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A rapid and safe middle fossa approach to the geniculate ganglion and labyrinthine segment of the facial nerve. (Original Article).

Abstract

The middle fossa approach provides neurotologic surgical access to lesions of the geniculate ganglion and the labyrinthine portion of the facial nerve as well as to the internal acoustic canal, and therefore helps preserve cochlear function. Although this approach is widely used, surgeons are still not certain which anatomic landmarks are best to locate the facial nerve and internal acoustic canal without causing labyrinthine damage. The purpose of this article is to describe a fast and safe technique to expose the geniculate ganglion and the labyrinthine portion of the facial nerve in two structures of the middle ear: the cochleariform process and the tympanic portion of the facial nerve. We prospectively evaluated 32 patients who underwent surgical facial nerve exploration via the middle fossa approach. Our goal was to determine the incidence of intraoperative difficulties and complications; we found none, and hearing levels could not be maintained in only one of the 32 patients. Our technique allowed us to decompress the first genu and the labyrinthine segment of the nerve. It also allowed us to reach and manipulate its tympanic segment in a very short amount of surgical time.

Introduction

The middle fossa approach was first used during the latter part of the 19th century as a means of exposing the trigeminal ganglion. In 1961, House became the first to report the use of this approach for the removal of an acoustic neuroma. (1) Five years later, Pulec was the first to describe the use of this technique for total facial nerve decompression. (2) Today, the middle fossa approach is used worldwide, primarily to access tumors in the internal auditory canal that have only a slight protrusion into the cerebellopontine angle. The middle fossa approach is used in patients who have useful hearing and to access the labyrinthine portion of the facial nerve in patients who have normal or near-normal hearing. (2-4)

The geniculate ganglion and the labyrinthine segment of the facial nerve can be reached via both the middle fossa approach and the transmastoid approach. (5) Both approaches have their surgical limitations, and it is important that surgeons be comfortable with both. One advantage of the transmastoid approach to the geniculate ganglion is that craniotomy is not necessary. However, in a poorly pneumatized temporal bone, it is not possible to reach the geniculate ganglion and the labyrinthine segment of the facial nerve with this approach. Also, the transmastoid approach exposes the entire labyrinthine segment of the facial nerve in only 60% of patients, mainly via the superior semicircular canal. (6,7)

The middle fossa approach provides a safe and direct route to the meatal, geniculate, and labyrinthine segments of the facial nerve. It has been used as a first-line approach in facial nerve surgery by most neurotologists because it allows for uniform visualization of and access to the facial nerve with minimal risk to the labyrinth. The two major concerns with this route are (1) the need for a wide temporal lobe retraction to permit a broad view of the anterior surface of the petrous pyramid and (2) the difficulty in finding anatomic landmarks that allow for the correct identification of and access to the internal acoustic canal and the fallopian channel without damaging the labyrinth. The distance from the labyrinthine portion of the facial nerve to the cochlea ranges from 0.5 to 1.0 mm, although early reports estimated that this distance might be much shorter in as many as 75% of patients. (8) Although extradural temporal lobe retraction today is safe and carries a minimal risk of injury to the temporal l obe, surgeons are still not certain which anatomic landmarks are best to locate the facial nerve and internal acoustic canal without causing labyrinthine damage. (9)

Because there are almost no topographic landmarks along the floor of the middle cranial fossa, orientation to the underlying structures can be a challenge. Several methods are available for localizing the internal acoustic canal and the facial nerve in the middle fossa floor:

* The Fisch method is based on identifying the arcuate eminence. The arcuate eminence is the major landmark used to safely locate the internal acoustic canal and its contents, which are often located at a 60[degrees] angle to the blue-lined superior semicircular canal. (10)

* The Pulec method involves tracing the greater superficial petrosal nerve to the geniculate ganglion and then exposing the labyrinthine segment of the facial nerve and the internal acoustic canal. (11)

* Jackler drilled a deep trough into the apical petrous bone anterior to the anticipated location of the canal. (12) Then, using the side of a diamond bur, he was able to delineate the anterior margin of the canal's dura and the porus acusticus internus.

* Garcia-Ibanez and Garcia-Ibanez proposed using the bisection of the angle formed by imaginary lines that pass through the greater superficial petrosal nerve and the arcuate eminence. (13)

* Sterkers' technique is based on identifying the internal acoustic canal by drilling bone 28 mm medial to the squama on the biauricular line. (14)

* Catalano and Eden reported using the head of the malleus and the lateral cortex of the temporal squama at the zygoma root as external references to identify the internal auditory canal. (15)

We have routinely used the middle fossa approach to decompress the facial nerve and to remove small facial nerve tumors. In this article, we describe a fast method used in our service to access the geniculate ganglion and the labyrinthine portion of the facial nerve.

Patients and methods

Thirty-two patients underwent surgery via the middle fossa approach from January 1997 through March2001 at the Hospital das Clinicas of the University of Sao Paulo. Of this group, 25 patients--18 men and seven women (mean age: 30 yr)--underwent a facial nerve exploration; 16 had experienced immediate facial nerve paralysis following a temporal bone trauma, and nine had experienced decompression of the labyrinthine segment of the facial nerve as a result of Bell's palsy. Some patients experienced a conductive hearing loss following their temporal bone trauma. Of the remaining seven patients, five had had a small meningioma, and two had had a facial nerve schwannoma.

During middle fossa surgery, each patient was placed in the supine position with the head rotated to one side. The surgeon sat at the head of the operating table. The skin incision began at the edge of the zygoma root and extended superiorly for approximately 7 cm (figure 1). After the temporal muscle flap was retracted, a craniotomy was made by opening a 3 x 4-cm rectangular bone flap (figure 2). The elevation of the dura allowed for the visualization of the greater superficial petrosal nerve anteriorly, the arcuate eminence, the superior semicircular canal, and the roof of the middle ear (figure 3). The roof of the middle ear was easily identified by its dark color. The surgeon opened a small hole with a diamond bur and identified the cochleariform process and the adjacent segment of the facial nerve (figure 4). The facial nerve was then followed until the geniculate ganglion could be visualized (figure 4). Once the geniculate ganglion was identified, the labyrinthine segment was easily exposed (figure 4).

Results

In this prospective series, we based our findings on three parameters: (1) the incidence of intraoperative difficulties or complications, (2) the incidence of immediate ([less than or equal to]1 mo) postoperative complications, and (3) the results of audiologic evaluations 90 days postoperatively.

Intraoperative difficulties. We did not experience any intraoperative difficulties or complications.

Postoperative complications. No patient experienced any postoperative complications, such as cerebrospinal fluid fistula, postoperative bleeding, or meningitis.

Audiologic testing. Our evaluation of patients 90 days after surgery found only one case of anacusis. This patient had a small facial nerve schwannoma at the first genu, and the patient's cochlea had been opened during tumor removal. No change in postoperative hearing was detected in any other patient.

We also found another positive aspect to the middle fossa approach; there was no need for a wide retraction of the temporal lobe to locate the tegmen tympani. We used the described technique for all 32 middle fossa operations. We did not have any difficulty finding and opening the tegmen tympani. The malleus and the cochleariform process were identified without difficulty, and no manipulation of the ossicular chain was needed to obtain a direct view of the second portion of the facial nerve. The next step was to reach the geniculate ganglion by following the tympanic segment of the facial nerve. After the geniculate ganglion was exposed, the labyrinthine segment was easily reached. This access allowed the surgeon to decompress the first genu and the labyrinthine segment of the nerve and to reach and manipulate its tympanic segment.

Discussion

The arcuate eminence is the major landmark used to safely locate the internal acoustic canal, which is often located at a 60[degrees] angle to the blue-lined superior semicircular canal. (10) This is the most common way of finding the internal acoustic canal and the labyrinthine segment of the facial nerve. However, there are some problems with this method. In about 15% of temporal bones, the arcuate eminence is not identifiable because it is located posterior to the superior semicircular canal rather than directly over it. As a result, this method relies on a landmark whose location is not invariable because its position can be altered by the amount of temporal bone pneumatization. (16)

Another option is to locate the geniculate ganglion by exposing the greater superficial petrosal nerve. However, this approach requires a wide temporal lobe retraction, which carries the risk of bleeding from the veins around the foramen spinosum. By exposing the greater superficial petrosal nerve up to the geniculate ganglion, this technique carries the risk of causing a lesion in the petrosal artery. The petrosal artery runs parallel to the greater superficial petrosal nerve and supplies the facial nerve at the area of its first genu and the labyrinthine segment. The bleeding caused by a lesion in this artery could result in a difficult surgery and possible compromise of the facial nerve blood supply. Most of the techniques that are useful in reaching the internal acoustic canal are not usually performed when the facial nerve is the only structure to be exposed. (12-15)

To avoid the difficulties encountered with other techniques, we use the cochleariform process in the middle ear as the primary landmark for identifying the tympanic segment of the facial nerve. We then follow this segment to reach the geniculate ganglion and the labyrinthine segment. The cochleariform process is easily found in the middle fossa approach by opening a small hole (0.5 cm) in the tegmen tympani. The second portion of the facial nerve can then be identified just adjacent to the cochleariform process, and it can be followed to the geniculate ganglion. This technique is safe and allows the surgeon to quickly decompress the first segment and the first genu. This strategy also makes it possible to reach the tympanic segment of the nerve. This is interesting because even when our goal is just to decompress the labyrinthine segment, as we do for Bell's palsy, it is possible for us to perform an amplified decompression, one that includes part of the second segment, without needing to perform a mastoidect omy.

The risk of creating an ossicular chain lesion or inducing a conductive hearing loss is minimal because no manipulation is required in the middle ear space. Even in cases when a conductive hearing loss is caused by temporal bone trauma, we do not attempt to restore hearing at that time. Instead, we perform a second procedure. In our view, the risk of complications (e.g., cerebrospinal fluid leak, meningitis, brain herniation) inherent in opening a space in the tegmen tympani is nonexistent once it becomes unnecessary to open the dura or the internal acoustic canal.

We conclude that the cochlearifrom process and the tympanic segment of the facial nerve can be routinely used as landmarks for the safe exploration of the labyrinthine segment of the facial nerve in the middle fossa approach.

References

(1.) House WF. Surgical exposure of the internal auditory canal and its contents through the middle cranial fossa. Laryngoscope 1961;71:1363-85.

(2.) Pulec JL. Total decompression of the facial nerve, Laryngoscope 1966;76:10 15-28.

(3.) Cokkeser Y, Aristegui M, Naguib MB, et al. Identification of internal acoustic canal in the middle cranial fossa approach: A safe technique. Otolaryngol Head Neck Surg 2001;124:94-8.

(4.) Bento RF, Miniti A, Tedesco-Marchese J. Indications and surgical techniques for lesions of the facial nerve due to temporal bone fractures. In: Proceedings of the 13th World Conference of Otolaryngology, 1985, Miami Beach, Fla. Amsterdam: Excerpta Medica, 1985;2:949-50 (International Congress Series, 680).

(5.) Pulec JL. The facial nerve: How to find it. Ear Nose Throat J 1993;72:677-85.

(6.) Salaverri MA. Transatical approach. A technique variation for total decompression of the facial nerve. Revista Brasileira de OtoRino-Laringologia 1974;40:262-4.

(7.) Bogar P, Bento RF. Paralisia facial de origem traumatica. Arquivos da Fundacao Otorrinolaringologia 1997;l:23-5.

(8.) Redleaf MI, Blough RR. Distance from the labyrinthine portion of the facial nerve to the basal turn of the cochlea. Temporal bone histopathologic study. Ann Otol Rhinol Laryngol 1996;105:323-6.

(9.) Driscoll CL, Jackler RK, Pitts LH, Banthia V. Extradural temporal lobe retraction in the middle fossa approach to the internal auditory canal: Biochemical analysis. Am J Otol 1999;20:373-80.

(10.) Fisch U. Transtemporal surgery of the internal auditory canal. Report of 92 cases, technique, indications and results. Adv Otorhinolaryngol 1970;17:203-40.

(11.) Pulec JL. Total facial nerve decompression: Technique to avoid complications. Ear Nose Throat J 1996;75:410-5.

(12.) Jackler RK. Middle fossa approach to the internal auditory canal. In: Jackler RK. Atlas of Neurotology and Skull Base Surgery. St. Louis: Mosby, 1996:79-99.

(13.) Garcia-Ibanez E, Garcia-Ibanez JL. Middle fossa vestibular neurectomy: A report of 373 cases. Otolaryngol Head Neck Surg 1980;88:486-90.

(14.) Pellet W, Cannoni M, Pech A. The Pyramid in Operative Position. Otoneurosurgery. Berlin: Springer-Verlag, 1990:67-9.

(15.) Catalano PJ. Eden AR. An external reference to identify the internal auditory canal in middle fossa surgery. Otolaryngol Head Neck Surg 1993;108:111-6.

(16.) Parisier SC. The middle cranial fossa approach to the internal auditory canal-an anatomical study stressing critical distances between surgical landmarks. Laryngoscope 1977;87(Suppl 4 Pt 2):l-20.

From the Department of Otolaryngology, University of Silo Paulo, Brazil.

Reprint requests: Ricardo Ferreira Bento, MD, Rua Pedroso Alvarenga 1255 cj. 22, 04531-012, Sao Paulo, Brazil. Phone: +55-11-3167-6556; fax: +55-11-3812-9129; e-mail: rbento@attglobal.net
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Comment:A rapid and safe middle fossa approach to the geniculate ganglion and labyrinthine segment of the facial nerve. (Original Article).
Author:Sanchez, Tanit Ganz
Publication:Ear, Nose and Throat Journal
Geographic Code:1USA
Date:May 1, 2002
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