Carcinoids are generally slow-growing neuroendocrine tumors found most commonly in the distal small intestine, proximal colon, and lung. Tumors can secrete up to 40 products (eg, serotonin, histamine, vasoactive intestinal peptide) that are usually metabolized by the liver and cause minimal symptoms in many patients. Gastrointestinal carcinoid tumors most commonly metastasize to the liver and prevent this hepatic inactivation, resulting in carcinoid syndrome. In contrast, extraintestinal tumors (eg, bronchial) have direct access to the systemic circulation without immediate hepatic clearance of their products. As a result, these tumors can cause the syndrome without liver metastases.
Carcinoid syndrome most commonly presents with episodic flushing (usually upper body for > 20-30 seconds) and secretory diarrhea. Other common features include cutaneous telangiectasias, bronchospasm, and tricuspid regurgitation. Wide metastases can increase tryptophan conversion to serotonin and its metabolite 5-hydroxyindoleacetic acid (HIAA). This may result in tryptophan and niacin deficiency causing pellagra (diarrhea, dermatitis, and dementia).
Elevated 24-hour urinary 5-HIAA (> 90% sensitive and specific) usually confirms the diagnosis in most patients.
Somatostatin analogs (eg, octreotide) provide symptomatic relief and can also prevent carcinoid crisis, a life-threatening complication due to tumor manipulation (eg, surgery, biopsy) or anesthesia causing release of large amounts of active products.