Recently, endoscopic techniques have been introduced for treating lateral skull base lesions with several advantages over classic techniques, including the use of natural orifice (i.e. EAC), access to the IAC and minimal soft tissue dissection, and bone removal (10). Despite fascinating results, further studies are needed for better understanding of its landmark and increasing surgical knowledge and skills. In this study, we successfully reached the IAC with a minimally invasive transcanal endoscopic technique, i.e. ETTA, on the cadaveric temporal specimen.
The ETTA provides good visualization of the IAC, with high magnification of every structure within IAC and outside it, including the whole facial nerve path, and it requires limited bone drilling compared to traditional methods (11). Facial nerve re-routing, which is used in traditional techniques, is not necessary for ETTA. Thus less manipulation of this nerve could reduce the probability of its paralysis after surgery. Variable endoscopic angles could ensure no blind spot and therefore, fully resection of the underlying pathology (12). Using a navigation system, an endoscope holder and special endoscopic ear surgical set (with simultaneous suction and dissection instrument) could be helpful with ETTA efficacy. According to Salami et al. study (13), a piezoelectric device using low-frequency ultrasonic waves (25 - 30 kHz), which could be used for platinotomy, antro-atticotomy, mastoidectomy, and endo-auricular osteotomy, with variable headpiece and inserts could be helpful when performing ETTA and other endoscopic approaches. Hearing preservation is not achievable via traditional techniques also it is not possible with the ETTA (11, 14).
Master et al. in their cadaveric study in 2016 (10) reported that IAC could be accessed via the endoscopic transcanal approach with a safe distance from vital neurovascular structures. During anatomical dissection of 19 temporal bones in this study, the mean distances from the carotid artery, jugular bulb, and middle fossa to the surgical path opening to the IAC were 4.1 ± 1.5, 6.4 ± 2.5, and 5.5 ± 1.9 mm, respectively. Marchioni et al. (11) conducted the first case series regarding exclusive ETTA access to the IAC and its efficacy for vestibular schwannoma removal. In their study, 10 patients with vestibular schwannoma (Koos Grade I or II) underwent exclusive ETTA and gross total resection was achieved in all the patients, with an average operative time of 192 min. Postoperatively, permanent facial nerve paresis/paralysis was observed in neither cases. According to another study by Marchioni et al. (15), although the hearing preservation is not possible with the ETTA, it is an effective approach for the resection of small intracanalicular acoustic neuroma with good post-surgical facial nerve function. Wick et al. (3) reported successful gross total resection of vestibular schwannomas in 4 patients (3 with Koos grade I and 1 with grade III) with a good long-term facial nerve function postoperatively. The mean hospital stay was 2.8 days in this study. Presutti et al. (16) concluded that expanded ETTA (using both endoscope and microscope, which allows bimanual dissection of the tumor and therefore, enhanced control of the surrounding neurovascular structures) was successful in the management of 10 patients with vestibular schwannoma Koos stage I or II with no facial dysfunction in the last follow-up. Marchioni et al. (9) described the expanded ETTA for the successful resection of vestibular schwannoma in patients with Koos grade II and III with no complication and good facial nerve result and concluded that this technique was a new approach that combined the advantages of a microscopic technique with the ones offered by the endoscope. Moon et al. (12) evaluated a modified ETTA with no EAC closure for the treatment of vestibular schwannomas and reported better cosmetic outcome compared to the original technique with total resection of the tumor in all cases.
The CSF leakage was observed less postoperatively with ETTA. Marchioni et al. (11) reported no CSF leakage after exclusive ETTA. In Presutti et al. study (16), 1 of 10 patients underwent expanded ETTA experienced postoperative CSF otorhinorrhea, which completely resolved in the follow-up period. Wick et al. (3) reported no postoperatively complication after ETTA on 4 patients with vestibular schwannomas. The mean percentage of this adverse effect was 10.3% for the retrosigmoid approach, 5.3% for the middle cranial fossa approach, and 7.1% for the translabyrinthine approach (17, 18). The lower risk of CSF leakage is because the ETTA is minimally invasive and with limited bone removal (9). The ETTA not only could minimize post-surgery complications but also could be a cost-benefit technique because it could shorten postoperative intensive care unit recovery and hospital stay (9, 11).
The important point of the present study is that for the lateral skull base surgeries through transcanal endoscopic techniques, increasing the surgeon’s skill and knowledge and better identification of the anatomical landmarks are necessary. The navigation system and other auxiliary tools could be used to increase the surgical accuracy and for better evaluation of the vital structures and tumor extension. It should be noted that the lack of articles in cadaveric temporal could cause some study limitations. Considering the probability of less manipulation and complications with transcanal endoscopic techniques, further studies should be conducted to verify their feasibility in a patient with lateral skull base lesions.
5.1. Conclusions
We tried to increase our learning curve and extend surgical experience in lateral skull base surgery via a minimally invasive ETTA. Access to the lateral skull base and IAC is possible with classic microscopic methods; however, it is accompanied by several complications and morbidities. With the advancement in the technology of endoscopic devices, the endoscopic approaches may be used to treat the pathologies of IAC through the EAC. This requires increased endoscopic skills and the use of auxiliary tools such as navigation guide for successful access to the IAC and decreasing morbidities.
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