The treatment for cases of vestibular schwannoma or Meniere's disease may require a translabyrinthine surgical approach. This surgical approach results in a complete loss of hearing in the surgical ear, leaving the patient with a unilateral hearing loss. Though assistive hearing technologies exist to route signals from the poorer hearing ear to the better hearing ear, affected patients continue to have limitations with localization and speech perception in noise. An alternative hearing device is a cochlear implant, which would provide the signal to the affected ear. This could potentially offer binaural cues, thus improving localization and speech perception in noise. Since the surgical procedures for a translabyrinthine approach parallel those for cochlear implantation, insertion of the cochlear implant could occur within the same surgery.
A vestibular schwannoma is a benign tumor on cranial nerve VIII that affects the vestibular and auditory systems. Hearing loss on the side of the vestibular schwannoma may result from degeneration of hair cells and spiral ganglia or growth of the schwannoma into the cochlear space. Treatment options include observation from routine imaging, radiation therapy, or surgical removal of the tumor. Despite treatment for the vestibular schwannoma, auditory sensitivity may be further reduced on the affected side as a result of the radiation therapy or compromises to the inner ear or cranial nerve VIII from surgical removal. Thus, in cases of unilateral vestibular schwannoma the patient is often left with a unilateral profound hearing loss.
Patients who are scheduled to undergo labyrinthectomy for intractable Meniere's disease are a second population with resulting unilateral profound hearing loss. These patients typically have non-functional hearing on the affected ear prior to the procedure. The main indication for the surgery is intractable vertigo and thus the loss of already non-functional hearing is typically well accepted.
Though hearing on the contralateral ear may be within normal limits, unilateral hearing loss is known to result in reduced speech perception in noise, variable abilities on localization tasks, increased report of hearing handicap, and reduced quality of life. Due to the severity of the hearing loss, these patient populations cannot utilize conventional amplification that would offer auditory input to the affected ear. The current hearing device options for this patient population include contralateral routing of signal (CROS) hearing aids and bone-conduction devices. With a CROS hearing aid, a microphone positioned near the affected ear picks up the signal and sends it to a hearing aid placed on the contralateral ear to present the signal to the unaffected side. Bone-conduction devices transmit the signal from the affected ear to the contralateral ear via transcutaneous vibrations. Though CROS hearing aids and bone-conduction devices provide the patient with auditory information from both sides, the ability to use binaural cues for localization and speech perception in noise is variable.
It is of interest as to the potential benefit of cochlear implantation in these populations considering the profound hearing loss resulting from surgical intervention. A cochlear implant is a two-part system, including the internal electrode array and external speech processor. The internal electrode array is surgically implanted into the affected cochlea. The external speech processor receives sounds and transmits this signal to the internal portion. The electrode array presents the signal via electrical pulses within the cochlear space, which is interpreted by the brain as sound.
Cochlear implantation has been reported as a viable treatment option in other cases of unilateral hearing loss, including sudden sensorineural hearing loss, and severe tinnitus. Further, cochlear implantation has been shown to offer superior speech perception in noise, localization abilities, and subjective report in cases of unilateral sensorineural hearing loss as compared to CROS hearing aids and bone-conduction devices.
There is limited evidence as to the success of cochlear implantation in patients with unilateral profound hearing loss resulting from vestibular schwannoma. One study reported variable speech perception outcomes in five cochlear implant recipients with a history of vestibular schwannoma due to neurofibromatosis type 2 (NF2) or sporadic growth. Limitations of this study are the subjects underwent a range of treatments prior to cochlear implantation and the cases reviewed had profound hearing loss in both ears. A second study reported on a case study of unilateral vestibular schwannoma removal and simultaneous cochlear implantation. This subject reportedly experienced an improvement in speech perception abilities and quality of life postoperatively. Determining the preferred treatment option for patients suffering from unilateral vestibular schwannoma is still needed.
Further, there is limited evidence of the preferred treatment option for patients suffering from unilateral profound hearing loss after undergoing a labyrinthectomy for intractable Meniere's disease. One study reported on a patient who underwent bilateral labyrinthectomies for Meniere's disease. They reported an improvement in speech perception abilities and subjective benefit; however, there was a delay between the two surgeries. Allowing for a waiting period between the two surgeries is not ideal as cochlear ossification may occur, limiting the ability to successfully insert the electrode array. Another study also reported successful outcomes from cochlear implantation in subjects with bilateral Meniere's disease. However, there is no published report investigating whether cochlear implantation improves speech perception and/or localization abilities in unilateral cases of Meniere's disease.
The goal of this project is to determine whether subjects who have undergone labyrinthectomy or a translabyrinthine surgical approach as the treatment for vestibular schwannoma or Meniere's disease benefit from cochlear implantation on speech perception and localization tasks. If the auditory nerve is able to transmit this signal effectively, then these two populations may be able to utilize the combination of electric (in the affected ear) and acoustic (in the non-affected ear) information for improved speech perception in noise and localization as reportedly experienced in other unilateral sensorineural hearing loss populations.
