TITLE: Parapharyngeal Space Neoplasms
SOURCE: UTMB Dept. of Otolaryngology Grand Rounds Presentation
DATE: February 18, 1998
RESIDENT PHYSICIAN: Kyle Kennedy, M.D.
FACULTY PHYSICIAN: Anna Pou, M.D.
SERIES EDITOR: Francis B. Quinn, M.D., F.A.C.S.

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INTRODUCTION

The parapharyngeal space (PPS) is located lateral to the upper pharynx and is a complex anatomical region through which pass many important neurovascular structures. Tumors arising in this region are rare and account for approximately 0.5% of head and neck neoplasms. These neoplastic processes represent a wide variety of both benign and malignant lesions arising from the structures within the PPS. The complex anatomical and pathological considerations within this region often present a substantial challenge to the otolaryngologist-head and neck surgeon in the evaluation and management of these lesions.

The majority of tumors arising in the PPS are benign, and surgical resection is the mainstay of therapy. An adequate understanding of the anatomy and pathology of the PPS is requisite in the systematic preoperative evaluation of PPS lesions and the choice of a safe and effective surgical approach for their removal. This review will address the following areas with regard to PPS neoplasms: pertinent anatomy of the PPS, presentation and evaluation, types of PPS neoplasms, surgical approaches, and complications.

ANATOMY

The PPS is typically conceptualized as a potential space in the shape of an inverted pyramid with its base at the skull base and its apex at the greater cornu of the hyoid bone. Various fascial layers within the PPS result in further compartmentalization of this complex anatomic region. These fascial compartments may limit, and in some cases, direct tumor spread within or beyond the PPS.

The superior limit of the PPS is represented by a small portion of the temporal bone. At the skull base, the superomedial limit of the PPS is defined by a layer of fascia extending from the medial pterygoid plate to the spine of the sphenoid. This fascial layer lies medial to the foramen ovale and foramen spinosum, which are considered to be structures within the infratemporal fossa or masticatory space.

The inferior limit of the PPS is represented by the junction of the posterior belly of the digastric muscle and the greater cornu of the hyoid bone.

The posterior limit of the PPS consists of the fascia overlying the vertebral column and paravertebral musculature, and the anterior limit is formed by the pterygomandibular raphe and medial pterygoid fascia.

The medial wall of the PPS is formed superiorly by the fascial connections between the tensor veli palatini muscle and the medial pterygoid muscle. Posteromedially, the pharyngobasilar fascia is present, and the superior pharyngeal constrictor complex lies inferiorly and inferomedially.

The lateral wall of the PPS is formed by, from anterior to posterior, the medial pterygoid fascia, ramus of the mandible, retromandibular portion of the deep lobe of the parotid, and the posterior belly of the digastric muscle.

Of note, the superior, posterior, and inferior borders of the PPS represent rigid structures which limit expansion of the PPS and tumor spread to the anterior, medial, and inferior directions.

As mentioned previously, fascial compartmentalization of the PPS is anatomically important in limiting and directing tumor spread. It is also important with regard to imaging the complex anatomical region of the PPS and preoperative treatment planning. With modalities such as computed tomography, a differential diagnosis may be established based on the location of a given lesion with respect to the various fascial planes and an appropriate surgical approach may be planned based on the character and extent of the lesion.

The fascia of the tensor veli palatini muscle extends posteriorly to connect with the styloid process and its associated muscle group. This fascia subdivides the the PPS into an anterolateral prestyloid space and a posterolateral poststyloid (or retrostyloid) space. Found within the prestyloid PPS are a portion of the retromandibular deep lobe of the parotid gland and, principally, fat. Lymph nodes may be found here, as well as a small branch of the mandibular division of CN V to the tensor veli palatini muscle. The poststyloid PPS contains the internal carotid artery, internal jugular vein, CNs IX to XII, sympathetic chain, and lymph nodes. Tumors may arise from any of the structures contained within these areas.

The stylomandibular ligament connects the styloid process with the ramus of the mandible and forms one of the boundaries of the stylomandibular tunnel. This a relatively rigid space bounded by the stylomandibular ligament, ramus of the mandible, and skull base which may permit primary tumors arising in the deep lobe of the parotid gland to enter the PPS. These lesions often have a characteristic dumbbell appearance on CT imaging due to constriction of the tumor in the confines of the stylomandibular tunnel.

The infratemporal fossa or masticatory space contains the pterygoid musculature, the mandibular division of CN V, and the mandibular condyle. Technically speaking, tumors originating within this region should not be considered with those of the PPS. The same holds true for those skull base lesions originating at the foramen ovale and those of the deep lobe of the parotid gland located lateral to the ramus of the mandible. Carotid body tumors are only included with those of the PPS if the extend superior to the posterior belly of the digastric muscle.

In addition to the previously mentioned lymph nodes contained within the PPS itself, many anastomotic lymphatic channels are present which connect the PPS with the oropharynx, oral cavity, sinonasal tract, and thyroid gland. These lymphatics connect with the node of Rouviere located superiorly within the retropharyngeal space and inferiorly with the jugulodigastric nodes.

PARAPHARYNGEAL SPACE NEOPLASMS

The neoplastic processes which may involve the PPS include primary PPS tumors, direct extension of tumors from regions adjacent to the PPS, and distant disease metastatic to the PPS. Only primary PPS neoplasms will be mentioned here. Approximately 80% of these tumors are benign and 20% malignant. Overall, approximately 50% of the neoplasms are of deep lobe of parotid or minor salivary gland origin, while 20% are of neurogenic origin.

Salivary Gland Neoplasms

Primary salivary gland neoplasms involving the PPS usually arise from the deep lobe of the parotid gland or minor salivary gland tissue. These lesions are typically benign pleomorphic adenomas. Approximately 10% of parotid tumors arise from the deep lobe and of these, less than 1% involve the PPS. Most deep lobe tumors arise lateral to the ramus of the mandible and present in fashion similar to that of of superficial lobe mass. Those lesions which do involve the PPS present in the prestyloid compartment.

Deep lobe parotid tumors may extend through the stylomandibular foramen to involve the PPS in the previously described dumbbell fashion. The external component presents as a pretragal mass and is usually larger than the pharyngeal component. Tumors arising in the retromandibular parotid may extend into the PPS via a fascial defect created where the external carotid artery pierces the parotid fascia. These lesions typically assume a more rounded appearance and may result in medial displacement of the tonsil and soft palate. If the lesions grow to considerable size, a palpable mass may also be noted externally at the angle of the mandible. Superficial lobe neoplasms in the tail of the parotid may extend superomedially to involve the PPS with a mass palpable at the angle of the mandible and a larger pharyngeal component.

Extraparotid salivary gland neoplasms may originate in ectopic rests of salivary tissue in lymph nodes of the PPS. Ectopic rests of salivary tissue may be located lateral to the superior pharyngeal constrictor muscle or may arise medial to the muscle from the mucosa of the pharynx. On CT, a fat plane between the deep lobe of the parotid and a PPS tumor may distinguish a lesion of extraparotid origin. Other benign salivary gland tumors such as Warthin's tumor have been found in the PPS, as well as malignant lesions such as adenoid cystic carcinoma, mucoepidermoid carcinoma, and acinic cell carcinoma.

The preferred treatment of benign pleomorphic adenomas of the PPS includes complete excision while avoiding disruption of the capsule and tumor spillage.

Neurogenic Neoplasms

The most common neurogenic tumor arising in the PPS is the neurilemmoma or schwannoma with the vagus nerve being the most frequent nerve of origin. The sympathetic chain is the second most common nerve of origin, and CNs IX through XII may give rise to these lesions as well. These benign, slow-growing lesions usually do not affect their nerve of origin, and they most often present as asymptomatic masses. Larger tumors may become symptomatic, and anteromedial displacement of the carotid artery may be evident on CT. Intracranial extension may occur via the jugular foramen. Neurilemmomas are the most common enhancing extraparotid tumors of the PPS by CT. Their preferred treatment is complete excision with preservation of the nerve of origin whenever possible. Unavoidable nerve sacrifice should be followed by nerve grafting if feasible.

Paragangliomas or chemodectomas may arise from the vagal or carotid bodies from cells of neural crest origin. The vagus nerve is the most common site of origin of these lesions in the PPS with carotid body tumors being much rarer. Vagal paragangliomas may present with medial displacement of the lateral pharyngeal wall with cranial nerve deficits being noted in approximately 30% of the cases at presentation. The malignant rate has been reported at 10%, and intracranial extension may occur via the jugular foramen. The lesions may be quite vascular and may be intimately associated with the internal carotid artery.

Carotid body tumors usually present as an asymptomatic mass at the angle of the mandible and are not considered as PPS neoplasms unless the extend superior to the posterior belly of the digastric muscle. Symptomatic and/or rapidly growing lesions in healthy patients less than 70 years of age may be considered for excision.

Paragangliomas may be multicentric in 10-20% of cases and should be evaluated for the production of catecholamines preoperatively.

Neurofibromas are the third most common form of neurogenic neoplasm to involve the PPS. The lesions are typically multiple and intimately associated with their nerve of origin, which makes neural preservation during excision difficult and often impossible. Cranial nerve deficits are more common with neurofibromas than with neurilemmomas.

A variety of malignant tumors of neural origin may arise within the PPS as well.

Other Neoplasms

A wide variety of other benign and malignant neoplasms may arise in the PPS including meningioma and lymphoma. Benign lesions of vascular and connective tissue origin, along with their malignant counterparts, are rare primary lesions of the PPS also. Importantly, squamous cell carcinoma of the upper aerodigestive tract such as the nasopharynx, oropharynx, hypopharynx, and oral cavity may metastasize to the lymph nodes of the PPS. However, this is not usually the initial manifestation of the primary tumor. On the other hand, metastasis from a primary thyroid carcinoma may indeed represent the presenting manifestation of the thyroid lesion.

PRESENTATION AND EVALUATION

Tumor Presentation

The signs and symptoms of PPS neoplasms are often subtle, and the anatomical configuration of the PPS often make accurate clinical assessment difficult. These tumors usually present as an asymptomatic mass, and the lesions must often grow to a considerable size before symptoms become apparent and clinical detection is possible. A diameter of approximately 2.3 to 3.0 cm is typically required for the neoplasm to be appreciated externally in the neck and/or intraorally. Patients may note a sensation of a lump in the throat and subtle fullness may be apparent at the angle of the mandible or intraorally in the lateral oropharyngeal wall or soft palate. Medial displacement of the tonsil may be noted. Superior tumor extension may produce nasopharyngeal obstruction with compression of the eustachian tube and resultant middle ear effusion. More medial displacement of the pharynx can produce dysphagia or aspiration, as well as changes in the voice.

As tumors become larger, compression of CNs IX through XII may cause hoarseness, dysphagia, or dysarthria. Involvement of the sympathetic chain and superior cervical ganglion may produce Horner's syndrome with ipsilateral ptosis, miosis, and anhydrosis. Cardiovascular instability or hypersensitivity may occur with involvement of CN IX. Significant pain, trismus, or cranial nerve deficits may also be indicative of malignancy.

A delay in diagnosis may occur as patients are often treated for other conditions with failure to recognize an underlying PPS neoplasm. The presence of these lesions may mimic tonsillar lesions or infections, otologic disease, nasal obstruction, and obstructive sleep apnea.

A detailed history and complete head and neck examination are requisite in the initial evaluation of PPS neoplasms. Notation of the previously mentioned physical findings may provide the clinician with clues as to the character and extent of the lesion. Again, the deeply situated anatomical location of the PPS make clinical assessment of a given lesion challenging. Bimanual palpation of these lesions is an important tool in the physical diagnosis and assessment of these tumors.

Evaluation

Radiographic imaging is essential in the assessment of PPS neoplasms, and CT and MRI have become tremendous aides in preoperative evaluation and treatment planning. Angiography and fine needle aspiration biopsy may be helpful in selected cases.

CT with and without contrast displays the soft tissues of the PPS well and can define the pre- or poststyloid location of a given lesion. Prestyloid lesions often result in the posterior displacement of the carotid artery, while poststyloid lesions often displace the carotid anteriorly. Displacement of the carotid by poststyloid neurogenic tumors may be somewhat variable depending on the location of the tumor with respect to the great vessels. The presence of a fat plane between the parotid gland and tumor distinguishes intraparotid from extraparotid masses. Lesions which may enhance with contrast on CT include neurilemmomas, paragangliomas, hemangiomas, and other vascular lesions. Malignant lesions may be suspected in the presence of irregular or infiltrative tumor margins, bone erosion, or cervical or retropharyngeal adenopathy.

MRI is said to distinguish soft tissues to a higher degree than does CT and is currently the most useful study to most thoroughly assess PPS neoplasms. Again, the direction of carotid displacement is useful in differentiating prestyloid pleomorphic adenomas from poststyloid neurilemmomas. The presence of signal flow voids helps distinguish paragangliomas from less vascular neurilemmomas.

If the presence of a paraganglioma is suspected, patients should be evaluated for the possible production of catecholamines by the tumor to facilitate perioperative management. Urinary vanillylmandelic acid, metanephrines, dopamine, epinephrine, and norepinephrine can be quantified alone with serum catecholamines.

Particularly vascular tumors or those with suspected carotid involvement can be evaluated with angiography when necessary. Subsequent embolization may assist in minimizing intraoperative blood loss. Carotid occlusions tests should be performed in those patients for whom carotid resection may be required.

Fine needle aspiration biopsy may be performed safely by the transoral route and may be helpful in confirming the presence of suspected pleomorphic adenoma. Other forms of transoral biopsy should be avoided due to the risk of hemorrhage and the potential for tumor spillage and scar formation which might hinder effective tumor removal and result in tumor recurrence.

SURGICAL APPROACHES

A wide variety of surgical approaches for access to the PPS have been described. External approaches are by far the most commonly used and should be planned to provide adequate exposure for complete tumor removal with identification and control of vital neurovascular structures in the region. As the majority of neoplasms encountered in the PPS are benign, it is essential that any surgical procedure in the PPS be designed to mininmize morbidity and mortality. Any approach should be designed to allow extension of the dissection in the event that additional exposure is necessary. The most commonly used approaches will be briefly described.

Cervical or Cervical-Parotid

The cervical approach employs a transverse incision in the submandibular region with subsequent removal or displacement of the submandibular gland. Incision of the fascia in deep posterior submandibular triangle provides access to the PPS. The digastric, stylohyoid, and styloglossus muscles as well as the stylomandibular ligament may be divided with subsequent anterior displacement of the mandible to provide additional access.

The cervical-parotid approach incorporates the previously described cervical approach with an extension of the submandibular incision into a standard preauricular parotidectomy incision. The main trunk of the facial nerve is identified with subsequent dissection of it various divisions and superficial and/or total parotidectomy.

Cervical or Cervical-Parotid with Midline Mandibulotomy

Either of the above approaches may be combined with a midline mandibulotomy performed in a step fashion to afford wider exposure for larger tumors. Other forms of mandibulotomy and mandibulectomy have been used as well. This approach minimizes morbidity from disruption of the inferior alveolar nerve. The mandibulotomy is reconstructed using plates or wires.

Transparotid

The transparotid approach is most often used for the removal of deep lobe parotid tumors (esp. dumbbell tumors). Superficial parotidectomy is followed by dissection and isolation of the facial nerve from the deep lobe. Dissection then proceeds posteroinferior to the mandible for access to the PPS. This approach may also be combined with mandibulotomies of various types.

Transoral and Transoral-Transcervical (Transparotid)

The transoral approach is typically only used for selected cases such as pleomorphic adenomas arising from minor salivary gland tissue and involving the palate or lateral pharyngeal wall. In the past, problems with this approach have included inadequate exposure, increased risk of tumor spillage or inadequate removal, and injury to neurovascular structures. This approach may be combined with an external or transcervical approach to improve exposure.

Cervical-Transpharyngeal

The cervical-transpharyngeal approach combines the cervical approach with midline mandibulotomy (mandibular swing) principally for access malignant or highly vascular lesions of the PPS.

Infratemporal Fossa

The preauricular-lateral infratemporal fossa approach provides access to lesions of the PPS which may involve the cranial base.

COMPLICATIONS

Complications as a result of surgery of the PPS are most likely to occur with removal of paragangliomas, neurogenic tumors, and malignant lesions. Problems may be minor, such as the presence of a seroma, or severe, such as serious neurovascular injury or even death.

Neurovascular injury is always a concern with surgery of the PPS due to the important neurovascular structures contained in the region. Carotid artery injury is a possibility when malignant or vascular lesions are associated with this vessel. Removal of neurogenic tumors may result in CN deficits for CNs IX-XII or injury to the sympathetic chain. The facial nerve is at risk with any dissection in the parotid region. These deficits may be well-compensated in some patients while others may require various forms of intervention to minimize morbidity.

Mandibulotomy complications such as loss of dentition, inferior alveolar nerve injury, or a mal- or non-union can be minimized with proper surgical technique, choice of the proper surgical approach, and attention to pre-existing dental disease.

Tumor recurrence has particularly been after removal of pleomorphic adenomas of the PPS. Disruption of the tumor capsule followed by spillage of tumor cells is the proposed mechanism. Recurrence may also be found after inadequate excision of vascular or malignant lesions, especially those involving the cranial base.

Other complications may include infection, hematoma or seroma formation, and leakage of cerebrospinal fluid.

CONCLUSIONS

The PPS is a complex anatomical region containing many vital neurovascular structures. The majority of tumors arising in this area are benign salivary gland or neurogenic neoplasms. Surgical resection is the mainstay of therapy for these lesions, and careful preoperative planning is essential. A cervical approach is adequate for removal of most of these tumors, but a flexible approach should be planned to provide additional exposure as necessary with minimal associated morbidity and mortality.

BIBLIOGRAPHY

Becker TS: Salivary Gland Imaging. In: Bailey BJ. Head and Neck Surgery- Otolaryngology. JB Lippincott, Philadelphia, 1993.

Carrau RL, Myers EN, Johnson JT: Management of Tumors Arising in the Parapharyngeal Space. Laryngoscope, 100:583-589, 1990.

Olson KD: Tumors and Surgery of the Parapharyngeal Space. Laryngoscope, 104 (suppl. 63):1-28, 1994.

Som PM, Biller HF, Lawson W: Tumors of the Parapharyngeal Space: Preoperative Evaluation, Diagnosis and Surgical Approaches. Ann Otol Rhinol Laryngol, 90:3-15, 1981.