|Figure 1: The outer ear consists of the auricle (unlabelled), the external auditory canal, and the lateral surface of the tympanic membrane (TM). The middle ear includes the medial surface of the eardrum, the ossicular chain, the eustachian tube, and the tympanic segment of the facial nerve. The inner ear includes the auditory-vestibular nerve, the cochlea and the vestibular system (semicircular canals). The auditory nerve, also called the cochlear nerve, transmits sound to the brain.|
Radiation to the area of the ear is often associated with a chronic, progressive hearing deterioration (Wang et al, 2004). Sometimes the radiation has been purposely delivered to treat a tumor of the inner ear area (such as an acoustic neuroma), but more commonly the radiation has been delivered to treat a cancer in the same region. For head and neck cancers, the temporal bone (which contains the inner ear), is often included in the radiation field.
The hearing loss is usually delayed -- with no hearing loss in the first month post surgery but as much as 50% hearing loss by one year (Yilmaz et al, 2008). Theunissen et al (2014) suggested that the deterioration was less severe, averaging about 5 dB 4.5 years post-treatment. It seems likely that the deterioration depends on the dose and other drugs given simultaneously. OAE testing in the first month can predict which patients are more likely to deteriorate. This is logical as OAE's are from a vulnerable set of cells in the inner ear, the outer hair cells.
Little is published regarding radiation effect on vestibular function. We have noticed ourselves that patients following gamma-knife radiosurgery often develop hyperventilation induced nystagmus (presumably due to a more fragile vestibular nerve). Strangely, Stavas et al (2014) recently reported that "there were no statistically significant associations or identifiable trends in vestibular function tests or DHI scores". It is difficult to see why radiation should injure hearing (see above) but not vestibular function, as they use similar sensors. We think that the Stavas et al study is misleading as it was underpowered (only 10 patients), and may also have been testing for the wrong vestibular paramater (i.e. no hyperventilation testing).
Chemotherapy may be combined with radiation, which can also independently cause reduced hearing (generally sensorineural hearing loss) due to ototoxicity (Meyer and Young, 2009). Cisplatin is the main ototoxic drug that is combined with radiation.