History: Chronic right neck swelling with fullness to palpation.
Exam: Long- and short-axis fat- and water-weighted images of the neck were performed.
Findings and Differential Diagnosis: Subjacent to the gel marker is a cystic lesion measuring 2.3 x 2.2 cm in size. This mass is located anterior to the sternocleidomastoid muscle, deep to the parotid gland, and posterior to the right submandibular gland. The cystic lesion demonstrates a thick rind of soft tissue that measures approximately 4 mm in thickness. The adjacent fat planes are sharp and well marginated. The differential diagnosis includes an infected second branchial cleft cyst, infectious mononucleosis or Epstein-Barr virus infection. The anatomic location and imaging characteristics favor an infected second branchial cleft cyst.
Discussion: Branchial cleft cysts are second only to thyroglossal duct cyst as a cause of a congenital cystic neck mass. By the end of the fourth week of fetal life, there are normally six separate branchial arches, which are derived from neural crest cells, separated by five endodermal branchial pouches. The pathologic origin of branchial cleft cysts is incompletely understood. The spectrum of branchial cleft anomalies includes sinuses, fistulas, and cysts. A sinus opens externally, usually on the neck. A fistula also has an external opening, but extends internally as well. A cyst communicates neither externally nor internally.
The majority (75%) of branchial cleft anomalies are cysts. Cysts usually present in older children or young adults, while sinuses and fistulae generally present earlier in life.
A parotid lymphoepithelial cyst arises in the first branchial cleft. This anomaly represents approximately 5% of all branchial cleft defects, and may communicate with the external auditory canal. On imaging, the most common appearance is that of a nonspecific cyst within or deep to the parotid gland.
The vast majority (95%) of anomalies arise from the second branchial cleft. These typically occur in patients between 10 and 40 years of age. The imaging appearance is usually that of a thin-walled cyst, although infection may produce wall thickening. As in this case, second branchial cleft cysts are classically located at the anteromedial margin of the sternocleidomastoid muscle, lateral to the carotid artery. MRI shows a cyst with variable T1 appearance and hyperintensity on T2-weighted sequences. The “beak sign”, a small rim of tissue pointing between the internal and external carotid arteries, is virtually pathognemonic for a second branchial cleft cyst.
Third and fourth branchial cleft cysts are extremely rare; a third branchial cleft cyst occurs in the posterior cervical space and a fourth from the pyriform sinus. The vast majority of fourth branchial cleft anomalies occur on the left.
