Middle Fossa Approach

Indications

The primary indications for the middle fossa approach are a small acoustic tumor, confined to the IAC or with moderate extension into the cerebellopontine angle, and good preoperative hearing. In contrast, the retrosigmoid approach is used for patients with good hearing and small tumors located mostly in the cerebellopontine angle, without extension into the lateral aspect of the IAC. For hearing conservation surgery, we use the arbitrary audiometric criteria of speech reception threshold of better than 50 dB and speech discrimination score of better than 70%, although these indications must be individualized to the needs of the patient.

Some patients have a subjective assessment about the usefulness of the preoperative hearing, such as the ability to use the telephone with the involved ear or to localize a sound source. These subjective valuations should also be considered when determining hearing preservation candidacy. Some authors advocate attempting hearing preservation in the removal of small acoustic tumors if any measurable preoperative hearing exists. Patients older than 65 years do not tolerate the middle fossa approach as well as younger patients because of the increased fragility of the dura and prolonged retraction of the temporal lobe.

Preoperative Evaluation

Several preoperative factors may predict postoperative hearing preservation. The most obvious is tumor size. Intuitively, the smaller the tumor, the easier it is to remove, and the more likely that hearing will be saved. This trend has been substantiated by several authors. ^{, ,} Some authors have also found that the better the preoperative hearing, the more likely it will be preserved, ^, whereas others have failed to identify such a relationship. ^{, ,} In addition, an intact preoperative stapedial reflex has been associated with successful postoperative hearing preservation.

Several authors have reported a relationship between preoperative auditory brainstem response (ABR) audiometry and hearing preservation. ^, In one report, hearing was preserved in 78% of patients with an interaural wave V latency difference of 0.4 ms or less. For latency differences of 0.5 to 2 ms, the hearing preservation rate decreased to 58%. In patients with no response on the ABR, postoperative measurable hearing remained in only 50%. Patients with a more normal preoperative ABR result apparently have a greater success rate for postoperative hearing preservation. This result may reflect less tumor involvement of the cochlear nerve. Others do not find preoperative ABR to be predictive of hearing outcome. ^,

Patient with tumors arising from the superior vestibular nerve have a higher rate of hearing preservation than do those with tumors arising from the inferior vestibular nerve. Acoustic tumors developing in the inferior portion of the IAC may involve the cochlear nerve earlier and more severely. ^, In a series of middle fossa acoustic tumor removals, 68% of patients whose tumors were found intraoperatively to arise from the superior vestibular nerve had measurable hearing preservation, whereas only 43% of patients whose tumors originated from the inferior vestibular nerve had measurable postoperative hearing. This difference was statistically significant.

Preoperative electronystagmography may predict tumor origin and hearing preservation. The caloric response reflects superior vestibular nerve function. In the presence of a small acoustic tumor, a normal caloric response indicates an inferior vestibular nerve tumor, whereas a decreased response suggests a tumor arising from the superior vestibular nerve. Of a group of 54 patients who had preoperative electronystagmography, hearing was preserved in 64% with hypoactive caloric responses, whereas postoperative hearing remained in only 45% of patients with normal caloric responses. The association of normal caloric tests with nonpreservation of hearing has also been reported by others, ^, although some series have not found this correlation. Vestibular evoked myogenic potentials may prove useful in determining the nerve of tumor origin. Early experience indicates that a combination of normal electronystagmography and a reduced vestibular evoked myogenic potential predicts the inferior vestibular nerve as the nerve of origin.

For intracanalicular tumors, the radiographic appearance may predict success at hearing preservation. Patients with small tumors that enlarge the IAC have a poorer prognosis for hearing preservation (RK Jackler, personal communication, 1990). In our experience, these small tumors that expand the canal are very adherent to the cochlear nerve, which adversely affects the hearing outcome. Fast spin echo MRI provides ultra-high-resolution images of the IAC anatomy. Using this imaging technique, it is possible to determine the nerve of origin for small tumors. Tumors that are impacted into the lateral end of the IAC, especially those that impinge on the cochlear nerve canal, have a lower rate of hearing preservation. The presence of a fundal cap of spinal fluid is a radiographic marker to determine the lateral extent of the tumor. A fundal cap of fluid has been reported to be a positive prognostic indicator for hearing preservation, though there is conflicting data in the literature about this.

Patient Counseling

After a thorough discussion of the relevant anatomy and the management options of observation and stereotactic radiation therapy, the choices regarding surgical approaches to remove acoustic tumors are described to the patient. For patients with small tumors and good preoperative hearing, the issues of hearing preservation are discussed. We tell such patients that there is approximately a 50% chance of saving hearing. It is also important for patients to understand that hearing rarely improves after tumor removal. If the preoperative electronystagmography and ABR data are available and are favorable (see earlier), patients are informed that the prognosis for hearing preservation is above average.

The patient is told that there is approximately a 90% chance that normal or near-normal facial nerve function will be obtained in the long term, but that there is a 20% to 30% chance of having temporary facial paresis in the early postoperative period. Although the facial nerve results for either the middle fossa or retrosigmoid approach are excellent, our best and most consistent facial nerve results for small tumors occur with the translabyrinthine approach.

Patients with preoperative tinnitus are counseled that the problem will likely get better, but will probably not disappear. Patients with no preoperative tinnitus have approximately a 25% chance of developing it postoperatively. Other important possible but rare complications are discussed, including cerebrospinal fluid leak, meningitis, serious brain complications, death, and blood transfusion options.

Recuperation can take weeks to months, and most patients return to work within 4 to 6 weeks. The patient should expect to be dizzy postoperatively, and the rapidity of the central compensation greatly influences the time course of the recuperation. Some patients will benefit from a course of vestibular physical therapy, particularly those with preoperative vestibular symptoms and the elderly.

Surgery