Carcinoid syndrome

• usually occurs when metastases are present in the liver and release serotonin into the systemic circulation
• may also occur with lung carcinoid as mediators are not 'cleared' by the liver

Carcinoid Syndrome

Carcinoid cells cause increased production of serotonin from tryptophan (required for niacin synthesis) resulting in niacin deficiency (ie, pellagra, with dermatitis, diarrhea, and dementia).

Localized tumors confined to intestine do not cause carcinoid syndrome as their secretory products are metabolized by the liver before entering the systemic circulation.

In contrast, intestinal carcinoids that metastasize to the liver and extraintestinal (eg, bronchial) carcinoids release vasoactive substances that avoid first-pass metabolism, resulting in carcinoid syndrome (eg, flushing, diarrhea, bronchospasm)

Features

• flushing (often the earliest symptom)
• diarrhoea
• bronchospasm
• hypotension
• right heart valvular stenosis (left heart can be affected in bronchial carcinoid)
• other molecules such as ACTH and GHRH may also be secreted resulting in, for example, Cushing's syndrome
• pellagra (niacin deficiency) can develop as dietary tryptophan is diverted to serotonin by the tumour

Investigation

• 24-h urinary 5-HIAA
• plasma chromogranin A y