Carcinoid syndrome

[2][1] Less common symptoms include malabsorption (leading to pellagra), fatigue, muscle loss, and cognitive impairment.

This is why carcinoid syndrome most often occurs in patients whom the neuroendocrine tumor has metastasized to the liver, which allows the substances to bypass the first pass metabolism.

[1] In the pulmonary neuroendocrine tumors or metastases, histamine release and kallikrein metabolism are the vasoactive mediators of flushing and the other symptoms of carcinoid syndrome.

[5] This mainly affects the right side of the heart unless there is anomalous circulation (i.e. patent foramen ovale) because the lungs will metabolize the substances released by the tumor similar to how the liver will.

[5] After initial tissue injury around the valves, plaque will develop and fibrosis will occur, possibly mediated by excess serotonin.

[5] With a certain degree of clinical suspicion, the most useful initial test is the 24-hour urine levels of 5-HIAA (5-hydroxyindoleacetic acid), the end product of serotonin metabolism.

[11] Nuclear medicine gamma camera imaging that utilizes radioactive somatostatin analogues such as indium-111 pentetreotide are used to localize the tumor.

[1] For patients with serotonin elevated 5x the upper limit of normal or more, an echocardiogram is recommended for evaluation of carcinoid heart disease.

[12] Peptide directed radiotherapy (PRRT) is another alternative treatment for patients who failed somatostain analogue therapy.

[12][2][7] The most important aspect of treating carcinoid heart disease is detecting its presence with echocardiography, likely with color doppler.

[14] Similarly to humans, the carcinoid syndrome is due to neuroendocrine tumors that arise mainly from the bowel but also from other organs.