Physiologic Anatomy

Intestine transplantation is reserved for those patients with permanent intestinal failure due to SGS requiring TPN with no possibility of discontinuation. These patients are at high risk for developing life-threatening complications such as TPN-induced liver disease, recurrent episodes of sepsis, and loss of venous access. In SGS, the patient's small intestine is unable to maintain an adequate nutritional status or fluid and electrolyte balance on enteral feedings, resulting in poor growth and development.[17,18] Due to extensive malabsorption, the intestinal contents proceed to the colon, causing an osmotic diarrhea and fermentation of fluids leading to a hypersecretory state.[19]

SGS is a result of extensive surgical resection or anatomic abnormalities that significantly decrease the length of the small intestine. It may also be the result of a "functional short gut" where, despite an adequate intestinal length, intestinal dysfunction associated with absorptive, secretory, or motility disorders of various etiologies and severity leads to malabsorption and TPN dependency.

Gastrointestinal (GI) Tract

The adult GI tract is approximately 4.5 meters (15 feet) in length. The absorptive surface of the GI tract is further increased by the folded mucosal layers, villi, and microvilli and consists of 3 layers: the mucosa, submucosa, and muscle layer. The mucosa, the outermost layer, is composed of the epithelium, lamina propria, and muscularis mucosa. The epithelium is a single layer of cells across which absorption takes place. Invaginations of epithelium into the underlying tissue form tubular exocrine glands that secrete mucus, acids, enzymes, water, and ions into the lumen of the intestine. The lamina propria is a layer of connective tissue below the epithelial layer that contains blood vessels, nerve fibers, and lymphatic ducts. The muscularis mucosa is a thin layer of smooth muscle that separates the mucosa from the submucosa.

The submucosa is another layer of connective tissue that contains larger blood vessels, lymphatics, and a network of nerve cells called the submucus plexus. The muscle layer is beneath the submucosa and consists of smooth muscle tissue responsible for the contractions that move gastric contents through the GI tract. An inner layer of smooth muscle in a circular pattern causes the lumen to constrict while the outer layer of longitudinal smooth muscle causes contractions of the tract. The myenteric plexus, an extensive network of nerve cells, is located between these 2 smooth muscle layers.

Duodenum

Over 9 liters of fluid may pass through an adult intestine each day, and about 95% of that fluid is absorbed by the small intestinal. The initial 20 cm (8 inches) of small intestine is the duodenum. The duodenum has a velvety appearance with numerous villi. The common bile duct and pancreatic duct drain into this section of the small intestinal. Loss of all or part of the duodenum in SGS results in iron, folate, and/or calcium malabsorption. The duodenum proceeds to the jejunum with the duodenal-jejunal flexure held in place by the ligament of Treitz.

Jejunum

The jejunum, particularly the midsection, is the primary site for digestion and absorption of most nutrients. This ability is a result of the increased surface area due to long villi found in this region and the presence of concentrated digestive enzymes and transport carrier proteins.[18] The jejunum has thick, large circular folds, is highly vascular, and has large villi. Its porous epithelium results in an osmotic gradient that allows for rapid flow of water and electrolytes from the vascular to the intraluminal space. The length of the jejunum and ileum, the distal section of the small intestine, is approximately 260 cm, with the ileum having a slightly narrower lumen.

Ileum

The ileum has a thinner wall with shorter villi and is less vascular and has more lymphoid tissue than the jejunum. The ileum also has a less porous epithelial layer, so it is less susceptible to massive fluid losses due to hypertonic feedings and reabsorbs fluid losses from the jejunum.[18] Depending on adaptation of the remaining intestine and the extent of ileal resection, removal of the ileum results in a loss of a primary reabsorptive site for jejunal secretions, massive fluid losses and impairments in vitamin B12 and bile acid absorption.[18,19] A reduction in bile acid absorption subsequently decreases the absorption of fats and fat-soluble vitamins. Patients with SGS secondary to jejunal resection have decreased absorption of most nutrients, but this loss is usually better tolerated than an ileal resection. However, if the remaining ileum is able to adapt with adequate retention of vitamin B12 and bile acid transport carrier proteins, jejunal resection is better tolerated. Synthesis of many GI hormones takes place in the ileum. These hormones have a role in the regulation of gastric emptying and intestinal transit. A significant resection of the ileum can cause delayed gastroduodenal emptying. Resection of the small bowel can also eliminate the negative feedback system that inhibits gastrin secretion and decreases gastric acid production, which may cause peptic ulcer disease, gastroesophageal reflux, and GI inflammation in patients with SGS.

Ileocecal Valve

The ileocecal valve separates the ileum of the small bowel and the colon. This valve regulates the movement of fluids from the small bowel to the colon and is also the major barrier for reflux of colonic bacteria. Loss of the ileocecal valve, particularly with significant concomitant ileal resection, places the patient at high risk for bacterial overgrowth in the small bowel and results in rapid transit of fluid and malabsorbed nutrients into the colon.[18,19]

Colon

The colon includes the remaining 120 cm (4 feet) of bowel and has a diameter of about 6 cm (2/5 inches) in an adult. It extends from the ileum to the anus and includes fairly straight segments described as the ascending, transverse, and descending colon. Mucus secretion occurs in the colon; its primary function is storage and concentration of fecal material. There are no villi in the colon, and it absorbs only about 4% of intestinal contents. Consequently, patients requiring massive ileal resection lose the primary site of reabsorption and the colon is unable to recover the fluid losses.

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