Carcinoid syndrome pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Carcinoid syndrome refers to the array of symptoms that occur secondary to carcinoid tumors. Carcinoid tumors are discrete, yellow, well-circumscribed tumors that can occur anywhere along the gastrointestinal tract (GI). They most commonly affect the appendix, ileum, and rectum. These tumors are unique in that they are endocrine in nature. They secrete hormones into the blood stream, which then travel to end organs and act upon them via appropriate receptors.
Pathophysiology
Carcinoid tumors produce the vasoactive substance, serotonin; it is commonly but incorrectly thought that serotonin is the cause of the flushing. The flushing results from secretion of kallikrein, the enzyme that catalyzes the conversion of kininogen to lysyl-bradykinin. The latter is further converted to bradykinin, one of the most powerful vasodilators known. Other components of the carcinoid syndrome are diarrhea (probably caused by serotonin), a pellagra-like syndrome (probably caused by diversion of large amounts of tryptophan from synthesis of the vitamin, niacin, to the synthesis of 5-hydroxyindoles including serotonin), fibrotic lesions of the endocardium, particularly on the right side of the heart resulting in insufficiency of the tricuspid valve and, less frequently, the pulmonary valve and, uncommonly, bronchoconstriction. The pathogenesis of the cardiac lesions and the bronchoconstriction is unknown. When the primary tumor is in the gastrointestinal tract, as it is in the great majority of cases, the serotonin and kallikrein are inactivated in the liver; manifestations of carcinoid syndrome do not occur until there are metastases to the liver. Carcinoid tumors arising in the bronchi, because their biologically active products reach the systemic circulation before passing through the liver, may be associated with manifestations of carcinoid syndrome without liver metastases.
Location
Carcinoid tumours are the most common malignant tumour of the appendix, but they are most commonly associated with the small intestine, and they can also be found in the rectum and stomach. They are known to grow in the liver, but this finding is usually a manifestation of metastatic disease from a primary carcinoid occurring elsewhere in the body. They have a very slow growth rate compared to most malignant tumours.