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

Summary

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.

Abstract

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.

Introduction

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.

Protocol

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.

Results

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 pal...

Discussion

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. ...

Materials

Name	Company	Catalog Number	Comments
2D endoscope HOPKINS Straight Forward Telescope 0, with 3, 2.7,1.9 mm diameter	Karl Storz, Germany	7220AA, 7220BA, 7220FA, 7229AA 1232A
3D medical LCD monitor LMD-2451 MT	Sony, Japan	22220055-3 9524 N 22201020-1xx	Image 1 Hub HD
computer-based 3D imaging system	Shinko Optical, Japan	HD-3D-A
Piezosurgery instrument	Mectron, Carasco/Genova, Italy	MP3-a30