Otoacoustic emissions can be evoked by introducing tones into the ear canal at two frequencies (f1, f2) that have a defined relation to each other (f1/f2=1.2). This type of evoked OAE is called a distortion-product otoacoustic emission (DPOAE). DPOAEs are frequency specific and usually can be detected in people with hearing thresholds of 50 dB or better. DPOAE testing can provide insight into potential hearing damage before pure tone audiometry thresholds change; it can potentially be used to identify noise-susceptible individuals. Reduction of DPOAE amplitude has been observed with short (hour) and long-term (years) noise exposure. Recent NASA-sponsored research has shown that DPOAEs can be reliably measured in a noisy environment using passive hearing protection, and they are repeatable over time.
Otoacoustic hearing assessments were evaluated as a screening tool to assess vulnerability to barometric pressure related hearing loss. Otoacoustic emissions are an objective, noise-tolerant technique and are therefore well suited for use in operational environments.
The objective of this project was to validate the procedures and verify the repeatability of self-administered tests in an operational setting. These data are important for successfully completing the planned otoacoustic emissions measurements on the International Space Station.
Crewmembers assessed their hearing using a PC-based otoacoustic hearing system. After the laptop computer (Thinkpad 760-Medical Equipment Computer functional equivalent) was unstowed, a soft, plastic probe was placed in one ear. Both ears were then covered with a passive noise-attenuating headset. Hearing was tested using the protocols developed as part of the NASA-funded "Otoacoustic Hearing Assessment of Space Station Crews" project. The protocols required 1-2 minutes to run. Supporting measurements included an assessment of ambient noise and pressure levels, an otoscopic exam of the ears, and a tympanogram.
Pre/Post Study: For each individual, initial ground-based testing incorporated a full audiometric profile including history, otoscopy, pure tone thresholds, DPOAEs and middle ear impedance (tympanometry). Tympanometry was performed to assess tympanic membrane integrity and middle ear pressure equilibration function. Baseline audiometry was performed within a sound proof booth.
Daily Assessment: Prior to data collection, the ear canal was visualized using an otoscope. A standard evaluation of Eustachian tube function using the Valsalva maneuver was then performed. Ambient noise levels were measured with a sound level meter. DPOAE testing was performed with the SMO 105 DPOAE protocol (f2 frequencies 1.5, 2, 3, 4, 5 kHz, L1=L2=70 dB SPL). Data was stored on computer hard drive, with a floppy disc drive backup.
Data were collected on the NEEMO missions and are located in this archive. However, data were not analyzed due to problems caused by the negative effect of high barometric pressure on the OAE hardware.
Hearing protocols have been developed and tested in the following settings: (a) with recorded ISS noise present (b)on the KC-135 aircraft, and (c) in the NEEMO underwater habitat.
The general conclusion is that DPOAE testing can be a reliable way to assess hearing during long duration stays on the ISS. Threshold hearing testing using Békésy audiometry may also be feasible.