fourth branchial cleft cyst
Fourth branchial cleft cysts are very rare and parallel the course of the recurrent laryngeal nerve. They are most commonly on the left side (80%) and usually form a sinus which extends from the apex of the piriform sinus, as do third branchial cleft sinuses, but passes inferiorly rather than superiorly to reach anterior left upper thyroid lobe. Cysts can be anywhere in the neck down to the mediastinum, however, in most cases, they are located adjacent to the thyroid gland.
The proximity of the third and fourth branchial arches makes distinguishing third from fourth branchial cleft abnormalities radiologically difficult. For an accurate diagnosis, the relationship of the sinus tract to the superior laryngeal nerve must be determined surgically.
The most common presentation is abscess within or anterior to the left thyroid lobe.