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.