Effects of unilateral eye closure on middle ear muscle contractions.

Middle ear muscle contractions (MEMCs) are most commonly considered a response to high-level acoustic stimuli. However, MEMCs have also been observed in the absence of sound, either as a response to somatosensory stimulation or in concert with other motor activity. The relationship between MEMCs and non-acoustic sources is unclear. This study examined associations between measures of voluntary unilateral eye closure and impedance-based measures indicative of middle ear muscle activity while controlling for demographic and clinical factors in a large group of participants (N=190) with present clinical acoustic reflexes and no evidence of auditory dysfunction. Participants were instructed to voluntarily close the eye ipsilateral to the ear canal containing a detection probe at three levels of effort. Orbicularis oculi muscle activity was measured using surface electromyography. Middle ear muscle activity was inferred from changes in total energy reflected in the ear canal using a filtered (0.2 to 8 kHz) click train. Results revealed that middle ear muscle activity was positively associated with eye muscle activity. MEMC occurrence rates for eye closure observed in this study were generally higher than previously published rates for high-level brief acoustic stimuli in the same participant pool suggesting that motor activity may be a more reliable elicitor of MEMCs than acoustic stimuli. These results suggest motor activity can serve as a confounding factor for auditory exposure studies as well as complicate the interpretation of any impulsive noise damage risk criteria that assume MEMCs serve as a consistent, uniform protective factor. The mechanism linking eye and middle ear muscle activity is not understood and is an avenue for future research.

Generalizability of clinically measured acoustic reflexes to brief sounds.

Middle ear muscle contractions (MEMC) can be elicited in response to high-level sounds, and have been used clinically as acoustic reflexes (ARs) during evaluations of auditory system integrity. The results of clinical AR evaluations do not necessarily generalize to different signal types or durations. The purpose of this study was to evaluate the likelihood of observing MEMC in response to brief sound stimuli (tones, recorded gunshots, noise) in adult participants (N = 190) exhibiting clinical ARs and excellent hearing sensitivity. Results revealed that the presence of clinical ARs was not a sufficient indication that listeners will also exhibit MEMC for brief sounds. Detection rates varied across stimulus types between approximately 20% and 80%. Probabilities of observing MEMC also differed by clinical AR magnitude and latency, and declined over the period of minutes during the course of the MEMC measurement series. These results provide no support for the inclusion of MEMC as a protective factor in damage-risk criteria for impulsive noises, and the limited predictability of whether a given individual will exhibit MEMC in response to a brief sound indicates a need to measure and control for MEMC in studies evaluating pharmaceutical interventions for hearing loss.

Acoustic reflexes are common but not pervasive: evidence using a diagnostic middle ear analyser.

OBJECTIVE: The objective of this study is to determine whether acoustic reflexes are pervasive (i.e. known with 95% confidence to be observed in at least 95% of people) by examining the frequency of occurrence using a friction-fit diagnostic middle ear analyser. DESIGN: Adult participants with very good hearing sensitivity underwent audiometric and middle ear testing. Acoustic reflexes were tested ipsilaterally and contralaterally in both ears across a range of elicitor frequencies. Reflex elicitors were 700 ms tones presented at maximum level of 100 dB HL. Two automated methods were used to detect the presence of an acoustic reflex. STUDY SAMPLE: A group of 285 adult volunteers with normal hearing. RESULTS: There were no conditions in which the proportion of participants exhibiting acoustic reflexes was high enough to be deemed pervasive. Ipsilateral reflexes were more likely to be observed than contralateral reflexes and reflexes were more common at 0.5 and 1 kHz elicitor frequencies as compared with 2 and 4 kHz elicitor frequencies. CONCLUSIONS: Acoustic reflexes are common among individuals with good hearing. However, acoustic reflexes are not pervasive and should not be included in damage risk criteria and health hazard assessments for impulsive noise.

Acoustic reflexes are common but not pervasive: evidence from the National Health and Nutrition Examination Survey, 1999-2012.

OBJECTIVE: To determine whether acoustic reflexes are pervasive (i.e. sufficiently prevalent to provide 95% confidence of at least 95% prevalence) and might be invoked in damage-risk criteria (DRC) and health hazard assessments (HHA) for impulsive noise. DESIGN: Cross-sectional analyses of a nationally-representative study. STUDY SAMPLE: National Health and Nutrition Examination Survey (NHANES) data collected between 1999 and 2012 were used. Over 60 thousand reflex traces obtained from 15,106 NHANES participants were used in the study, along with demographic, audiometric, health and exposure variables obtained in that study. RESULTS: Acoustic reflexes were not sufficiently prevalent to be deemed pervasive by any detection method or in any subgroup defined by age or audiometric characteristics. The odds of observing acoustic reflexes were greater for women, young adults, and people with better hearing sensitivity. Abnormally high tympanometric admittance and "Other" race/ethnicity (i.e. people who do not self-identify as exclusively Non-Hispanic White, Non-Hispanic Black, Mexican-American, or Hispanic) were associated with lower odds. CONCLUSIONS: Acoustic reflexes are not sufficiently prevalent to be included in DRC and HHA for impulsive noise.

Auditory risk estimates for youth target shooting.

OBJECTIVE: To characterize the impulse noise exposure and auditory risk for youth recreational firearm users engaged in outdoor target shooting events. The youth shooting positions are typically standing or sitting at a table, which places the firearm closer to the ground or reflective surface when compared to adult shooters. DESIGN: Acoustic characteristics were examined and the auditory risk estimates were evaluated using contemporary damage-risk criteria for unprotected adult listeners and the 120-dB peak limit suggested by the World Health Organization (1999) for children. STUDY SAMPLE: Impulses were generated by 26 firearm/ammunition configurations representing rifles, shotguns, and pistols used by youth. Measurements were obtained relative to a youth shooter's left ear. RESULTS: All firearms generated peak levels that exceeded the 120 dB peak limit suggested by the WHO for children. In general, shooting from the seated position over a tabletop increases the peak levels, LAeq8 and reduces the unprotected maximum permissible exposures (MPEs) for both rifles and pistols. Pistols pose the greatest auditory risk when fired over a tabletop. CONCLUSION: Youth should utilize smaller caliber weapons, preferably from the standing position, and always wear hearing protection whenever engaging in shooting activities to reduce the risk for auditory damage.

Distributions of pure-tone hearing threshold levels among adolescents and adults in the United States by gender, ethnicity, and age: Results from the US National Health and Nutrition Examination Survey.

Cumulative distributions of audiometric pure-tone thresholds for a non-occupationally noise-exposed population vary with demographic characteristics (e.g. gender, ethnicity, age), tested ear, and stimulus frequency. However, commonly-used audiometric databases either do not take these differences into account, or account for them using data not easily generalized to the US population. The objective of this study was to obtain distributions that are generalizable to the US population without significant history of exposure to occupational noise. Cumulative pure tone threshold distributions from the National Health and Nutrition Examination Survey (NHANES) III and the 1999-2004 data from the continuous NHANES were fitted with an asymmetric sigmoid function with reverse asymmetry, by gender, ethnicity, age, ear, and the stimulus frequency. Results indicated that conditional distributions based on these factors are warranted. Percentiles on the cumulative distribution functions can be transformed into standard normal variates (i.e. z-scores) to facilitate combination of results over time or across individuals with different demographic characteristics. However, combinations across frequency could obscure meaningful differences between the reference group and the hearing test results under analysis.