Previous studies have reported that the incidence of final tumor enlargement after gamma knife surgery is about 2%-9%[18–20]; however, as only about 26% of these patients choose surgical resection[21], such cases are rare. Nonetheless, with the extended follow-up, the number of patients who failed gamma knife treatment may continue to increase. Significant adhesions, thickening, and fibrosis have been reported during surgery for VS after gamma knife treatment [22, 23], which is similar to our findings. Nevertheless, there are only a few studies on the effect of preoperative gamma knife therapy on VS resection's clinical efficacy and safety. Recent studies on the efficacy of microsurgical resection of VS after gamma knife (Table 5[23–27]) included a small number of cases, with a maximum of 16 cases[27]. To the best of our knowledge, the current study has the largest number of cases to date. Lee et al[26] included 13 patients in their study, some of whom underwent gamma knife and craniotomy prior to microsurgical resection of VS, and even some of them were NF-2. As these factors may have interfered with the final results compared with the study, which enrolled patients with VS who only underwent gamma knife before surgery[28], these factors were excluded from the present study.
Facial paralysis is one of the most serious diseases because the loss of facial expression is a physically and mentally debilitating condition[29]. Protecting facial nerve function is the basic goal of the treatment of vestibular schwannoma and also the key factor affecting the treatment decision[24, 30]. In their study, Gerganov et al[28] found that the long-term facial nerve function retention rate in patients with no preoperative gamma knife was better than in patients with preoperative gamma knife, which is consistent with our findings. This could be explained by the following: 1) microsurgical resection was more likely to cause facial nerve injury in group A; 2) postoperative facial nerve function was more difficult to recover in group A. The main cause of postoperative facial paralysis is the direct intraoperative injury to the facial nerve[31]. In this study, there was no significant difference in facial nerve function between the two groups one week after surgery, indicating that a preoperative gamma knife does not make facial nerve more likely to be intraoperatively injured. Previous studies with long-term follow-up of facial nerve function after VS surgery[32, 33] showed that worse early postoperative facial nerve function of patients, and within the last follow-up after surgery, patients would experience A long recovery period of facial nerve function. However, during follow-up, we found that patients in group A did not experience a recovery period of neurological function (Figure 1). The House Ear Clinic (Los Angeles, California) reported that once the facial nerve has been radiated, the regeneration potential is diminished, and the recovery from microsurgical trauma is not as robust[22]. The results of our clinical study confirmed the validity of their theory. Therefore, it is more important to protect the nerve during surgical resection of patients with a preoperative history of gamma knife treatment. Advanced microsurgical techniques and neurophysiological monitoring have been reported to be effective for intraoperative protection of the facial nerve [34].
In this study, the incidence of postoperative disequilibrium in patients with a preoperative history of gamma-knife was significantly higher compared to patients without preoperative gamma-knife treatment, which might be due to the following reasons: 1) the vestibular nerve is involved in the regulation of the balance in human body[35]; it adheres to the tumor and is vulnerable to damage by gamma knife. The recovery ability of the vestibular nerve is weakened after microresection. 2) After gamma knife treatment, vestibular schwannoma strongly adheres to the cerebellum, and the cerebellum can be easily damaged during operation. Disequilibrium is one of the most significant factors that can negatively affect the daily life of patients[36]. The early postoperative period is critical for appropriate treatment to alleviate postoperative symptoms[37]. Therefore, we believe that after the resection of vestibular schwannoma with a preoperative gamma knife, attention should be paid to whether the patients experience disequilibrium in the early postoperative period, which should be actively managed via the implementation of effective measures. A previous prospective randomized clinical study suggests that specific exercises with visual biofeedback improve vestibulospinal compensation and balance function in patients after vestibular schwannoma microsurgical removal[38], which could serve as an effective way to treat disequilibrium.
As shown in Table 6, the postoperative control rate of tumors after surgical resection for patients with preoperative history of gamma-knife treatment was as high as 91.7-100% in previous studies. In this study, the tumor control rate in patients from group A reached 100% during the mean follow-up time of 52 months, possibly because 82.5% of patients underwent total tumor resection, which was far higher compared to previous studies[25, 26]. Currently, there is still controversy about whether VS should be completely excised. Gerganov et al[28] suggested that every VS should be completely excised, especially in those patients with a previous history of gamma knife therapy. Gurge's preferred option is to protect neurologic function versus complete resection of the tumor, as residual tumors can then be treated with stereotactic radiation therapy[39]. In the present study, seven patients in group A did not have total resection of the tumor; however, their tumor did not increase during long-term follow-up. Therefore, we believe that when the total resection of the tumor is difficult for a patient with a history of gamma knife surgery, total resection is not necessary.
Lee et al[23] suggested that gamma knife treatment before microdissection may increase the probability of postoperative complications, which is in line with our results. We found that the incidence of postoperative pulmonary infection in group A was higher than that in group B, which may be due to the following: 1) prolonged operation time tends to increase the probability of pulmonary infection after craniocerebral surgery[40]. A previous study has reported that the average operation time of vestibular schwannoma patients with gamma knife treatment history takes 95 minutes longer[24]. 2) Because of the severe adhesion between tumor and nerve after gamma knife operation, the posterior cranial nerve can be easily damaged during operation, which might be associated with the occurrence of postoperative pulmonary infection[41]. The meticulous surgical technique helps surgeons to avoid unnecessary nerve injury and prolongation of surgery duration.
Limitation
There are some limitations in the present study. First, this is a single-center retrospective study. Surgical technique and management of patients vary among hospitals. Second, there is a lack of quantitative indicators used to describe the degree of tumor adhesion. Finally, in the process of retrospective follow-up, some data were missing, such as hearing test results. The conclusion of this study needs to be further verified via multi-center research with a larger sample size and more detailed data.