Acoustic neuromas (vestibular schwannomas) arise in the vestibular portion of the eighth cranial nerve at the interface of its central neuroglial and peripheral Schwann cell components. After initial intracanalicular growth the tumor protrudes from the auditory canal into the cerebellopontine angle. The stage at which an acoustic tumor is detected is related to the nature of the symptoms and signs, the physician’s alertness, and the availability of modern diagnostic imaging modalities. Toynbee1 first described a primarily intracanalicular acoustic tumor in 1853. With the widespread availability of high-resolution magnetic resonance imaging (MRI) and sophisticated hearing tests, intracanalicular acoustic tumors are now diagnosed more easily. Purely intracanalicular neuromas differ from large cerebellopontine angle neuromas with regard to the effect of their growth. Asymptomatic tumors have also been discovered during imaging performed for other reasons. Because there is no consistent growth rate, there is no general consensus regarding the identification of candidates for therapeutic intervention. In general, management outcomes are better for smaller tumors, regardless of the strategy chosen. Owing to the small number of intracanalicular cases reported in most acoustic tumor series, it is difficult to define the exact rate of hearing preservation after intracanalicular acoustic tumor surgery.

Intracanalicular acoustic tumors as a separate entity have been addressed in relatively few reports.^2–5 In addition, there is wide variability in the criteria used to define hearing preservation. The criteria for preserved hearing in various series varies from as loose as any measurable hearing to as strict as having a pure-tone average (PTA) of <30 dB and a speech discrimination score of ≥70%. Terms like measurable, valid, useful, usable, serviceable, and good hearing are used to report post-operative hearing. A variety of classification systems have been used to document postoperative hearing. Although MRI has eliminated most diagnostic uncertainty, controversy still exists regarding the best management.

Background

Preservation of hearing has become the most challenging goal of intracanalicular acoustic tumor surgery. The reported hearing preservation after microsurgical excision of intracanalicular tumors varies from 12% to 82%.^{2, 5–10} In 1969, House and Hitselberger⁶ first reported that they obtained functional hearing preservation in four out of five patients with intracanalicular tumors excised via the middle fossa approach. Ten years later, Brackmann¹¹ reported “some” hearing preservation in 5 out of 11 patients using this approach. Subsequently many surgeons further refined the middle fossa approach and reported better hearing preservation rates.¹² Both Silverstein et al.⁹ and Jannetta et al.¹³ used the retrosigmoid approach in order to achieve hearing preservation. Sterkers et al.¹⁰ had a 66.7% hearing preservation rate using the retrosigmoid approach. Nadol et al.¹⁴ reported a 50% useful hearing preservation rate but included patients with a speech discrimination score (SDS) as low as 15% and a speech reception threshold (SRT) of <70 dB in the useful hearing group. They also reported that 36% (5 14) had hearing preservation. Preserved hearing was defined as a change of < 15 dB in SRT and < 15% in SDS, as compared with preoperative levels.^{14, 15}

Samii et al.⁵ recommended early surgery by the retrosigmoid approach in order to improve hearing preservation rates. In a review of 1000 acoustic tumors, Samii et al.⁵ reported a 46% (17 37) hearing preservation rate in the subgroup of patients who had intracanalicular tumors. Samii et al.⁵ advocated the Hannover classification, which included patients with PTA levels as poor as 80 dB and speech discrimination scores as low as 10%, in the preserved hearing group. Useful hearing preservation (defined as PTA <40 and SDS ≥70%) was noted to be 29%.¹⁶ Haines et al.² reported an 82% (9 11) hearing preservation rate using both middle fossa (5 6) as well as retrosigmoid approaches (4 5), although in the next few patients hearing was not preserved so readily (personal communication). Irving et al.⁷ compared hearing preservation rates (based on the 50 50 rule) using either a middle fossa or a retrosigmoid approach. They reported a 44% useful hearing preservation rate in the middle fossa group as compared with 12% in the retrosigmoid group. They concluded that the middle fossa approach has enhanced lateral exposure that facilitated lateral to medial dissection and promoted development of tumor arachnoid planes at the fundus. They hypothesized that this results in less traction on the distal cochlear nerve at its weakest point where it forms fine filaments in the modiolus and also less traction on the distal labyrinthine artery at its foraminal end.⁷

Most of the reported hearing preservation rates reflect immediate postoperative hearing status but do not document long-term hearing outcome at 1 or more years after surgery. Long-term follow-up of patients with preserved postoperative hearing showed delayed hearing loss in a significant number of patients.17, 18 Shelton et al.¹⁸ reported further loss of preserved postoperative hearing in 56% of patients over a mean follow-up of 8 years.¹⁸

Facial nerve function has not been documented in most of the hearing preservation studies. Nadol et al.¹⁴ observed postoperative facial paresis in 4 of 14 patients of smaller acoustic tumors that recovered in a year. Atlas et al.¹⁹ described 3 patients with House-Brackmann grade II results and 2 with grade III results. Cohen and Ransohoff²⁰ reported a single intracanalicular tumor patient with preserved hearing; this patient had partial facial weakness with incomplete recovery. Irving et al.⁷ noticed an increased incidence of transient facial nerve palsy after middle fossa surgery in the immediate postoperative phase.

Evolution of Radiosurgery

Lars Leksell coined the term stereotactic radiosurgery in 1951. He developed the technique for delivery of a single high dose of precisely focused radiation to achieve closed skull destruction of a small intracranial target. Acoustic tumor stereotactic radiosurgery using the Gamma knife unit was first performed by Leksell in 1969.²¹ In a review of his 20 years’ experience with treatment of acoustic neuromas by radiosurgery, Norén et al.¹⁵ reported that more than 90% of patients had tumor growth control. The initial patient treated with the first North American 201-source Cobalt-60 Gamma knife unit at the University of Pittsburgh in 1987 had an acoustic neuroma.²² Radiosurgery technique has evolved steadily during the past decade.^{4, 15,22–29} The results after radio-surgery of acoustic tumors have established it as an important minimally invasive alternative to microsurgery.^{24, 26},²⁷ Pollock et al.²⁹ performed a retrospective matched cohort analysis of micro-surgical and radiosurgical outcomes at the University of Pittsburgh and found radiosurgery as an effective and less costly management strategy. Ogunrinde et al.³ described 10 patients with intracanalicular tumors managed by radiosurgery; no patient had a delayed facial or trigeminal neuropathy.

RADIOSURGICAL TECHNIQUE

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