The differential diagnosis for a cystic abnormality in the neck is limited. The key consideration is that of a branchial cleft cyst versus a cystic node, the latter either typically due to a squamous cell carcinoma or due to mycobacterial disease. Other diagnostic considerations include the possibility of thyroglossal duct cysts (midline), epidermoid/dermoid cysts (usually floor of mouth), cystic hygromas (usually posterior triangle) and laryngoceles representing lateral cystic changes arising from the laryngeal ventricles that are often associated with laryngeal carcinoma.
In this location the appearances are highly typical of a branchial cleft cyst. Although congenital these frequently present in adulthood, often due to a palpable mass or following an infective episode. Although generally thin walled during an acute infection the boundaries at CT may become thicker, ill-defined or rim enhancing. Aspiration often demonstrates denatured squamous cells which can cause difficulty in differentiation from squamous cell carcinoma so depiction of the typical anatomy is helpful in diagnosis.
The branchial cleft embryology and anatomy is complex, yet in the adult the pathological manifestations are less confusing. Overall there are 6 branchial arches, between them on the internal (endodermal) surface these result in five pouches, on the external surface (ectodermal) these result in five clefts. The first branchial cleft, is the only one to give rise to adult structures, the eustachian tube, tympanic cavity, mastoid antrum, and contributes to the external auditory canal. The second branchial pouch, lined by endoderm, gives rise to the palatine tonsil and tonsillar fossa. The third branchial pouch forms the inferior parathyroid gland, thymus, and pyriform sinus; the fourth branchial pouch leads to the formation of the superior parathyroid gland and apex of the pyriform sinus.
During the third to fifth week of embryonic development the second arch grows caudally over the third fourth and sixth arches. The edges of the cervical sinus fuse and gradually the trapped ectoderm resorbs. The aetiology for branchial cleft abnormalities is thought to be either incomplete fusion of this cervical sinus or trapped ectoderm cell rests. Branchial cleft abnormalities are usually (>75%) cysts, although sinuses or fistulae can also occur. By far the commonest branchial cleft abnormalities are 2nd branchial cleft abnormalities Only 2-3% are bilateral.
Second branchial cleft abnormalities are most common type (~95%), typically presenting as cysts. These occur as in this case anterior to sternocleidomastoid and posterior to the submandibular gland. They can be classified according to the Bailey classification. Type I are deep to platysma, type II abut the internal carotid artery and are adherent to the internal jugular vein (most common), type III extend between the internal and external carotid arteries and finally type IV abut the pharyngeal wall and may extend to the skull base.
First branchial cleft cysts, account for 5-8% of branchial cleft cysts, result in lymphoepithelial cysts involving the parotid or external auditory canal extending into the submandibular gland. Third and fourth branchial cleft defects are very rare and can be difficult to differentiate. The former usually presents as a posterior triangle cyst, the latter most often as a sinus tract.