Учебники / Computer-Aided Otorhinolaryngology-Head and Neck Surgery Citardi 2002

the ampullate end is found. This then indicates the position of the superior vestibular and facial nerves [5]. This approach relies on the position of the arcuate eminence on the floor of the MCF to orient dissection. Although in 98% of cases the lateral aspect of the arcuate eminence predicts the underlying lateral limb of the superior semicircular canal, the relationship can vary and the arcuate eminence may be absent in 15% of patients [6]. Yet another approach is to begin drilling in a ‘‘safe area’’ 28 mm from the inner surface of the squamous temporal bone and locating the IAC medially [7].

17.2.1 Technique

Patients with vestibular schwannomas that do not extend more than 5 mm beyond the lip of the porus acusticus, evidence of a superior vestibular nerve tumor, and good preoperative hearing are offered MCF removal of their tumors. Other patients in whom the technique is appropriate include patients with intractable vertigo and unilateral vestibulopathy who desire vestibular nerve section. The author does not use the MCF approach as his primary approach for vestibular nerve section; the retrosigmoid approach is the preferred technique in this instance, unless the patient has failed a prior attempt at nerve section.

On the day prior to surgery, five to seven adhesive markers (fiducial markers) containing a contrast agent are applied to the skin surface in locations that encompass the area of the temporal bone, and a high-resolution noncontrast CT is performed. The dataset is sent to the surgical workstation and patients are instructed to leave the fiducials undisturbed overnight.

On the day of surgery, the patient is positioned in a slightly lateral recumbent position on a bean-bag. Early in our experience, a Mayfield head holder to which we attached the reference arc stabilized the head (Figure 17.1). However, because MCF surgery is performed without the use of muscle relaxant (to allow continuous electromyographic monitoring of the facial nerve), patient movement occasionally caused enough slippage of the head to render the localizer inaccurate. For that reason, the author now uses a small reference arc anchored directly to the calvarium with screws. This method provides reliable registration that cannot be disturbed by patient movement. After anchoring the reference arc, the external fiducial markers are registered and a supplemental registration based on surface contours is performed. The attachment of the reference arc and registration of the fiducial points are performed while other preparations are made for surgery (i.e., starting an arterial line, placing the urinary catheter, placing EMG electrodes) and does not add significantly to the surgical time.

After performing a MCF craniotomy, the brain is retracted with a HouseUrban MCF retractor. The floor of the MCF is mapped with the position of the superior semicircular canal, the basal turn of the cochlear, the geniculate ganglion, and IAC marked. Three orthogonal views (sagittal, axial, and coronal)

(A)

(B)

FIGURE 17.1 (A) Patient positioned for middle cranial fossa surgery with image guidance. (B) Operating room arrangement for middle cranial fossa surgery. (From Ref. 1.)

allow three-dimensional orientation of the instrument tip. The IAC is then ‘‘bluelined’’ with the dura left undisturbed until it has been uncovered for 180°. The dura is then opened with an ophthalmic knife and the tumor removed or vestibular nerve cut using standard techniques.

CAS, in the experience of the author, reduces the time required to orient the dissection in the temporal bone during MCF surgery. With attachment of the reference arc directly to the calvarium, the accuracy of the system is increased and remains stable throughout the procedure, even during the rare times when the patient moves.

17.3PETROUS APEX DRAINAGE PROCEDURES

Petrous apex lesions are relatively rare, usually benign, and typically cystic. The most common lesions are cholesterol granulomas. These locally destructive, expansile lesions can invade the cochlea and labyrinth, causing hearing loss and vertigo. They can also involve Meckel’s cave, causing fifth nerve symptoms. Cholesterol granulomas typically arise in well-pneumatized temporal bones as a result of air cell obstruction and foreign body reaction.

Infections of the petrous apex (petrous apicitis) may require drainage. Symptoms of petrous apicitis include boring retro-orbital pain and abducens palsy due to inflammation of Dorello’s canal (Gradenigo’s syndrome). The condition often arises after mastoidectomy is performed on a draining ear.

A number of approaches can access the petrous apex: transcochlear, infracochlear, infralabyrinthine, transethmoidal/transsphenoidal, or the anterior approaches of Ramadier, Eagleton, and Lempert [8]. The MCF approach can also access the petrous apex, but because it is difficult to establish a drainage tract, it has been largely abandoned for purposes of drainage [9].

For most lesions of the petrous apex, the author favors the infracochlear approach because it readily accesses the air cells anterior to the cochlea. The infracochlear air cells (a region known as the infracochlear triangle) are bounded anteriorly by the carotid artery, posteriorly by the jugular bulb, and superiorly by the basal turn of the cochlea. For patients with nonserviceable hearing, the approach can be extended to include the cochlea (the transcochlear approach). If dissection in the infracochlear approach remains inferior to the round window, the cochlea will not be violated and hearing can be preserved. The infracochlear approach can establish a wide tract into the cyst with dependent drainage.

As in MCF surgery, conventional techniques can be used to reliably approach the area of interest. The advantage of CAS is in the fairly common scenario of either an absent or poorly developed infracochlear air-cell tract or a deeply situated or small cyst. In these situations, the author has found that image guidance allows the trajectory of dissection to be monitored and vital structures, such as the horizontal segment of the carotid artery, to be avoided.

17.3.1 Technique

The following case of a petrous apex granuloma illustrates the benefits of CAS. A 35-year-old woman developed numbness, tingling, and intermittent sharp pains on the right side of her face one year before presenting to our institution. Sudden loss of vision in her right eye that spontaneously recovered brought her to the attention of physicians 8 months before seeing the author. Her past medical history included a right myringoplasty 14 years prior to her presentation. About a year before presenting, she had a severe right ear infection. The hearing in her right ear was subjectively impaired.

Evaluated by another surgeon, she was found on MRI (Figure 17.2) to have a right petrous apex cyst with a signal characteristic of a cholesterol granuloma (hyperintensity on T1 and T2 and nonenhancement with gadolinium infusion). The cyst impinged the horizontal carotid artery and cavernous sinus. Middle cranial fossa drainage was planned after a temporal bone CT showed no infracochlear air-cell tract. Concerned about the temporal lobe retraction required, she sought evaluation at our institution.

She reported continued fluctuating vision in her right eye and right facial hypesthesia. An audiogram demonstrated normal hearing in the left ear and a moderate neurosensory hearing loss in the right ear with 80% word recognition. Infracochlear drainage was discussed, and the day prior to surgery she underwent a high-resolution CT of the temporal bones with adhesive fiducials applied. On the day of surgery, a reference arc was anchored with three 4 mm long selftapping screws through a small scalp incision (Figure 17.3). The arc has four detachable reflecting balls that allowing tracking head position during surgery. Registration of the arc and fiducial points was then performed (Figure 17.4). A trajectory for orienting dissection was planned on the workstation (Figure 17.5).

FIGURE 17.2 MRI of petrous apex cyst. (A) T1-weighted without contrast; (B) T1- weighted with gadolinium; (C) T2-weighted.

FIGURE 17.7 Skeletonizing the carotid in the infracochlear triangle.

the bone over the chorda tympani was removed with an otologic drill and continuous irrigation. The chorda tympani was followed inferiorly. The inferior external auditory canal bone was removed until the level of the jugular bulb was identified.

Before beginning dissection of the infracochlear triangle, accuracy of the registration of the navigational system was confirmed using landmarks in the middle ear such as the lip of the round window, the stapes, and the bone of the promontory. Drilling then exposed the internal carotid artery and jugular bulb. In this patient’s case, the cyst was both deeply situated and the infracochlear air cells were poorly developed, making CAS particularly helpful. (Figures 17.7, 17.8, and 17.9 show the progress of dissection.)

Following surgery, the patient was discharged after an overnight stay. Her visual and facial symptoms resolved completely. An audiogram 2 months after surgery showed no change from her preoperative hearing.

17.4INSTRUMENTATION

Initially, neurosurgical instruments adapted for the navigational system were used (Figure 17.10) but proved unwieldy in the confines of the ear under the otologic microscope. To overcome these drawbacks, otologic instruments were modified by the author with attachments for the LED array (Figure 17.11). These shorter instruments allow simultaneous dissection and localization under

FIGURE 17.12 Drill fitted with mount for LEDs. (Courtesy of Medtronic Xomed, Jacksonville, FL.)

the otologic microscope. The angled straight pick is most useful because the angled handle places the LED array out of the line of sight. Otologic drills have also been fitted with LEDs to allow use with the navigational system (Figure 17.12).

As mentioned previously, early use of the Mayfield head-holder has given way to the direct attachment of the reference arc to the calvarium. The petrous apex drainage case described employed a pedestal stabilized with three screws through a small scalp incision. A recently designed pedestal requiring only one percutaneous screw promises to reduce the time involved with this step as well as eliminating the need for a separate incision with its attendant risk of bleeding and infection (Figure 17.13).

FIGURE 17.13 Single screw pedestal for attachment of reference arc. (Courtesy of Medtronix Xomed, Jacksonville, FL.)