Ruptured spleen: Rupture of the capsule of the spleen that is a potential catastrophe and requires immediate medical and surgical attention. Splenic rupture permits large amounts of blood to leak into the abdominal cavity, and it is severely painful and life threatening. Shock, and ultimately death, can result. Patients typically require immediate surgery. Rupture of a normal spleen can be caused by trauma, such as an accident. If an individual's spleen is enlarged, as is frequent in mononucleosis, most physicians will not allow participation in major contact sports or other activities because injury to the abdomen could be catastrophic.