It is difficult to conceive that most of the noise generated by drilling the mastoid would not be conveyed to the contralateral cochlea, by direct transmission through the skull bone, where the attenuation factor is only 5-10 dB. Only a few studies have been conducted to ascertain this with minimal variables in terms of the duration of drilling, speed of the drill and the size and nature of the drills used – whether diamond or cutting. In this study the authors assessed 30 patients who underwent drilling of the mastoid for cholesteatoma and had normal hearing in the contralateral ear. The hearing was tested by DPOAE and TEOAE preoperatively, in the recovery and the first two consecutive days. Pure tone audiometry was used preoperatively and in the first and second consecutive postoperative days. The speeds of the drill were 60,000 and 35,000 rpm. There was no statistically significant difference observed in the pure tone audiometry assessed as average threshold for low and high frequencies in the pre and postoperative periods. However, there were statistically significant changes in the pre and postoperative TEOAEs over both low and high frequency ranges. The DPOAE changed only in the high frequencies. The follow-up period was only 72 hours, therefore it has not been established whether or not these changes were temporary or permanent. A few studies from the literature support these findings in contrast to one which does not, but the authors explain that in that particular study only a small diamond burr (1-4mm) was used. Diamond burrs produce less noise. There were also statistically significant correlations between the burr times and changes in DPOAEs at high frequencies and TEOAEs at both high and low frequencies for cutting burrs, and changes in high frequency DPOAE only for diamond burrs. The speed of drilling did not matter. Quite understandably, this study has used a number of variables but the sample size is rather small and the assessment is collective which excludes variables such as individual susceptibility. Studies with larger sample size and longer follow-ups are required.

The effect of mastoid drilling on hearing of the contralateral ear.
Goyal A, Singh PP, Vashishth A.
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Madhup K Chaurasia

United Lincolnshire Hospitals NHS Trust; University of Leicester, UK.

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