Thyroglossal duct cyst

General:
Most common congenital neck mass (70% of congenital neck anomalies)
2nd most common benign neck mass, after benign lymphadenopathy
50% present before 20 years of age
Rarely hereditary (AD) - occur in prepubertal girls

Embryology:
Median outgrowth from floor of primitive pharynx at the level of foramen cecum (junction of anterior 2/3 and posterior 1/3 of tongue)
Thyroid gland descends in the neck, penetrates the underlying mesoderm of tongue and floor of mouth musculature, passes anterior to the developing hyoid and laryngeal cartilages.
During migration, the anlage of thyroid gland is connected to tongue by narrow tubular thyroglossal duct, which involutes by 8th–10th week of gestattion
The original opening of thyroglossal duct persists as foramen cecum of tongue and the inferior end of canal persists as pyramidal lobe of thyroid gland
Persistance of any duct gives cystic lesion
Ectopic thyroid tissue in 60%

Location and size:
80% occur at or just inferior to hyoid bone (80%), 20% above the hyoid
75% are located in midline or 25% slightly off-midline in anterior neck (within 2 cm of midline)
rarely as a mass in the floor of mouth
Commonly 1.5-3 cm (0.5-6cm)

Clinical presentation:
Painless enlarging mass in teen or young adult
Moves with tongue protrusion
commonaly present due to infection

Imaging:
Cystlike mass in anterior midline or off-midline at or above hyoid bone level

US:
Anechoic mass with thin wall in 40%
More commonly, hypoechoic with increased through-transmission
May have fine to coarse internal echoes
No correlation between US appearance and infection and inflammation, because heterogeneity is more likely to be due to proteinaceous content rather than infection

CT:
Smooth, well-circumscribed mass with thin wall
Attenuation of 10–18 HU, increased HU due to increased protein content
May have septations
Peripheral rim enhancement is usually seen and is not an indication of infection

MRI:
Cystic, may vary due to protein content
Enhancement may represent inflammation

Complications:
Infection: common presenting symptom
Malignancy: 1%. 80% are papillary, but every thyroid cancer has been reported. Nodal spread is less common

Treatment:
Drainage or partial always leads to recurrence
Complete excision - resecting of central portion of hyoid bone and a core of tissue following the expected course of the thyroglossal duct to foramen cecum

Prognosis:
Recurrence 2.6%

References:
1. Koeller KK et al. Congenital Cystic Masses of the Neck: Radiologic-Pathologic Correlation. Radiographics. 1999;19:121-146