Short-Bowel Syndrome: Background, Anatomy, Pathophysiology

splenic flexure syndrome diet, short-bowel syndrome overview

Short-Bowel Syndrome

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Background

Each year in the United States, many patients undergo resection of long segments of small intestine for various disorders, including inflammatory bowel disease (IBD), malignancy, mesenteric ischemia, and others. Juvenile survivors of necrotizing enterocolitis, midgut volvulus, and other abdominal catastrophes are becoming more common. Various nonoperative procedures can leave patients with a functional short-bowel syndrome. An example of this clinical scenario is radiation enteritis.

Those patients who are left with insufficient small bowel absorptive surface area develop malabsorption, malnutrition, diarrhea, and electrolyte abnormalities. The subset of patients with clinically significant malabsorption and malnutrition are said to have developed short-bowel syndrome.

The average length of the adult human small intestine is approximately 600 cm, as calculated from studies performed on cadavers. According to Lennard-Jones and to Weser, the range extends from 260 to 800 cm. [1] Any disease, traumatic injury, vascular accident, or other pathology that leaves less than 200 cm of viable small bowel or results in a loss of 50% or more of the small intestine places the patient at risk for developing short-bowel syndrome.

Short-bowel syndrome is a disorder clinically defined by malabsorption, diarrhea, steatorrhea, fluid and electrolyte disturbances, and malnutrition. The final common etiologic factor in all causes of short-bowel syndrome is the functional or anatomic loss of extensive segments of small intestine so that absorptive capacity is severely compromised. Although resection of the colon alone typically does not result in short-bowel syndrome, the condition’s presence can be a critical factor in the management of patients who lose significant amounts of small intestine. [2, 3]

Massive small intestinal resection compromises digestive and absorptive processes. Adequate digestion and absorption cannot take place, and proper nutritional status cannot be maintained without supportive care. Today, the most common causes of short-bowel syndrome in adults include Crohn disease, [4] radiation enteritis, mesenteric vascular accidents, trauma, and recurrent intestinal obstruction. In the pediatric population, necrotizing enterocolitis, intestinal atresias, and intestinal volvulus are the most common etiologic factors.

Other conditions associated with short-bowel syndrome include congenital short small bowel, gastroschisis, and meconium peritonitis.

Not all patients with loss of significant amounts of small intestine develop short-bowel syndrome. Important cofactors that help to determine whether the syndrome will develop or not include the premorbid length of small bowel, the segment of intestine that is lost, the age of the patient at the time of bowel loss, the remaining length of small bowel and colon, and the presence or absence of the ileocecal valve.

Several operative or invasive procedures and therapies have been designed for and applied to the treatment of short-bowel syndrome. These include the establishment of central venous access for delivery of total parenteral nutrition (TPN), intestinal transplantation, and nontransplantation abdominal operations. The history of these various treatment strategies is discussed in this section.

TPN was developed successfully by Dudrick et al. [5] Their landmark paper included laboratory studies in a canine model and clinical results in 30 adult patients with a variety of gastrointestinal (GI) maladies ranging from achalasia to traumatic pancreatitis to regional enteritis. The animal model clearly demonstrated efficacy. Beagle puppies fed entirely intravenously surpassed their littermate controls in weight gain and were equal in terms of activity level, skeletal growth, and other developmental landmarks.

In the clinical arm of the study, [5] the 30 subjects receiving TPN were able to achieve positive nitrogen balance, maintain weight, heal wounds, and close fistulae. Wilmore and Dudrick reported positive nitrogen balance, growth, and development in an infant born with diffusely atretic small bowel who was fed entirely parenterally. [6]

After these initial successes, the new technique was introduced into the clinical mainstream, and indications for its use have expanded tremendously. Patients with short-bowel syndrome are now routinely treated with TPN, especially early in their course. New therapeutic strategies that may allow patients to discontinue or curtail the use of TPN are discussed in subsequent sections.

Numerous nontransplant operative approaches have been used in the treatment of patients with short-bowel syndrome. The creation of reversed intestinal segments was popular in the 1960s and 1970s. The aim of this operation was to produce a sort of functional partial small-bowel obstruction that would slow intestinal transit time, thereby encouraging greater nutrient absorption and decreasing diarrhea and nutrient loss.

Around that time, recirculating small bowel loops were also being created, with the same idea in mind. The results were mixed to questionable, and these procedures are rarely used today. Intestinal lengthening (Bianchi procedure), the creation of artificial enteric valves, strictureplasty, and intestinal tapering procedures continue to be employed at some centers today.

Intestinal transplantation was first attempted in dogs in 1959. The procedures failed because of the great concentration of immune system cells associated with the gut. The discovery and clinical implementation of powerful immunosuppressive drugs, such as FK506 (tacrolimus) and cyclosporine A, made successful small bowel transplantation possible.

The first successful combined transplantation of small intestine and liver in a human was performed in 1990. Since that time, the technique of isolated small intestinal transplantation has been developed and applied. Better graft survival rates are achieved when patients receive their transplant before complications secondary to short-gut syndrome occur, especially that of cirrhosis. [7]

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