Gallstone ileus is a rare form of small bowel obstruction caused by an impaction of a gallstone within the lumen of the small intestine. Such a gallstone is hypothesized to enter the gut lumen via cholecysto-enteric fistula. Large stones, >2.5 cm in diameter, are thought to predispose to fistula formation by gradual erosion through the gallbladder fundus. Most commonly, obstruction occurs at the distal ileum (i.e. ileocecal valve), but can occur at almost any other location in the GI tract.Classically there is obstruction/impaction about 60 cm proximal to ileocaecal valve.
The name "gallstone ileus" is a misnomer because an ileus is, by definition, a non-mechanical small bowel motility failure.
Diagnosis of gallstone ileus requires radiographic studies. Classic findings of pneumobilia (air within the biliary tree), small bowel obstruction, and radiolucent gallstone on abdominal plain films is known as Rigler's triad.
Initial management involves fluid resuscitation and potentially nasogastric suctioning.  Since gallstone ileus constitutes a form of mechanical small bowel obstruction, it can be a surgical emergency and requires laparoscopic removal or propulsion of impacted stone. 
Bouveret's Syndrome: is reverse gallstone ileus where the gallstone propagates proximally and causes gastric outlet obstruction by being impacted in first part of duodenum.
- Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson JL, Loscalzo J. Chapter 311. Diseases of the Gallbladder and Bile Ducts. In: Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson JL, Loscalzo J, eds. Harrison's Principles of Internal Medicine. 18th ed. New York: McGraw-Hill; 2012. http://www.accessmedicine.com/content.aspx?aID=9135488. Accessed December 17, 2013.
- Gaillard F. Radiopaedia.org - Gallstone Ileus. http://radiopaedia.org/cases/gallstone-ileus. Accessed December 17, 2013
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- Jones J, Gaillard F. Radiopaedia.org - Rigler's Triad. http://radiopaedia.org/articles/riglers-triad. Accessed December 17, 2013