"En-Bloc" Removal of Small- to Medium-Sized Acoustic Neuromas With Retrosigmoid-Transmeatal Approach

Abstract

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.

Intraoperative Monitoring for Hearing Preservation and Restoration in Acoustic Neuroma Surgery

“…Auditory monitoring has enabled us to improve hearing preservation significantly. 9,10 In selected patients, preservation of some hearing (classes A to C) was achieved in 72% of patients, whereas useful hearing (classes A and B) was obtained in 52% of patients. These results are similar to those reported by Slattery et al 25 using the middle fossa approach and intraoperative monitoring.…”

“…None of them, however, received implants because they retained some degree of hearing on completion of surgery, as evidenced by intraoperative preservation of CNAPs. 8 The surgical technique of RS-TM removal of AN has been described in previous articles, 9,10 and details will be omitted here. Recently, we used this approach for ABI placement and, in agreement with the latest reports in the literature, 11,12 found it to be the most direct route to the cochlear nucleus area.…”

“…It is able to improve the incidence of hearing preservation from approximately 30% to 70%, when direct recording techniques (CNAPs) are used. 9,10,[16][17][18] This improvement is obtained through the identification of ongoing hearing deterioration in real time that allows the surgeon to interrupt damaging maneuvers. As a result of this information, refinement of the surgical technique may be achieved, such as the use of the en-bloc removal technique rather than debulking.…”

“…As a result of this information, refinement of the surgical technique may be achieved, such as the use of the en-bloc removal technique rather than debulking. 9,10 One particular aspect of hearing preservation in AN surgery is represented by patients who undergo total deafness, either for an NF2 disease or because of a solitary AN in the only hearing ear. In these subjects, when hearing preservation is not feasible, restoration of hearing is possible by implanting a bionic ear.…”

Advantages of the Retrosigmoid Approach in Auditory Brain Stem Implantation

“…The posterior wall of the internal auditory canal is drilled out, and the tumor is freed and removed "en bloc" or, in the case of large tumors, dissected after debulking. 9,10 Intraoperative auditory and facial nerve monitoring is performed using direct recording techniques. [11][12][13] After tumor excision, microsurgical dissection is necessary to reach the cochlear nucleus area positioned on the surface of the lateral recess of the fourth ventricle.…”