Excision of Neural Tumors of the Parapharyngeal Space

Introduction

To safely resect neural tumors of the parapharyngeal space (PPS) and to counsel patients about potential consequences and complications of surgery requires that the surgeon has a detailed knowledge of the surgical anatomy of the PPS, requests appropriate radiological studies from which the likely pathology can be predicted (80% benign), and understands the possibilities and limitations of the various surgical approaches.

Neural tumors in the PPS include solitary schwannomas, neurofibromas, and malignant nerve sheath tumors. There are at least three major types of neurofibromas: neurofibromatosis-1 (NF1), neurofibromatosis-2 (NF2), and schwannomatosis. They all represent genetic abnormalities. With schwannomas, the nerve fibers of the parent nerve may be draped over the tumor, making it possible to preserve the nerve, although transient postoperative paresis is common. However, in most cases the schwannoma is inseparable from the nerve and the nerve must be sacrificed. Neurofibromas are not encapsulated, and because the nerve fibers are part of the tumor, the nerve cannot be preserved. Malignancy occurs in about 10% of patients with NF1 and may be signified by invasion of adjacent tissue, or metastases.

The PPS has prestyloid and poststyloid compartments. It is important to distinguish preoperatively between a prestyloid and a poststyloid mass, as this affects the differential diagnosis and may determine the surgical approach. Poststyloid neural tumors arise from cranial nerves IX–XII or from the sympathetic nerves. Prestyloid tumors are likely to originate from branches of the mandibular division of cranial nerve V. However, the nerve of origin may not be apparent even at surgery if a tumor has originated from minor, unnamed nerves. Vagal and carotid paragangliomas also arise in the poststyloid PPS and may be difficult to differentiate from a schwannoma on computed tomography (CT) and magnetic resonance imaging (MRI), as flow voids that are typical of a paraganglioma may not be apparent in a small paraganglioma, and schwannomas may also appear vascular. Carotid angiography may be helpful in such situations to draw a distinction. Salivary gland tumors and metastases (e.g., from papillary thyroid carcinoma) should also to be considered in the differential diagnosis.

The PPS extends as an inverted pyramid from the skull base superiorly, to the hyoid bone inferiorly. Fig. 72.1 presents an axial view of the prestyloid (yellow) and poststyloid (pink) components of the PPS separated by the styloid process, tensor veli palatini muscle, and its fascia (brown). The poststyloid space contains the internal carotid artery and the internal jugular vein, as well as the lower cranial nerves IX–XII, and the sympathetic trunk. It is confined medially by the pharyngobasilar fascia and the superior constrictor muscle of the pharynx.

Prestyloid tumors may present with bulging of the lateral wall of the oropharynx, which displaces the tonsil medially ( Fig. 72.2 ). The prestyloid PPS is bordered anterolaterally by the medial pterygoid muscle, and posterolaterally by the deep lobe of the parotid gland (see Fig. 72.1 ). It contains mainly adipose tissue. Radiologically, prestyloid tumors typically displace the adipose tissue of the PPS anteromedially ( Fig. 72.3 ).

Poststyloid masses typically present as a mass extending into the superior lateral neck or the oro- or nasopharynx, dysfunction of cranial nerves IX–XII, or Horner’s syndrome. The poststyloid PPS is limited medially by the pharyngobasilar fascia and the superior pharyngeal constrictor. It contains the internal carotid artery and the internal jugular vein, as well as the lower cranial nerves IX–XII, and the sympathetic trunk (see Fig. 72.1 ). Radiologically, poststyloid tumors typically displace the adipose tissue of the PPS anterolaterally (see Fig. 72.3 , Fig. 72.4 ).

Key Operative Learning Points

Selecting the appropriate surgical approach is very important in achieving adequate access and in minimizing morbidity. In order to plan the correct surgical approach and before embarking on surgery, the surgeon needs the following information:

• Is the mass pre- or poststyloid? This is determined clinically and radiologically with CT/MRI. This information permits the surgeon to narrow down the differential diagnosis, to plan the best approach, and to counsel patients preoperatively about possible sequelae.

• Is the mass benign or malignant? This may influence the surgical approach, and even affect the decision whether or not to do surgery. It is generally determined by Fine needle aspiration cytology (FNAC) done transcervically or transorally. The surgeon should not be concerned about puncturing the internal carotid artery with a small-caliber needle, as bleeding can be controlled with digital pressure.

• Is the mass a paraganglioma? This is important, as paragangliomas may rarely be hormonally active and may require preoperative embolization. When it is difficult to distinguish a paraganglioma from a schwannoma on CT or MRI, proceed to a diagnostic angiogram.

• What are the anatomical relations of the internal and external carotid arteries to the mass? Particularly with a poststyloid mass, knowledge of the position of the arteries in relation to the mass is important in planning and doing the surgery (see Fig. 72.4 ).

• What is the cerebral cross-over blood flow like should the internal carotid artery have to be sacrificed? Should this be a concern, it can be determined by angiography ± balloon occlusion testing.

Preoperative Period

• What neurological deficits may be caused by surgical excision? Causing permanent cranial nerve deficits, a cerebrovascular accident, or first bite syndrome can be devastating for the surgeon and patient alike, and may significantly impact on a patient’s quality of life.

• Does the patient have NF1, NF2 or schwannomatosis?

• Does the patient require surgery, radiation, or watchful waiting? Not all benign neural tumors require surgical excision, as they are often slow growing, and growth may be arrested by radiation. Therefore careful consideration should be given to nonsurgical treatment options, especially in older patients.

Operative Period