Fig. 8.1
Diagram of positions of meningiomas in axial and sagittal plane. APFM (a, b), MPFM (b, e). PPFM (c, f) (Published with permission)
Tumors of the middle petrous face straddle the IAC and usually have a point of origin above the internal acoustic meatus and can be referred to as “middle petrous face” meningiomas (MPFM) (Fig. 8.1b, e). These are the tumors that Cushing referred to as “those simulating acoustic tumors ,” as they present with audiovestibular symptoms such as tinnitus, hearing loss, dizziness, and vertigo [9]. The hearing loss tends to manifest as decreased pure tone audiometry and impaired speech audiometry. Interestingly, high-frequency hearing is preserved, in contrast to the high-frequency sensorineural hearing loss that occurs with vestibular schwannoma [2].
Tumors arising from the region from the IAC posteriorly to the sigmoid sinus can be referred to as “posterior petrous face” meningiomas (PPFM) (Fig. 8.1c, f). In the lower 1/3 of the petrous face lies the vestibular aperture, an oblique opening in the bone for the vestibular aqueduct. The vestibular aqueduct terminates in a blind sac called the endolymphatic sac that lies partially embedded in folds of posterior petrous face dura. Even small tumors overlying the endolymphatic sac have been associated with audiovestibular symptoms resembling Meniere’s syndrome . It is likely that compromise of the as yet poorly understood function of the endolymphatic sac gives rise to alterations in the fluid spaces of the inner ear, thus potentially giving rise to cochlear and vestibular dysfunction [8]. Large tumors of the posterior petrous face are associated with symptoms of cerebellar dysfunction, ataxia, or elevated intracranial pressure. Figure 8.1 shows the proposed anatomic locations for each of the three petrous face locations and the approximate relationships to cranial nerves 5 and 7–12. Large tumors that span the entire petrous face can be referred to as AMPPF meningiomas.
Surgical Approaches
Most petrous face meningiomas can be exposed and resected via a standard retrosigmoid craniotomy. Preoperative preparation includes a volumetric magnetic resonance (MR) imaging study with 1.25 mm slices for image guidance during surgery. Since the approach is behind the mid-coronal plane of the skull, we use scalp-based fiducials for registering the image to physical space. An MR venogram can be included and is used to assess patency and dominance of the venous sinuses preoperatively.
The patient is positioned supine with a padded 1 Liter intravenous fluid bag under the ipsilateral shoulder, the head is secured with pins in the head holder, and the head is rotated 60° to the opposite side and then gently laterally extended down toward the floor. Kidney rests are placed along the side opposite of the exposure to secure the patient during rotation of the body away from the surgeon, which is used to obtain optimal lateral and superior views. Intraoperative neuromonitoring of cranial nerves 5, 7, 8 (Auditory Brain Responses (ABR)), and 11 is established. For larger tumors, somatosensory and motor-evoked potentials may also be monitored. Tumors with adjacent brainstem edema are usually not completely resectable due to the obscured pial-tumor interface. We have tried to move away from the routine use of a lumbar drain. A preoperative lumbar spinal tap for drainage of 30 mL of cerebrospinal fluid (CSF) is one option for decompression. The image guidance reference arc should be positioned anterior to the surgical field. After image registration, the position of the transverse-sigmoid sinus junction is marked on the scalp. A C-shaped incision is marked behind the ear using the top of the pinna and mastoid tip as the superior and inferior end points. The skin flap is dissected subcutaneously and rotated forward over the ear. Then the fascia over the mastoid process is incised with electrocautery from near the tip superiorly to the base of the mastoid process and then posteriorly along the superior nuchal line. The posterior aspect of the temporalis muscle is dissected subperiosteally forward and the suboccipital muscles posteriorly and inferiorly. Initially a self-retaining retractor is used but after the soft tissue dissection can be replaced by scalp hooks so as not to interfere with access to the operative field once under the microscope.
Once the soft tissue dissection is complete, image guidance is used to select burr hole positioning, and then a small craniotomy or craniectomy is completed exposing the inferior surface of the transverse sinus and the posterior surface of the sigmoid sinus. The mastoid emissary vein is a good landmark to follow toward the sigmoid sinus as it is posterior to the sinus. It can be isolated by removing the surrounding bone and can be coagulated and cut close to the sigmoid sinus, or it can be left in a bony canal up to the back edge of the sinus. Care must be taken to not use a volume of bone wax so as not to stuff in too much wax sufficient to encroach upon the lumen of the sigmoid sinus and thus cause thrombosis. In addition to bone wax, excessive use of any hemostatic agent, including liquid ones, can also lead to sinus thrombosis, so these should be avoided as well. If bleeding is encountered, a simple Gelfoam placed over the sinus followed by a cotton patty should be sufficient to control bleeding without thrombosing the sinus. We typically avoid the “extended retrosigmoid approach” advocated by others [14] due to our experience with two cases of intraoperative sinus thrombosis and cerebellar swelling. During the standard retrosigmoid approach, opening the dura toward the foramen magnum and subsequently opening the cisterna magna arachnoid allow for drainage of CSF, cerebellar relaxation, and exposure of the tumor.
There are some nuances for each location, which will be covered below based on an experience of over 115 cases.
APFM
These tumors reside high and anteriorly in the CPA , and we perform suprameatal drilling in nearly every case to facilitate exposure of their base of attachment. It is our practice to do these cases with our neuro-otologists who perform the drilling. The removal of this bone is completed using a combination of round cutting and diamond burrs. The bone is removed over the midportion internal auditory canal until the dura is exposed to delimit the inferior portion of the dissection, and the bone is progressively removed superiorly. The subarcuate artery is drilled through and controlled with bone wax or drilling and can be taken without adverse sequelae. Posterolaterally, care must be taken to not enter the superior semicircular canal. Medially the dissection may be continued until the entry of the trigeminal nerve or tumor is clearly seen into Meckel’s cave. Removal of this bone carries with it the advantage of removing a common site of the attachment of the meningioma to its dural base, which may devascularize the tumor and simplify further dissection. We typically preserve the superior petrosal vein when possible. The fifth nerve is usually displaced superiorly and medially to the tumor. Larger tumors have variable extension into Meckel’s cave. While ultrasonic aspirators are efficient at tumor debulking, their size can pose a problem due to the depth of the tumor in the CPA. The CO2 laser is small enough to fit easily into the angle and can be used to cut pieces of tumor out, debulk the central portion of the tumor, and vaporize the base of the tumor attachments.
MPFM
The main challenge in removing MPFMs is their relationship to the cranial nerve 7–8 complex and the risk of hearing loss or facial weakness. Similar to tumors around the optic apparatus, we have found it beneficial to debulk the tumor first before attempting to dissect the nerves from the surface of the tumor. Often there is a tongue of tumor or hypervascular tissue that extends into the internal auditory canal (IAC). Removal of the bone 270° surrounding the IAC with round diamond and cutting burrs permits the removal of these tumor extensions allowing more complete removal. If hearing preservation is a priority, then care should be taken to not enter the vestibule laterally. This limits the dissection of the lateral most few millimeters of the IAC. The preoperative scans should also be evaluated for evidence of a high-riding jugular bulb which may compromise the bony removal over the IAC. All of the drilling of the IAC should be completed prior to opening the dura of the IAC. If it appears that the meningioma extends into the IAC like a carpet, we accept near total removal while monitoring evoked potentials from the seventh and eighth nerve. It should be noted that there is often enhancement that extends into the IAC visible on preoperative MRI scans that may correspond to hypervascular dura rather than frank tumor involvement. Indeed, often residual enhancement seen on early post-op scans fades over the subsequent 12–18 months, suggesting in these cases that the enhancement was simply hypervascular tissue and not residual tumor.
PPFM
Small tumors in this location may be operated on when there is a consistent and disabling audiovestibular syndrome presumably related to the function of the endolymphatic sac. Hyperostotic bone should be drilled down in this region when encountered. When the tumors are very large in this location, a modified far lateral approach can assist with CSF release in the upper cervical spine as reported previously by Sanai et al. [14].
Case Examples
Case 1: APFM
A 73-year-old woman presented after several years of left facial numbness and atypical facial pain, multiple dental procedures, and root canal surgery with no relief. She was evaluated by a neurologist who identified trigeminal sensory loss. MR imaging revealed a small/medium APFM impinging on the fifth nerve root entry zone (Fig. 8.2a). A retrosigmoid craniotomy was done with neuromonitoring, suprameatal drilling, and the use of the CO2 laser to complete a Simpson Grade 2 removal (Fig. 8.2b). Her postoperative course was uncomplicated with relief of her facial pain syndrome and gradual improvement in facial numbness over the next 18 months.
