Answer: Thyroglossal duct cyst

Histology: The lesion is a cyst lined by pseudostratified ciliated epithelium associated with lymphoid tissue and acute and chronic inflammation. The cyst is in close proximity to the hyoid bone.

Discussion: Thyroglossal duct cysts represent epithelial remnants of the thyroglossal duct. They are the most common congenital cysts in the neck. The cyst may occur anywhere along the thyroglossal duct tract and therefore may be found in tongue or above or below the hyoid. In the majority of patients, the cyst is at or just below the hyoid bone adjacent to the thyrohyoid membrane, usually within two cm of the midline. The typical cyst is found in close association with the hyoid bone, thyrohyoid membrane or thyroid cartilage. The cyst is lined by pseudostratified ciliated or squamous epithelium. Normal thyroid tissue is usually seen in the wall of the cyst, but is not required for the diagnosis.

Resection of the cyst and the midportion of the hyoid bone in continuity and resection of a core of tissue from the hyoid upwards towards the foramen caecum is the standard operation (Sistrunk procedure).

The differential diagnosis includes other cysts that may occur in the neck, including dermoid cysts, sebaceous cysts, medially placed branchial cleft cysts. Dermoid cysts are the second most common to occur in the neck. Dermoid and sebaceous cysts tend to be superficial and less tethered to the underlying structures, including the hyoid bone, in contrast to thyroglossal duct cysts. Branchial cleft cysts are usually more laterally located, but can be confused with thyroglossal duct cysts if they are somewhat medially located. In contrast to thyroglossal duct cysts, branchial cleft cysts are often associated with a sinus tract or fistula.

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