In the adult, the length of the small bowel varies from 300 to 600 cm and correlates directly with body surface area. Several factors determine the severity of short-bowel syndrome, including the extent of resection, the portion of the GI tract removed, the type of disease necessitating the resection, the presence of coexistent disease in the remaining bowel, and the adaptability of the remaining bowel. Generally, resection resulting in less than 120 cm of intact bowel leads to short-bowel syndrome (<150 cm if ending in an ileostomy, <75 cm if colon remains). Infants may survive resection of up to 85% of bowel owing to the enhanced ability of the bowel to adapt and grow with the child. Because of its specialized absorptive function, resection of the ileum is usually not well tolerated. However, the entire jejunum can be resected without serious adverse nutritional sequela.
A. Etiology and pathophysiology
Short-bowel syndrome is characterized by dehydration, electrolyte derangements, acidic diarrhea, steatorrhea, malnutrition, and weight loss. Congenital anomalies leading to short-bowel syndrome include intestinal atresia, midgut volvulus with intestinal necrosis, and necrotizing enterocolitis. In middle-aged adults, inflammatory bowel disease and trauma are the leading causes of massive intestinal resection. In the elderly, prominent causes include mesenteric ischemia and strangulated hernia.
Adaptation. The distal small intestine has the greatest adaptive potential and can assume many of the absorptive properties of the proximal GI tract. Cellular hyperplasia and bowel hypertrophy occur over a 2- to 3-year period, increasing the absorptive surface area. Fat absorption is the metabolic process most likely to be permanently impaired; other functions adjust and normalize fairly well.
Fluid and electrolyte response. Of the 8 to 10 L of fluid presented daily to the small intestine, only 1 to 2 L are delivered into the colon. Significant quantities of electrolytes are absorbed in this process. With short-bowel syndrome, this physiology is altered. Strict intake and output records and close monitoring of serum electrolytes are critical in the early management of patients with short-bowel syndrome.
Malabsorption and malnutrition
Gastric hypersecretion, seen early in the postoperative period, can persist for prolonged periods. Increased acid load may injure distal bowel mucosa, leading to hypermotility and impaired absorption. The severity of hypersecretion correlates directly with the extent of bowel resection. This generally is more pronounced after jejunal than after ileal resection. Loss of an intestinal inhibitory hormone has been implicated.
Cholelithiasis. Altered bilirubin metabolism after ileal resection increases the risk of gallstones secondary to a decreased bile salt pool, which causes a shift in the cholesterol saturation index. Chronic total parenteral nutrition (TPN) also increases risk of cholelithiasis.
Hyperoxaluria and nephrolithiasis. Excessive fatty acids within the colonic lumen bind intraluminal calcium. Unbound oxalate, normally made insoluble by calcium binding and excreted in the feces, thus is absorbed readily, resulting in hyperoxaluria and calcium oxalate urinary stone formation.
Diarrhea and steatorrhea. Rapid intestinal transit, presence of hyperosmolar enteric contents in the distal bowel, disruption of the enterohepatic bile acid circulation, and bacterial overgrowth all promote diarrhea and steatorrhea. Fat absorption is most severely impaired by ileal resection. The delivery of bile acids into the colon produces a reactive watery diarrhea that may be severe. Unabsorbed fats in the colon further inhibit absorption and stimulate secretion of water and electrolytes.
Intestinal microflora. Loss of the ileocecal valve permits reflux of colonic bacteria into the small bowel. Intestinal dysmotility further promotes bacterial colonization. Bacterial overgrowth and changes in the indigenous microbial population result in pH alteration and deconjugation of bile salts, with resultant malabsorption, fluid loss, and decreased vitamin B12 absorption. Infectious diarrhea (bacterial or viral) is a major cause of morbidity.
Acutely, the primary goal is to stabilize the metabolic, respiratory, and cardiovascular parameters related to the fluid shift and sepsis that frequently accompany massive small-bowel resection.
Deranged motility patterns and changes in intraluminal milieu may produce a prolonged ileus. Parenteral nutrition should be provided until GI function resumes. If ileus persists for an unduly prolonged period, mechanical obstruction or sepsis may be the cause.
Gastric hypersecretion requires H2-receptor antagonists or proton-pump inhibitors to reduce the hypersecretion response and protect against peptic ulceration. Antacids neutralize acid on contact and should be administered for nasogastric aspirate pH of less than 5.
Nutritional support should be instituted early to maintain positive nitrogen balance and to promote wound healing and adaptation of the remaining bowel. Enteral nutrition has a positive trophic effect on the bowel mucosa and should be started as soon as possible. Feeding tubes placed at laparotomy can be very helpful. Even if caloric goals are not met, enteral formula stimulates the remaining intestine and facilitates adaptation. Feeds should initially be low volume, low fat, and isosmotic.
C. Chronic Treatment
Diarrhea has many causes in short-bowel syndrome. Frequently, dietary modification improves symptoms. H2-receptor antagonists reduce acid production and the volume of enteric contents. Chelating resins, such as cholestyramine, reduce intraluminal bile salts and subsequent diarrhea but affect the available systemic bile salt pools. Antisecretory medications, such as loperamide and somatostatin analog, may be beneficial. Low-dose oral narcotics, such as diphenoxylate hydrochloride and atropine (Lomotil) or codeine, are efficacious but addictive. Bacterial overgrowth should be evaluated by stool culture and prophylactic antimicrobials administered as needed.
Nutritional support with supplemental vitamins, trace elements and minerals, and essential fatty acids should be given parenterally until adequate enteral absorption is established. The absorption of fat-soluble vitamins A, D, E, and K is especially likely to be compromised. Vitamin B12 and calcium absorption are also affected by altered fat absorption and should be supplemented. If required, chronic TPN can be administered nightly to permit normal daytime activities.
Late complications, mostly secondary to metabolic derangements, are common. Problems include nephrolithiasis, cholelithiasis, nutritional deficiency (e.g., anemia, bone disease, coagulopathy), liver dysfunction, TPN-related complications, and central venous catheter–related problems, for example, sepsis or thrombosis. Anastomotic leak, fistula, stricture, and obstruction can also occur well beyond the early postoperative period. Late obstruction (partial or complete) is fairly common, and reoperative rates are high.
D. Surgical therapy
Various surgical procedures have been described for the management of short-bowel syndrome, although they have not been widely adopted. Most important is the prevention of complications by minimizing the extent of initial bowel resection. Bowel transplantation is not widely performed but is an alternative in some patients, especially those with massive resection and virtually no remaining bowel.