The carcinoid syndrome is due to secretion of systemically active substances by typically well differentiated neuroendocrine tumours. It is most commonly seen in patients with advanced mid-gut carcinoid tumours.
Whether carcinoid syndrome is a true paraneoplastic phenomenon (given that the malignant cells may normally produce these hormones) is somewhat debatable. I couldn't find anywhere better to put it.
Symptoms and Signs
The most common symptom is cutaneous flushing (90% +) with red or violet discolouration. Watery diarrhoea is also very common and impacts of quality of life. Less common findings are bronchospasm and heart murmurs.
These symptoms are due to production of systemically active substances in carcinoid tumours. These substances are degraded in the liver and therefore gastrointestinal carcinoids are less likely to give rise to carcinoid early in the clinical course. The exact substances that are responsible for individual symptoms is debatable.
- Tryptophan is metabolised to serotonin in vastly increased amounts in most carcinoid tumours. Serotonin is a pro-motility agent which would explain the copious diarrhoea. It does not cause flushing directly but may promote this as a secondary effect.
- Histamine is produced by many carcinoid tumours. Histamine causes vasodilatation and flushing.
Other products may also be involved.
The diagnosis is established by measurement of urinary or serum serotonin or its products. Chromogranin is often elevated in the blood and urine of patients with neuroendocrine malignancies.
Localisation of the tumour is best accomplished by CT scan of the chest and abdomen and targeted endoscopy (or small bowel pill cam).
Carcinoid syndrome is most commonly associated with tumours of the mid-gut, but only once liver metastases have developed. It is less commonly seen with bronchial or rectal carcinoid tumours despite the lack of liver inactivation of metabolites.
Removal of a non-metastatic carcinoid should resolve the syndrome, but the majority of carcinoid syndrome occurs in already-metastatic lesions within the liver. Liver resection of oligometastases is also a potential method of resolving the syndrome.
Somatostatin analogues (octreotide) bind to the somatostatin receptor which is present on the majority of carcinoid tumour cells, and reduce their release of hormonal agents.
Symptomatic treatment (eg. loperamide for diarrhoea) is also possible for refractory cases.