Welcome back and thanks for joining me on this discussion of carcinoid syndrome, a favorite of the
USMLE examination under the section of small bowel. Before going further, I like to give you some background
information which are high-yield in nature for carcinoid and in fact, neuroendocrine tumors in general.
First, carcinoid tumors are primitive stem cell derivatives. They usually occur in the small bowel but they
can happen anywhere, appendix, colon, or the lung. They are usually considered indolent or slow
growing tumors. But they can be aggressive and metastatic. Shortly, we’ll discuss what happens when
the carcinoid tumor becomes metastatic. There is some association between the size of the carcinoid
tumor and the rate of metastasis. What are some common symptoms of carcinoid tumors? Well, when
we talk about carcinoid tumor symptoms, it usually means that the carcinoid tumor has become
metastatic. We’ll get to this. Flushing, asthma, abdominal pain, these are all symptoms due to a very
important serotonin secretion of the carcinoid tumor. What are some other findings? Potentially
weight loss, early satiety, chronic anemia that’s not explained. Rarely, if the carcinoid tumor is in the
small bowel, it can lead to small bowel obstruction and be the cause or lead point of an intussusception.
Intussusception is the topic of another lecture. What are some other symptoms? Diarrhea,
bronchospasm, cardiac complications, all very high-yield associations with carcinoid tumor symptoms.
Again, dyspnea on exertion, edema, ascites, plaque like deposits of fibrous tissues, right heart valve
is the most likely affected so new murmurs are potentially heard, and severe tricuspid regurgitation.
Interestingly, patients with carcinoid tumors and carcinoid syndrome can develop niacin deficiency
due to the very important tryptophan metabolism to serotonin. Because tryptophan is a precursor
to serotonin production, it then decreases the substrate for niacin production and vitamin B3 deficiency.
Here’s some schematic showing you where tryptophan is a precursor to many things, proteins, 5-HTP.
5-HTP is a precursor to serotonin. Tryptophan is a precursor to niacin. Back to my point about how
because of the serotonin increased secretion or the need for additional substrate that there is not
enough tryptophan potentially to go down the niacin pathway. Very importantly to point out, you’ll
notice from the slide that serotonin then creates further metabolites including 5-HIAA which is very
important for our diagnosis of carcinoid tumors. This is a high-yield topic in a syndrome of pellagra.
This is a constellation of dermatitis, dementia, diarrhea, and death. Again, dermatitis, dementia,
diarrhea, and death.