4. Discussion:
Benign tumors are the most common entity (80-90%) found in parapharyngeal space (PPS) with parotid gland as the most common site of origin [19, 20-21]. The parapharyngeal space is divided into 2 compartments by an osteomuscular aponeurotic sheath originating at the styloid process. The pre-styloid space contains components such as deep lobe of the parotid gland, fat and lymphatic nodes, inferior, lingual and temporal auricular alveolar nerves. The retro-styloid space encloses neurovascular structures such as carotid artery, jugular vein, cervical sympathetic chain, cranial nerves IX, X, XI and XII suggesting more diverse histology from this compartment [22]. Parapharyngeal space tumors must be clearly identified as erroneous anatomical boundaries may have resulted in inclusion of adjacent tumors in some of the published literature such as inclusion of deep lobe tumors should only be considered if the location is retro-mandibular. Similarly, lesions at foramen ovale must be a part of infratemporal fossa tumors while carotid body paragangliomas below the posterior body of digastric must not be considered a part of PPS tumors [23]. Due to complex anatomical location and diverse histology, precise approaches to the PPS tumors have remained a controversial debate [24, 25]. Complete excision with minimal morbidity should be the aim of the operating surgeon particularly when dealing with benign PPS tumors. Size, location (pre / post styloid), proximity to the skull base and vascular bundle, extension to the deep lobe of the parotid and imaging based position (superior/middle/inferior) of the lesions are some of the key factors considered pre-operatively to select a surgical approach [7, 13, 15].
The inclusion of mandibulotomy have raised many concerns such as facial scar, oro-cervical fistula, prolonged operating time, malocclusion, trismus, delay in resuming regular diet and risk for exposure of fixation plate and temporomandibular joint (TMJ) dysfunction [26-28]. The most used approach to access PPS tumors published in literature has been trans-cervical either alone or in combinations. In the cumulative series of 631 patients, this approach has been utilized for 60% of cases addressing both salivary and extra parotid neurogenic tumors. The facial nerve may not need to be exposed to further extend the dissection and exposure. Due to excessive push and pull, neuropraxia may occur in the post-operative setting. The facial nerve identification may increase the access to large sized and superiorly placed PPS tumors. The rationale behind the use of trans-cervical approach in our cumulative search was small (preferably less than 8cm), benign, pre-styloid extra parotid tumors in the lower parapharyngeal space. Further exposure can be achieved by division of posterior belly of digastric muscle and removal of submandibular gland on occasions.
The cervico-parotid approach uses formal identification of facial nerve trunk and indicated for tumors in proximity to the deep lobe of the parotid gland having the risk of adherence to the facial nerve. Trans-cervical approach only may jeopardize the preservation of facial nerve. In our cumulative results, it is the second most used approach well in accordance with previous published data. The most commonly used indications in the data for cervico-parotid access were large, pre or retro-styloid, deep lobe parotid or minor salivary gland as well as neurogenic tumors with or without facial nerve involvement locating middle to lower PPS but not involving the base of skull. Access may be further enhanced by prognathic mandibular dislocation dividing the stylomandibular ligament and styloid muscle.
The transmandibular approach with its modifications (Midline / paramedian or lateral splits) is usually performed for massive, large vascular or recurrent benign tumors placed more superiorly in the PPS. In the cumulative series, transmandibular approach was only used in 6% of the patients and more so because of the large size (mostly > 8cm), vascular nature of the tumor, superior location in the PPS and proximity to skull base. The use of transmandibular approach has been further limited as indicated by the recent studies showing decline in the use of transmandibular approach (9% to 6%) when compared with previous reports [29-31]. Prolonged hospital stays, delay in resuming normal diet, tracheostomy covering, trismus, associated TMJ and Occlusal disturbances are some of the drawbacks of this technique [32, 33].
The cumulative studies in our cohort have depicted significantly high number of neurogenic complications in both salivary and neurogenic tumors combined. Detailed analysis has shown these complications to be more common in tumors with neurogenic tissue of origin. Literature has described five major histological subtypes of neurogenic tumors with more than 90% having benign histology. Riffat et al. have reported paraganglioma to be the most common subtype while John et al. and Danke et al. have found Schwannomas to be the commonly found histological entity [34, 35]. Schwannomas arise usually from cranial nerves IX to XII or cervical sympathetic chain with decreased risk of nerve injury when small in size. On the other hand, paragangliomas are derived from vagal nerve with a potential for intracranial extension or malignant transformation. The list of complications in our combined series included vocal cord paralysis (73%), Horner’s syndrome (9.3%), hypoglossal nerve injury (6.7%) and first bite syndrome (4%). The tumors with salivary histology had a complication rate 12.8% as compared to neurogenic tumors (47.7%) which is significantly lower than neurogenic tumors (t value=2.42, p=0.023). This preemptive assessment of anticipated range of neurologic complications must be considered to educate the patients on the post-operative sequelae and simultaneously preparation to facilitate rehabilitation.
Our systematic analysis may have its share of limiting factors. Most of the studies included are retrospective series with inherent biases for selection of surgical approaches, clinical expertise, intraoperative and post-operative care with a variable support for post-operative rehabilitation. Majority of studies have omitted data on some critical factors when applying these surgical approaches such as length of hospital stay, cost effectiveness, return to normal diet or oral feeding in trans-mandibular approach, infection, time to healing and length of the surgery.