Magnetic resonance imaging (MRI) and computed tomography (CT) scanning play a significant role in the diagnostic workup of masses located in the retrostyloid parapharyngeal space. Retrostyloid neoplasms frequently arise from neural crest origin, and both paragangliomas and schwannomas are common considerations in the differential diagnosis of such masses. Although paragangliomas and schwannomas share several similarities on imaging, it is often the unique characteristics of each lesion that allow for a definitive radiologic diagnosis.
The carotid body tumor is the most frequently identified paraganglioma in the head and neck and has a typical appearance on imaging that is rarely replicated by other lesions [
A 59 year-old female presented to a tertiary-care university medical center with a two-year history of a progressively enlarging right-sided neck mass. Symptoms on presentation were limited to neck discomfort and mild dysphagia. The patient had an otherwise unremarkable past medical history. On physical examination, a large, mobile, pulsatile level II neck mass was identified. A comprehensive cranial nerve examination was normal. An MRI of the neck demonstrated a mass at the level of the carotid bifurcation, which on postgadolinium imaging demonstrated heterogeneity consistent with internal flow voids (Figure
Postcontrast MRI of the neck demonstrating a mass at the level of the carotid bifurcation.
MRA of the neck demonstrating splaying of the internal and external carotid arteries, that is, the “lyre sign.”
Given the radiographic characteristics of the mass and findings on physical examination, a preliminary diagnosis of a carotid body tumor was established. The patient was subsequently brought to the operating suite for a definitive surgical excision. Intraoperatively, a grey, gelatinous-appearing mass was identified at the carotid bifurcation. Upon further dissection, the tumor was unequivocally identified as originating from the proximal segment of hypoglossal nerve, independent of the carotid artery. The distal segment of the hypoglossal nerve was identified as it exited the mass (Figure
Intraoperative photograph demonstrating a mass originating from the proximal segment of hypoglossal nerve, with the distal segment clearly seen exiting the mass.
Initial attempts at subcapsular dissection with preservation of the hypoglossal nerve proved unsuccessful, as fibers of the nerve became progressively attenuated and inseparable from the tumor. The decision was thus made to transect the hypoglossal nerve and excise the mass. After excision, a neurorrhaphy was performed via primary anastomosis using epineural 8-0 nylon sutures in simple interrupted fashion. Final surgical pathology confirmed the working diagnosis of a hypoglossal schwannoma.
Carotid body paragangliomas are uncommon parapharyngeal space neoplasms that arise from chemoreceptive and sustentacular cells at the carotid bifurcation. These benign lesions display an insidious growth pattern and have been suggested to have an increased incidence in populations living at high altitudes, speaking to the chemoregulatory function of the carotid body [
The differential diagnosis for masses located in the parapharyngeal space can be refined based on preoperative imaging characteristics and physical examination findings. Although paragangliomas and schwannomas are both lesions of neural crest origin potentially located in the retrostyloid space, the unique pathophysiology, tumor biology, and management considerations for each lesion make an accurate diagnosis of significant importance. Currently, a standard component of the diagnosis and workup for parapharyngeal space lesions includes MR or CT angiography. At many institutions, formal angiography is only obtained preoperatively based on CTA or MRA findings, with subsequent embolization performed for highly vascular lesions. Although digital and conventional angiography modalities are of great utility, they are not universally available, and conventional MRI and CT play a significant role in the initial diagnosis of these lesions. In the classic 1988 description by Som and colleagues, the use of CT and MR imaging is presented in its ability to narrow the diagnosis of parapharyngeal space lesions based on fat planes, signal characteristics, and the displacement of the carotid artery [
Despite the efficacy of CT and MRI in establishing a diagnosis for typical parapharyngeal space lesions, several imaging similarities between paragangliomas and schwannomas exist that can be confounding. Both schwannomas and paragangliomas are benign retrostyloid lesions that may enhance avidly on MRI following gadolinium infusion, and both have variable signal intensity on T2-weighted images. Imaging characteristics that are distinct to each lesion are therefore employed to assist with establishing a diagnosis. Imaging principles frequently rely on the characteristic blood flow expected for each lesion, or on the anatomic origin of the neoplasm in question [
In a similar fashion to that of paragangliomas, the origin of a cervical schwannoma can also be predicted based on the displacement of the great vessels seen on imaging. Schwannomas arising from the sympathetic chain characteristically displace great vessels anteriorly, while vagal schwannomas are known to separate the carotid artery from the internal jugular vein. Large paragangliomas and those arising from the nodose ganglion, however, may occupy the same anatomic location typical to some schwannomas, and therefore share a similar appearance on noncontrast imaging. In this regard, the importance of contrast enhancement becomes increasingly valid. Because only 1/3 of schwannomas are found to enhance with equal avidity to that of paragangliomas and few produce a similar “salt and pepper” appearance on T2-weighted MRI, a diagnosis can still be accurately made under most circumstances [
In the patient presented here, a diagnosis of carotid paraganglioma was established preoperatively based on the presence of enhancement with contrast infusion, apparent large internal vascular flow voids, and the splaying of the internal and external carotid artery on MRI. Despite the preoperative imaging characteristics typical of a carotid body paraganglioma, the lesion was indeed determined to be originating from the hypoglossal nerve intraoperatively. In retrospect, the heterogeneous appearance of the lesion misconstrued as vascular flow voids was likely secondary to variable density of the enlarging neoplasm. Considering the natural location of the hypoglossal nerve, inferior growth of this neoplasm also allowed for the internal and external carotid artery to splay as it further occupied the carotid bifurcation, resulting in the “lyre sign” typical of a carotid body tumor. To our knowledge, this is the first hypoglossal schwannoma mimicking a carotid body paraganglioma to be described in the literature.
Hypoglossal schwannomas are thought to represent only 5% of all nonvestibular schwannomas, with approximately 110 cases reported in the English literature to date. Of these cases, the vast majority of cases originate intracranially, with or without extracranial extension [
Treatment for both the carotid body paraganglioma and the hypoglossal schwannoma is primarily surgical. Resection of a cranial nerve schwannoma requires dissection from the nerve fascicles, and complete removal is attempted when possible. Intraoperative monitoring of the hypoglossal nerve has been described as a reliable technique; however monitoring is not a routine procedure at most centers. If sacrifice of the nerve is required for complete excision, great auricular nerve cable graft may be utilized if a tension-free anastomosis is not otherwise possible [
While majority of parapharyngeal space neoplasms are amendable to diagnosis with a high level of certainty based on preoperative patient history and imaging characteristics, circumstances exist where standard diagnostic techniques may be misleading. As this report demonstrates, the clinical and radiologic presentation of a hypoglossal schwannoma may closely mimic that of the more common carotid body tumor. As such, a high level of suspicion must be maintained whenever an aberrancy in the usual presentation of a carotid body tumor is encountered. Though rare, the hypoglossal schwannoma should remain a consideration when evaluating any patient with a mass in the parapharyngeal space.
None of the authors has financial disclosures or conflict of interests this paper was presented at the Combined Otolaryngology Spring Meetings (COSM), April 18th–22nd, 2012, San Diego, CA.
M. Lee, MD: analysis and interpretation of data, and drafting of paper. D. Sidell, MD: analysis and interpretation of data, and drafting of paper. A. Mendelsohn, MD: critical review of paper. K. Blackwell, MD: study design, critical review of manuscript, and supervision.