Carcinoid syndrome is a paraneoplastic syndrome comprising the signs and symptoms that occur secondary to neuroendocrine tumors (formerly known as carcinoid tumors). The syndrome is caused by neuroendocrine tumors most often found in the gut releasing biologically active substances into the blood causing symptoms such as flushing and diarrhea, and less frequently, heart failure, vomiting and bronchoconstriction. Signs and symptoms The carcinoid syndrome occurs in approximately 10% of all neuroendocrine tumors or about 30-40% of more advanced/well developed neuroendocrine tumors. The biologically active substances that are released by the tumors cause the symptoms of the carcinoid syndrome. These substances act on the vessels to produce the symptoms of the carcinoid syndrome. Flushing: The most common finding is flushing of the skin, usually of the head and the upper part of thorax in about 85% of people. The flushing may come and go and may also be triggered by various factors such as diet (i.e. alcohol intake), activity, and stress. Diarrhea: The second most common finding occurring in about 80% of people. It may also be associated with abdominal cramping and pain. Bronchoconstriction: A relatively rare symptom affects about 15% of those having carcinoid syndrome and often accompanies flushing, sneezing, and shortness of breath. Heart Disease: About 60-70% of the those affected by carcinoid syndrome develop cardiac complications. This mainly affects the right side of the heart causing fibrosis of the tricuspid and pulmonic valves. This may be heard as a murmur and may contribute to fatigue. Less common symptoms include malabsorption (leading to pellagra), fatigue, muscle loss, and cognitive impairment. Late complications may include mesenteric and retroperitoneal fibroses as well. Pathophysiology The carcinoid syndrome occurs secondary to neuroendocrine tumors. These tumors occur mostly in the gut and less commonly in the lungs, but may also occur in other places in the body such as the pancreas, kidneys, and other organs. Neuroendocrine tumors produce several biologically active substances, mainly amines and peptides. There are over 40 substances known to be secreted by these tumors but the exact effect of each and their contribution to the carcinoid syndrome is unknown. The most common substances found to be released and contribute to the syndrome include serotonin, histamine, tachykinins, kallikrein, and prostaglandins with the greatest contribution appearing to be from serotonin. The symptoms of the carcinoid syndrome result from the action of these substances largely on the blood vessels. These biologic substances are often