Carcinoid syndrome pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Parminder Dhingra, M.D. [2]
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.
Pathogenesis
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 and manifestations of carcinoid syndrome do not occur until there are metastases to the liver. 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. Large amounts of tryptophan from synthesis of the vitamin, niacin, to the synthesis of 5-hydroxyindoles including serotonin produces pellagra like features including diarrhea. Carcinoid tumors arising in the bronchi, reach the systemic circulation before passing through the liver, may be associated with bronchoconstriction and manifestations of carcinoid syndrome without liver metastases. They have a very slow growth rate compared to most malignant tumors.
Embryology
Carcinoid tumours originate from neuroendocrine cells (Enterochromaffin or amine precursor uptake and decarboxylase [APUD] cells), which embryologically are of neural crest origin. Gastrointestinal carcinoids derive from cells that migrate from the neural crest to the foregut, midgut and hindgut.
Location
Carcinoid tumors are normally found throughout the gastrointestinal tract from mouth to anus, with the highest concentration of cells in the appendix and small intestine. The pancreas contains a large number of these cells, the biliary tree only a few and the liver normally contains none. Fibrotic lesions are found on endocardium, particularly on the right side of the heart.
Associated Conditions
Goblet Cell Carcinoid: This is considered to be a hybrid between an exocrine and endocrine tumor derived from crypt cells of the appendix. They behave in a more aggressive manner than do classical appendiceal carcinoids. Spread is usually to regional lymph nodes, peritoneum, and particularly the ovary. They do not produce sufficient hormonal substances to cause the carcinoid or other endocrine syndromes. In fact, they more closely resemble exocrine than endocrine tumors. The term 'crypt cell carcinoma' has been used for them, and though perhaps more accurate than considering them carcinoids, has not been a successful competitor.
Gross Pathology
In the gastric or intestinal wall, carcinoids may occur as firm white, yellow, or gray nodules and may be intramural masses or may protrude into the lumen as polypoid nodules; the overlying gastric or intestinal mucosa may be intact or have focal ulceration.
Microscopic Pathology
Goblet Cell Carcinoid
Histologically, globet cell carcinoid forms clusters of goblet cells containing mucin with a minor admixture of paneth cells and endocrine cells. The growth pattern is distinctive: typically producing a concentric band of tumour nests interspersed among the muscle and stroma of the appendiceal wall extending up the shaft of the appendix. This makes the lesion difficult to suspect grossly and difficult to measure. Small tumor nests may be camouflaged amongst the muscle or in periappendiceal fat; cytokeratin preparations best demonstrate the tumor cells; mucin stains are also helpful in identifying them.
Gastric or Intestinal Carcinoid
Neuroendocrine cells have uniform nuclei and abundant granular or faintly staining (clear) cytoplasm, and are present as solid or small trabecular clusters, or are dispersed among other cells, which may make them difficult to recognize in sections stained with hematoxylin and eosin; immunostaining enables their exact identification. At the ultrastructural level, neuroendocrine cells contain cytoplasmic membrane-bound dense-cored secretory granules (diameter >80 nm) and may also contain small clear vesicles (diameter 40–80 nm) that correspond to the synaptic vesicles of neurons.
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