There is probably not a more controversial topic in all of neurosurgery or neurotology than the optimal treatment for small- to medium-sized acoustic neuromas.

Acoustic neuromas usually arise from the superior vestibular nerve at the transition point between myelin production from oligodendrocytes (central myelin) and myelin production from Schwann cells (peripheral myelin). This transition point, the Obersteiner–Redlich zone, is usually within the internal auditory canal (IAC) near the transverse crest. Therefore, acoustic neurinomas tend to originate within the IAC and grow through the porus acusticus into the cerebellopontine angle. In general, about 14% of acoustic neuromas are intracanalicular.

Small vestibular schwannomas are defined generally as Koos grade I or IIa, either purely intracanalicular or protruding from the IAC no more than 10 mm, respectively. 

Patients usually present with tinnitus, vertigo, and unilateral sensorineural hearing loss. Surgical approaches include the middle fossa and retrosigmoid approaches and are selected on the basis of tumor size and location and the possibility of hearing preservation. My preferred surgical approach to small acoustic neurinomas in patients with useful hearing is the retrosigmoid craniotomy.

The increased adoption of magnetic resonance imaging (MRI) has resulted in an increased incidence in the diagnosis of small vestibular schwannomas, particularly within the elderly population. However, the management of small asymptomatic to minimally symptomatic vestibular schwannomas presents a challenge within the spectrum of care for patients diagnosed with a vestibular schwannoma. Surgeons must avoid increasing the patient’s symptoms with the proposed plan of care. 

Therefore, this management topic is quite controversial within the neurosurgical literature and is subject to frequent debate. The perspective presented in this chapter is mine. 

Three management options should be considered for small vestibular schwannomas:

1. Conservative observation and serial radiographic follow-up
2. Stereotactic radiosurgery
3. Microsurgical resection

These options are described in detail as applied to small acoustic neuromas. In general, conservative observation is pursued after the initial diagnosis. 

Determining the optimal course of action for these small lesions should include consideration of the natural course of growth and symptomatic capabilities of the lesion versus the risk of intervention. This philosophy provides a baseline for justifying the recommendation of more permanent treatment options. 

Some authors and our otolaryngology colleagues have advocated for immediate microsurgery or radiosurgery. However, variables such as tumor size and patient age, hearing status, and treatment preference heavily impact those recommendations. The average growth rate is generally 1 to 2 mm/year; however, these lesions can be prone to rapid expansion, so the initial observation intervals should be conservative. Many authors recommend an initial 6-month interval for follow-up imaging. 

A lack of obvious tumor growth on imaging should not be assumed to indicate a lack of symptomatic progression, given that even stable lesions can result in gradual hearing deterioration. Therefore, audiometric testing for speech discrimination and decibel loss is appropriate.

Conservative observation is recommended by many for small vestibular schwannomas that are asymptomatic or mildly symptomatic. The obvious argument against this recommendation is the possibility for sensorineural hearing loss, even in the absence of tumor progression. The possibility of sensorineural hearing loss and early progression should be weighed against the risks associated with microsurgical resection or stereotactic radiosurgery. The surgeon must take the patient’s unique anatomical and clinical factors into consideration when making this determination. 

A large series by Tveiten et al. (1) of patients with an average of 8 years of follow-up and serviceable hearing at diagnosis revealed a 68% chance of maintaining functional hearing via conservative management compared to 40% via Gamma Knife radiosurgery and 14% via microsurgical resection. This study carried an innate bias given that the treated tumors were more likely to have undergone progressive growth, but nonetheless, the results of this study highlight the value of conservative measures. 

Multiple study authors have recommended stereotactic radiosurgery for small vestibular schwannomas, which secures serviceable hearing in 40% to 75% of patients 3 to 7 years after diagnosis. However, a randomized controlled study comparing observation to radiosurgery has not been performed. Therefore, recommendations for this surgery are typically conservative. 

It is important to note that the cochlear radiation dose should be less than 4.2 Gy (2). Initiating Gamma Knife radiosurgery before the onset of subjective hearing loss is preferrable (3). 

Surgery for small vestibular schwannomas must result in the preservation of not only facial nerve function but also hearing. Resection can be performed via either a retrosigmoid or middle fossa approach. 

The initial hearing-preservation rate after these approaches ranges from 50% to 100%, but the possibility of delayed decline in functional hearing persists. The vast range in success rates of resection depends on patient selection and the surgeon’s skill. Therefore, these considerations must be a factor when recommending this treatment to patients. 

Surgery may offer an opportunity for saving the hearing of patients with serviceable hearing. This depends highly on the surgeon’s hands and the patient’s baseline hearing, tumor size, and extent of tumor extending toward the fundus of the IAC. Patients who have excellent hearing before surgery, a small tumor, and a good fundal cap of cerebrospinal fluid (CSF) are the best candidates for hearing preservation surgery. Similarly, these are also the patients who fare best with observation or radiosurgery.

The techniques for resecting small acoustic neuromas follow those for the resection of larger ones. The following series of illustrations demonstrates nuances of the technique for resection.

The opened IAC air cells are sealed with bone wax to prevent CSF leakage through peritubular air cells. The closure is performed in standard fashion. 

If the 3-month MRI confirms gross-total resection, I order MRI at 2 years to look for early recurrence and again 5 years after that (7 years postoperatively) to investigate for later recurrence. If the 7-year scan is negative for tumor recurrence, I plan to order MRI in 7 to 10 years to look for very late recurrence.