Hearing loss in vestibular schwannoma
Hearing loss is one of the most common complaints in patients with vestibular schwannoma. Hearing loss in vestibular schwannoma is most often slowly progressive, with patients not even noticing it at first. In some cases, however, hearing loss may be sudden, irreversible and severe to profound.
Hearing loss in vestibular schwannoma occurs due to different mechanisms. It may be due to the destruction of the hearing cochlear (hearing) division of the cochleovestibular nerve. The tumour can primarily originate from the cochlear division, causing early hearing loss. Luckily this is rare. In most cases however, the vestibular schwannoma affects the vestibular portion of the cochleovestibular nerve, before it eventually affects the cochlear division. Hearing loss is then usually delayed. It should be appreciated that the tumour originates within a bony canal, the internal auditory canal (IAC). The non-involved part of the nerve has no place to displace to when the tumour starts getting bigger. Injury therefore is imminent.
In some other rare cases, the schwannoma may originate within the cochlea, the organ of hearing, with early onset hearing loss. This type of schwannoma and the others that originate in the labyrinth of the inner ear is revered to as an inner ear schwannomas.
In patients with good hearing it is often advised to observe the tumour with watchful waiting/observation and follow up magnetic resonance imaging(MRI). This decision is taken based on the assumption that some tumours, when discovered may not necessarily grow. Unfortunately, this is not a guarantee that the hearing may not deteriorate. (For more on this). In about 15% of patients the hearing loss in vestibular schwannoma may occur suddenly. In these patients it is believed that the tumour may irritate the blood vessels and cause spasm so that blood supply to the inner ear is compromised. The mechanism of infarction of the inner ear is the same as that of a heart attack.
Surgery is a treatment aimed in removing or reducing the size of the tumour. Surgery may injure the residual hearing nerve fibres but most likely hearing loss during surgery occurs when blood supply to the inner ear is damaged. The risk of hearing loss is less if the tumour to be removed is small. In these cases, a middle cranial fossa approach will most likely be selected.
Radiation therapy is aimed at stopping growth of the tumour, not to remove the tumour. Radiation may damage the residual functioning cochlear nerve fibres causing a radiation induced hearing loss. In many cases the tumour tends to swell after radiation and swelling may compress blood vessels in the bony canal with subsequent sudden hearing loss as described above.