Thyroglossal duct cyst (TGDC) is the most common congenital cervical anomaly and the second most common neck mass in children (after lymphadenopathy). It accounts for approximately 70% of all midline neck masses and 7% of all neck masses in children. Embryology: during the 3rd-7th week of gestation, the thyroid gland develops from the foramen cecum at the base of the tongue and descends through the developing tongue and neck musculature, passing in front of, through, or behind the developing hyoid bone, finally reaching its adult location anterior to the trachea by 7 weeks. The thyroglossal duct, which guides this descent, normally involutes by week 8-10. Failure of complete obliteration leaves a remnant tract that can develop into a cyst at any point along this path. Most cysts (65-80%) lie in the infrahyoid region, 15-20% in the suprahyoid region, and 1-2% intralingual.
Clinical presentation: TGDC most commonly presents in childhood (50% before age 10, 70% before age 30), but can present at any age. Classic presentation is a painless, smooth, mobile, midline (or near-midline, more commonly slightly to the left) neck mass that elevates with tongue protrusion or swallowing (due to attachment to the hyoid bone and tract to the tongue base). Size typically 1-3 cm, but can range from a few millimeters to several centimeters. The cyst may be soft and fluctuant or firm. Skin overlying the cyst is normal unless infected. Approximately 10-15% present after acute infection (sudden onset of pain, redness, swelling, fever) due to bacterial superinfection of cyst fluid (most often Staphylococcus aureus or oral anaerobes), and may rupture spontaneously forming a draining sinus or fistula. Differential diagnosis includes dermoid cyst (more superficial, doesn't elevate with tongue protrusion), ectopic thyroid (firm, may be only functioning thyroid tissue), lymphadenopathy, branchial cleft cyst (typically lateral), lipoma, and cervical thymic cyst.
Workup: Diagnosis is primarily clinical. Ultrasound is the imaging modality of choice in children — confirms cystic nature (anechoic with posterior acoustic enhancement, may have internal echoes if infected or hemorrhagic), demonstrates relationship to hyoid bone, and CRITICALLY confirms presence of normal orthotopic thyroid gland (essential to avoid inadvertent removal of the only functioning thyroid tissue, which would result in lifelong hypothyroidism). If ultrasound is non-diagnostic or thyroid is not visualized, thyroid scintigraphy with Tc-99m or I-123 confirms thyroid location. CT or MRI is reserved for atypical cases, suspected malignancy, or surgical planning of complex anatomy. Fine needle aspiration is generally not recommended (low yield, may seed tract, complicates surgery), but may be considered in adults to rule out malignancy. Treatment is the Sistrunk procedure (described by Walter Sistrunk in 1920): en bloc excision of the cyst, the entire tract from cyst to foramen cecum, the central one-third of the hyoid bone (where the tract typically passes through), and a small core of tongue musculature extending to the foramen cecum. This radical en bloc resection reduces recurrence rate from 50% (with simple cystectomy) to less than 5%. Pre-operative antibiotics if infected. Recurrence risk factors: prior incision and drainage, multiple recurrences, inadequate hyoid bone resection, missed branches of the tract. Malignancy occurs in less than 1% of TGDC, almost always papillary thyroid carcinoma (occasionally squamous cell), typically in adults; treated with Sistrunk procedure plus consideration of total thyroidectomy and adjuvant radioiodine if high-risk features.