.[4] In addition, gastric and intestinal volvulus as a complication of bariatric surgery is becoming a more frequent finding.

Children with intestinal atresia, midgut volvulus, intussusception, or necrotizing enterocolitis undergo bowel resection to treat their disease, and can develop SBS as a complication of surgery.[9] Those with gastroschisis may also develop SBS as a result of bowel ischemia.

Pathophysiology

The adult small bowel measures approximately 3 to 8 meters with different portions having different functions.[10][11] The density and depth of intestinal crypts and their associated enzymes changes from the proximal to the distal small bowel, creating a characteristic gradient.

  • Proximal small bowel: the majority of macronutrient (including fat), micronutrient, and medication absorption occurs in the first 100 to 150 cm of small intestine.[12] Yet, if a large proportion or all of the jejunum is resected, the ileum can adapt to absorb most of these nutrients. Iron is absorbed in the duodenum and, thus, deficiency is uncommon. However, calcium absorption requires vitamin D; therefore, deficiency may be observed despite the absorption of calcium in the duodenum. The loss of enteric hormones (e.g., cholecystokinin) leads to a decrease in biliary and pancreatic secretion, impairing digestion. At the same time, an increase in gastrin concentration stimulates gastric hypersecretion, which can injure the mucosa and create unfavorable pH conditions for digestion and absorption. The unabsorbed nutrients increase the osmolarity of the intestinal contents, triggering diarrhea when delivered to the ileum and colon. Jejunal electrolyte and fluid losses often exceed absorption; in fact, patients with proximal jejunostomy may secrete more fluid than they ingest. Jejunal motility is stimulated by oral intake and for a person on a normal diet, at least 100 cm of jejunum is necessary to maintain positive fluid and electrolyte balance.[13] If less than 100 cm residual jejunum is present, increased fluid and electrolyte losses can occur.

  • Distal small bowel: ileal resection severely impairs water and electrolyte reabsorption. The terminal ileum has fewer macronutrient absorption roles, but has specific irreplaceable functions such as bile salt and vitamin B12 absorption. Terminal ileal resection interrupts the enterohepatic circulation of bile salts, and therefore retards fat absorption in the jejunum. Specialized cells in the terminal ileum are also critically important in the absorption of vitamin B12, which cannot be absorbed in the jejunum.[10][11] However, vitamin B12 malabsorption is rare if less than 60 cm of terminal ileum is resected. Nevertheless, even relatively modest resections can result in bile salt malabsorption and spillage to the colon. This increases colonic permeability, which in turn increases colonic fluid secretion and diarrhea. If more than 100 cm of terminal ileum is resected, patients can deplete their bile salt pool resulting in fat malabsorption and fat-soluble vitamin deficiencies.[14][15] The ileum, and possibly the right side of the colon, are also locations for the secretion of meal-stimulated glucagon-like peptide 2 (GLP-2), which is important in intestinal mucosal repair and thus absorption.

  • Large bowel: the colon has a critical role for the absorption of sodium and water, with a more minor role for the absorption of amino acids and medium-chain triglycerides. However, in patients with substantial small bowel resection, the colon becomes an important organ for energy absorption. Unabsorbed starches, soluble fiber, and other complex carbohydrates are fermented by endogenous colonic bacteria to short-chain fatty acids (butyrate, acetate, and proprionate), which serve as an important energy source for colonocytes. This process is referred to as carbohydrate salvage. Vitamin K and B9 (folate) are also synthesized by colonic bacteria. In addition, the ileocecal valve provides a crucial means of slowing intestinal transit and maximizing the time available for the reabsorption of fluid, electrolytes, and nutrients. SBS occurring in patients who have had a colectomy, or with a residual colon that is not in continuity with the small bowel, is more challenging to manage as these crucial abilities of the colon are lost.[14]

In patients with more extensive bowel resections, the secretion of the motility-regulating factors GLP-1 and GLP-2, and peptide YY (PYY) is lost, leading to rapid gastric emptying, which further contributes to diarrhea and malabsorption.[16] GLP-2 is also important for the development of small bowel hypertrophy during the adaptive period.

Diarrhea results from increased osmolarity of the intestinal contents (mostly due to fat malabsorption), bile salt-induced irritation of the colon, decreased intestinal transit time, increased gastric acid production, and decreased gut surface area for water reabsorption.

Classification

Intestinal failure functional classification[1]

Type 1

  • Acute, short-term, and usually self-limiting condition. Occurs perioperatively after abdominal surgery, or in association with a critical illness such as acute pancreatitis or head injury. Parenteral nutrition (PN), including parenteral fluids, is necessary for short-term fluid or nutritional support.

Type 2

  • Prolonged, acute conditions, often in metabolically unstable patients, requiring complex multidisciplinary care and intravenous supplementation over periods of weeks or months. Usually the result of extensive bowel resection, due to an acute event (e.g., mesenteric ischemia, volvulus) or due to a complication of abdominal surgery (e.g., anastomotic leak). Associated with septic, metabolic, and complex nutritional complications. PN weaning is possible, with time, during and following intestinal adaptation.

Type 3

  • Chronic condition, in metabolically stable patients, requiring intravenous supplementation over months or years. May be reversible or irreversible. May occur as an evolution of type 2 intestinal failure, as the result of progressive diseases requiring multiple resections, as a consequence of congenital disease (e.g., gastroschisis) or at the end stage of intra-abdominal or pelvic cancer. Patients with irreversible type 3 intestinal failure require long-term PN or bowel transplantation for survival.

Intestinal failure pathophysiologic classification[1]

Intestinal failure may be classified into five major pathophysiologic conditions:

  • short bowel

  • intestinal fistula

  • intestinal dysmotility

  • mechanical obstruction

  • extensive small bowel mucosal disease.

Clinical classification of chronic intestinal failure[1]

Chronic intestinal failure is categorized into 16 subtypes (categories A1-A4, B1-B4, C1-C4, and D1-D4) on the basis of the requirements for energy and volume of intravenous supplementation.

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