Diagnostic Pitfall in the Carotid Space: Accessory Nerve Schwannoma Simulating Cystic Metastasis—A Case Report

Objectives: The aim of this case report is to highlight the diagnostic challenges of carotid
space masses, share clinical experience, and educate clinicians by presenting a case of a
rare disease. Introduction: Accessory nerve schwannomas are rare, benign peripheral
nerve sheath tumors. They make up only a small percentage of all cervical schwannomas.
Given their rarity and varying appearance on imaging, these tumors can be difficult to
accurately diagnose. Schwannomas may mimic other carotid space pathologies, such as
metastatic lymphadenopathy, paragangliomas, or sympathetic chain tumors. Accurately
identifying the nerve of origin before surgery is important for effective surgical planning
and neurological function protection. Case Description: A 50-year-old woman presented
with an asymptomatic left-sided neck mass. Computed tomography (CT) revealed a
cystic lesion with a thick, contrast-enhancing capsule in the left carotid space, causing
internal jugular vein compression and partial thrombosis. Subsequent MRI showed a
28 mm×23mm×38mmwell-definedmasswithcharacteristic schwannoma features,
including T2/Short tau inversion recovery (STIR) hyperintensity, peripheral enhancement,
central cystic degenerative components, and peripheral diffusion restriction with corre
sponding lower apparent diffusion coefficient (ADC) values. Split-fat sign and fascicular
sign were also seen on the MRI. Despite these imaging findings, the radiological interpreta
tion suggested a sympathetic chain schwannoma as the most likely diagnosis. The correct
diagnosis of accessory nerve schwannoma was established intraoperatively when the mass
wasvisualized to be attached to the accessory nerve. Conclusions: This case highlights that even with suggestive MRI features, the rarity of accessory nerve schwannomas can lead
to misidentification of the nerve of origin. Accurate diagnosis may require intraoperative
visualization, thus marking the importance of including accessory nerve involvement in
the differential diagnosis of carotid space masses.