Short Bowel Syndrome Flashcards
Short bowel syndrome (SBS)
is characterized by malabsorption due to congenital absence or resection of large portions of the small intestine, typically leaving the adult with 150 to 200 cm of functional small bowel.
The major causes of SBS in adults are:
Crohn disease in which multiple intestinal resections have been performed; mesenteric infarction from venous or arterial thrombosis, arterial embolism, or midgut volvulus; massive enterectomy performed to manage traumatic injuries or tumor resection; and radiation injury
The major consequence of extensive intestinal resection
is loss of absorptive surface area, which results in malabsorption of macronutrients, micronutrients, electrolytes, and water.
Three types of intestinal resections typically are encountered
limited ileal resection for Crohn disease, often with cecectomy or right hemicolectomy; extensive ileal resection with or without partial colectomy; and extensive small intestinal resection and total colectomy resulting in proximal jejunostomy
The length of the small intestine is estimated at 3 to 8 m, and nutrient absorption is preserved until more than one-half of the small intestine is resected.
Most macronutrients (carbohydrate, fat, and nitrogen) are absorbed in the proximal 100 to 150 cm of intestine
Villi are taller and crypts are deeper in the jejunum than in the ileum, and the activity of microvillus enzymes and nutrient absorptive capacity per unit length of intestine are several-fold higher in the proximal than in the distal small intestine; loss of part of the jeju- num initially compromises nutrient absorption more than does loss of an ileal segment of similar length because of these morphologic and functional differences.
The ileum, however, eventually is able to compensate for jejunal loss, whereas the jejunum is unable to compensate for ileal absorption of bile salts and vitamin B1
Patients with a proximal jejunostomy have rapid gastric emptying of liquids and rapid intestinal transit because of impaired release of glucagon like peptide-1 (GLP-1), GLP-2, and Peptide YY (PYY), which can compromise the gastric phase of digestion and result in inadequate mixing with biliary and pancreatic secretions, insufficient enzymatic digestion, and nutrient maldigestion.
Patients with a proximal jejunostomy are net secretors of salt and fluid, because jejunal fluid secretion is stimulated by oral intake and subsequent gastric emptying of nutrients; these patients excrete more fluid than they ingest and absorb, and accordingly, their fluid management may be challenging.
Most patients whose jejunal length is less than 100 cm and who have no colon require long-term PN.
the ileocecal valve acts as a brake to slow intestinal transit, thereby increasing nutrient-enterocyte contact time and enhancing absorption.
On unrestricted diets, patients with a proxi- mal jejunostomy cannot reabsorb such large volumes, a con- sequence of which is that voluminous diarrhea develops complicated by hypovolemia, hyponatremia, and hypokalemia.
Vitamin B12 malabsorption usually is demonstrable when more than 60 cm of ileum has been
resected
Resection of less than 100 cm of ileum causes moderate
bile acid malabsorption and increased bile acid loss to the colon or in stomal effluents
The increased loss of bile acids into the
colon induces electrolyte and water secretion and can exacerbate diarrhea, a condition called
cholerrheic enteropathy.
More extensive ileal resections (>100 cm) cause severe bile acid malabsorption, which, if bile acid loss exceeds hepatic bile acid synthesis, can result in a reduced bile acid pool size, insufficient micellar solubilization of lipolytic products and resultant steatorrhea; fat
malabsorption is accompanied by fat-soluble vitamin deficiency.
Patients with SBS and a preserved colon have increased GLP-1 and GLP-2 con- centrations and demonstrate normal gastric emptying
Loss of the Ileocecal Valve
The primary functions of the ileocecal valve are to separate ileal and colonic contents, thereby minimizing bacterial colonization of the small intestine, and to regulate emptying of ileal contents into the colon.
The ileocecal valve is removed in extensive or dis- tal ileal resections, as a consequence of which intestinal transit time decreases, and SIBO is risked if the ileum is anastomosed to the colon.
After jejunectomy and duodenoileal anastomosis, the ileum attains the morphologic characteristics of the jejunum, with taller villi and deeper crypt with time, an increase in ileal diameter and length also occurs.
Patients with a limited ileal resection (<100 cm), with or without right hemicolectomy, may resume intake of solid food in the late postoperative phase.
Secretory diarrhea without steatorrhea is the typical finding in limited ileal resec- tions.
Treatment with a bile acid-binding resin, such as chole- styramine (2 to 4 g with meals) or colestipol (1 to 2 g with meals), often ameliorates diarrhea if bile acid malabsorption is the main cause
The serum methylmalonic acid level will be elevated in the presence of B12 deficiency as B12 is a cofactor necessary for the metabolism of methylmalonic acid to succinyl co-enzyme A.
Patients with ileal disease or resection who are at risk for vitamin B12 malabsorption (those with >60 cm of terminal ileum resected) should be treated for life with parenteral B12, usually in a dose of 1 mg intramuscularly each month.
Massive enterectomy is associated with gastric hypersecretion for about the first 6 months postoperatively.
These patients benefit from IV H2RAs or oral or IV PPIs; absorption of orally ingested medications may be impaired, and more than the usual doses of these agents may be required
Use of anti-motility agents is important to control fluid losses; such agents include loperamide hydrochloride (4 to 6 mg 4 times daily) and diphenoxylate-atropine (2.5 to 5 mg 4 times daily), codeine (15 to 60 mg 2 to 4 times daily) tincture of opium (0.6 mL [2.5 mg] 2 to 4 times daily), and the somatostatin analog octreotide (50 to 100 μg 2 to 3 times daily).
Octreotide can slow intestinal transit and increase Na+ and water absorption,but it also decreases splanchnic protein synthesis, thereby inhibiting post-resectional intestinal adaptation; the risk of cholelithiasis also is increased with octreotide
Patients with SBS whose colon is in continuity with the remaining small intestine should be provided a high complex-car- bohydrate diet that includes starch, non-starch polysaccharides, and soluble fiber.
When more than 100 cm of terminal ileum has been resected, fat maldigestion can develop because bile salt malabsorption leads to decreased micelle formation, which results in poor fat solubi- lization.
The bile acid- sequestering agent cholestyramine may be useful to decrease bile salt-related diarrhea in patients with less than 100 cm of terminal ileum resected, but it can worsen steatorrhea in patients who have undergone a more significant resection fat-soluble vitamin defi- ciency can also develop by further reducing an already reduced bile salt concentration because of its binding to dietary lipids.
The diet composition for patients with a
jejunostomy who are taking food by mouth can be more liberal because the percentages of energy absorption are similar for a low-fat, high-carbohydrate and a high-fat, low-carbohydrate diet.
Liver disease often develops in patients who require long-term PN. Formerly known as parenteral nutrition-associated liver disease, this complication is now known as intestinal failure-associated liver disease.
Interruption of the enterohepatic circulation of bile acids by ileal resection results in decreased hepatic bile acid secretion and altered composition of hepatic bile in terms of its organic com- ponents: bile acid, cholesterol, and phospholipids
d-Lactic acidosis is a rare complication of SBS and in this set- ting is observed only in patients with a preserved colon. The episodes of acidosis usually are precipitated by increased oral intake of refined carbohydrates and can be induced in the patient with SBS by carbohydrate overfeeding.
The most important surgical procedure is re-anastomosis of residual small bowel to residual colon. This procedure carries relatively low mortality and morbidity rates and allows enhanced energy absorp- tion from SCFAs produced from bacterial fermentation of unab- sorbed carbohydrate.
The main indication for transplan- tation in children and adults is PN-dependent SBS complicated by progressive liver disease.
