Branchial cleft cysts are a benign anomaly caused by incomplete obliteration of a primordial branchial cleft. They typically appear in childhood or adolescence, but can appear at any age. They present as a non-tender, fluctuant mass following an upper respiratory infection, most commonly at the anterior border of the sternocleidomastoid muscle. These lesions are thought to originate during the 4th week of gestation when the branchial arches fail to fuse. The second branchial cleft is the most common site (95%) and cysts from in this distribution can affect cranial nerves VII, IX, and XII.
