PP506 - Evaluating Electrical Hearing Thresholds in Unilateral and Bilateral Hearing Loss Patients

Abstract:
Auditory electrostimulation is a non-invasive method for auditory perception that utilizes surface electrodes on the skin to transmit an electric field. Electrical thresholds were evaluated in 13 patients with varying degrees, types, and configurations of bilateral or unilateral hearing loss. Acoustic and electrical thresholds were measured across octave frequencies from 0.25 - 20 kHz and compared against 29 normal hearing controls with established reference levels. Electrical thresholds followed patients’ best bone conduction thresholds, and there was no difference between individuals with unilateral hearing loss and normal hearing. This research has implications for alternative methods for sound transduction.

Summary:
Auditory electrostimulation involves placing surface electrodes proximal to the auditory system to generate an electric field. High-frequency audio carrier signals and modulation signals are combined and transmitted through external electrodes, resulting in high-fidelity auditory perception. Compared to conventional air and bone transduction, electrostimulation offers access to a broader frequency range. By bypassing the outer and middle ear, electrostimulation allows for auditory perception in the presence of conductive hearing loss or with hearing protection. The purpose of the study was to assess equivalency between air, bone, and electrical thresholds. We determined how electrical hearing thresholds differ in patients with various degrees, types, and configurations of hearing loss. Our objectives were: (1) to assess how bone conduction thresholds and electrical thresholds were related, and (2) to compare electrical and acoustic thresholds in patients with unilateral hearing loss and patients with bilateral sensorineural hearing loss.

This data was collected as part of a pilot study. There were 13 participants (6 female, 7 male). Of the patients, 7 had bilateral sensorineural hearing loss (4 with symmetrical hearing loss, 3 with asymmetrical hearing loss). The other 6 patients had unilateral hearing loss (3 with unilateral sensorineural hearing loss, 1 with unilateral conductive hearing loss, and 2 with single-sided deafness). Twenty-nine normal hearing participants (14 female, 15 male) served as controls, for whom equivalent reference levels have already been established.

Acoustic thresholds (dB HL and dB SPL) and electrical thresholds (mA) were obtained at the following frequencies: 0.25, 0.5, 1, 2, 4, 8, 10, 12.5, 16, and 20 kHz.  Acoustic thresholds were obtained with ER 3A inserts at the octave speech frequencies and high frequency thresholds were completed with DD450 headphones. An electrical headset was used to obtain electrical hearing thresholds. Testing was performed in a single channel placement of two electrodes on the mastoid processes with steady-state pure tones. A descending method, decreasing in increments of 0.5 mA and then increasing in increments of 0.2 mA, was used to obtain electrical thresholds at each frequency.

Preliminary results indicate that patients with unilateral hearing loss had electrical thresholds followed their best bone thresholds and, consequently, the air thresholds of their normal hearing ear. There was no significant difference in electrical thresholds between patients with unilateral hearing loss and patients with normal hearing in both ears. Participants with bilateral sensorineural hearing loss had electrical thresholds that were greater than participants with unilateral hearing loss. The configuration of bilateral patients’ electrical thresholds was similar to that of their hearing loss.

This early research promotes equivalency between acoustic and electrical thresholds. This could potentially promote the clinical use of an electrical audiometer. This research has clinical applications, as noninvasive electrical stimulation may be an effective means of auditory stimulation for specific populations, such as those with unilateral hearing losses. For military purposes, electrostimulation presents as an alternate mode of communication that will provide better security in field applications while allowing for use of hearing protection.