Thymic cyst

General features:
Uncommon lesions
1st decade (2/3rd)

Embryology:
Persistence of thymopharyngeal duct remnants
May be acquired

Clinical features:
Present in 1st 3 decades
Most are asymptomatic
Slow growing painless mass

Location:
Along the path of thymopharyngeal duct, adjacent to carotid sheath
From angle of mandible to thoracic inlet
Anterior or deep to SCM
More common on left
Uncommonly present with hoarseness, dysphagia, stridor, respiratory distress in newborns Sudden enlargement may be with Valsalva, hemorrhage or infection
Not associated with myasthenia gravis or neoplasm
Connected to thymus by a fibrous cord in 50%
May extend through thyrohyoid membrane into pyriform sinus (similar to 3rd and 4th branchial cleft cyst)

Imaging:
Unilocular (may be multilocular) cyst parallel to SCM, adjacent to carotid space
May extend into mediastinum

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