Possible Causes of Treatment Related Hearing Loss

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Mechanical damage to the axons or ischemia of the cochlea or auditory nerve may lead to immediate postoperative hearing loss during microsurgery. Stretch injury to the transitional Obersteiner-Redlich zone of the cochlear nerve has been suggested as the cause of hearing loss during tumor removal.32 We did not observe immediate hearing loss in any patient after radiosurgery. If hearing impairment is noted, usually it is gradual. Early hearing loss after radiosurgery (within 3 months) is rare and may result from neural edema or demyelination.

Delayed hearing loss after microsurgery could result from the vascular or fibrotic changes or the development of endolymphatic hydrops after damage to the endolymphatic duct during surgery. The precise mechanism of delayed hearing loss after radiosurgery is still unclear. Perhaps delayed obliteration of microvessels or even direct radiation injury to the axons, in patients treated with higher radiosurgical doses, could account for hearing loss. Linskey et al.26 hypothesized that length of the nerve irradiated predicts radiation injury to the nerve. The transitional Obersteiner-Redlich zone of the cochlear nerve may represent the most radiosensitive part of the cochlear nerve and may be responsible for hearing loss if a sufficiently high dose is given. The effect of radiation on normal microvessels supplying the cochlear nerve is still unclear. However, our experience suggests that doses of 13 or 14 Gy are well tolerated by auditory nerve and effectively prevent further tumor growth.23'24 The conservation of hearing associated with the exclusive use of 4-mm collimator most likely is due to the sharp fall-off of the radiation field at the tumor margin. The 4-mm beam spares a substantial number of nerve fibers of the cochlear division which likely is compressed and pushed anteriorly by the tumor. A larger beam diameter not only covers the tumor but also encompasses most of the cochlear and facial nerve fibers within the 50% isodose volume, as the average diameter of normal internal auditory canal is only 4 mm (range 2 to 8 mm).33

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