Continuous retrograde monitoring of the facial nerve during cerebellopontine angle surgery: normative data

"En-bloc" removal of small- to medium-sized acoustic neuromas with retrosigmoid-transmeatal approach

Surgery of acoustic neuroma (AN) has significantly refined over the past years due to a series of advances in diagnostics and surgical technique. Electrophysiologic investigation performed during surgery has greatly contributed to this progress, increasing the surgeon's understanding of the mechanism of damage and suggesting various changes in his or her surgical strategy.In this context, the advantages of the retrosigmoid "en-bloc" removal of small to medium size ANs have been examined in the present study. At the ENT Department of the University of Verona, 103 subjects with AN were operated on, from January 1990 to December 1995, with a retrosigmoid-transmeatal approach. Eighteen subjects (17.4%) presented pure a intracanalar (IC) tumor and 85 (82.6%) had both IC and extracanalar (EC) involvement. All the IC tumors (n = 18) and 70 of the IC-EC neuromas with an EC size less than 25 mm are reported in this paper for a total of 88 patients. The first 48 patients were operated on via the classic procedures described in the literature, characterized by removal of the tumor after "debulking" and limited exposure of the internal auditory canal (IAC). The following 40 subjects were operated on according to the technique of "en-bloc" removal of the tumor and wide exposure of the IAC.In the "en-bloc" group the tumor was first detached from the cerebellar flocculus and the pons, when necessary. The tumor was not debulked to preserve the anatomic relationship with the nerves and to facilitate identification, cleavage and dissection of the tumor from the neural structures. Thereafter, the posterior wall of the IAC was drilled out and opened in a circumferential range from 180 to 270 degrees . The IAC dura was subsequently opened, and the distal end of the AN along with the vestibular nerves were identified. The vestibular nerves were sectioned in the distal portion of the IAC and dissected with the tumor from the underlying facial and cochlear nerves. Dissection continued medially to the IAC porus. The AN was progressively dissected from the cochlear and facial nerves in the cerebellopontine angle (CPA) with multiple direction maneuvers, as required by the characteristics and degree of adherence to the neural structures.The anatomic and functional results obtained with this new procedure ("en-bloc" removal) were compared with the classic "debulking" technique. The statistical analysis shows an improvement in postoperative outcome for both auditory and facial nerve function. The "en-bloc" removal procedure along with the wide exposure of the content of the IAC and electrophysiologic monitoring of the seventh and eighth cranial nerves are, in our experience, the recommended strategies for improving outcomes in small to medium size ANs.

Continuous retrograde monitoring of the facial nerve: preliminary experience during acoustic neuroma surgery

Continuous electromyographical (EMG) monitoring of the facial nerve is widely used during acoustic tumor surgery. Mechanical stimulation of the facial nerve is capable of eliciting synchronous and asynchronous EMG responses alerting the surgeon to damaging maneuvers performed on the nerve. Mechanical stimulation, however, elicits EMG responses only when the nerve has been injured by the underlying pathology or previous surgical maneuvers, and the technique is sensitive to administration of muscular blockers. In addition, EMG is unable to furnish quantitative information about the damage. The present paper illustrates an alternative technique for intraoperative facial nerve monitoring, that is, the recording of facial nerve antidromic potentials (FNAPs).Eleven subjects operated on by acoustic neuroma surgery via a retrosigmoid approach (tumor sizes ranging from 12 to 28 mm) participated in the investigation. Bipolar electrical stimulation of the marginalis mandibulae was performed to elicit FNAPs. Stimulus intensity ranged from 2 to 6 mA with a delivery rate of 7/second. A silver-wire electrode positioned on the proximal portion of the acoustic-facial bundle was used to record action potentials. Changes in latency and amplitude of FNAPs were analyzed as a function of the main surgical steps. FNAP monitoring provided quantitative real-time information about damaging maneuvers performed on the nerve and allowed prediction of postoperative facial function.

Retrosigmoid-transmeatal en bloc removal of small to medium-sized acoustic neuromas

One hundred three subjects with acoustic neuroma (AN) underwent surgery--involving a retrosigmoid-transmeatal approach--between January 1990 and December 1995. All the intracanalar tumors (n = 18) and 70 of the intra- and extracanalar neuromas with an extracanalar neuroma size less than 25 mm--a total of 88 patients--constituted the final study population. The first 48 patients were operated on with the use of classic procedures described in the literature, characterized by limited exposure of the internal auditory canal and removal of the tumor after debulking. Surgery in the next 40 subjects was conducted according to the technique of en bloc removal of the tumor. The main features of this technique are continuous direct recording of cochlear and facial-nerve action potentials, wide opening of the internal auditory canal with lateral extension to the fundus, and removal of the tumor following the capsular lining without debulking. Statistical analysis of the results of the en bloc removal, compared with the previously used debulking technique, showed improvement in postoperative outcome for both auditory and facial-nerve function.