A 6-year-old boy is brought to the pediatrician by his mother who noticed a lump in the middle of his neck two weeks ago. The boy has never complained of pain in this area and has not had any fevers. The mother states that the mass seems to become more prominent when the child has a cold. Past medical history is otherwise unremarkable. On physical examination, there is a 2-cm, non-tender, fluctuant, mobile mass located in the midline of the neck, just inferior to the hyoid bone. When the patient is asked to protrude his tongue, the mass is observed to move upward. There is no associated lymphadenopathy, and the thyroid gland is not palpable in its normal anatomical position.
Which of the following is the most likely diagnosis?
The correct answer is:
E) Thyroglossal duct cyst
A thyroglossal duct cyst is the most common congenital neck mass in children. It results from the failure of the thyroglossal duct to involute after the thyroid gland migrates from the foramen cecum at the base of the tongue to its final position in the lower neck. Because the duct is anatomically associated with the hyoid bone and the base of the tongue, the pathognomonic physical exam finding is a midline mass that moves superiorly with tongue protrusion or swallowing.
Answer choice A: Branchial cleft cyst, is incorrect. A branchial cleft cysts is also congenital remnant, but it is almost always located lateral to the midline, typically anterior to the sternocleidomastoid muscle. It does not move with tongue protrusion.
Answer choice B: Dermoid cyst, is incorrect. While a dermoid cyst can present as a midline neck mass, it is typically firm (containing sebaceous material or hair follicles) rather than fluctuant. Crucially, it does not have a functional connection to the hyoid or tongue and therefore do not move with tongue protrusion.
Answer choice C: Ectopic thyroid gland, is incorrect. While an ectopic thyroid can present as a midline mass, it represents the patient's only functioning thyroid tissue in 70% of cases. In this clinical vignette, the movement with the tongue points specifically to a cystic remnant of the duct. However, it is standard practice to perform an ultrasound or thyroid scan before excision to ensure a normal thyroid gland exists elsewhere.
Answer choice D: Laryngocele, is incorrect. A laryngocele is a dilation of the laryngeal saccule filled with air. It typically presents as a lateral neck mass that increases in size with increased endolaryngeal pressure such as blowing a trumpet or performing a Valsalva maneuver.
Key Learning Point
A thyroglossal duct cyst presents as a midline, mobile neck mass that moves upward with tongue protrusion. It is a remnant of the thyroid's descent path. The definitive treatment is surgical excision of the cyst, the entire duct tract, and the central portion of the hyoid bone to minimize the high rate of recurrence.
