Background

The parapharyngeal space is a complex area containing many vital structures. A wide variety of pathology may arise from or involve the parapharyngeal space; thus, head and neck surgeons are generally familiar with the anatomy of this region via the transcervical route. However, this space lies immediately lateral to the tonsillar fossa and is frequently encountered during transoral robotic radical tonsillectomy. This necessitates a thorough understanding of the anatomy of this region through the unique perspective of transoral robotic surgery (TORS).

Basic Anatomy

The parapharyngeal space is generally shaped like an inverted pyramid ( Fig. 3.1 ), with its base at the skull base adjacent to the temporal and sphenoid bones; within this small area lie the carotid canal, jugular foramen, and hypoglossal canal. The apex of the parapharyngeal space is located at the greater cornu of the hyoid bone. The anterior wall of the parapharyngeal space is composed of the pterygomandibular raphe and pterygoid fascia. Posteriorly it is bounded by the prevertebral fascia and the carotid sheath; the posteromedial portion of the parapharyngeal space also communicates with the retropharyngeal space. The pharynx marks the medial border of the parapharyngeal space. The lateral border is the most complex; superiorly this is composed of the ramus of the mandible, the medial pterygoid muscle, and the deep lobe of the parotid gland; inferiorly it is composed of the posterior belly of the digastric muscle.

The parapharyngeal space is divided into prestyloid and retrostyloid compartments by the fascia extending from the styloid process to the tensor veli palatini and medial pterygoid plate (also known as the stylopharyngeal aponeurosis or aponeurosis of Zuckerkandl and Testut). The retrostyloid compartment contains the glossopharyngeal, vagus, accessory, and hypoglossal nerves and the cervical sympathetic trunk as well as the carotid artery, jugular vein, and carotid body. The prestyloid compartment contains fat, minor salivary glands, the internal maxillary artery, and branches of the mandibular branch of the trigeminal nerve ( Fig. 3.2a ). The sagittal and coronal views are shown in Fig. 3.2b and Fig. 3.2c.

Surgical Anatomy of the Parapharyngeal Space

The parapharyngeal space lies directly lateral to the oropharynx and tonsillar fossa; thus, this space is frequently encountered during TORS for oropharyngeal lesions. Although head and neck surgeons are generally quite familiar with the lateral approach to the parapharyngeal space ( Fig. 3.3 ), it is important to understand the anatomical relationships of this space when it is approached from the medial side. TORS may also be utilized to access the parapharyngeal space directly for removal of tumors in this area. Access to the parapharyngeal space is accomplished via incision of the pterygomandibular raphe and division of the musculature of the anterior tonsillar pillar and superior pharyngeal constrictor muscle. Once through the constrictor muscle, the fatty contents of the parapharyngeal space are encountered.

An understanding of the vascular anatomy of the parapharyngeal space is critical for safely performing surgery in this area. Intraoperative and postoperative hemorrhage are the most feared complications of TORS and can be life threatening. Two muscles, the stylopharyngeus and styloglossus, are critical landmarks for the vascular anatomy of this region.

The styloglossus muscle originates from the anterolateral aspect of the styloid process near the apex. It then passes inferiorly and medially between the internal and external carotid arteries and inserts into the lateral aspect of the tongue near its dorsal surface. The ascending palatine artery crosses the styloglossus muscle at the distal third, closest to a transoral surgical field. The stylopharyngeus is a long, slender muscle, cylindrical superiorly, flattened inferiorly. It originates on the medial aspect of the styloid process near the base, then passes inferiorly between the superior pharyngeal constrictor and the middle pharyngeal constrictor, finally inserting beneath the mucous membrane of the pharynx. The glossopharyngeal nerve runs on the lateral side of this muscle and crosses over it to reach the tongue. Together, these two muscles define an important plane during transoral surgery ( Fig. 3.4 ). Anterior to this plane is a relatively safe area, with only small vascular branches present in this area. Posterior to this plane lie major vessels such as the carotid artery and jugular vein; thus, this plane, which is defined by the styloglossus and stylopharyngeus, should be transgressed only with extreme caution.

The internal carotid, located in the retrostyloid compartment, is protected by the styloid diaphragm during the transoral approach. In the parapharyngeal space, it is separated from the external carotid artery by the styloid process, styloid diaphragm, glossopharyngeal nerve, and pharyngeal branch of the vagus. In most cases, the internal carotid is relatively straight as it traverses the parapharyngeal space; however, in less than 10% of individuals it may display a tortuous course close to the pharynx near the tonsillar fossa. Superiorly, the glossopharyngeal, vagus, accessory, and hypoglossal nerves lie between the internal carotid artery and the internal jugular vein, which lies posterior to the carotid at the skull base.

The external carotid artery is located in the prestyloid space. It is separated from the internal carotid by the styloid diaphragm, pharyngeal venous plexus, and glossopharyngeal nerve. This typically lies lateral to the parapharyngeal fat pad. Thus, during TORS, dissection that remains medial to the parapharyngeal fat pad will avoid exposure and injury of the external carotid artery. However, in a small number of individuals the external carotid may bulge into the parapharyngeal fat, between the styloglossus and stylopharyngeus muscles, and lie adjacent to the pharyngeal constrictors. The external carotid also gives off several branches in the area of the parapharyngeal space that may be at risk during TORS.

The ascending palatine artery supplies the tonsil and superior pharyngeal constrictor muscle and effectively supplies the parapharyngeal space ( Fig. 3.5 ). Thus, knowledge of the anatomy of this artery is important to avoid surgical injury. In most individuals the ascending palatine artery originates from the facial artery, although it may also originate directly from the external carotid artery. In two-thirds of individuals it then crosses the styloglossus muscle, then enters the prestyloid parapharyngeal space. In the other third of individuals it crosses between the stylopharyngeus and styloglossus muscles before entering the prestyloid compartment. This vessel crosses the styloglossus muscle at approximately a 65° angle and then gives off two or three branches to the medial pterygoid muscle. It then divides near the levator veli palatini muscle, with one branch supplying this muscle before entering and supplying the soft palate. The other branch runs through the superior constrictor muscle to supply the palatine tonsil; distal branches of this may anastomose with the ascending pharyngeal artery.

The ascending pharyngeal artery typically originates from the medial aspect of the external carotid artery, although other variants have been described, including originating from the internal carotid artery, the occipital artery, or complete absence of the ascending pharyngeal artery. In most cases, this artery then runs vertically between the internal carotid artery and the lateral pharyngeal wall up to the skull base, lying on the longus capitis muscle. However, this artery may occasionally ascend between the internal carotid artery and the jugular vein or may separate into several branches. Branches of this artery supply the middle pharyngeal constrictor, the stylopharyngeus, and neuromeningeal structures within and adjacent to the jugular and hypoglossal foramina.

Several cranial nerves also run though the parapharyngeal space, including the glossopharyngeal, vagus, accessory, and hypoglossal nerves. The relationship of these nerves to the other structures in the parapharyngeal space is important for avoiding injury to these structures.

The glossopharyngeal nerve provides sensation to the posterior tongue and pharyngeal wall and has the most involved anatomy of all the cranial nerves within the parapharyngeal space. The upper portion of the glossopharyngeal nerve exits the jugular foramen and then travels between the internal carotid artery and internal jugular vein, descending deep to the styloid process. Branches to the carotid body and carotid sinus originate from this segment of the nerve and travel along the medial aspect of the internal carotid artery. The glosspharyngeal nerve then passes posteriorly to the stylopharyngeus muscle, giving off branches to the stylopharyngeus and pharyngeal wall. The nerve then emerges from the lower border of the stylopharyngeus, at which point it is covered by fascia and the hyoglossus muscle. From a transoral perspective, this portion of the nerve inferior to the stylopharyngeus is located in the plane formed by the styloglossus and stylopharyngeus muscles. The lingual branch of the glossopharyngeal nerve then enters the tongue base by traveling through the constrictor muscles in the area of the glossotonsillar sulcus. This nerve branch may be at risk during palatine tonsillectomy, and injury to this nerve may contribute to postoperative dysphagia and dysgeusia.

The vagus nerve exits the skull base through the jugular foramen. Just after it exits the skull base, the superior (jugular) ganglion is encountered; at this point small branches from the vagus travel to the auricular nerve and to the accessory nerve. Just inferior to the jugular ganglion is the inferior (nodose) ganglion. Additional branches, including the pharyngeal nerve and the superior laryngeal nerve, arise at this point. The main trunk of the vagus then runs inferiorly in the neck within the carotid sheath. Aside from the area just inferior to the skull base, this nerve is relatively protected during transoral surgery of the parapharyngeal space by the jugular vein and carotid artery.

The accessory nerve briefly enters the parapharyngeal space as it exits the jugular foramen at the skull base. It then rapidly travels posteriorly and inferiorly, crossing the jugular vein (anterior to the vein in approximately 80% of case, posterior to the vein in approximately 20% of cases) and courses laterally in the neck toward the trapezius muscle. Like the vagus nerve, it is usually not encountered in transoral surgery of the parapharyngeal space.

The hypoglossal nerve exits the hypoglossal foramen of the skull base and travels posterior to the vagus nerve. It then passes between the internal carotid artery and the jugular vein to run deep to the posterior belly of the digastric muscle in the lateral neck. It is typically not encountered in transoral approaches to the parapharyngeal space unless extensive dissection occurs around and behind the jugular foramen at the skull base.