A Study Of Drill Induced Hearing Loss In The Contralateral Ear Following Mastoid Surgery.

Abstract

Background: Chronic otitis media is the inflammation of the mucoperiosteal lining of the middle ear space and mastoid cavity. Mastoidectomy is considered to be the mainstay of treatment of COM. The usage of micro motor drill on the ear has effect on the contralateral ear due to the noise induced by the drill and the sound-conducting characteristic of the intact skull. The ipsilateral cochlea is exposed to a 100 dB sound during drilling, while the opposite cochlea to levels 5 to 10 dB lower. This in turn can lead to dysfunction of the outer hair cells, thus causing temporary or permanent hearing loss. Aims and Objectives: 1. To identify the drill induced hearing loss in the contralateral ear, by transient evoked otoacoustic emissions. following mastoidectomy. 2. To identify the relation between the type of burr tip used and the amount of hearing loss. 3. To identify that hearing loss is found to be more, if drilling is done for a longer duration of time. Methodology: It is a hospital – based prospective study, from November 2018 to April 2020. A total of 63 patients that underwent mastoidectomy were included in this study, with age ranging from 8 to 50 years. For each patient a thorough clinical history was taken with a detailed otologic examination was done, to make sure the contralateral ear was normal. Routine blood investigations along with a x-ray mastoid was done for all patients. Each patient was subjected to a pre-operative PTA and TEOAE. PTA was repeated on POD-1 and POD-7. TEOAE was done on POD-1,3 and 7 for each patient. If any changes in TEOAE readings are detected, a repeat OAE was done until normal values were obtained. A repeat TEOAE will be done on day 15, 30, 60 and 90 postoperatively, if required. TEOAE was recorded at 1000, 2000, 3000 and 4000 Hz. Intraoperatively, the type of surgery, the type of burrtip used as well as the individual drilling time for each type of drill bit was recorded, using a stopwatch. Results: Out of the 63 patients enrolled in this study, 30 patients (47.6%) developed transient and temporary SNHL on POD-1 and 7 patients (21.2%) had impaired hearing by POD-3. 4 patients (10.8 %) recovered by POD-3, 19 patients (51.4 %) had normal TEOAE readings by POD-7, 10 patients (27.0 %) were normal by POD-15, while the remaining 4 (10.8 %) recovered by POD-30. It was observed that higher frequencies of 3000 Hz and 4000 Hz were more commonly affected. All patients recovered by POD-30. No change was detected on pre and post-operative PTA. It was also detected that drilling with a cutting burr for an average of 45.4 minutes resulted in drill induced hearing loss, whereas when used for an average of 37.5 minutes, normal TEOAE readings were achieved. It was established that usage of a diamond burr for an average of 13.8 minutes resulted in hearing loss in the contralateral ear. However, no hearing impairment was noted when drilling was done for an average of 10.8 minutes. Conclusion: Thus, drilling with a cutting burr for more than 40.2 minutes, can lead to hearing loss. This has a sensitivity of 70% and a specificity of 69%. On the other hand, drilling with a diamond burr for over 12.5 minutes, can lead to hearing impairment, with a sensitivity of 68% and a specificity of 65%. The drill is not only a source of noise but is also a strong vibration generator, and a strong oscillation is transmitted into the cochlea. Thus surgeons should pay more attention to the vibrations and lessen the intensity of drill induced hearing loss by an appropriate selection of burrs and drills, thus minimizing the vibrations of the temporal bone.