Transpetrosal Approaches to the Posterior Fossa

Ramachandra P. Tummala, M.D.; Ernesto Coscarella, M.D.; Jacques J. Morcos, M.D., F.R.C.S.

Disclosures

Neurosurg Focus. 2005;19(2) 

In This Article

Posterior Transpetrosal Approaches

The posterior temporal bone dissection includes the retrolabyrinthine, translabyrinthine, and transcochlear approaches. All three of these are extensions of the basic mastoidectomy. The retrolabyrinthine approach is the most minimal of the three, whereas the other two approaches represent a graduated increase in the extent of petrous bone resection (Fig. 1). The increased bone resection involved in the translabyrinthine and transcochlear approaches Results in a corresponding increase in anterior exposure. Nevertheless, the progressive gain in exposure is obtained at the expense of a risk to seventh and eighth cranial nerve function.

Photograph of a cadaveric dissection showing an overview of the temporal bone and depicting the posterior surface of the petrous part. The sphenoid bone, which articulates anteriorly with the petrous and squamous temporal bone, has been removed in this specimen. The pyramidal petrous part, located between the sphenoid and occipital bones, has a base, apex, and three surfaces. The sigmoid sinus descends along the posterior surface of the mastoid part and turns anteriorly toward the jugular foramen. The posterior transpetrosal approaches involve progressive degrees of resection of the petrous temporal bone. The retrolabyrinthine (green outline) dissection exposes the area between the superior petrosal sinus, the sigmoid sinus, and the posterior semicircular canal. The translabyrinthine approach (pink outline) extends more anteriorly to remove all three semicircular canals and to expose the anterior wall of the IAC. The transcochlear (blue outline) dissection extends even more anteriorly to the petrous apex, resulting in an almost complete petrosectomy with the widest and most direct exposure of all the posterior transpetrosal approaches. PET. = petrous/petrosal; POST. = posterior; RETROLAB = retrolabyrinthine; S.C. = semicircular canal; SIG. = sigmoid; SUP. = superior; TRANSLAB = translabyrinthine.

We routinely place the patient supine with the ipsilateral shoulder elevated and the head turned to the opposite side so that the mastoid process is at the highest point in the field. We do not hesitate to use the lateral position for patients with large or inflexible necks. The abdomen is also draped in preparation for an autologous fat graft harvest. A retroauricular C-shaped incision is made, and the scalp flap is retracted anteriorly. If a combined approach with a subtemporal craniotomy is planned, as is most common, the incision is curved anteriorly to expose the root of the zygoma just anterior to the tragus (Fig. 2). The translabyrinthine and transcochlear approaches are dependent on and are progressions of the retrolabyrinthine exposure.

Intraoperative photograph showing the standard retroauricular incision for a posterior transpetrosal combined subtemporal approach. The incision is curved anteriorly to allow exposure of the root of the zygoma just anterior to the tragus. The presumed locations of the transverse and sigmoid sinuses (dashed lines), based on the surface anatomy, are also indicated.

The mastoidectomy is initiated by wide removal of the cortical bone overlying the mastoid. Detailed descriptions of this procedure are found elsewhere.[12] The anterior boundary of the cortical bone removal is the posterior wall of the EAC. The remaining boundaries are the temporal line (linea temporalis or supramastoidal crest) superiorly and the tip of the mastoid inferiorly. The sigmoid sinus is skeletonized inferiorly through the infralabyrinthine air cells to the jugular bulb.

After exposure of the sinodural angle between the middle fossa dura, the superior petrosal sinus, and the sigmoid sinus, the next task is identification of the mastoid antrum.

Following the middle fossa dural plate and posterior EAC wall deep into the Macewen triangle leads to the antrum. The short process of the incus can now be identified in the fossa incudis and helps localize the lateral semicircular canal, which lies immediately below the antrum. The external genu of the facial nerve is located at the inferior surface of the lateral semicircular canal. Removal of air cells inferiorly also exposes the digastric ridge, and removal of the remaining air cells posterior to the lateral semicircular canal exposes the crossing posterior semicircular canal. Removal of the remaining inferior mastoid and retrofacial air cells skeletonizes the vertical segment of the facial nerve, which is located within the fallopian canal anterior to the digastric ridge. Complete skeletonization is unnecessary, and a layer of cortical bone is left intact over the facial nerve to prevent injury. Drilling through the supralabyrinthine air cells and following the sinodural angle to the petrous ridge will lead to the SSC. All three semicircular canals are therefore skeletonized but preserved. Removal of the bone plates on the middle and posterior fossa dura mater and the sigmoid sinus completes the extradural part of the retrolabyrinthine approach (Fig. 3). This approach provides exposure of the CPA but does not give the surgeon adequate visualization of the anterior brainstem or petroclival region (Fig. 4). Adequate exposure of the IAC is possible in most cases.[1] Visualization of the CPA may be compromised if the sigmoid sinus is larger than normal or anteriorly displaced.

Photographs of a cadaveric dissection showing the retrolabyrinthine approach, right side. This dissection was done in the context of a combined presigmoid subtemporal approach. A basic mastoidectomy was the fundamental step in this dissection. A: The boundaries of the cortical bone removal are the posterior wall of the EAC anteriorly, the middle fossa dural plate superiorly, and the sigmoid sinus posteriorly. B: The mastoid antrum lies deep to the suprameatal or Macewen triangle, which is bound superiorly by the posterior root of the zygomatic arch, posteriorly by a vertical line extending along the posterior border of the external auditory meatus, and inferiorly by the superior margin of the external auditory meatus. C: The sigmoid and superior petrosal sinuses, the middle fossa dura, and the posterior fossa dura have been skeletonized to reveal the sinodural angle. The bone labyrinth has been revealed without violating the individual semicircular canals. D: Overview of the retrolabyrinthine approach after dural opening, showing the limited exposure of the CPA and brainstem. AICA = anterior inferior cerebellar artery; CN = cranial nerve; J. = jugular; LAT. = lateral; PICA = posterior inferior cerebellar artery; V. = vein; ZYG. = zygomatic.

A and B: Axial and coronal T2-weighted MR images obtained in a 50-year-old woman presenting with a 1-year history of headaches, dysarthria, and imbalance. Her neurological examination revealed normal hearing and mild right dysmetria. To preserve hearing, this large petroclival meningioma was resected via a combined retrolabyrinthine–subtemporal approach. C: Intraoperative view obtained after completion of the extradural portion of the procedure. The middle and posterior fossa dural plates and the sigmoid sinus have been skeletonized. Note the limited but adequate working space between the sigmoid sinus and the bone labyrinth, although it averages 1 cm. D and E: Contrast-enhanced T1-weighted axial and coronal MR images demonstrating gross-total resection. Postoperatively, the patient experienced a right sixth cranial nerve palsy and transient worsening of her right-sided dysmetria. Her facial strength and hearing remained intact.

This dissection is an anterior extension of the retrolabyrinthine approach (Fig. 5). The initial part of the translabyrinthine dissection includes all of the steps outlined for the retrolabyrinthine approach. The limits of the dissection are the mastoid segment of the facial nerve anteriorly, the jugular bulb inferiorly, and the middle fossa superiorly.[2]

Photographs of cadaveric dissections showing the translabyrinthine approach, right side. A: The bone labyrinth has been opened to expose the individual semicircular canals. The tip of the instrument is in the ampulla of the SSC, which lies directly beneath the arcuate eminence. The lateral semicircular canal is the most laterally projecting one and the first one encountered in the translabyrinthine approach. It is the landmark for identifying the underlying labyrinthine segment of the facial nerve. B: Magnified view. The two instruments lie within the ampullae of the superior and posterior semicircular canals. These landmarks represent the superior and inferior aspects of the deep lateral portion of the IAC, respectively. C: Orientation of the IAC in relation to the seventh and eighth cranial nerve complex. D: Overview of the translabyrinthine dissection showing a wider exposure of the anterior CPA and prepontine cistern. In this specimen the exposure is relatively limited because of a high jugular bulb. GANG. = ganglion; LAB = labyrinthine; N. = nerve; SEG. = segment; STYLOMAST. = stylomastoid; TYMP. = tympan; VEST. = vestibular.

In the translabyrinthine approach, the semicircular canals are removed and the vestibule is opened to reveal the nerves to the lateral, inferior, and superior ampullae. The superior ampulla represents the lateral wall of the IAC fundus, and additional bone is removed until the IAC is skeletonized. The superior vestibular nerve is encountered first when the IAC is opened; it lies posterosuperiorly within the canal and is separated from the facial nerve by the thin vertical crest (the Bill bar). One advantage of the translabyrinthine approach is that the vestibular nerves are encountered before the facial nerve; this decreases the risk of injury to the facial nerve. Removal of bone from the superior wall of the IAC must be done cautiously because the facial nerve lies superficially under the dura mater and can be injured. The result of the additional bone resection in the translabyrinthine exposure is increased access to the anterolateral brainstem and inferior clivus (Fig. 6). Nevertheless, resection of the labyrinth and sacrifice of hearing are necessary to produce this increased anterior access.

This 67-year-old woman presented with a 4-year progression of left hemiparesis, hoarseness, and left facial palsy (House–Brackmann Grade III). She had become increasingly lethargic in the days prior to her presentation. Her disease had been inappropriately labeled as a stroke based on her computerized tomography scan. An audiogram confirmed absence of hearing, and laryngoscopy showed left vocal cord paresis. A–C: Contrast-enhanced T1-weighted axial (A), coronal (B), and sagittal (C) MR images revealing a heterogeneously enhancing mass occupying the left middle and posterior fossae and causing severe compression of the brainstem. The mass appears to invade the cavernous sinus as well as extending through the foramen ovale and the jugular foramen. This chondrosarcoma was resected and the brainstem was decompressed via a combined translabyrinthine subtemporal approach. D–F: Postoperative contrasted axial (D), coronal (E), and sagittal (F) views confirm complete resection and show the abdominal fat graft extradurally. Gratifyingly, the patient's mobility improved after surgery.

The partial translabyrinthine approach is a valuable variant. It represents an attempt at combining improved exposure of the petroclival junction compared with the retrolabyrinthine approach, without the inevitable deafness incurred by the full translabyrinthine approach. It requires for its success a meticulous drilling of either the superior or posterior semicircular canal (depending on whether the upper or the lower clivus, respectively, needs to be exposed), with special precautions taken to minimize loss of endolymph. Hearing can indeed be preserved in most instances.

This dissection, in turn, is an anterior extension of the translabyrinthine approach. The transcochlear approach necessitates resection of the cochlea, division of the GSPN, and posterior transposition of the facial nerve. In the original description, the anterior limits of this dissection were the posterior wall of the EAC and the middle ear.[5] Variations of this procedure, however, routinely require resection of the middle ear. The initial part of the dissection is the same as for the translabyrinthine approach, except that the EAC is transected, and the skin of the cartilaginous canal is sutured closed. The facial nerve is skeletonized from its entrance into the IAC to the stylomastoid foramen. The facial recess, an area of air cells contained between the vertical facial nerve medially, the chorda tympani laterally, and the fossa incudis superiorly, is opened to enter the middle ear. In the modified transcochlear approach, the middle ear ossicles are removed. Both the chorda tympani and the GSPN are sectioned at their origins from the facial nerve. These maneuvers free the descending segment of the facial nerve and allow its transposition posteriorly.

Next, the cochlea is resected, starting at the basal turn and proceeding anteriorly to the petrous ICA. The septum between the basal turn and the ICA is also resected along with the carotid ridge, which is a thin plate of bone separating the ICA from the jugular bulb. This maneuver exposes the jugular bulb completely. Therefore, the surgeon must be aware of the ninth, 10th, and 11th cranial nerves as they exit the jugular foramen.

Completion of the transcochlear approach as described earlier Results in removal of the posterior petrous and tympanic temporal bone (Fig. 7). The resulting wide exposure creates direct views of the clivus, primarily its middle half, the anterior brainstem, and the BA (Fig. 8). Extensions of this approach through the middle fossa can be used to access lesions of the infratemporal fossa and posterior nasopharynx.

Photographs of cadaveric dissections showing the transcochlear approach, right side. A: The GSPN has been sectioned to mobilize the facial nerve posteriorly. B: Posterior transposition of the facial nerve exposes the tympanic bone. C: Resection of the tympanic bone exposes the carotid ridge, a thin plate of bone separating the carotid canal from the jugular foramen. The cochlea lies just medial to the horizontal segment of the petrous ICA. D: Overview of the transcochlear approach after removal of the carotid ridge and cochlea. Of the posterior transpetrosal approaches, this dissection provides the widest and most direct view of the petroclival region and the prepontine cistern. INF. = inferior; VERT. = vertical.

This 60-year-old man had undergone radiation therapy without surgery at another facility 4 years earlier for a petroclival meningioma. He presented to us with very poor balance, dysphagia, complete hearing loss, and severe facial weakness on the right side. A–C: Contrast-enhanced T1-weighted axial (A), coronal (B), and sagittal (C) MR images revealing that the enhancing lesion had grown significantly in 4 years. These images (as well as the T2-weighted sequences that are not shown) predicted a poor dissection plane between the tumor and the brainstem. Because of the hearing loss and nearly complete facial palsy, we resected this lesion subtotally via a combined transcochlear– subtemporal approach. D and E: The postoperative MR images demonstrate a small amount of tumor that was left attached to the brainstem, which is markedly decompressed.

A rarely used variant of the transcochlear approach is the transotic approach. In it, the facial nerve is not transposed and tumor resection is performed in front of and behind the undisturbed facial nerve.

Closure of the posterior transpetrosal approach involves meticulous inspection and obliteration of potential cerebrospinal fluid fistulas. Any remaining air cells are sealed with bone wax. The middle ear and IAC are filled with autologous muscle or fat grafts. The cavity created by the mastoidectomy is filled with autologous abdominal fat, and the wound is closed in layers, including a rotation flap of the temporalis fascia, with application of a pressure dressing at the end of the procedure.

Comments

3090D553-9492-4563-8681-AD288FA52ACE

processing....