Thyroglossal duct cyst and fistula are congenital developmental abnormalities caused by incomplete degeneration of the thyroglossal duct. A fistula forms when the cyst becomes infected and ruptures or is surgically incised. Most cases appear before the age of 5, with males slightly outnumbering females. Approximately 40% of cases are complicated by infection.

bubble_chart Pathological Changes

On the 3rd a cycle of day and night of the embryo, a diverticulum-like thyroid primordium forms in the midline between the first and second pairs of pharyngeal pouches at the base of the primitive pharynx. This primordium migrates downward along the midline in front of the larynx to the neck, forming a slender duct known as the thyroglossal duct. The hyoid bone develops from both sides toward the midline, enclosing the duct either anteriorly or posteriorly, with its lower end forming the thyroid gland. By the 5th a cycle of day and night, the thyroglossal duct degenerates into a solid fibrous cord, leaving a remnant at the oral end as the foramen cecum at the base of the tongue. If the epithelial cells within the duct fail to degenerate during development, a thyroglossal cyst can form at any point along the midline from the foramen cecum to the suprasternal notch.

The inner wall of a thyroglossal cyst is lined with stratified squamous or columnar epithelial cells, and the cyst or fistula wall is entirely composed of connective tissue without lymphoid tissue. The cyst contains a pale yellow mucoid fluid.

bubble_chart Clinical Manifestations

At the midline of the neck, corresponding to the thyroid cartilage below the hyoid bone, a round mass with a diameter of 1–2 cm can be observed. It has a smooth surface, clearly defined edges, and a cystic nature that feels solid due to tension from filling. The mass is relatively fixed and cannot be moved up, down, left, or right, but it may shift slightly during swallowing or tongue protrusion. In smaller cysts, a cord-like band connecting to the hyoid bone can be palpated. If not infected, it does not adhere to the skin and is neither tender nor painful. If it ruptures spontaneously or is incised and drained, a thyroglossal duct fistula forms, frequently discharging clear or turbid mucus. Over time, the fistula may temporarily close and scab over, but it often reopens and discharges fluid again, recurring persistently without healing. Deep within the fistula, a cord-like tissue extending upward toward the hyoid bone can be palpated.

1. It is commonly seen in children and young adults. There is a round mass below the hyoid bone in the anterior neck, with a smooth surface, clear boundaries, cystic consistency, no adhesion to the skin, and it moves up and down with swallowing. A cord-like structure can be palpated along the direction of the hyoid bone, and the mass retracts and elevates when opening the mouth and extending the tongue.

2. When the cyst becomes secondarily infected, there is local redness, swelling, and tenderness. If it ruptures spontaneously or is incised and drained, it may form a persistent fistula.

3. The mucoid secretion often contains columnar and squamous epithelial cells.

bubble_chart Treatment Measures

Opinions differ as to whether small cysts require excision, but considering the complexity of surgery after infection and the increased recurrence rate, early surgery is advisable once diagnosed. The surgeon must be thoroughly familiar with the following anatomical features: ① the fistula is tightly adherent to and traverses the hyoid bone; ② the portion of the fistula behind the hyoid bone is extremely fine and fragile; ③ the fistula may have diverticulum-like protrusions or branches. The key to surgery is to excise a portion of the hyoid bone and completely remove the cyst and fistula to prevent postoperative recurrence. The recurrence rate is approximately 4–5%, all due to incomplete excision. For infected cases, incision and drainage should be performed first, followed by antibiotic therapy, with radical surgery undertaken after the inflammation subsides.

bubble_chart Differentiation

10-20% of cysts are located above the hyoid bone and should be differentiated from submental lymphadenitis and dermoid cysts, which commonly occur in this area. Cysts located between the sternum and the thyroid should be distinguished from tracheogenic cysts, dermoid cysts, thyroid cysts, softened subcutaneous nodal lymph nodes, and ectopic salivary gland cysts. Special emphasis should be placed on identifying ectopic thyroid tissue, as literature reports cases of hypothyroidism following its mistaken removal, since 70% of such cases lack normal thyroid tissue. Therefore, thyroid scanning and functional tests should be performed when necessary. Cysts slightly deviating from the midline should be differentiated from branchial cleft cysts.