A variety of therapeutic approaches including follow-up observations, surgical treatments, and stereotactic radiotherapy could be recommended for VS patients. Vestibular schwannoma surgery is the first choice for symptomatic patients. Although the microsurgical and imaging technology have made much progress in recent years, the postoperative facial nerve injury in patients receiving vestibular schwannoma surgery is still inevitable, which greatly reduces the quality of life [1, 2]. To intraoperatively improve the protection of facial nerve, we identified the factors that might affect the long-term facial nerve function of patients receiving vestibular schwannoma surgery.
Previous studies have shown that the intraoperative anatomical retention rate of facial nerve in VS patients is about 86.9%-98.1% [3, 11-13] while the incidence of long-term facial paralysis was 26% to 67.1% (HB Ⅲ-Ⅵ) [3, 5, 11]. In this study, 53 patients (59.6%) suffered from facial nerve injury (HB Ⅲ-VI) 6 months after operation, which is consistent with previous reports.
During the surgery, we found that due to the difference growth pattern and locations of epicenter of the tumor, the facial nerve location and deformation degree were differences. Despite some tumors volume were small, but due to their growth direction facing the face nerve which could cause severe pushing of the facial nerve, and some tumors were large, but their growth direction is opposite to the facial nerve, which makes the facial nerve receive lighter pushing. Therefore, this study innovative used the concept of the facial nerve elongation in order to explore the relationship between the facial nerve deformation degree and post-operative facial nerve function. The results of logistic regression analysis showed that facial nerve elongation was an independent risk factor and positively correlated to facial nerve injury 6 months after operation, whichresults were not founded in the past . Besides, the ROC curve analysis showed that the possibility of facial nerve injury increases largely when the facial nerve elongation is larger than 2.925cm.
The fact that tumor size affects the postoperative facial nerve functional outcomes has been widely recognized [6, 14, 15]. Most of previous studies evaluated the tumor size by calculating the maximum tumor diameter. However, a big deviation exists when the maximum diameter is used to represent the actual tumor size . Here, we used the tumor volume to measure the tumor size. The results of logistic regression analysis showed that tumor volume was an independent risk factor and positively correlated to facial nerve injury 6 months after operation, which is consistent with published studies [4, 16]. Besides, the ROC curve analysis showed that the possibility of facial nerve injury increases largely when the tumor volume is larger than 10.965cm³.
The relationship between the enlargement of IAC and the postoperative facial nerve injury remains controversial [4, 17]. In previous study, the IAC was mostly divided into enlarged or not even though the IAC sizes in patients differ greatly. Distinguishing whether the IAC enlarges or not according to one standard is not suitable. In this study, the enlarged degree of IAC was used to evaluate IAC size to reduce the potential error. The results showed that the possibility of long-term postoperative facial nerve injury increased as the enlarged degree of IAC grew. When enlarged degree of IAC exceeded the cut-off point of 1.88, the risk of facial nerve injury 6 months after the operation would largely increase.
The compressional deformation of facial nerve has rarely been included as a risk factor to be analyzed in previous studies. In clinical practice, we found that the postoperative facial nerve injury was prone to appear when the facial nerve was compressed and deformed seriously. Thus, the facial nerve elongation was introduced and statistically analyzed in this study. The univariate logistic regression analysis showed that the facial nerve elongation was significantly correlated with facial nerve injury after vestibular schwannoma surgery, while multivariate logistic regression analysis revealed that facial nerve elongation was not an independent risk factor for facial nerve injury. These suggest that facial nerve elongation could be used as a potential predictive factor for postoperative facial nerve injury in patients receiving vestibular schwannoma surgery. However, the distributional course of facial nerve was not limited to the aforementioned four types. Using the MRI to evaluate the facial nerve elongation was not precise, which lead to potential error. Diffusion tensor image-based three-dimensional reconstruction could be used to reconstruct the distribution of facial nerve [18-20] and to accurately evaluate the facial nerve elongation in the future.
In addition, this study showed that the facial nerve adhesion to tumor was an independent risk factor for facial nerve injury, which was consistent with the findings of Renato Torres et al. [3, 21]and with the clinical experience of surgeons.There are still some defects in this study. First, the number of patients enrolled in this study is not sufficient to get a precise conclusion. Second, several potential confounding risk factor, such as the blood pressure, blood glucose and so on, are not included to be analyzed.