At least 13 types of neuroendocrine cells exist in the gastrointestinal (GI) tract, all of which secrete bioactive peptides or amines.2
Neuroendocrine tumors (NETs) can arise from these cells in the small intestine and proximal colon and exhibit secretory activity.3
NETs arising in the distal colon and rectum, conversely, are usually nonfunctional and cause symptoms owing to increasing tumor mass.2-4
All of these GI NETs can produce nonspecific symptoms that are uncomfortable for patients and challenging for physicians.3
Carcinoid, or functional, syndrome occurs most frequently when certain types of gastrointestinal (GI) NETs metastasize to the liver, which allows secreted serotonin and other vasoactive substances to bypass hepatic metabolism and directly enter systemic circulation.2,5 Other substances include corticotropin, histamine, dopamine, substance P, neurotensin, prostaglandins, kallikrein, and tachykinins.4
Oversecretion of these substances can cause the cluster of symptoms known as carcinoid syndrome. These include flushing, diarrhea, cramping, and wheezing.6 Additionally, cardiac disease is one of the most serious aspects of carcinoid syndrome, occurring in approximately 2/3 of patients.7,8
Because these symptoms are nonspecific they are often attributed to other more common conditions, such as irritable bowel syndrome. However, by distinguishing certain clues, physicians can make a differential diagnosis to rule out or preliminarily identify a GI NET as a possible cause.3,9
GI NETs may be associated with other symptoms and syndromes, depending on the tumor’s site of origin. Click on the figure to learn more.