Conservative Posterior Petrosectomy
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Posterior petrosectomy is the key component of the posterior petrosal approach. This skull base osteotomy is a combination of 1) temporal craniotomy, 2) posterior petrosectomy, and 3) suboccipital craniotomy.
Posterior petrosectomy involves partial resection of the petrous pyramid to allow access to the petroclival region, anterior cerebellum, and brainstem at the level of cranial nerves (CNs) V-X. In other words, this route facilitates access to the petroclival region bounded by the middle fossa dura superiorly and the jugular bulb inferiorly.
Petroclival lesions above this level are more suited for an anterior petrosectomy. Lesions below the level of the jugular tubercle can be readily reached through an expanded retromastoid exposure.
In essence, posterior petrosectomy is centered over the petrous ridge and is comparable to pterional approach that is based on the sphenoid ridge.
There are several variations of posterior petrosectomy that differ in the quantity of temporal bone resected. Stepwise removal of the temporal bone expands the ventral operative trajectory, but also increases the risk of CN injury, particularly to CNs VII and VIII.
Several expansive modifications of posterior petrosectomy can be defined based on a progressive increase in bone removal to allow a wider and more direct approach to the anterior brainstem along the cerebellopontine angle: retrolabyrinthine, transcrusal, translabyrinthine, transotic, and transcochlear. The transotic and transcochlear approaches sacrifice hearing and are associated with a risk of facial nerve injury (Table 1).
|Stage||Temporal Bone Structures Removed|
|Mastoidectomy with skeletonization of the lateral petrous bone, semicircular canals are preserved.|
|Transcrusal (partial labyrinthectomy)||Superior and posterior semicircular canals are removed from the ampullae to the common crus|
|Horizontal canal and vestibule are removed and the lateral internal auditory canal is opened.|
|Transotic||Otic capsule is removed, including the cochlea, vestibule, and semicircular canals. Facial nerve is skeletonized, but left covered by bone. The wall of the external auditory canal may be divided, the ear canal sutured, and ear structures reflected forward. Carotid is exposed and the eustachian tube is plugged.|
|Same as transotic plus posterior mobilization of the facial nerve by cutting its anterior branches from the geniculate ganglion.|
When combined with the subtemporal approach, posterior petrosectomy expands the exposure as listed in Table 2.
|Variations of Posterior Petrosectomy||Anatomic Structures Accessible|
Middle half of the clivus, middle fossa, cerebellopontine angle, lateral aspect of the brainstem
|Middle half of the clivus, middle fossa, cerebellopontine angle, anterolateral aspect of the brainstem|
|Transcochlear||Middle half of the clivus, middle fossa, cerebellopontine angle, central clival depression, anterior aspect of the brainstem|
Most operative extra-axial lesions displace the brainstem and provide additional working space after their decompression; therefore, the retrolabyrinthine and translabyrinthine approaches are usually adequate to access the anterior lesions crossing the midline.
In this chapter, I attempt to describe a limited posterior petrosectomy involving the retrolabyrinthine approach in combination with subtemporal and suboccipital craniotomies. This modification of posterior petrosectomy is the most conservative osteotomy and carries the lowest risk of hearing loss and facial nerve injury.
The main limitation of this approach is that in most patients, it does not provide direct access to the region of the clivus known as the central clival depression. The central clival depression is the area of the clivus nearest the midline between the intermeatal plane above and the jugular tubercles below. Access to this area decreases via the retrolabyrinthine approach in individuals with more acute petroclival angles.
In my experience, in most cases, intracapsular tumor decompression provides an ample amount of space to reach the anterior brainstem, with no need for more extensive bone removal as described for the other variations of posterior petrosectomy. In patients with a very pronounced central clival depression, the retrosigmoid or transcondylar approaches can, in fact, provide a more desirable operative working angle.
The next chapter of this volume focuses on a more extended posterior petrosectomy that via additional petrous drilling skeletonizes the hearing apparatus to provide a more anterolateral trajectory to the brainstem. The more expanded variations of this skull base approach through a radical petrous pyramid resection are rarely necessary and may lead to avoidable morbidity.
Indications for Posterior Petrosectomy
Partial resection of the petrous pyramid facilitates exposure of the inferior two-thirds of the petroclival region and its associated extra-axial lesions such as meningiomas.
Vascular pathologies such as aneurysms of the middle third of the basilar artery and ventrolateral pontine cavernous malformations can be accessed as well. However, lesions along the lower third of the clivus and foramen magnum are more effectively exposed through the retrosigmoid and/or transcondylar corridors.
The supratentorial ventrolateral brainstem or anterior midbrain may be accessed through this approach via an inferior-to-superior working trajectory. Retrochiasmatic craniopharyngiomas were previously reached through this route. However, the endoscopic transnasal corridor has obviated the use of petrosectomy for this purpose.
Epidermoid tumors can be removed with relative ease through the extended retrosigmoid approach even if they cross the midline and center themselves along the ventral brainstem. These tumors’ soft consistency and relative lack of adherence to the surrounding neurovascular structures allow the more limited approaches to perform adequately.
I use posterior petrosectomy and its extended variant for large meningiomas of the middle to lower two-thirds of the clivus. Almost all other lesions can be handled via the extended retrosigmoid craniotomy or the conservative transcondylar osteotomy.
The presence of parenchymal edema in the brainstem is an ominous sign—it signifies a high risk of pial violation if the operator attempts gross total resection. Extension of the tumor to the midline or the contralateral side increases the technical complexity of the procedure. Encasement of the basilar artery is not uncommon, and does not necessarily correlate with the absence of arachnoid planes around the artery.
A computed tomography scan can define the degree of tumor calcification. Highly calcified meningiomas present a daunting challenge, and their encasement of the surrounding vital vessels should warn the surgeon against radical resection. Surgery on these tumors should be limited to conservative debulking and brainstem decompression.
I do not use preoperative embolization for meningiomas, but I do so for glomus jugulare tumors and hemangiopericytomas within the posterior fossa. The operator is often unable to devascularize deep-seated posterior fossa tumors early in surgery. They may be out of reach due to the surrounding cranial nerves and limited working space to mobilize the tumor and expose its base without its initial generous enucleation. This feature may encourage one to embolize the tumor preoperatively.
Brainstem auditory evoked responses (BAERs) and somatosensory evoked potentials (SSEPs) monitoring are used during the petrosectomy procedures. Facial nerve electromyography and lower cranial nerves’ monitoring (including endotracheal tubes with electromyographic sensors) may be needed for select patients.
A CT or catheter angiogram is beneficial in localizing displaced vascular anatomy and the location of the anteriorly displaced vein of Labbé that can affect the subtemporal trajectory. In addition, the presence of major veins traversing the tentorium discourages a posterior approach during which the tentorium would need to be sectioned. Alternatively, combined approaches obviating the need for tentorial sectioning are recommended.
The detailed anatomy of the temporal bone is relevant. The surgeon needs to gain extensive experience in the laboratory with temporal bone drilling to effectively perform transtemporal approaches. The participation of our otologist colleagues during the procedure and specifically during the petrosecotmy stage is recommended.
CONSERVATIVE POSTERIOR PETROSECTOMY
The operative corridor through the posterior transpetrosal approach is narrow, requiring extradural retraction of the temporal lobe and cerebellum during bone work before reaching the cerebellopontine angle cisterns to drain cerebrospinal fluid (CSF). Consequently, intraoperative implantation of a lumbar drain before skin incision should be strongly considered.
I have not experienced any complication such as herniation syndromes related to the use of a lumbar drain, regardless of the large size of the tumor and the associated mass effect. Gradual CSF drainage is used during burr hole placement and elevation of the bone flap. This decrease in intracranial tension facilitates mobilization of the dura and its associated venous sinuses away from the inner skull surface and protects them during bone work.
I drain 20cc of CSF upon completion of the skin incision and then drain up to 40 cc more (in 10-cc aliquots) as necessary to achieve adequate dural relaxation during bone work and manipulation of the dura along the middle and posterior fossae. This maneuver obviates the need for fixed retractors and diuretics, including mannitol.
The patient is positioned supine on the operating room table. The thorax should be slightly elevated to promote venous drainage. A shoulder roll is placed under the patient’s contralateral shoulder and the patient's head is rotated until the sagittal suture is nearly parallel with the floor.
If the patient has a supple neck, up to 70 degrees of rotation may be applied. If the patient has limited neck mobility, the size of the shoulder roll can be increased to compensate for the limited neck rotation.
I collaborate with my otology colleagues for matoidectomy.
In this form of nonextended retrolabyrinthine approach, hearing preservation is an important goal, and aggressive skeletonization of the bony labyrinth is not attempted. If an accidental disruption of the bony labyrinth occurs but the membranous labyrinth is intact, a piece of fascia may be used to cover the defect to preserve vestibular function.
However, if both the bony and membranous components of the labyrinth are transected, a small piece of wax must be inserted in the defect. In this situation, postoperative vertigo and/or hearing loss can be expected. For an extended retrolabyrinthine approach that includes skeletonization of the labyrinth, refer to the chapter on Extended Posterior Petrosectomy.
Dural opening involves the transection of the tentorium.
Following surgical treatment of the pathology at hand, closure begins. The dura is approximated primarily, but watertight closure is not feasible and additional measures are taken to prevent CSF leakage. All exposed air cells are meticulously waxed, and strips of adipose tissue or fascia lata are placed across the dural opening to seal the dural defect.
Additionally, the vascularized periosteal flap that was harvested during exposure is used to cover the defect in the dura. The bone flap is replaced and secured using miniplates, and the rest of closure is conducted in a standard fashion.
A lumbar drain is continued to remove 8cc/hour of CSF for 48 hours after surgery. Patients are mobilized as soon as possible.
Pearls and Pitfalls
- Posterior petrosectomy is rarely needed, even in a busy skull base surgery practice. Retromastoid craniotomy and its expansion through unroofing of the sigmoid sinus and the transtentorial routes typically allows adequate exposure of some petroclival tumors. Nonetheless, a petrosectomy provides the surgeon with multiple flexible operative working angles to handle a fibrous and vascular meningioma. This corridor leads to tumor devascularization early in surgery while minimizing brain retraction.
- The use of lumbar CSF drainage facilitates intracranial decompression, therefore expediting dissection of the dural venous sinuses away from the inner skull during craniotomy and bone work.
- Collaboration with our otology colleagues is paramount during execution of mastoidectomy while protecting the labyrinth.
- Unlike the transverse sinus, the sigmoid sinus is embedded in the skull bone; patience and meticulous bone drilling is require for its preservation. If necessary, a thin shell of bone should be left on the sigmoid sinus to maximize its protection.
- The incision along the tentorium should be completed as close to the petrous ridge as possible to avoid creation of a sizable tentorial flap that could interfere with the operative corridor.
For additional illustrations of combined transpetrosal-middle fossa approaches, please refer to the Jackler Atlas by clicking on the image below:
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