MANAGEMENT OPTIONS FOR SKULL BASE

* Clinical Research Fellow

** Consultant, Dept. of Otorhinolaryngology

The James Cook University Hospital

South Tees NHS Trust, Middlesbrough, UK.

 

Corresponding Author:

Anirvan Banerjee

Consultant, Dept. of Otorhinolaryngology

The James Cook University Hospital

Marton Road

Middlesbrough TS4 3BW

Tel/Fax: +44 164 2854040

Email: Anirvan.Banerjee@stees.nhs.uk

MANAGEMENT OPTIONS FOR SKULL BASE/ PETROUS APEX LESIONS - REVIEW ARTICLE

Abstract:

Surgeons approach lesions of the skull base with understandable reluctance. The problems being surgical inaccessibility, the obstacles of vital neural and vascular anatomy and the overwhelming surgical mortality rate as a result of haemorrhage and sepsis. There has been a technical revolution in microsurgery, anaesthesia and neurodiagnosis. Armed with technology surgery has become the mainstay in management of these dreaded lesions.

This article focuses on glomus tumour and its associated lesions. Their diagnosis and new treatment concepts are discussed. The various choices available for these lesions are discussed.

Key Words:

Skull base surgery, management, interventional neuroradiology, skull base tumour, diagnosis, conservative treatment (or therapy), petrous apex

MANAGEMENT OPTIONS FOR SKULL BASE/ PETROUS APEX LESIONS - REVIEW ARTICLE

The management of skull base and petrous apex lesions is uniquely challenging as they arise in an area which is anatomically complex and involves critical neurovascular structures. The lesions are usually histologically benign but they behave as if locally malignant with extensive invasion of bone, soft tissue and nerves.1 The skull base can be classified into three distinct areas. The lateral skull base - temporal bone and cerebellopontine angle (CPA); anterior skull base - cribriform plate and the anterior cranial fossa; and central skull base - greater and lesser wings of sphenoid, sella turcica and the clivus.2 The petrous apex is a pyramid shaped structure that is the most medial aspect of the temporal bone. When viewed from above this region can be divided into anterior and posterior segments by drawing a parallel line through the internal auditory canal.3 Disease processes most frequently involve the much larger anterior portion, which lies anteromedial to the cochlea and internal auditory canal.  The smaller, and clinically less significant posterior portion lies between the semicircular canals and the IAC.

Presentation

Petrous apex lesions present with hearing loss as the most common symptom followed by vestibular dysfunction, headache, tinnitus, facial spasm, diplopia, facial paralysis, and otorrhea. These symptoms often present months or years before diagnosis, and incidental discovery is not uncommon.4

Investigations: Audiology - Air and bone conduction pure tone audiometry is the minimum requirement for patients with lateral skull base tumours. Speech audiometry is essential when contemplating hearing preservation surgery.  

Vestibular function tests – Caloric testing is by far the most common vestibular function test described in relation to skull base lesions.2

Tests of facial function – Electroneurography (ENoG) is used as a predictive test both pre- and post-operatively in patients with skull base lesions.

Radiology – Principal reasons for imaging include: (1) screening; (2) investigation of suspicious symptoms or signs; (3) surgical planning and navigation and (4) for monitoring size, extent or recurrence of lesions. In several instances both CT and MRI are complementary and essential.5, 6

Some patients may also require carotid and vertebral artery angiography. Magnetic resonance angiography and venography is a low risk technique of assessing the skull base vasculature. Although the resolution of these studies is good, the primary disadvantage is the inability to perform interventional measures.7 CT angiography and venography allow for another detailed means of assessing petrous apex lesions and vessels. This technique uses ultra-thin slices with timed administration of contrast and provides incredible detailed information. An additional advantage of temporal bone CT angiography compared with its counterparts is the ability to define the vascular structures and their relationship with the lesion and other structures in or adjacent to the petrous apex.3, 8  

Positron emission tomography (PET) and single photon emission computerized tomography (SPECT) can be extremely helpful in certain circumstances. PET is useful in differentiating granulation or scars from recurrent or residual disease.2 SPECT is superior to CT in detecting skull base erosion9 and in vascular tumours the radiotracer shows specific uptake followed by rapid ‘wash-out’.10 However PET and SPECT have limited resolution and can fail to detect small lesions.11

The detection of multiple paragangliomas or metastatic disease has been facilitated by radionucleotide scintigraphy with metaiodobenzylguanidine (MIBG) or indium-11-octreotide.12

General investigations – Endocrine activity has been reported in a small subset (1%-3%) of head and neck chemodectomas.13 24 hour urinary vanillylmandelic acid (VMA) detects a secreting tumour that could cause problems during surgery.

Biopsy is the only diagnostic technique that allows tissue diagnosis of skull base tumors. In many cases, the clinical presentation and diagnostic imaging provide sufficient information upon which to base a treatment decision. Because surgical excision is the treatment of choice for the vast majority of benign skull base tumors, a histologic tissue diagnosis is unnecessary prior to definitive resection. However, an attempt at biopsy is indicated for cases in which diagnostic imaging has not sufficiently narrowed the diagnostic possibilities, in very slow-growing lesions for which observation is contemplated, or when the patient is not a good surgical candidate. Depending on the tumor location, both open biopsy and needle biopsy have indications. Open biopsy of tumors manifesting with palpable masses close to the surface can be approached through a simple skin incision. Biopsy of sinonasal masses can be performed with endoscopic guidance. Needle aspiration biopsy using either CT or MRI guidance can be used to safely biopsy deeper skull base tumors.14

Diagnostic and Therapeutic Angiography

Many skull base/ petrous apex tumours are either intrinsically hypervascular or secondarily involve arteries and veins of the brain that enter and exit the cranium through the skull base. Angiography, embolization and major artery occlusion have a pivotal role in investigation and treatment of these conditions. The potential benefits that these procedures offer must outweigh the risks of their possible complications.

Diagnostic angiography is considered especially when evaluating the suitability of tumour to embolization. It also provides the surgeon with information regarding the anatomy of the circle of Willis and the likely tolerance to potential carotid sacrifice.15

Embolization is carried out to reduce intraoperative blood loss and improve visualization of the operative field during surgery. It is rarely curative but reduces the rate of radical tumour removal, surgical complication rate and incidence of recurrence. The tumours that benefit include glomus tumours, meningiomas, chordomas, sarcomas, oestrogenic tumours, metastases, olfactory neuroblastomas and junvenile angiofibromas.15

Internal carotid artery (ICA) test and permanent occlusion is carried out in tumours intimately related to the ICA or tumours that receive significant arterial supply from extradural ICA branches. The assessment of cerebral circulation to tolerate permanent sacrifice of a major vessel is warranted in these cases. Permanent balloon occlusion can follow successful test occlusion of the ICA if the patient does not experience any neurological dysfunction during the test and if there is adequate venous phase opacification when injecting contralateral carotid artery. An external-internal carotid bypass should be considered for those patients who fail the test procedure yet need carotid occlusion.16

Treatment

As a result of specific challenges presented to the surgeon by the lesions of the skull base, it is understandable that several therapeutic options for their treatment exist. However they are all not equally successful in accomplishing a curative end. Total surgical excision is the only treatment modality that offers cure for patients with tumours of skull base.17

Surgical approaches to the Skull Base

Infratemporal fossa approach (ITF)

            This approach provides access to the cavernous sinus, clivus, nasopharynx, and petrous apex. Fisch described several lateral ITF approaches centered around the subtemporal exposure and rerouting of the facial nerve.

The Fisch A approach is indicated for lesions within the temporal bone, such as glomus tumors. This approach involves the exenteration of the middle ear, a subtotal petrosectomy, and a permanent anterior transposition of the facial nerve.

The Fisch B and C approaches are designed to approach more anterior pathology involving the petrous apex and clivus. The critical maneuvers in the type B ITF approach are the reflection of the zygomatic arch and temporalis muscle inferiorly and removal of the bone of the skull base floor to provide access to the ITF. A key to this extradural exposure is the subtotal petrosectomy. This step includes a canal-wall down mastoidectomy including complete skeletonizing of the labyrinth, facial nerve, sigmoid sinus, middle and posterior fossa dura, and the jugular bulb, as well as exenteration of all hypotympanic air cells and skeletonizing of the ICA.

The type C approach is an extension of the type B and is used for lesions of the anterior ITF, sella, and nasopharynx. The feature distinguishing the type C from the type B approach is resection of the pterygoid plates. This permits exposure of the lateral wall of the nasopharynx, eustachian tube orifice, posterior maxillary sinus, and posterior nasopharyngeal wall past the midline.

The type D approach is a preauricular ITF approach that uses orbitozygomatic osteotomies and resection of the floor of the middle fossa to expose the medial middle cranial fossa without a lateral temporal craniotomy. Subtype D1 addresses tumors of the anterior ITF, while the subtype D2 is designed for lateral orbital wall lesions and high pterygopalatine fossa tumors.18

Retrolabyrinthine

            The retrolabyrinthine approach is a true skull base approach that preserves hearing by following a direct route through the temporal bone to expose the cerebellopontine angle without manipulation of neural structures. The most common indications for this approach are resection of cerebellopontine angle and posterior petrous ridge tumors, vestibular neuronectomy, partial section of the sensory root of the fifth cranial nerve, fenestration of symptomatic arachnoid cysts, and biopsy of brain stem lesions.19

Transcochlear

            This approach is accomplished by forward extension of the translabyrinthine opening into the cerebellopontine angle. The facial nerve is mobilized in the temporal bone from the stylomastoid foramen to its entrance into the internal auditory canal. Having removed the barrier of the facial nerve, additional bone removal can be carried forward to the internal carotid artery, which now becomes the forward limit for temporal bone resection. The access attained through this exposure allows removal of tumors arising from the petrous tip, as well as tumors arising directly from the clivus.20

Translabyrinthine approach

            This was reintroduced by Hitselberger and House.21 This approach primarily used in cerebellopontine angle lesions provides wide access to the posterior fossa with little or no need for brain retraction. It is also used in the surgical management of petrous apex lesions when hearing is poor or the tumour is large.22 The bony exposure is performed in three stages: complete mastoidectomy, labyrinthectomy, and IAC dissection. The primary disadvantages of this approach are the loss of any residual hearing, worsening balance function, and vertigo that occurs immediately after the surgery.

Combined

            The combined approaches provide exposure for lesions that extend in the middle and posterior fossa. These approaches use a transtemporal approach (retrolabyrinthine, translabyrinthine, and transcochlear) in addition to a middle fossa craniotomy. The primary advantages of these combined approaches are decreased brain retraction, improved exposure and the possibility for hearing preservation (retrolabyrinthine). The primary disadvantage is sacrifice of hearing if either translabyrinthine or transcochlear approach are used.3, 22

Suboccipital

            The areas exposed in this approach are the jugular foramen, occipital condyle, lower clivus to the midline, petrous apex, tympanic cavity, the vertical portion of the intrapetrous carotid artery below the level of the eustachian tube, cerebellopontine angle, the jugulocarotid space in the upper neck. The approach is indicated for extra-, intra-, and transdural lesions of the jugular foramen area. The main advantages are no cerebrospinal fluid leak, preservation of the facial nerve, middle and inner ear functions. Lower cranial nerve deficit formed the major morbidity in the present series and is still an unsolved problem in such cases.23

Anterior craniofacial

            A Weber-Fergusson incision is employed and the approach is transmaxillary24

Middle fossa

             This is most commonly used in Vestibular Schwannoma surgery when attempting to preserve hearing. The primary disadvantages being limited accessible tumour size, temporal lobe retraction, limited posterior fossa exposure and increased risk of tumour recurrence.25

Surgical Approaches to the petrous apex

Infracochlear

            This approach is utilized in cystic lesions of the petrous apex in patients with serviceable hearing. The main advantages being a dependant drainage in a well aerated middle space adjacent to the eustachian tube, adequate access to the petrous apex despite a high jugular bulb, simple revision if required and preservation of the normal middle ear mechanisms.3, 26          

Subtemporal

            This approach provides variying degrees of exposure to the petrous apex, clivus, ventral brainstem and anterior cerebellopontine angle.27

Infralabyrinthine

            This is the most common approach to cystic lesions of the petrous apex in patients with serviceable hearing. A high-riding jugular bulb necessitates the use of infracochlear approach. The advantages of the infralabyrinthine approach include an anatomy familiar to most otologists, direct route to most cysts of the petrous apex, and avoidance of entering the middle ear. 3

Subarcuate and sinodural angle approaches

            These are mainly used in suppurative processes of the petrous apex.3

Supracochlear approach

            This approach allows for drainage or biopsy of lesions in the anterior superior aspect of the petrous apex. The advantage being the preservation of the external auditory canal and the labyrinth. Unfavourable aspects include a significant risk to the labyrinthine facial nerve and the potential need for removal of the malleus head and incus.3

Retrosigmoid approach

            This approach gives excellent access for tumours that arise in the cerebellopontine angle and involve the posterior cranial fossa. The suprameatal extension of the retrosigmoid approach allows improved access to the petrous bone anterior to the internal auditory canal.3

Transnasal endoscopic approach

            An endoscopic approach requires a wide corridor created with the removal of the middle turbinate ipsilateral to the lesion, a posterior septectomy, and bilateral wide sphenoidotomies. This allows a two-surgeon, four-hands technique, which involves the introduction of the scope, suction, and dissection instruments through both nares.28

Non-Surgical Management:

Advanced age, poor physical condition and extremely slow progress of the disorder are some of the indications for a non-surgical management to be considered.

Wait-and-see policy – A good alternative in glomus tumour, schwannoma or meningiomas which are small in size and are slow growing on meticulous follow-up examinations.29

Medical therapy – There are few indications for medical treatment in skull base lesions. In meningiomas that may be unresectable or refractory, complete tumour resection may not be possible, or surgery may be contraindicated adjuvant therapy is helpful. Trials have been undertaken but with disappointing results with tamoxifen, thalidomide glucocorticoids, hydroxyurea and several chemotherapeutics.29

Petrous apicitis is an inflammatory process often secondary to suppurative otitis media. Medical therapy is aimed at eliminating bacterial infection and promoting drainage with aggressive antibiotic therapy. Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus are the primary pathogens responsible for petrositis. Steroids may help decrease inflammation, pain, and swelling. Early surgical intervention is critical because of the severe complications secondary to chronic otitis media and petrous apicitis.

Skull base osteomyelitis is primarily a medical disease requiring long-term antimicrobial therapy directed against Pseudomonas aeruginosa. Severe otitis externa with granulation tissue in an immunosuppressed patient is the most common clinical setting. Often, the pain is described as deep and boring, with this symptom seeming out of proportion to physical findings. Aminoglycosides, coupled with an antipseudomonal penicillin derivative, are the antibiotics of choice. Quinolone antibiotics offer enteral therapy with encouraging results. Gallium-67 scanning is used to monitor the course of the disease. Technitium-99 scanning is more specific in the diagnosis, but findings remain positive after the course of the disease so they cannot be used to monitor therapy.30

Somatostatin therapy for glomus tumours – Histologically these tumours present a high concentration of somatostatin hormone-binding sites on their cell surface.31 Somatostatin 20 milligram deep intramuscular injections given once every three months can be considered as a further treatment option in recurrent or inoperable tumours.32

Radiotherapy – Radiotherapy alone has a significant role in the management of these tumours. Radiotherapy however carries the disadvantage of damaging the surrounding structures such as the cochlea, the facial nerve and adjacent brain tissue. Consequently radiotherapy is only applied to patients whose tumour is impossible to remove surgically or those who refuse surgery, are elderly or have a poor physical condition.29

Stereotactic Radiosurgery - Gamma knife radiosurgery has been proposed as an alternative to conventional external beam radiotherapy as it has the capability of delivering high-dose single fraction radiation while sparing adjacent neurovascular structures.

            This modality is used as an alternative to surgical treatment in vestibular schwannoma (tumours < 3.0 cm size),33 paragangliomas.34 Its role in meningioma is much more as an adjuvant to surgery, especially valuable in parts of the disease that is surgically inaccessible.35

 

SUMMARY

A technical revolution in microsurgery, anaesthesia and neurodiagnosis has made the management of skull base and petrous apex lesions less intimidating for the modern surgeons. There is need for more knowledge on the biological behaviour of the most common pathologies in the skull base. The role of radiotherapy and stereotactic radiosurgery still have to be defined. Prospective randomized studies are difficult due to the rarity of the lesions.

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