What is carcinoid syndrome?

Carcinoid syndrome refers to a characteristic set of signs and symptoms including:

• flushing of the skin (90% cases)
• diarrhoea (75% cases)
• wheezing (20% cases)
• problems with the right side of the heart (33% cases) - this includes fibrosis (thickening) of the heart muscle and problems with the heart valves

Other possible features include fibrosis (thickening and scarring) of the pleural, peritoneal and retroperitoneal spaces (potential spaces around the lungs, the guts and behind the gut respectively).

Pellagra, a condition characterised by dermatitis, diarrhoea and depression, may also occur. This is because the carcinoid cells convert the essential amino acid tryptophan into a variety of hormones (see 'What is a carcinoid tumour?'), greatly reducing the conversion of tryptophan into the compound niacin. Niacin is a molecule used in wide variety of metabolic reactions and deficiency in it causes pellagra.

The syndrome is caused by carcinoid tumours (see 'What is a carcinoid tumour?') arising in the liver (normally as secondary lesions) or outside the gut, that secrete chemicals into the bloodstream, producing such symptoms.

Carcinoid tumours affect approximately 2/100,000 of the population. Only 10% of these actually cause carcinoid syndrome. They tend to present from middle age onwards.