Thyroglossal cyst-CT

Case description- Thyroglossal duct/fistula.

History – 24year male with swelling in neck since long duration.

Long well defined regular fluid sigal intensity track in the anterior soft tissue in midline & to the left (less than 2 cm from midline from foramen cecum to the thyroid isthmus with supra & infra hyoid components with no bleed /septae /secondary tracks / intra laryngeal component /significant lymphadenopathy-suggesting thyroglossal duct cyst /fistula.

DD- unusual presentation of branchial cleft cyst/ delphian adenopathy /epidermoid cyst.

Teaching Points by – Dr MGK Murthy & Dr GA Prasad

Technician: Mr Venkat

1. Embryology- Thyroid follicular cells originate as the median thyroid anlage, an endodermal thickening between the first and second pharyngeal arches . At day 24, this tissue forms a small pit, the thyroid bud which elongates into a bilobate diverticulum and descends caudally. A small channel, the thyroglossal duct, temporarily persists as a connection between the tongue and the caudal migration of the thyroid primordium and then involutes between the 8th and 10th weeks of gestation. The inferior portion of the thyroglossal duct differentiates into the pyramidal lobe of the thyroid gland.
2. The most common congenital anomaly related to the thyroglossal duct is the throglossal duct cyst(TDC). It is thought to represent segments of the duct that fail to regress and consequently differentiate into epithelial-lined cysts. TDCs develop anywhere along the course of the duct remnant, from the base of the tongue to the suprasternal region. Cysts located near the foramen caecum are lined by stratified squamous epithelium, whereas cysts located near the thyroid gland are lined by cells similar to thyroidal acinar epithelium. 
3. Functional thyroid tissue within the TDC has been described  and more than half contain normal thyroid tissue in their walls .
4. TDCs can be  off midline. Off-midline cysts are characteristically located adjacent to the outer surface of the thyroid cartilage, deep to the strap muscles. TDCs are located in the region of the hyoid bone. About 20% to 25% are suprahyoid, 15% to 50% occurring at the level of the hyoid bone, where they may be anterior or posterior to the hyoid bone, and 25% to 65% occurring in the infrahyoid part of the neck
5. The presence of a solid component should alert to the possibility of a TDC carcinoma, as malignant degeneration of the epithelial lining of a TDC (usually into a papillary carcinoma) has been reported as a rare complication.
6. Irrespective of the site or the size and appearance of a TDC, a Sistrunk procedure is the recommended procedure of choice . This entails resection of the cyst and any remaining tract, and excision of the middle third of the hyoid bone. Incomplete resection invariably results in recurrence.
7. The differential diagnosis of TDC in adults includes dermoid cyst, branchial cleft cyst, lymphadenopathy, and a cystic nodule arising from the thyroid gland.