Fourth branchial cleft anomalies

Extremely rare
Present during young age
3rd and 4th together form 2nd most common congenital lesion of posterior space, posterior to SCM
Commonly present as sinus tract rather than cyst or fistula
Most are on left side
Arises from pyriform sinus, pierces thyrohyoid membrane, descends into mediastinum and may extend under aortic arch
Most are short and sinus tracts
Seen below superior laryngeal nerve
May be associated with ectopic or undescended parathyroid

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