Editorial: Hearing after Gamma Knife surgery

Abstract

Yomo and colleagues4 review the extensive vestibular schwannoma (VS) experience from Marseille. They paid special attention to the longitudinal audiometric data both before and after Gamma Knife surgery (GKS) in 154 patients with unilateral VS over an 8-year period. Their hypothesis was that radiosurgery would worsen the expected rate of hearing deterioration in a patient with VS in comparison with the natural course. Recently, there have been a number of articles on conservative management for VS that have included analyses of hearing loss. A recent report from the Marseille group compared radiosurgery outcomes to observation in patients with intracanalicular tumors and found that failure (tumor growth plus or minus hearing loss) was worse in the patients who were observed.3 In the present analysis, Yomo and colleagues found that the annual hearing reduction was 5.39 dB/year before radiosurgery and 3.77 dB/year afterward. The data were collected over a mean of 22 and 52 months, respectively. A cochlear dose of less than 4 Gy was found to be important for hearing preservation. The authors noted a trend toward reduction in the annual hearing loss after radiosurgery but called for more clinical information to identify a possible protective effect of radiosurgery for hearing. It is important to note that their radiosurgical technique was excellent and included use of stereotactic frame-based GKS, careful anatomical dose delineation of the tumor and regional structures, and precise tumor dose delivery. Radiosurgery was followed by meticulous serial assessments. Why does hearing loss occur in the setting of VS? In the natural state, one may hypothesize that effects on cochlear nerve axons due to local compression, invasion, or ischemia may be important. Certainly, tumor volume correlates with hearing function at presentation, although the extent of the tumor in the auditory canal is also important. Patients with larger tumors who have only a small component within the canal may maintain excellent hearing. Hearing loss after radiosurgery may be due to additional mechanisms. Radiation may affect axonal physiology or the function of the cochlea, causing them to be different from the pretreatment state. In addition, the induced tumor injury is mediated through acute and later chronic inflammation, which in some patients can lead to a transient expansion of the tumor capsule and release of cytokines. These changes may affect hearing. Although tumors usually regress over time, transient intratumoral changes may have permanent effects on hearing. In the background of this analysis, the authors importantly point out the simple effects of aging on hearing. An annual hearing loss of 1 dB per year continues to occur in the background of the tumor and its treatment. Perhaps as important a conclusion as any other in the report is the need for long-term evaluations of hearing, since shortterm analyses tend to overstate the rates of hearing preservation. Prior reports with a mean duration of follow-up of even less than 1 year have been published with conclusions on hearing “preservation rates.”1 Longer-term analyses from the same institution showed lower rates.2 In summary, there is much work to do to further our understanding of the physiology and pathology of hearing function and preservation in the setting of VS and the available management options. Twenty years ago hearing preservation was the “Holy Grail” of the VS story. Now it is an expected outcome for many patients. Recently, there has been some resurgence in interest for the wait-andsee strategy. Although this makes sense for the elderly, I believe that it is a lesser option for most patients. This important report will encourage other centers to obtain meticulous audiometric follow-up. (http://thejns.org/doi/abs/10.3171/2011.2.JNS11222)