What are carcinoid tumors?
Carcinoid is a slow-growing tumor arising from enterochromaffin cells identified by silver staining of cytoplasmic granules within the cells. These cells release vasoactive substances, which have amine precursors. The substances include serotonin, prostaglandin, histamine, and kallikreinins. Histologically, carcinoid tumors that arise from the ileum demonstrate dense nests of cells with uniform size and nuclear appearance. Histochemically, they typically exhibit an argentaffin reaction in which the cells convert silver salts to a metallic silver color. A positive argentaffin reaction is not required for diagnosis. Tumors arising from the embryonic foregut usually contain few argentaffin cells.
Bronchogenic carcinoid can range from having typical carcinoid features to being indistinguishable from oat cell carcinoma of the lung. Tumors arising from the ileum or jejunum produce more common manifestations of carcinoid syndrome, such as cutaneous flushing, intestinal hypermobility, and hypotension caused by serotonin release. Tumors arising from the stomach usually tend to produce histamine, causing hypotension and less commonly wheezing. Carcinoid syndrome was originally thought to be due to serotonin and histamine only. These agents predominate in the syndrome; however, more recently other hormone and vasoactive polypep-tides have been identified. The mediators include prostaglandins, bradykinins, tachykinins, adrenocorti-cotropic hormones, and vasoactive intestinal peptide (VIP). Serotonin is produced from tryptophan by a hydroxylation and decarboxylation reaction. It is broken down ultimately to 5-HIAA by the enzymes monoamine oxidase and alcohol dehydrogenase (Figure 41.1). The measurement of 5-HIAA in the urine is used to monitor the disease process clinically.
Histamine release is seen more commonly with foregut carcinoid tumors and is thought to cause bronchospasm associated with flushing. Serotonin causes vasodilation and vasoconstriction; therefore, both hypotension and hyperten-sion can be seen. Serotonin has no effect on cardiac function itself at normal levels; however, at elevated levels it can cause positive chronotropic and inotropic responses. Other effects due to high serotonin levels are increased gut motility, and secretion of sodium chloride (NaCl), potassium (K+), and water by the small intestine. Hyperglycemia can also occur from elevated adrenergic levels as well. Patients may exhibit prolonged effects from anesthesia with excessive postopera-tive drowsiness. Bradykinins can cause severe hypotension secondary to extreme vasomotor relaxation or vasodilation. Flushing can occur due to enhanced nitric oxide synthesis. Bronchospasm can follow, especially in patients with cardiac disease or asthmatics.