Multiple endocrine neoplasia type 2B--genetic basis and clinical expression.

Multiple endocrine neoplasia (MEN) type 2B is a heritable endocrine disorder characterized by medullary thyroid carcinoma (MTC), pheochromocytoma, multiple mucosal neuromas, and a marfanoid habitus. Intestinal ganglioneuromatosis, corneal nerve thickening and skeletal abnormalities are also often present. The disease is inherited in an autosomal dominant fashion and is caused by a single mutation in the RET proto-oncogene, with a methionine to threonine substitution at codon 918. The MTC in MEN 2B presents at an earlier age and tends to be more aggressive than the MTC in MEN 2A. It is multicentric and bilateral and occurs as young as age 3, with early lymph node metastases. Pheochromocytoma is also often bilateral but is rarely malignant. If pheochromocytoma is detected, adrenalectomy should precede thyroidectomy to avoid intraoperative catecholamine crisis. Patients at risk for MEN 2B should undergo genetic screening in infancy. Total thyroidectomy should be performed on all patients positive for RET mutations even prior to the onset of clinical symptoms.