[Differential diagnosis of children's enlarged necks--cervical thymic cysts].

We present a cervical thymic cyst in an otherwise healthy 11 year old boy. Ultrasonography revealed a smooth, medium echogenic cystic mass of 62 x 28 x 12 mm with weak acoustic enhancement lying between the carotid artery and the internal jugular vein. The mass extended from the hyoid bone to the inferior pole of the thyroid gland. CT scans also revealed the displacement of the carotid artery and the internal jugular vein by an atypically dense cystic mass. At operation the vagus nerve had to be dissected from the posterior wall of the cyst. A solid band of fibrous tissue extended to the inferior pole of the thyroid gland; there was no connection to the pharyngeal wall. On histological examination Hassal's bodies, cholesterol crystals and a mild round-cell infiltration were observed. The postoperative course was normal. There were no immune defects or signs of lymphogranulomatous disease. Cervical thymic cysts may be recognised before operation and differentiated from second arch cysts by their atypically low infrahyoid position, their intimate relation to the vessels of the neck and their extension into the upper mediastinum. Surgery can be difficult due to the intimate relation to the vessels of the neck and the vagus nerve, and to extension into the upper mediastinum.