telescoping of a segment of the intestine (large intestine) into an adjacent segment, producing a mechanical obstruction of the alimentary canal. Primary intussusception is sometimes congenital and rarely appears later than the third year of life; it arises in the course of intestinal development, but the mechanism producing it is unknown.

Secondary intussusception may occur at any age and appears to be triggered by the development of a growth such as a tumour or polyp projecting into the intestinal lumen. If the wall at that point is weakened, it may buckle when peristalsis forces the growth toward the anus and the segment of the gut immediately anterior to the growth is pulled along and becomes ensheathed by a posterior segment. The blood supply to the engulfed segment may become impaired and result in gangrene and severe constriction or complete blockage of the canal. Treatment is usually surgical removal of the growth and of the affected part of the intestine.