Retrosigmoid Transmeatal RS Approach

The most common indication for this approach is the removal of vestibular schwannoma or cerebellopontine angle tumors with hearing preservation. The approach is also useful for selective vestibular neurectomy in unilateral Meniere's syndrome. Another indication is for microvascular decompression of the seventh and eighth nerve complex in those patients with hemifacial spasm or severe vertigo. Presenting symptoms are similar to those mentioned in the previous section.

studies determining a patient's candidacy for hearing preservation have already been described. ABR and ECoG are always performed as a baseline for intraoperative monitoring. it should be noted that MRi with gadolinium enhancement may not predict actual tumor extent into the IAC.8 The position of the vestibular labyrinth in relation to the posterior aspect of the porus acusticus is important in calculating the lateral extent of meatal bone removal. In those patients with more than 1 cm of tumor extension into the cerebellopontine angle and good hearing, I perform the retrosigmoid transmeatal approach. With larger tumors, patients are informed of the lower success rate with hearing preservation so that they do not have unrealistic expectations.

The patient is positioned supine with a large roll under the ipsilateral shoulder to facilitate rotation of the head and neck. In this approach I always use pin fixation of the head. The head is turned laterally at 45 degrees, the neck is flexed, and the vertex is tilted slightly down. In obese patients, or those with limited mobility of the cervical spine, a modified park bench or lateral position may be planned. All auditory and facial nerve monitoring electrodes are placed. The active electrode for ECoG can be placed on the tympanic membrane or middle ear promontory. The scope of this chapter does not allow a detailed description of intraoperative monitoring techniques. Usually direct eighth nerve monitoring, ECoG, and ABR are used together.

There are some differences in surgical equipment between the TL and RS approaches. The pneumatic dissection tool should have a craniotome attachment in addition to the standard burs. The working distance for bone and tumor dissection is longer in the RS approach. I use a longer set of drill attachments and longer microinstruments for dissection.

The curved hockey-stick incision is made and an anteriorly based flap is secured with fishhooks. Initial bone work is performed with complete identification of the sigmoid sinus and transverse sinus. The posterior emissary vein is identified, bipolared, and sectioned. The mastoidectomy is limited and as few air cells as possible are opened to lessen the possibility of CSF leak. Skeletonizing the venous structures facilitates turning the bone

incision

Retrosigmoid

Mastoid lip

Limited mastoidectomy and hone waxed celis

Mastoid lip

Limited mastoidectomy and hone waxed celis

Csf Leak Mastoid

Durai incisions

Durai incisions

FIGURE 27-4 (A,B) Illustrations of initial exposure and dural incision for the retrosigmoid approach.

flap, which is removed with the craniotome and preserved for later replacement (Fig. 27-4).

Neurosurgical colleagues open the dura, release CSF from the cistern, and gain tumor exposure (Fig. 27-5). They may also perform additional debulking in large neoplasms to provide better exposure of the posterior aspect of the temporal bone.

The neurotologist returns to the operating room for the temporal bone dissection. A U-shaped incision is made in the dura extending from the jugular fold to below the level of the tentorium. Small pledgets of absorbable sponge are placed in the subarachnoid space to limit bone dust exposure. Labyrinthine landmarks including the vestibular aqueduct and singular canal are useful indicators during dissection.9 Once established, IAC dissection is continued from the porus laterally to the level of the vestibule. The vestibular labyrinth must be avoided. The common crus and posterior canal are at greatest risk. The endolymphatic sac is also left undisturbed. Normally, several millimeters of the IAC can be carefully exposed (Fig. 27-6). It is very important to completely skeletonize the IAC circum-ferentially, at least 180 degrees, rather than simply unroofing the internal auditory meatus.

After the bone work is completed, an inferior incision is made along the meatal dura. If distal tumor extent is present, lateral dissection may require the use of endoscopes and angled internal canal dissectors. Sharp dissection is required to identify the appropriate cleavage plane. Sharp technique is also important to limit traction on the cochlear nerve in hearing preservation. If the tumor is small, it should be removed en bloc if possible. Identification of the normal proximal eighth nerve allows placement of an active electrode for direct eighth nerve monitoring. Frequent communication between the audiologist and surgeon is important during the tumor work. In larger tumors, additional debulking is frequently required once the tumor has been removed from the IAC and the course of the distal facial and cochlear nerves has been determined. The scope of this chapter does not allow for detailed description of the vasculature of the posterior cranial fossa, which is well described else-where.10 This is a crucial part of the procedure, and all significant vessels should be carefully dissected from the tumor in the proper plane and preserved.

Closure is initiated with a careful search and occlusion of exposed air cells in the region of the meatus. Endoscopic control may reveal distal tumor or previously undiscovered air cells.11 The internal auditory meatus is sealed with a large piece of prepared free temporalis muscle. The dura is normally closed primarily, but may require augmentation with autogenous grafts or manufactured dura substitutes. The bone flap is returned over a large pad of absorbable gelatin sponge and secured with miniplates. Routine closure is then performed. Primary cranioplasty is not usually required.

As previously mentioned, the incidence of complete facial paresis is now uncommon, although patients with larger tumors may have a significant delay in neural recovery. Cerebrospinal fluid leak is somewhat more common in this approach than in

FIGURE 27-5 Photograph of small tumor in cerebellopontine angle prior to transmeatal bone work. Arrow indicates porus acusticus. Arrowheads indicate eighth nerve.

Bone drilled from 1AC

Retrosigmoid Appraoch

FIGURE 27-6 (A-C) Illustrations of completed bone work prior to meatal tumor removal.

Bone drilled from 1AC

FIGURE 27-6 (A-C) Illustrations of completed bone work prior to meatal tumor removal.

the TL approach. If initial measures including bed rest and short-term lumbar drainage are ineffective in closing the leak, then reoperation can be performed. If hearing has been maintained, a transmas-toid approach without closure of the eustachian tube can be used.

Hearing loss may occur due to direct mechanical injury to the cochlear nerve or more likely due to interruption of the blood supply of the cochlea. Sharp dissection, preservation of microvasculature, intraoperative monitoring, one-piece tumor removal, and limiting cautery are all important in lowering the potential for hearing loss. It is also possible that early bone work with decompression of the IAC may improve perfusion pressure during extended tumor dissection for larger schwannomas.12 Other serious complications including stroke or hemorrhage are rare. These may occur early in the postoperative period, and the surgeon must be vigilant and immediately evaluate any questionable change in mental status or the neurologic examination. Addi tional postoperative procedures are similar to those described in the previous section. If sutures or staples are used for the skin closure, they are normally removed between 10 and 14 postoperative days. Facial function is assessed and eye care is initiated.

Audiometric testing and vestibular evaluation are performed at 1 month. Patients having difficulty with vestibular compensation are referred for postoperative platform-based balance retraining therapy. Patients who have lost hearing may be candidates for transcranial amplification. All patients undergoing a retrosigmoid approach are followed with imaging. I recommend a 1-year postoperative MRI study and a final study done 4 years postprocedure. In the uncommon case of residual or recurrent schwannoma, radiosurgical treatment is the primary mode of therapy. This may not be required unless significant growth is demonstrated. Translabyrinthine removal of residual growing neoplasm can also be entertained if there is a nonhearing ear.

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