An Endoscopic Transcanal Transpromontorial Approach for Vestibular Schwannomas using a Computer-Based Three-Dimensional Imaging System

Here, we present a transcanal transpromontorial approach for vestibular schwannomas using a computer-based three-dimensional (3D) imaging system combined with a two-dimensional (2D) endoscope. This system provided stereoscopic vision, better depth perception and reduced visual fatigue. This 3D imaging system enabled the application of 3D vision technology in endoscopic lateral skull base surgery.

A 2D monocular endoscope has been used in transcanal transpromontory vestibular schwannoma surgery instead of craniotomy. However, the absence of depth perception is the limitation of this approach. With the loss of depth perception, the surgeon will be not able to perform delicate and particularly complicated surgery. A binocular endoscope has been developed to provide stereoscopic vision with better depth perception for complicated anatomic structures and has been applied in some endoscopic surgeries. However, the diameter of the endoscope is a limitation in the performance of transcanal otologic surgeries. A small diameter endoscope facilitates easier surgery in a restricted space. A computer-based 3D imaging system can obtain 3D images in real-time using a small monocular endoscope. In this study, to evaluate the feasibility of a computer-based 3D imaging system for endoscopic lateral skull base surgery, we applied this 3D imaging system in a transcanal transpromontorial approach in two patients with vestibular schwannomas. The surgical procedure was completed without complication in these two cases. There was no mortality, perioperative complications, nor notable postoperative complications. Using this computer-based 3D imaging system, a better depth perception and stereoscopic vision was observed compared to a conventional 2D endoscope. The improvement in depth perception offers superior management of the complicated surgical anatomy.

Minimally invasive surgery has become mainstream. Many techniques have been developed, such as the da Vinci robot system and the endoscope. However, the equipment and cost of da Vinci robotic surgery are bulky and very high, respectively. Compared to the conventional craniotomy surgery, the endoscopic transcanal transpromontorial approach for resection of vestibular schwannoma has been developed to decrease the risks of vestibular dysfunction and cerebrospinal fluid leak¹. However, lack of stereoscopic vision is still the main limitation of endoscopic surgery, especially for complicated ear surgeries². Hence, the 3D endoscope was developed to imitate the binocular disparity to generate stereopsis of operative vision^3,4. However, the caliber of the currently available 3D binocular endoscope is equal to or greater than 4 mm, making its application in transcanal endoscopic ear surgeries difficult. In addition, when the 3D binocular endoscope is used at close range, its large binocular parallax may lead to double vision.

A monocular 3D endoscope was first introduced in sinus surgeries in 2013⁵. This monocular 3D endoscope system incorporates a microscopic array of lenses in front of a single video chip in the endoscope, acting as separate visual receptors. This method mimics “insect eye” technology, which in turn generates 3D vision. A novel computer-based 3D imaging system was first applied in transurethral endoscopic surgery in 2015⁶. The processor simulates a 3D image by converting the conventional 2D endoscopic image into a pair of images, as received from two viewpoints. The major advantage of this computer processing system is that it can be adapted to conventional monocular endoscopes of any diameter. Both abovementioned 3D imaging systems have not been previously used in otologic surgery. We applied the computer-based imaging processor to endoscopic ear surgeries, including tympanoplasty, mastoidectomy, ossiculoplasty and cochlear implant². This image system has some advantages for transcanal endoscopic ear surgeries. First, we can use all the equipment from the 2D endoscope system and do not need to change the whole system. Second, the caliber of the scope is no longer a concern. The average diameter of the external ear canal is 7 mm in width⁷; the caliber of the instruments (e.g., hook, dissector, and forceps) is approximately 1–2 mm. Thus, the proper caliber of the endoscope is restricted for transcanal ear surgeries. The common calibers of the 2D endoscope for otologic surgery are 3, 2.7 and 1.9 mm, and all of them could be used with this computer-based processor. Therefore, a smaller diameter 2D endoscope equipped with a novel 3D imaging system can be easily and conveniently applied in otologic surgery and enable ear surgeons to operate with 3D vision. In our previous work, we also found that there is no time delay and no visual fatigue when performing ear surgeries using this computer-based 3D endoscopic system².

In this study, to evaluate the feasibility of the computed-based 3D imaging system for endoscopic lateral skull base surgery, we applied this 3D imaging system to the transcanal endoscopic transpromontorial approach for two patients with vestibular schwannomas with nonserviceable preoperative hearing.

The protocol follows the guidelines of Chang Gung Memorial Hospital’s Human Research Ethics Committee. Ethical approval for the experiment was obtained from the Institutional Review Board of the hospital (IRB No. 201600593B0).

1. Patient position and skin marking

1. After general anesthesia, place the patient in a supine position on the operating table, with the head gently rotated to the contralateral side and elevated 15–30°.
2. Elevate the head of the bed approximately 15–30° to avoid blood recruitment to middle and inner ear and decrease bleeding.
3. Use an electrophysiologic facial nerve monitor to assist the surgeon in facial nerve location and dissection.
   1. Use the detector probe to touch the suspected facial nerve or tissue to make sure that the operating direction is correct.
   2. Set a current of 1 A on the monitor. If the monitor alarms, stop the procedure. Then, decrease the current to 0.5 A and 0.2 A to ensure that the facial nerve will not be damaged.

2. Local anesthesia and incision in the ear canal

1. Using a 3 mL syringe with a 21 G needle, provide local anesthesia by injecting the anesthetic (2% lidocaine with 1:100,000 epinephrine) subcutaneously to the external auditory meatus until the skin of ear canal become blanching.
2. After sterilizing the surgical area, including the external auditory canal, use a round knife to make a circumferential skin incision of the ear canal (EAC) at the osseo-cartilaginous junction.
3. Use a round knife to carefully elevate the lateral EAC skin to form a skin flap for postoperative closure of the ear canal.
4. Use the cotton ball soaked with epinephrine or electric cauterization to control the wound bleeding.

3. Canaloplasty

1. Remove the medial side of the EAC skin and tympanic membrane.
   1. Under the endoscope, use a round knife to elevate the EAC skin connected to the tympanic membrane.
   2. Use an alligator clamp to completely remove the skin and tympanic membrane.
      NOTE: Try not to retain any epithelium in the external ear canal or middle ear cavity to avoid possible risk of external ear and middle ear cholesteatoma postoperatively.
2. Widen the ear canal transmeatally with a 2 mm diamond burr.
   1. Using an endoscope with the four-hand technique, enlarge the diameter of the canal to directly visualize all of the middle ear cavity. The assistant holds the endoscope with two hands, and the surgeon can also perform the surgical procedure with two hands.
   2. Otherwise, under the microscope, use both hands of the surgeon to enlarge the diameter of the canal with a 2 mm cutting burr.
   3. Use a silicone sheet or cotton ball to separate the middle ear and external auditory canal in order to avoid getting bony chips or the epithelium of the canal into the middle ear cavity.

4. Insertion of the endoscope and setting of the 3D imaging system

1. Hold a 3.0 mm endoscope with the left hand and insert it into the canal after the bleeding is well-controlled.
2. Place both monitors of the 2D and 3D images in front of the operative table. Click Open to open software.
   NOTE: The 2D and 3D monitors provide 2D and 3D images, respectively, from different machines.
3. Have the surgeon and all observers wear stereoscopic glasses for 3D vision.
   NOTE: Real time 3D reconstruction of the endoscopic ear image is carried out throughout the surgery by the processor. There is simultaneous display of the 2D (shown on one monitor) and 3D (shown on the other one) images. With or without goggles, observers can compare the 2D and 3D images of the surgical field. Any change in brightness, sharpness and color, and time delay could be perceived.
4. Use a 45° 3 mm endoscope to confirm that the residual skin of the external auditory canal and remnant of the eardrum have been completely removed to avoid possible cholesteatoma after the surgery.
5. To avoid heat injury, keep the light resource under 40% throughout the surgery, and frequently move the endoscope forward and backward in the canal.
6. Use antifog solution to clear the endoscope if the endoscope lens is contaminated with blood.

5. Approach to the inner ear and tumor resection

1. Cut the chorda tympani nerve with the scissors and remove the remnant chorda tympani nerve with the retrieval device (e.g. Alligator) and suction.
   1. Remove all the ossicular chain (malleus, incus and stapes).
   2. Under the endoscope, carefully remove the incus, malleus and stapes by the retrieval device, respectively.
2. Carefully preserve the function and the path of the facial nerve with a facial nerve monitor.
   1. Under the endoscope, observe the facial nerve canal, and avoid touching or damaging the facial canal.
3. Remove the outer portions of the basal and middle turn of the cochlea and some of the lateral wall of the modiolus to expose the tumor with a piezosurgery instrument.
   NOTE: Similar to the surgical procedure introduced by L. Presutti⁸, the vestibular schwannoma can be visible after entering the fundus of the IAC.
4. Remove the tumor carefully.
   1. When the tumor is visible, separate the tumor from the facial nerve and the cochlear nerve and remove the tumor.
   2. Set a stimulus of 0.05-0.1 mA, and use the probe to touch the suspected tissue to result in a facial nerve response. Be careful not to use the suction tube to touch the nerve.
5. Pack the defect with abdominal fat and hemostatic agents (e.g. Surgicel and Floseal).
6. Suture the lateral EAC skin flap to the tragal skin in a watertight fashion for cosmetic reasons.

6. Post-operative procedure

1. Admit the patient postoperatively to the intensive care unit for 24-48 hours.
2. Transfer the patient to general ward if no postoperative complications occur.

We had performed two cases of vestibular schwannoma resection through the transcanal endoscopic transpromontorial approach in our hospital.

Case 1
A 35-year-old male was diagnosed with neurofibromatosis type II with multiple cranial nerve schwannomas and a left side vestibular schwannoma. He had almost complete hearing loss for 1 year before the operation. He underwent the transcanal endoscopic transpromontorial approach because of the sudden worsening of left facial palsy to HB grade V⁹: grossly asymmetric face at rest and slight movement of mouth angle. The surgery was completed without difficulty. There were no intraoperative or postoperative complications noted. The patient had an uneventful postoperative course. His facial function improved to HB grade 3 at his first postoperative visit 2 weeks after surgery and almost completely recovered after 1 month (Table 1).

Case 2
A 78-year-old female had a right side Koos grade I vestibular schwannoma¹⁰. The tumor was approximately 1 cm and located within the internal auditory canal. She had no serviceable hearing before the operation (Table 1). This means that a pure tone audiometric test reported profound sensorineural hearing loss (mean 95 dB threshold). Because her vertigo became worse, she underwent the transcanal endoscopic transpromontorial surgery for tumor resection as shown in Figure 1. There were no intraoperative or postoperative complications noted. There was no facial palsy postoperatively. At the end of the surgery, we placed the sutures between the EAC flap and tragal skin. She was regularly followed in the outpatient department, and she had uneventful and good cosmetic postoperative results.

During the surgery, the surgeon carried out the procedures of the transcanal endoscopic approach without difficulty. The surgeon did not have to switch to conventional 2D endoscopic surgery to complete the surgeries. Additionally, there was no mortality, perioperative complications nor notable postoperative complications. The patients stayed in the neurosurgery intensive care unit 1 day postoperatively for observation and then were transferred to their original ward. The patients were discharged 5 days later.

There were three assisting doctors, one neurosurgeon, 1 ENT doctor, 2 anesthesiologists, 2 interns and 3 nurses participating in the operation. Questionnaires were completed by these 12 participants after the surgery. Three main questions regarding 3D image system for endoscopic ear surgery were asked in response to our previous study². All participants agreed that the 3D imaging system enabled them to perceive stereoscopic vision and provided fair depth perception without experiencing visual fatigue or discomfort when observing the 3D images.

This system provided 3D vision. The depth perception offered superior management of the complicated surgical anatomy. In addition, there was no time delay and no visual fatigue noted by the surgeon. The endoscope has the benefit of visualizing the complex anatomy of the ear and lateral skull base around the tumor. The sharp image and stereoscopic vision offered by the computer-assisted system also provided visual depth perception, which is vital to the operation.

Figure 1: Endoscopic image of the transcanal transpromontorial surgery for the left side vestibular schwannoma. (A) Conventional 2-dimensional (2D) endoscopic image. (B) A 3D image obtained from the computer-based processing system. The vestibular schwannoma (black arrow) of case one patient was exposed after drilling the basal, middle, and apical turn of the cochlea. Please click here to view a larger version of this figure.

Table 1: Patients demographic characteristics, preoperative and postoperative results.

Endoscopic ear surgery has become more popular. However, the main limitation is the lack of stereoscopic vision when compared to a microscopic surgery. The use of a 3D endoscope may be difficult in transcanal ear surgery because of the limited space in the external ear canal. In this study, we applied a 3D computer-based processing system with a conventional 2D endoscope in the transcanal transpromontorial approach for vestibular schwannoma resection and evaluated its clinical feasibility for lateral skull base surgery. Both patients underwent resection of their vestibular schwannomas with the transcanal transpromontorial approach. There were no intraoperative or postoperative complications noted. All the surgical procedures were carried out without difficulty.

This computer-based processing system produces 3D video images from conventional 2D endoscopy images. The advantage of this 3D imaging system is that it can be combined with any conventional 2D endoscope to perform various transcanal ear procedures. In addition, this 3D imaging system provided stereoscopic vision and better depth perception, especially for the complicated and delicate surgical anatomy. In addition, no time delay and no visual fatigue were noted by the surgeon.

Transcanal endoscopic transpromontorial approaches for vestibular schwannoma resection have been published for patients with small vestibular schwannomas with nonserviceable preoperative hearing^1,8,11,12,13. This approach through the natural surgical corridor (EAC) is a minimally invasive procedure with low morbidity when compared with other traditional approaches for vestibular schwannoma removal. However, this approach destroys the cochlea, resulting in hearing loss and loss of the chance for cochlear implantation for hearing reconstruction.

As far as simple tympanoplasty, this 3D system may provide a few benefits, but the cost of the 3D imaging system is very high. However, the 3D imaging system is beneficial in performing complicated and meticulous procedures, such as lateral skull base surgery.

The advantage of endoscopic surgery includes seeing around the corner of the target, which can help preserve more tissue and prevent tissue injury, as 3D images offer stereoscopic images that make precise operations. Combining the microscope and endoscope is promising for the transcanal transpromontory resection of vestibular schwannomas. However, there are disadvantages of this hybrid method. First, the surgeon must be familiar with performing both the microscopic and endoscopic techniques. Second, the equipment is expensive. The hybrid technique for the transcanal transpromontory resection of vestibular schwannomas needs more practice and experience with the complicated anatomy, especially from different viewpoints. The anatomy related to the traditional craniotomy and transcanal transpromontorial approaches is different. The limitation of this approach is the tumor size must be less than 2 cm and be of Koos grade I or II¹¹.

In conclusion, our preliminary results indicate that the proposed computer-based 3D imaging system provides superior depth perception compared to the 2D endoscope. Transcanal endoscopic transpromontory approach is a feasible method for resection of small vestibular schwannoma.

The authors have nothing to disclose.

The present study was supported, in part, by Chang Gung Memorial Hospital under Grant Nos. CMRPG3J0701, CORPG3F0851 and by the Ministry of Science and Technology (Taiwan) under Grant No. MOST-108-2314-B-182A-109.