Intestinal resection and short bowel syndrome

Intestinal resection is usually well tolerated, but massive resection is followed by the short-bowel syndrome. 
The effects of resection depend on the extent and the areas involved. 
Because the gut is long, a 30-50% resection can usually be tolerated without undue problems. 
Shortened bowel depends on several factors:
-the length of the bowel resected
-the location of the bowel resected
-the integrity of the bowel remaining
-the presence of the colon

Residual jejunum shows less capacity for structural and functional adaptation than residual ileum.

The ileum has specific receptors for the absorption of bile salts and vitamin B12, so that relatively small resections will lead to malabsorption of these substances. Removal of the ileocaecal valve increases the incidence of diarrhoea. The following occur in ileal resection: Bile salts and fatty acids enter the colon and cause malabsorption of water and electrolytes leading to diarrhoea. Increased bile salt synthesis can compensate for loss of approximately one-third of the bile salts in the faeces. Greater loss than this results in decreased micellar formation and steatorrhoea, and lithogenic bile and gallstone formation. Increased oxalate absorption is caused by the presence of bile salts in the colon. This gives rise to renal oxalate stones. There is a low serum B12 and macrocytosis. Glucagon-like peptide 2 (GLP-2) is low following ileal resection. GLP-2 is a specific growth hormone for the enterocyte and this deficiency may explain the lack of adaptation with an ileal resection. Investigations include: small bowel follow-through, measurement of B12, bile salts and occasionally fat absorption. hydrogen breath test will show rapid transit. Many patients require B12 replacement and some need a low-fat diet if there is steatorrhoea. If diarrhoea is a problem, colestyramine, which binds bile salts, often helps.

Jejunal resection:

The ileum can take over the jejunal absorptive function. Jejunal resection may lead to gastric hypersecretion with high gastrin levels; the exact mechanism of this is unclear. Structural and functional intestinal adaptation take place over the course of a year, with an increase in the absorption per unit length of bowel.

Massive intestinal resection (short-bowel syndrome):

This most often occurs following resection for Crohn's disease, mesenteric vessel occlusion, radiation enteritis or trauma. There are two types of short-bowel syndrome:

1-Shortened small intestine ending at a terminal stoma:

The major problem is of sodium and fluid depletion and the majority of patients with 100 cm or less of jejunum remaining will require parenteral supplements of fluid and electrolytes, often with nutrients. Sodium losses can be minimized by increasing salt intake, restricting clear fluids between meals and administering oral glucose-electrolyte mixture with a sodium concentration > 90 mmol/L. Jejunal transit time can be increased and stomal effluent loss of fluids and electrolytes reduced by treatment with the somatostatin analogue octreotide, and to a much lesser extent, with loperamide, codeine phosphate or co-phenotrope. There is no benefit of a low-fat diet, but fat assimilation can be increased on treatment with colestyramine and synthetic bile acids.

2-Shortened small intestine in continuity with colon

Only a small proportion of these patients require parenteral supplementation of fluid, electrolytes and nutrients because of the absorptive capacity of the colon for fluid and electrolytes. Unabsorbed fat results in impairment of colonic fluid and electrolyte absorption so patients should be on a low-fat diet. A high carbohydrate intake is advised as unabsorbed carbohydrate is metabolized anaerobically to short-chain fatty acids (SCFAs). SCFAs are absorbed and act as an energy source (1.6 kcal/g) and stimulate fluid and electrolyte absorption in the colon. Patients are often treated with colestyramine, which binds dihydroxy bile acids which otherwise have a deleterious effect on colonic fluid and electrolyte absorption and increase colonic oxalate absorption to form renal stones.

Note:
if less than 100 cm if the ileum is resected, the liver can compansate for the loss of absorptive capacity by producing an increased amount of bile salts, which enter the colon and cause bile irritant diarrhea. this type of diarrhea is treated with cholestyramine, which bind the excess bile salts and improves diarrhea.note:

if more than 100 cm is resected, the liver can no longer compansate. the resulting bile salt deficiency leads to steatrrhea.this can be managed by prescribing diet that consist of medium chain triglycerides.