Intussusception: Infolding (prolapse) of a portion of the intestine within another immediately adjacent portion of intestine, which predominantly affects children. Intussusception decreases the supply of blood to the affected part of the intestine and frequently leads to intestinal obstruction. The pressure created by the two walls of the intestine pressing together causes inflammation and swelling, and it reduces the blood flow. Death of bowel tissue can occur, with significant bleeding, perforation, abdominal infection, and shock occurring very rapidly. Most cases of intussusception occur in children between 5 months and 1 year of age. Symptoms begin with sudden, loud crying in an infant, with the baby drawing the knees up to the chest due to abdominal cramping. The pain and crying are intermittent but recur frequently and increase in intensity and duration. Fever is common. As the condition progresses, the infant becomes weak and then shows signs of shock, including pale color, lethargy, and sweating. About half of afflicted infants pass a bloody, mucousy ('currant jelly') stool. The cause of intussusception is not known, although viral infections of the intestine may contribute to intussusception in infancy. In older children or adults, the presence of polyps or a tumor may trigger intussusception. Early diagnosis is very important. On examining the abdomen, a physician may feel a mass. Abdominal X-rays may suggest intestinal obstruction, but a barium enema is needed to show the characteristic telescoping of the bowel. In some cases, the intestinal obstruction can be relieved with a barium or air enema. If the obstruction cannot be reduced by a barium enema, surgery is required for treatment.