Anatomical and physiological considerations

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The proximal 100 to 200 cm of the jejunum is the primary site of carbohydrate, protein and water-soluble vitamin absorption [14]. Fat absorption occurs over a larger length of small bowel, a length that increases as the amount of fat ingested increases. The junctions between jejunal epithelial cells are relatively large compared to other areas of the bowel, allowing rapid flux of fluids and nutrients so that the jejunal contents can become iso-osmolar. As a result, the concentration of sodium in jejunostomy fluid is about 100mEq/l (range, 90-140). Sodium absorption in the jejunum can only occur against a concentration gradient, is dependent upon water fluxes and is coupled to the absorption of glucose [15].

In contrast to the jejunum, the ileum has tighter intercellular junctions [15]. This leads to less movement of water and sodium. The active transport of sodium chloride allows for significant fluid reabsorption and the ability to concentrate its contents. The ileum is also the primary site of carrier-mediated bile salt and B12 absorption and is the site of production of many gastrointestinal (GI) hormones such as glucagon-like peptides 1 and 2 and peptide YY. These GI hormones are important for control of bowel motility and growth [16]. Although it is commonly believed that the ileocecal valve is beneficial in slowing transit and preventing reflux of colonic contents into the small bowel, studies in patients who have previously undergone ileocecal valve excision have not confirmed these benefits [17, 18]. Reports suggesting a benefit due to the presence of the ileocecal valve may in fact reflect the retention of a significant length of terminal ileum.

Knowledge of the remaining small bowel length can be useful to predict the clinical outcome in SBS patients. Nevertheless, establishing an accurate estimation of bowel length is often difficult. While information from operative reports is preferred, such notes frequently record the amount of bowel removed rather than the amount remaining. A barium contrast small bowel series may provide an estimate of bowel length and is useful to delineate other structural features such as the presence of bowel dilatation [23]. The normal small bowel length depends upon the measurement method used, but typically ranges from about 300 to 800 cm in adults and between 200 and 250 cm in full-term infants at birth [24]. The large range of small bowel length in humans underscores the importance of being aware of the small bowel length remaining following a resection rather than the length of small bowel removed. Importantly, when considering small bowel length, measurements begin at the duodeno-jejunal flexure.

Anatomical factors that affect the outcome of the SBS patient include not only the length, but also the region of the remaining small intestine and the presence of the colon. The ileum is capable of both structural and functional adaptation, while the jejunum mainly adapts functionally (see Intestinal Adaptation) [25, 26]. As a result, a jejunal resection is generally better tolerated [13]. Unfortunately, in most patients with SBS, the ileum has been resected, leaving only a portion of jejunum, often in combination with a portion of the colon. There is evidence to support functional small bowel adaptation in those with a jejuno-colic anastomosis, but not an end-jejunostomy [26, 27]. The presence of the colon has clearly been shown to be beneficial in SBS patients given its ability to absorb water, electrolytes and fatty acids, slow intestinal transit and stimulate intestinal adaptation. It has been suggested that, in terms of need for PN, the presence of at least half of the colon is equivalent to about 50 cm of small bowel [27]. It has also been suggested that those adult SBS patients with a jejuno-colic anastomosis who have at least 100 cm of jejunum may not require long-term PN, while most adult SBS patients who have <50 cm of jejunum attached to colon will require long-term PN [13,27]. Similarly, those without a colon and <100 cm of jejunum are likely to require permanent PN. In comparison to adults, infants with less than 30 cm of small bowel are unlikely to be weaned from parenteral nutrition.

The colon plays a vital role in fluid and electrolyte reabsorption with a capacity to absorb up to 61 daily [19]. Complete loss of the colon often leads to problems with dehydration and electrolyte abnormalities for SBS patients. Enteroglucagon, neurotensin and peptide YY are produced in the proximal colon (and ileum) and are responsible for the jejunal, ileal and colonic brake phenomena that slow small intestinal transit in response to fat intake [16, 20, 21]. In addition to the resorptive capabilities of the colon, bacterial fermentation of malabsorbed carbohydrates to short-chain fatty acids with subsequent absorption in the colon provides an additional energy source which can be substantial, up to 1,000 kcal daily [22].

In general, a SBS patient will have one of the following bowel anatomies: jejuno-colic anastomosis, end-jejunostomy or jejuno-ileocolonic anastomosis. Patients with a jejuno-colic anastomosis rarely have an ileocecal valve. Patients with a jejuno-ileal anastomosis have the best prognosis; however, this anatomy is the least common. Patients with an end-jejunostomy are the most difficult to manage and are most likely to require permanent parenteral support [13].

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