Definition of Carcinoid syndrome:

Carcinoid syndrome is a group of symptoms associated with carcinoid tumors -- tumors of the small intestine, colon, appendix, and bronchial tubes in the lungs.

Causes, incidence, and risk factors:

Carcinoid syndrome is the pattern of symptoms sometimes seen in people with carcinoid tumors. These tumors are rare, and often slow growing. About 70% of carcinoid tumors are found in the gastrointestinal tract.

Carcinoid syndrome occurs in about 1 in 10 people with carcinoid tumors, usually after the tumor has spread to the liver or lung.

These tumors release too much of the hormone serotonin, as well as several other chemicals that cause the blood vessels to open (dilate).