Endoscopic Transcanal Transpromontorial Removal of an Intracochlear Schwannoma and Traditional Cochlear Implantation

Intracochlear (intralabyrinthine) schwannomas are rare. These develop within the bony labyrinth. Typically, vestibular schwannomas develop on the vestibulocochlear nerve (CN VIII) which originates from the brainstem posterior to the facial nerve (CN VII). Vestibular schwannomas may reside in the CPA or extend into the internal auditory canal which can be classified by the Koos grading system. Intracochlear schwannomas develop on the intracochlear branches of the vestibulocochlear nerve. Intralabyrinthine schwannomas account for about 10% of vestibular schwannomas in centers that specialize in temporal bone imaging, but probably have a lower prevalence in non-specialized imaging centers. Intracochlear schwannomas are the most common type of intralabyrinthine schwannomas. These intracochlear schwannomas are usually found anteriorly between the basal and second turn of the cochlea, as seen in this case. Most intracochlear schwannomas develop from the scala tympani, likely due to the proximity of the nerve to the scala tympani near the habenula perforate. Intracochlear schwannomas can be surgically removed or radiated. The type of procedure involved depends on the size of the tumor and the patient’s hearing on the contralateral side. A cochlear implant can be placed at the time of tumor removal or be inserted through the tumor. Surgical removal can also improve dizziness and intractable vertigo. In this case, an endoscopic transcanal transpromontorial approach is used for tumor removal. An endoscope was used to provide the surgeon with an optimal visual field. Lidocaine with 1:10000 epinephrine is injected into the external auditory canal. Next, the tympanomeatal flap is elevated anteriorly off the temporal bone using a suction elevator to access the middle ear space. For this surgical approach, the tympanomeatal flap must be elevated. Once the middle ear is visualized, immediately medial is the cochlear promontory. The promontory is the bony prominence that covers the basal turn of the cochlea. Superiorly, the incus, stapedial tendon, and pyramidal eminence are visible. A skeeter drill is used to access the intracochlear space. Once the schwannoma is visualized, debulking of the mass can begin by using a curved pick and suction. After drilling of the promontory, the basal turn of the cochlea is visualized inferiorly, and the second turn of the cochlea is located slightly superior to the basal turn. The vestibule also becomes visible slightly posteriorly. Prior to the procedure, the physician discussed the possibility of using CROS hearing aids versus BAHA versus CI to achieve binaural hearing. The patient preferred CI after discussing their options. After complete removal of the tumor, a mastoidectomy was performed and a 612 cochlear implant was placed through the facial recess into the round window. A 612 CI was used because it had a stiff stylet that would allow the physician to push through any scar tissue and/or help push any remaining tumor from the cochlea. The placement of the cochlear implant was visualized in the second turn of the cochlea. Small pieces of cartilage graft and then fascia were placed over the exposed cochlear duct. Gel foam was then used to fill the middle ear space. The tympanomeatal flap is replaced and Bactroban ointment is placed throughout the external auditory canal. Final pathology revealed benign schwannoma. The spindle cells were small to intermediate in size and fairly uniform. There was a suggestion of Verocay bodies. Immunohistochemical stains demonstrate spindle cells positive for S-100 and were negative for AE1/3 and smooth muscle myosin. The Ki-67 proliferation index was less than 5%.

In patients with schwannomas that are growing at a fast rate, surgery may be indicated to preserve function of the vestibulocochlear and facial nerves. Surgical intervention for intracochlear schwannomas is indicated in patients with intractable vertigo, extension of the tumor into the cerebellopontine angle, concern for pathologic diagnosis, or attempting to preserve hearing prior to deterioration. Patients with unilateral deafness are expected to regain some hearing function with the use of an implant, CROS hearing aid, or a bone-anchored hearing aid.

If the tumor is not growing, and the patient is asymptomatic with aidable hearing, it may be preferred to observe the tumor for a period with repeated imaging exams. The risk of surgical intervention is unnecessary in these situations. Serious medical illness may also be considered a contraindication to operating due to increased risk of adverse outcomes, but this depends on the size and location of the tumor. If the patient retains some hearing function in the diseased ear, this approach would not be indicated due to the likelihood of total hearing loss post-op. Rather, surgery through the middle cranial fossa or a retrosigmoid approach may be more appropriate if the tumor is in the inner auditory canal.

The patient is placed supine with the head slightly rotated so that the diseased side is easily accessible. General anesthesia is administered for patient sedation. Intraoperative facial nerve monitoring is utilized.

The patient experienced sudden hearing loss in the right ear in January 2021. Audiometry performed in March 2021 showed severe right sided hearing loss with 0% speech recognition on the right side. Repeat audiometry one week later showed findings of no changes in the right ear. There was no improvement in hearing after 1 month. They also complained of worsening hearing loss in their left ear and an increased difficulty understanding speech with background noise. Cochlear implant evaluation was performed after 6 months of no improvement in hearing. The AzBio Sentence Test showed 0% perception in the right ear and the CNC Word Test showed 0% for word and 0% for phoneme recognition in the right ear. The patient had an MRI-IACS in March 2021 showing asymmetric enhancement involving the mid and basal turns of the right cochlea along with abnormal signal with FLAIR hyperintensities involving the right vestibule suggestive of labyrinthitis. A repeat MRI-IACS was performed in June 2021 showed no changes, which suggested an intracochlear schwannoma, rather than hemorrhage.

After lifting the tympanomeatal flap, the middle ear space is visualized. The incus, pyramidal eminence, cochlear promontory are easily visible. Once the cochlea is drilled, the basal and second turns of the cochlea and the vestibule become visible as well.

Advantages of an endoscopic transcanal transpromontorial approach include allowing the surgeon to have a direct lateral to medial view of the IAC and only minor superficial tissue dissection being needed. Also, little drilling is required to reach the mass. A disadvantage to this approach is that preservation of hearing in patients that retain some hearing function in the diseased ear is not feasible.

The primary complications of this procedure include total hearing loss, infection, injury to the chorda tympani nerve, and possible injury to the facial nerve. Regarding complication rates of endoscopic ear surgeries (EES), a study of 825 patients who underwent exclusive EES showed minor intraoperative complications in 4.1% of cases and early post-op complications in 1.3% of cases.

No conflicts to disclose.

Thank you to Dr. James Prueter for offering his time, expertise, and guidance on this project.

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