Thyroglossal duct cysts, typically seen in paediatric populations, are the most common congenital anomaly in the neck, originating from remnants of the thyroglossal duct, histologically presenting a pseudostratified columnar epithelium, and appear on ultrasound as well-defined anechoic or hypoechoic lesions in the midline of the neck.
Description
Thyroglossal duct cysts (TDCs) are benign congenital anomalies that occur due to incomplete involution of the thyroglossal duct. They are the most common congenital anomaly in the neck and most often present in children, although they can be detected at any age. The thyroglossal duct is a structure present during embryological development that guides the downward migration of the thyroid gland from the base of the tongue to its final pre-tracheal position.
Pathogenesis
During embryological development, the thyroid gland begins at the base of the tongue and migrates downwards in the neck through a tract called the thyroglossal duct. By the 10th week of gestation, the thyroid reaches its final position in the neck, and the thyroglossal duct normally involutes and disappears. When sections of the thyroglossal duct fail to involute, they may form cysts, known as thyroglossal duct cysts.
Subtypes
The location of TDCs along the tract of the thyroglossal duct can vary:
- Sublingual: Least common (<1%).
- Suprahyoid: Most common (60-65%).
- Infrahyoid: Less common (20-25%).
- Intrathyroidal: Rare.
Epidemiology, Risk Factors & Associations
- Predominantly occur in children and young adults, but can present at any age.
- No significant gender preference.
Clinical Features
- Painless, midline neck mass that moves on swallowing or protrusion of the tongue (due to its connection with the base of the tongue).
- The mass may become infected and present with pain, redness, and fever.
Complications
- Infection, abscess formation.
- Rare risk of malignancy (papillary thyroid carcinoma), reported in 1% of cases.
Pathological Features
Histopathology
- Macroscopic: Well-defined cystic mass, size can vary.
- Microscopic: Lined by respiratory (pseudostratified columnar) and/or squamous epithelium. May contain thyroid follicles.
Radiological Features
General Features
- Well-defined midline neck mass that may contain fluid, soft tissue, and sometimes calcifications.
US
- B-mode: Well-defined anechoic or hypoechoic lesion, possibly with internal debris. May show posterior acoustic enhancement.
- Colour Doppler: No internal blood flow.
CT
- Non-contrast: Well-defined cystic mass, possibly with calcifications. Soft tissue stranding if infected.
Diagnosis
Diagnosis is typically based on clinical presentation and imaging findings. Fine-needle aspiration or surgical excision may be used for histological confirmation.
Differential Diagnosis
- Dermoid cyst: More common in females, usually off midline, contains fat which can be seen on CT/MRI.
- Branchial cleft cyst: Typically located along the anterior border of the sternocleidomastoid muscle, rather than midline.
- Cystic thyroid nodule: Located within the thyroid gland.
Prognosis
The prognosis is excellent. Although TDCs can recur after surgery, this is relatively rare.
Management
The standard treatment for symptomatic or large thyroglossal duct cysts is surgical removal using a procedure called the Sistrunk operation. This operation involves removing the cyst and the tract of the thyroglossal duct up to the base of the tongue to minimise the chance of recurrence. If the cyst is infected, antibiotic treatment is initiated before surgery.