- Carcinoids are hormone-secreting tumors which likely originate from neuroendocrine cells present diffusely throughout the GI and respiratory system. Although they can occur anywhere within the alimentary tract, they most commonly arise in the small or large intestine, appendix, and rectum. Within the respiratory system they most commonly arise in bronchi.
- Carcinoids generally grow as sheets of monotonously similar, small, round cells. Electron microscopy reveals numerous secretory granules filled with the tumor's bioactive hormone or hormones. Although slow-growing, carcinoids are malignant and can metastasize to distant organs.
- The primary clinical consequences of carcinoids arise due to synthesis and secretion of bioactive hormones. In total, carcinoids have been observed secreting a variety of hormones and sometimes more than one simultaneously; however, in most cases tumors are noted to secrete high levels of serotonin and additionally some histamine. The clinical consequences of carcinoid, known as "Carcinoid Syndrome" discussed below are thought to be a direct result of serotonin and histamine release.
- Carcinoid Syndrome
- Carcinoid Syndrome is characterized by episodic flushing of the skin as well as diarrhea. Skin flushing is thought to be a result of histamine release and is often associated with pruritis. Diarrhea appears to be due to high plasma serotonin levels which enhance GI motility and small intestine secretion, resulting in a combined dysmotility diarrhea and secretory diarrhea. A minority of patients also develop valvular heart disease which is characterized by fibrosis of the heart valves, especially the tricuspid valve. Although the pathogenesis of cardiac disease is not well-understood it can lead to the cardiovascular complications.