Hearing loss, the most common symptom of vestibular schwannoma
Hearing loss is the most common symptom experienced by individuals with vestibular schwannoma, with more than 90% of patients reporting this issue. This symptom typically presents gradually, but in some cases, sudden sensorineural hearing loss can occur, causing concern for patients. Additionally, hearing loss in vestibular schwannoma can fluctuate over time, eventually leading to total deafness in the affected ear.
With the advancements in medical imaging technology, such as MRI scans, tumours are being detected earlier when they are smaller. This early detection can help preserve hearing function in some cases and influence management.
Word recognition
Word recognition is the ability to identify spoken words correctly. Patients with vestibular schwannomas often experience hearing loss and difficulty with word recognition. This can lead to challenges in communication and understanding speech. For example, a patient with a vestibular schwannoma may have trouble following conversations in noisy environments or understanding spoken instructions from healthcare providers. They may also experience difficulty understanding speech when using a phone.
Mechanism of hearing loss in vestibular schwannoma
Hearing loss in vestibular schwannoma can occur through various mechanisms. One way is through pressure on the cochlear nerve, eventually leading to its destruction. Sometimes, the tumour may even infiltrate the nerve, especially in conditions like neurofibromatosis type two (NF2). Vascular factors can also play a role, with the tumour affecting the arteries supplying the nerve and cochlea. Additionally, the vestibular schwannoma may infiltrate the cochlea itself.
Another mechanism involves increased pressure and reduced flow of cerebrospinal fluid in the internal auditory canal, impacting the function of the auditory system. Furthermore, the tumour may secrete toxic proteins that can damage the inner ear, contributing to hearing loss.
Despite the absence of radiological evidence of tumour growth on MRI, patients may experience progressive hearing loss due to some of these underlying mechanisms.
Hearing assessment in vestibular schwannoma
Hearing in vestibular schwannoma is typically tested through various methods. Pure tone audiometry involves using headphones to present tones at different frequencies and intensities to determine the individual’s hearing threshold. This test measures air and bone conduction to evaluate the degree and type of hearing loss.
Word recognition testing assesses the individual’s ability to understand spoken words, which vestibular schwannoma can affect disproportionality. This testing can be done in background noise to simulate real-world listening environments and provide a more accurate assessment of hearing abilities. The result is expressed in percentage at a specific sound intensity, and mention is made if there is a “roll-over”.
A “roll-over” in word recognition testing occurs when a patient’s ability to identify words peaks at a specific volume correctly and then declines with further volume increases. This can signify retrocochlear hearing loss, often associated with vestibular schwannoma. Identifying and addressing poor word recognition and roll-over in testing is crucial as early as possible to prevent further hearing loss and complications.
Additionally, auditory brainstem response (ABR) measurement is commonly used to evaluate the function of the auditory nerve and brainstem in response to sound stimuli. This test involves placing electrodes on the scalp to measure the electrical activity along the auditory pathway.
Classification of hearing loss in vestibular schwannoma
There are two ways to classify hearing loss in vestibular schwannoma: the AAO-HNS classification system and the Gardner-Robertson system. The AAO-HNS system categorizes hearing loss based on speech discrimination scores and pure tone averages. Meanwhile, Gardner-Robertson describes five grades based on pure tone averages and word recognition in noisy environments.
Serviceable hearing refers to the ability to benefit from hearing aids or assistive listening devices. According to the AAO-HNS classification system, Grade A and Grade B and according to Gardner-Roberson, Grade I and II constitute serviceable hearing. Both classification systems help healthcare professionals determine the extent of hearing loss and guide treatment decisions for patients with vestibular schwannoma.
Hearing loss in the "wait and scan" observation group
When observing a vestibular schwannoma, hearing loss can still occur despite the absence of growth. This is a significant consideration in managing patients with these tumours, as hearing loss is often the most common presenting symptom. Research has shown that the likelihood of a patient maintaining serviceable hearing is around 66% at five years and 44% at ten years. Additionally, the risk of sudden sensorineural hearing loss, defined as a loss of at least 30dB in at least three consecutive frequencies over three days, is around 15%.
Comparing the risk of further hearing loss in the wait and scan group with the risk after microsurgery or radiation is crucial in determining each patient’s best course of action. Administering steroids has been shown to recover sudden sensorineural hearing loss potentially, highlighting the importance of timely intervention in cases of hearing loss associated with vestibular schwannoma.
The following table by Hunter and coworkers may help to estimate the likelihood of hearing loss with the wait and scan management option.
Hearing loss after microsurgery
During the immediate operative period, the risk of further hearing loss is at its highest. This risk is not solely dependent on the size of the tumour being operated on but instead on the potential injury to the nerve, its blood supply, and direct trauma to the inner ear and cochlea during surgery. The approach taken during the procedure also plays a significant role in the outcome, as the translabyrinthine approach results in total loss of residual hearing. This approach is typically chosen for patients with unserviceable hearing loss.
Alternatively, the middle fossa and retrosigmoid approaches may be selected if hearing preservation is a priority. However, even with these approaches, the likelihood of maintaining serviceable hearing after surgery is around 50-75% at 2 and 5 years and 25-50% at ten years. It is crucial for patients to be aware of the risks involved in surgery and to discuss with their healthcare provider the best approach for their case to help mitigate the potential for further hearing loss.
Hearing loss in the radiotherapy group
After radiation treatment for vestibular schwannoma, the immediate risk for hearing loss is the least compared to microsurgery. Risk factors for the development of hearing loss include a larger tumour, marginal tumour dose of more than 12 Gy, a cochlear dose of more than 4 Gy, a smaller size of the lateral cerebrospinal fluid fundal cap, rapid pre-treatment growth rate and pre-treatment hearing loss, especially with word recognition.
However, delayed hearing loss can occur during pseudo-progression, typically 4-12 months after treatment. Regarding hearing loss risk, there is no significant difference in the type of radiosurgery administered (gamma knife vs LINAC). Overall, the risk of hearing loss or an increase in hearing loss is lower with radiotherapy than with microsurgery only in the short term.
The likelihood of maintaining serviceable hearing after radiotherapy is approximately 75% at two years, 50-75% at five years, and 25% at ten years. While radiotherapy may be beneficial in the short term, the long-term outcomes regarding hearing preservation are comparable to microsurgery at five and ten years.
Hearing loss in neurofibromatosis type 2
In Neurofibromatosis type 2 (NF2) patients, vestibular schwannomas behave differently than solitary tumours. Tumours tend to be more aggressive. NF2 patients are more likely to experience hearing loss, and the risk of hearing loss increases after undergoing radiotherapy or microsurgery compared to those with solitary vestibular schwannomas. Reports have shown tumour cells infiltrating the cochlear nerve in NF2 cases. Additionally, bilateral hearing loss is common in NF2, presenting a significant therapeutic challenge. It may be an option to attempt hearing preservation surgery earlier.
References
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Grant KJ, Parthasarathy A, Vasilkov V, Caswell-Midwinter B, Freitas ME, de Gruttola V, Polley DB, Liberman MC, Maison SF. Predicting neural deficits in sensorineural hearing loss from word recognition scores. Sci Rep. 2022 Jun 23;12(1):8929. doi: 10.1038/s41598-022-13023-5. PMID: 35739134; PMCID: PMC9226113.
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