Although these tumours occur more commonly in the stomach, they can occur in the duodenum, small intestine, colon and rectum
They arise from pacemaker cells of Cajal in the intestinal wall.
These are the most common tumours affecting the small intestine. Historically, they are known as "Carcinoid Tumours", but we are trying to move away from this term.
Most commonly they occur in the ileum (distal half of the small intestine) or appendix, but can occur in jejunum (proximal small intestine), colon and rectum.
They may present with small bowel obstruction. Usually when this happens, the obstruction is caused, not so much by the bulk of the tumour itself, but more by a dense fibrotic (desmoplastic) response that is evoked around a small tumour. This response is probably due to secretion of serotonin by the tumour.
Appendiceal "carcinoids" are often found incidentally by the pathologist examining an appendicectomy specimen after the patient presents with symptoms of appendicitis.
Often they are only diagnosed after the tumour has metastasized to lymph nodes and the liver. In such cases the metastases are often multiple and bulky, while the small intestinal primary is small and easily overlooked. There can also be multiple small bowel tumours.
Secretion of serotonin by intestinal neuroendocrine tumours (iNET) results in a constellation of symptoms (diarrhoea, facial flushing and bronchospasm (asthma)) known as "carcinoid syndrome". Another consequence of the excessive secretion of serotonin by iNET hepatic metastases is "carcinoid heart disease" in which the serotonin provokes fibrosis under the endothelium (inner lining membrane) of the right-sided chambers of the heart, which leads to damage to the tricuspid and pulmonary valves of the heart, making the valves incompetent and causing right-sided heart failure.
The other important functioning neuroendocrine tumour is the gastrinoma, which occur most commonly in the duodenum, the first part of the small intestine just beyond the stomach and intimately related to the pancreas. Gastrinomas can also arise from the pancreas. They secrete large volumes of the hormone, gastrin. Gastrin normally stimulates the acid-producing parietal cells of the stomach. When secreted in excess and uncontrolled by tumours the excessive acid secretion results in ulceration of stomach and duodenum, severe acid reflux disease and profuse diarrhoea. This constellation of symptoms is known as Zollinger-Ellison Syndrome. These tumours can also be multiple, are also usually very small and, when they metastasize, the metastases are usually much more bulky than the primary tumour.