Vestibular schwannomas (a.k.a. acoustic neuromas) are slow-growing and benign tumours arising from the balance (vestibular) nerve. Most patients present with progressive symptoms of hearing loss, tinnitus, aural fullness, and imbalance. Over 90% of vestibular schwannomas are unilateral. Bilateral tumours almost exclusively occur in patients with Neurofibromatosis type 2 (NF2). Most patients with small and stable tumours can be observed. For larger or growing tumours, surgical resection or radiation are both effective in achieving tumour control. With any treatment strategy, patients are confronted with hearing loss and hearing rehabilitation in every stage of the disease is, therefore, an essential part of our treatment.
Hearing loss can be divided into two types: conductive hearing loss and sensorineural hearing loss. In a normal hearing ear, sound waves travel through the ear canal towards the eardrum. The eardrum and the middle ear bones (ossicles) transduce the sound energy to the hearing organ, the cochlea. In conductive hearing loss, sound waves are hindered to reach the cochlea by either obstruction in the ear canal or conditions affecting the middle ear or ossicles. Hearing loss caused by conditions in the cochlea or the vestibulocochlear nerve results in sensorineural hearing loss.
With pure tone audiometry (hearing testing) we can differentiate between these two types of hearing loss. Auditory thresholds are assessed by presenting stimuli via headphones and bone oscillators. With the headphones the entire auditory pathway is assessed, while with the bone oscillators the sound energy is transmitted through the vibration of the skull directly to the cochlea, bypassing the middle ear and the ossicles. Combining these two thresholds gives us an impression of the degree and type of hearing loss, i.e. conductive, sensorineural or a combination of both. Besides the pure tone audiogram, we routinely also perform tests of speech discrimination or speech understanding.
In someone with a unilateral vestibular schwannoma, we typically see a sensorineural hearing loss in the affected ear and normal hearing thresholds in the other ear. An asymmetric sensorineural hearing loss on the audiogram is, therefore, one of the reasons to perform additional imaging or additional hearing testing (auditory brainstem response or ABR) to identify or exclude a vestibular schwannoma. In patients with sensorineural hearing loss due to conditions in the cochlea, like excessive noise exposure, the patient’s speech discrimination ability is usually retained. In the typical patient with a vestibular schwannoma, the speech discrimination is poorer than what is expected based on the hearing loss measured with the pure tone audiogram. The presence of the tumour on the vestibulocochlear nerve leads to a distortion of speech.
Expected Natural History of Hearing
The best hearing outcomes occur in patients who are able to have their tumours simply followed with serial MRI scans. In these people, the hearing will remain the best the longest. Unfortunately, over time the hearing can deteriorate, even if the tumour does not change in appearance on subsequent MRI scans. In whom the hearing will deteriorate and in whom it will not is not possible to predict at this time.
Radiation therapy can sometimes stop a tumour from growing and preserve residual hearing. This is particularly possible if the radiation can be directed at the tumour while avoiding the cochlea. Similarly, it is sometimes possible to surgically remove the tumour and preserve the residual hearing. This is somewhat dependent on the size and the location of the tumour, but predictable preservation of hearing even in small, ideally situated tumours is not possible. In no instance is the hearing improved with treatment of the tumour.
Hearing Rehabilitation
Rehabilitation of the patient’s hearing loss is important at every stage. Patients with a mild to moderately severe hearing loss in one ear might benefit from a hearing aid. This hearing aid will amplify the sound enhancing their hearing ability. However, patients with severe-to-profound hearing loss may expect little benefit from a hearing aid on their affected side. For these patients, a contralateral routing of signals (CROS) hearing aid might be a better option.
The CROS device consists of two hearing aids. On the affected side, the hearing device consists primarily of a microphone that picks up the sound and transmits the signal wirelessly to the receiver of a hearing aid placed on the better hearing ear. With this setup, the CROS hearing aid can pick up sound from the impaired ear and transmit it to the better hearing ear. This enables patients to hear sounds from their impaired side with their better hearing ear. A CROS hearing aid can improve hearing ability especially in a quiet environment, but not all users experience these benefits, and some have difficulties integrating the sounds from both sides. As this process takes time and is different for every individual user, a thorough trial period with a CROS device is therefore advised before committing to purchase.
A different, but more expensive option for patients with unilateral severe-to-profound hearing loss (single-sided deafness) is a bone conduction device (BCD) or bone anchored hearing aid (BAHA). This system consists of a titanium screw that is surgically fixated in the bone behind the ear, and an external bone oscillator which can be attached to the screw. The microphone, integrated into the bone oscillator, receives the sound signal. The sound processor converts this signal to an oscillation which is transmitted via the screw to the skull. The vibrating skull will activate the cochleae on both sides as they are embedded in the bone of the skull. In a patient with a unilateral severe hearing impairment due to a vestibular schwannoma, the impaired side is unable to adequately process these signals, but the vibrations are also registered by the cochlea on the normal hearing side resulting in activation of this cochlea. As with a CROS hearing aid, the BCD enables patients with unilateral hearing loss to hear sounds from their impaired side with their better hearing ear. The main difference is that the BCD uses vibration of the bone to activate the cochlea on the better hearing side, whereas the CROS hearing aid uses air conduction.
The CROS hearing aids and BCDs are only viable options for patients with a properly functioning contralateral ear. In patients with bilateral severe-to-profound sensorineural hearing loss caused by either a unilateral vestibular schwannoma and contralateral impaired cochlear functioning, or bilateral vestibular schwannomas, cochlear implants (CIs) and auditory brainstem implants (ABIs) are indicated.
A cochlear implant consists of an external sound processor and a surgically implanted device with an electrode array that is placed in the cochlea on the impaired side. The sound received by the sound processor is converted to an electrical signal which is directed through the electrode array. In the cochlea, the electric current activates nearby nerves resulting in activation of the nerve fibers which propagate this signal up to the brain. Although with vestibular schwannomas the problem is located behind the cochlea, in the vestibulocochlear nerve, cochlear implants are able to activate enough neural tissue to produce a signal, which can still be propagated through the nerve and past the tumour. Cochlear implantation in an ear with a vestibular schwannoma is mainly indicated for patients with bilateral tumours due to NF2. Because the tumour properties and extent are different in each patient, the hearing result of cochlear implantation in patients with a vestibular schwannoma is more variable than in traditional cochlear implant patients with hearing loss not due to a vestibular schwannoma.
If, despite cochlear implantation, patients do not fare well, or if the tumor is growing and requires surgical resection, an ABI might be indicated. Just like a CI, an ABI consists of an external sound processor and an internal device with an electrode array. However, this electrode array is not placed within the cochlea but instead is placed on the cochlear nucleus in the brainstem. The ABI enables patients to perceive sounds, even after surgical removal of the tumour and the vestibulocochlear nerve. However, the hearing result is in general poorer and more variable compared to cochlear implant users. Most ABI users are able to detect sounds and obtain measurable assistance with lip-reading, but a minority are able to gain significant speech understanding. Vancouver has recently been approved for an ABI program.
Conclusion
Hearing rehabilitation is an integral part of vestibular schwannoma treatment. Given the variation in hearing impairment among patients and the progressive nature of the hearing loss, an individually tailored approach is required in every stage of the disease. We advise patients to talk with their audiologists and ENT surgeons about all different rehabilitation options to aid them in their current situation and in their future.
Article by Dr. Marc Lammers, Dr. Emily Young, Dr. Jane Lea, Dr. Brian Westerberg
BC Rotary Hearing and Balance Centre, Division of Otolaryngology, St. Paul’s Hospital, University of British Columbia.
Dr. Brian Westerberg, specializing in Otology and Neurotology is based at the B.C. Rotary Hearing and Balance Centre in Vancouver. He completed a Fellowship at Stanford University and a Masters in Health Care and Epidemiology at the University of British Columbia (UBC) and has served the Royal College of Physicians and Surgeons as Program Director, and member of the Examination Committee and Chair of the Specialty Committee in OHNS at UBC.
Dr. Westerberg's interest in global health is apparent in his involvement in Zimbabwe and Uganda Hearing Health Care Programs. He is a clinical professor in the Department of Surgery and Director of the Branch for International Surgical Care at UBC.