An Evaluation of the World Health Organization and American Medical Association Ratings of Hearing Impairment and Simulated Single-Sided Deafness

BACKGROUND:According to the World Health Organization (WHO), a pure-tone threshold average (PTA) ≤25 dB HL for the better ear represents "no impairment." This implies that patients with single-sided deafness (SSD) would have "no or very slight hearing problems." According to the American Medical Association (AMA), a patient with SSD would receive a binaural hearing impairment rating of 16.7%. The premise of the WHO and AMA methods is that PTA is related to the ability to perceive speech in everyday environments. PURPOSE:The overall goal of the study was to evaluate the WHO and AMA criteria for the rating of hearing impairment. The purpose of this study was to quantify the impact of simulated SSD on the ability to recognize speech in the presence of background noise in terms of binaural and spatial advantage measures. RESEARCH DESIGN:Study participants were tested using the standard protocol for the Hearing in Noise Test (HINT) in both binaural and monaural conditions using a simulated soundfield environment under headphones. The target sentences were presented at 0°. Binaural thresholds were obtained for the Noise Front (0°), Noise Left (270°), and Noise Right (90°) listening conditions. Monaural thresholds were collected for each ear for the Noise Front condition and for the unshadowed ear for the Noise Left and Noise Right conditions. Binaural advantage was determined by subtracting the binaural from the monaural thresholds. Spatial advantage was determined by subtracting the Noise Side from the Noise Front thresholds. STUDY SAMPLE:Twenty-five young native speakers of English with normal pure-tone thresholds (≤25 dB HL, 250-8000 Hz) participated in the study. DATA COLLECTION AND ANALYSIS:Pure-tone threshold data were collected using the modified Hughson-Westlake procedure. Speech recognition in noise data were collected using a Windows-based HINT software system. The binaural and spatial advantage measures were calculated from the HINT thresholds. Statistical analyses included descriptive statistics, correlation coefficients, and matched-pairs t-tests. RESULTS:The average binaural advantage for the Noise Front conditions was 1.21 dB (p < 0.01) or a maximum estimated intelligibility improvement of 12.01% when the speech and noise were presented at 0°. The average binaural advantage across the Noise Side conditions was 11.25 dB (p < 0.01) or a maximum estimated intelligibility improvement of 84.09% when the noise was spatially separated from the speech. The average spatial advantage for the binaural conditions was 6.72 dB (p < 0.01) or a maximum estimated intelligibility improvement of 60.03%. The average spatial advantage for the monaural conditions was -3.32 dB or a maximum estimated decrease in intelligibility of 32.27%. CONCLUSIONS:The results do not support the WHO or AMA hearing impairment ratings for SSD. The WHO and AMA criteria for the determination of hearing impairment should be updated to include speech recognition in noise testing with and without the spatial separation of the speech and noise stimuli. In this way actual, as opposed to inferred perceptions of speech in noisy environments, may be determined. This will provide a much-needed improvement in the ratings of hearing impairment.