Carotid sinus syncope caused by a unique multilocular second branchial cleft cyst: a case report

Second branchial cleft cysts (SBCCs) are common congenital neck anomalies that typically present as asymptomatic lateral masses. While they occasionally cause local pressure symptoms, systemic hemodynamic complications are extremely rare. We report an unusual case of carotid sinus syndrome (CSS) caused by a multilocular SBCC, where the patient's primary complaint was syncope triggered mechanically by neck movement. An 18-year-old Palestinian Arab female presented with a 4.4 cm right lateral neck mass and a history of recurrent syncope specifically triggered by turning her head. On physical exam, the mass felt like a single lesion. However, a contrast-enhanced CT scan revealed an atypical structure: a large ovoid cyst accompanied by a smaller, 1.3 cm inferoposterior cyst that appeared radiographically separate. During surgery, the lesion was identified as a single, septate multilocular cyst located deep to the sternocleidomastoid muscle (Bailey Type II), directly compressing the carotid bifurcation. Following surgical excision, the patient's syncopal episodes stopped entirely. Pathology confirmed the diagnosis of a second branchial cleft cyst. Benign neck masses can cause significant systemic effects by mechanically compressing carotid sinus baroreceptors and triggering an exaggerated vagal response. This case also highlights a rare multilocular variant of an SBCC that can mimic dual pathology on imaging. Clinicians should consider underlying structural lesions, including branchial cleft cysts, in the differential diagnosis for patients presenting with position-dependent syncope.