Late-Onset Gastrointestinal Manifestations of Multiple Endocrine Neoplasia Type 2B (MEN2B): Diffuse Ganglioneuromatosis Causing Megacolon.

Multiple endocrine neoplasia type 2B (MEN2B) is a rare autosomal dominant disorder caused by RET proto-oncogene mutations, classically associated with medullary thyroid carcinoma (MTC), pheochromocytoma, and gastrointestinal ganglioneuromatosis. Gastrointestinal symptoms, including constipation and megacolon, typically present in infancy or childhood; late-onset presentation is rare. We report a 66-year-old woman with MEN2B, previously treated with total thyroidectomy for MTC and bilateral adrenalectomy for pheochromocytoma, who presented with absolute constipation, abdominal distension, nausea, and vomiting. Imaging demonstrated marked colonic dilatation without obstruction. She underwent emergency total colectomy with end ileostomy, and histopathology confirmed diffuse ganglioneuromatosis. This case represents one of the oldest reported presentations of megacolon in MEN2B and highlights the importance of recognising gastrointestinal features in all age groups to allow timely surgical intervention and optimise patient outcomes.