Discussion: Thyroglossal duct cysts are usually found in children but are not uncommon in adults. They usually present in the second decade of life with a noticeable midline mass. The thyroglossal duct cyst is the most common cystic embryologic remnant in the head and neck.[1] The location of the thyroglossal duct cyst in the floor of the mouth is a result of the embryology of the normal thyroid. The thyroid begins as the thyroid primordium which forms from an epithelial lined tubular structure, the thyroglossal duct at about three weeks of gestation. The duct elongates from the foramen cecum which is located behind the V-shaped row of circumvallate papillae at the base of the tongue. The duct penetrates the underlying mesoderm and descends as a diverticulum through the base of the tongue and mylohyoid muscle into the floor of the mouth. A posterior and superior extension occurs in the region of the hyoid bone before descent is resumed, and the thyroid gland duct reaches its normal final location at the base of the neck. By seven weeks of gestation the thyroid gland has traversed the length of the duct and has reached its final position. The migration of the thyroid gland can be arrested anywhere along the course of the thyroglossal duct.[2] If the duct fails completely to migrate the thyroid gland will develop within the floor of the mouth (lingual thyroid). When the thyroid gland is located in its normal position in the lower neck, fragments of thyroid tissue may still be found anywhere along its course.[3] Because the duct is lined with secretory epithelium a cyst may form if any portion of the thyroglossal duct fails to involute. Twenty percent of thyroglossal duct remnants occur in the suprahyoid region, 15% are at the level of the hyoid bone itself as in the case presented here (Fig. 2) and may be insinuated into the osseous structure, and 65% of thyroglossal duct remnants occur in the infrahyoid region.[1]
Most patients present with asymptomatic neck masses within the anterior triangle. About 75% of thyroglossal duct cysts occur in the midline. A carcinoma coexists in the thyroglossal duct cyst in fewer than 1% of cases but this is invariably carcinoma of thyroid gland origin. Papillary carcinoma accounts for 85% of carcinomas which arise from the thyroglossal duct.[3] Usually these are discovered as incidental findings at the time of surgery for the cyst. The reported incidence of ectopic thyroid tissue along the thyroglossal duct varies widely between less than 1% to more than 35%.
Major different diagnostic considerations for thyroglossal duct cyst include obstructed laryngoceles and branchial cleft cysts.[4] Laryngoceles, since they arise within the larynx can be identified as there is some connection back to the larynx itself whereas thyroglossal duct cysts are usually totally extralaryngeal. Since both of these lesions may occur deep to the strap muscles, some confusion may exist. Branchial cleft cysts occur along the anterior border of the sternocleidomastoid muscle and should never be confused with thyroglossal duct cysts which are much closer to the midline and associated with midline structures as described above.
References:
1. Reede DL. CT of thyroglossal duct cyst. Radiology 1985; 157:123-127.
2. Frisa J, Bellin M. CT mediastinum examination in recurrent nerve paralysis. JCAT
1984;8:901-911.
3. Noyek AM, Friedberg S. Thyroglossal duct and ectopic thyroid disorders. Oto Clin
of North America 1981;14:187-212.
4. Sironi M, Assi A, Andruccioli M. Identification of ectopic thyroid tissue. Clin Nuc
Med 1996;21:585-593.
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