Despite its importance and ubiquity in treating skull base pathology, many skull base surgeons are uncomfortable performing this powerful approach. At many institutions the neuro-otologists are solely responsible for this component of the surgery. Furthermore, an adequate, concise description of this technique with anatomically correct illustrations has not been available to skull base surgeons in training, especially neurosurgeons. Mastering this approach will help surgeons to become a more vital component of their interdisciplinary skull base team. This article therefore details the microsurgical anatomy and operative nuances of the retrolabyrinthine approach.

Figure 1 (A) Patient position for the retrolabyrinthine approach (left-sided approach). The inion (1), mastoid process (2), zygomatic arch (3), external auditory meatus (4), (more ...)

Although not required, a preoperative temporal bone computed tomography (CT) scan may be obtained with 1 mm high-resolution cuts in both the axial and coronal planes for planning the surgical approach. Considering the high interpatient variability of anatomic structures in the temporal bone, this study provides useful information for the surgeon in training, especially the degree of mastoid aeration, size and orientation of the sigmoid sinus and jugular bulb, and limits of the semicircular canals.

The incision line for the periosteal flap is created 1 cm under the initial skin incision using skin rakes to provide exposure. Once this incision is completed, the periosteum and muscular fascia are detached from the cranium with a periosteal dissector and also reflected forward. The scalp (superficial) and periosteal (deep) flap incisions do not override, thus providing a barrier against postoperative cerebrospinal fluid (CSF) leakage. During this dual-layer flap elevation, a mastoid emissary vein is usually encountered 1 cm inferior to the asterion along and slightly anterior to the occipitomastoid suture. This vein connects the sigmoid sinus to the cutaneous venous plexus and may be a source of bleeding and potential air embolus if torn. Bleeding from the emissary vein is controlled by firmly placing bone wax in the mastoid emissary foramen. The attachments of the sternocleidomastoid and splenius capitus muscles to the mastoid tip are freed using a periosteal dissector or electrocautery. The two-layered scalp flap is padded underneath with a surgical sponge and then retracted anteriorly with multiple fishhooks. Hemostasis is achieved with bipolar cauterization.

Figure 2 Serial intraoperative photomicrographs of a left-sided retrolabyrinthine approach. (A) Mastoid surface after scalp elevation. The mastoid tip (mt), posterior margin of the external acoustic meatus (eam), (more ...)

The mastoid bone removal is performed using a high-speed cutting burr and continuous suction irrigation. Shallow troughs outlining the superior and anterior perimeters of the mastoidectomy are created. The superior trough extends from the zygomatic root to the asterion, just inferior to the linea temporalis. The anterior trough is perpendicular to the superior one and extends from the posterior–superior rim of the external auditory meatus down to the mastoid tip. Working both posterior and inferior to these margins, cortical bone is removed with brushlike strokes exposing a variable number of mastoid air cells. A uniform depth of dissection is maintained as more air cells are exposed and the cortical bone covering the sigmoid sinus becomes apparent (Fig. 2B). A thin, depressible layer of bone may be left on the surface of the sigmoid sinus for protection. Approximately 1 cm of posterior fossa dura is exposed posterior to the sinus. This additional bone removal behind the sigmoid sinus allows retraction of the sinus posteriorly, providing improved exposure medial to the bony labyrinth.

Once the sigmoid sinus is fully skeletonized, additional air cells anterior to the sigmoid sinus are removed to expose the reflection of the dura of the middle and posterior fossae. This bone removal is continued until the middle fossa dura is completely exposed below the linea temporalis. Successively smaller cutting burrs are used as the mastoidectomy proceeds. The sigmoid sinus is followed inferiorly and the air cells in the mastoid tip are removed exposing the inferior segment of the sigmoid sinus and digastric ridge. The mastoid antrum is opened, and the dense cortical bone overlying the horizontal semicircular canal (Koerner's septum) is identified and preserved (Fig. 2C). The short process of the incus is identified in the incudal fossa. Care is taken to avoid bur contact with the incus, which may conduct to the inner ear causing sensorineuronal hearing loss.

Higher magnification and a diamond bur are used to skeletonize the bony labyrinth, fallopian canal (containing the facial nerve), and presigmoid dura. The bone covering the presigmoid dura at the sinodural angle (of Citelli) is removed. Removal of this bone is paramount to achieve adequate dural exposure. The horizontal semicircular canal on the medial wall of the mastoid antrum and the posterior semicircular canal on the postero-medial aspect of the antrum are also skeletonized. The bony labyrinth must not be violated during this critical stage of the retrolabyrinthine approach or hearing loss may occur. Changing the angle of view by moving the microscope and depressing the dura away from the thinned bone covering the presigmoid dura facilitate safe bone removal and maximize the final exposure.

If the bony labyrinth is accidentally violated with the drill but the membranous labyrinth remains intact, a small amount of bone wax can be used to occlude the defect. However, if both the bony and membranous components of the labyrinth are transected, the surgeon must act quickly and cork each end of the defect with a small piece of muscle to prevent excessive leakage of the peri- and endolymph, which may cause significant vertigo after surgery.

The surgeon now removes the remaining aerated bone overhanging the antero-medial rim of the mastoidectomy, which contains the vertical segment of the facial nerve in the fallopian canal (Fig. 2D). A thin layer of cortical bone is left covering the fallopian canal. Neurophysiologic monitoring of the facial nerve helps prevent inadvertent injury of the facial nerve during this stage of the dissection. Osseous dissection of most temporal bones can also be carried medially and anteriorly from the posterior semicircular canal to the porus acousticus. The retrofacial air cells are also removed (Fig. 2E). These maneuvers provide further exposure of the presigmoid dura.

The dura is opened with an anteriorly based C-shaped flap using a No. 11 blade and microscissors. The dura is tacked up with 4-0 braided nylin sutures. The endolymphatic sac will be visible inferior to the posterior semicircular canal as a whitish, thickened area of dura emerging from the vestibular aqueduct. To maintain its integrity, the endolymphatic sac often defines the anterior limit of the dural opening. Routinely, the middle fossa dura is not opened and the superior petrosal sinus is not sacrificed.

The cerebellum resting against the petrous temporal bone is visualized after the dura is opened. Using microsurgical technique under high magnification, the arachnoid adhesions between the cerebellum and petrous ridge are placed under tension by gently retracting the cerebellum posteriorly with a cottonoid-padded microsuction. These adhesions are dissected sharply. The cerebellopontine angle and prepontine arachnoid cisterns are opened sharply and CSF is released. With transection of cerebellar adhesions and release of CSF from the subarachnoid space, the cerebellum falls away from the petrous ridge providing adequate exposure of the cerebellopontine angle and posterior face of the petrous ridge without static retraction on the cerebellum.

Through the retrolabyrinthine exposure, both the brain stem origin and cisternal segment of the VII–VIII cranial nerve complex are well visualized in the cerebellopontine angle (Fig. 1D). A loop of the anterior inferior cerebellar artery (AICA) is often found intimately associated with this complex. A microdissector and hand-held nerve stimulator are used to identify the components of the VII–VIII cranial nerve complex: the facial nerve, acoustic nerve, and vestibular nerve. In most patients the porus of the internal auditory canal is inaccessible for exposure through this approach because the sigmoid sinus limits a more obtuse viewing angle in relation to the posterior petrous ridge. The ninth and tenth cranial nerves are visible inferiorly, anterior to the level of the jugular bulb. The fifth cranial nerve and basilar artery can also be examined through this exposure.

The periosteal flap is closed with interrupted 3-0 vicryl sutures spaced 5 mm apart, which firmly cinches the fat graft in the mastoid defect. In a similar manner, the galeal-skin flap is closed with inverted 3-0 vicryl sutures. As already described, the two flap incisions do not overlap, providing an additional barrier to CSF leakage. The skin is closed with a running, locked 3-0 nylon suture. A mastoid compression dressing is placed and maintained 3 days after surgery to promote wound healing and to prevent pseudomeningocele formation by assisting the fresh tissue planes to fix.

Initially, the retrolabyrinthine approach was popularized for functional procedures of the cerebellopontine angle, including vestibular and sensory trigeminal neuronectomies.^1,2,3,4 With popularization of the “keyhole” concept in neurosurgery, where adjustments in trajectory through a relatively small dural opening were used to create a significantly larger operative field while minimizing exposure of nearby eloquent structures, the retrolabyrinthine approach was subsequently reported useful for the management of other cerebellopontine angle pathologies.^5,6,7,8 For example, Darrouzet et al⁶ used the retrolabyrinthine approach to resect 60 acoustic neuromas, with facial nerve and hearing preservation outcomes similar to those of previous series using more extensive retrosigmoid and translabyrinthine approaches. In their series, 63% of tumors were larger than 2 cm and 15% were larger than 3 cm. These authors concluded that the retrolabyrinthine approach may be used to resect acoustic neuromas of almost any size.

Although we believe the exposure provided by the retrolabyrinthine approach is too limited for large cerebellopontine angle tumors, it provides adequate exposure for smaller tumors, especially petrous ridge and jugular fossa meningiomas, epidermoids, and arachnoid cysts, and for biopsy of intrinsic brain stem lesions. Despite the risk of hearing loss, some authors advocate partial labyrinthectomies to widen the exposure.⁹ Our philosophy is that even a small chance of hearing loss is unacceptable when using the retrolabyrinthine approach. If a wider exposure is required, we would opt either to sacrifice hearing with a translabyrinthine approach, when indicated, or to perform a retrosigmoid craniotomy, thus preserving hearing and obtaining a wider surgical corridor.

A retrosigmoid craniotomy can be used to expose the same region and anatomical structures as the retrolabyrinthine approach. However, for small lesions located adjacent to the presigmoid dura, we believe the retrolabyrinthine approach has certain advantages. First, it provides a direct approach through the temporal bone. It preserves hearing, which may be compromised with cerebellar retraction during the retrosigmoid approach. It also requires no retraction or manipulation of neural elements. Finally, using a small craniectomy without transection of the musculature avoids postoperative cervicalgia.

The most frequent approach-related complication associated with the retrolabyrinthine exposure is CSF leakage. For all major clinical series, the incidence of CSF leakage after this approach has ranged from 3 to 7%.^1,2,3,4,6,7 Although uncommon, injury to the facial nerve in the fallopian canal and damage to the labyrinth causing hearing loss can occur during bone removal.

The retrolabyrinthine craniectomy is an important technique for skull base surgeons to understand and master. It not only provides safe and direct exposure to select pathology of the cerebellopontine angle as a primary surgical approach, it also is the foundation for more common and extensive cranial base exposures of the posterior and middle fossae. It is the initial step when performing a translabyrinthine, transcochlear, infratemporal, or combined petrosal approach to both infra- and supratentorial compartments. When indicated, it can provide additional exposure for retrosigmoid and far-lateral craniotomies. Day et al⁷ used the retrolabyrinthine approach with transection of the sigmoid sinus “as a simple alternative to more involved strategies” to manage 11 aneurysms of the lower basilar trunk and AICA.

Despite frequent use of the retrolabyrinthine approach alone, with modification, or as the foundation of a more extensive cranial base approach, the skull base literature has lacked an adequate description of the procedure. We hope this communication also encourages renewed interest in this approach in the neurosurgical community.

Porter, Randall W. (Interdisciplinary Skull Base Section, Division of Neurological Surgery, Barrow Neurological Institute, Phoenix, Arizona).

The presigmoid retrolabyrinthine approach is very useful for small lesions involving the cerebellopontine angle and vestibulocochlear nerve complex and for vestibular neurectomy. It also can be combined with a supratentorial craniotomy to access lesions of the petroclival region that may extend above or below the tentorium. The benefit of the retrolabyrinthine approach is that the surgeon primarily works in the presigmoid region. Consequently, the posterior extent of the craniotomy and possibly some of the soft tissue dissection can be limited. The disadvantage is that access to the medial cerebellopontine angle is limited. An extension of this approach, the partial labyrinthectomy, has been reported to be associated with good preservation of hearing while improving access to the cerebellopontine angle. Although we have not attempted this technique at our institution, we are interested in the experience of other centers. Overall, this excellent article reminds both neurosurgeons and neuro-otologists of the versatility and usefulness of this approach.

We thank Benjamin Kress for creating Figure 1 and Sigrid Hahn, M.D. for editorial assistance.