When Is Temporary Threshold Shift Injurious to Marine Mammals?

Evidence for synaptopathy, the acute loss of afferent auditory nerve terminals, and degeneration of spiral ganglion cells associated with temporary threshold shift (TTS) in traditional laboratory animal models (e.g., mice, guinea pigs) has brought into question whether TTS should be considered a non-injurious form of noise impact in marine mammals. Laboratory animal studies also demonstrate that both neuropathic and non-neuropathic forms of TTS exist, with synaptopathy and neural degeneration beginning over a narrow range of noise exposures differing by ~6–9 dB, all of which result in significant TTS. Most TTS studies in marine mammals characterize TTS within minutes of noise exposure cessation, and TTS generally does not achieve the levels measured in neuropathic laboratory animals, which have had initial TTS measurements made 6–24 h post-exposure. Given the recovery of the ear following the cessation of noise exposure, it seems unlikely that the magnitude of nearly all shifts studied in marine mammals to date would be sufficient to induce neuropathy. Although no empirical evidence in marine mammals exists to support this proposition, the regulatory application of impact thresholds based on the onset of TTS (6 dB) is certain to capture the onset of recoverable fatigue without tissue destruction.

Temporary threshold shift after noise exposure in hypobaric hypoxia at high altitude: results of the ADEMED expedition 2011

Objectives To evaluate whether there is an increased risk for noise-induced hearing loss at high altitude rsp. in hypobaric hypoxia. Methods Thirteen volunteers got standard audiometry at 125, 250, 500, 750, 1000, 1500, 2000, 3000, 4000, 6000, and 8000 Hz before and after 10 min of white noise at 90 dB. The system was calibrated for the respective altitude. Measurements were performed at Kathmandu (1400 m) and at Gorak Shep (5300 m) (Solo Khumbu/Nepal) after 10 days of acclimatization while on trek. Temporary threshold shift (TTS) was analyzed by descriptive statistics and by factor analysis. Results TTS is significantly more pronounced at high altitudes. Acclimatization does not provide any protection of the inner ear, although it increases arterial oxygen saturation. Conclusion The thresholds beyond which noise protection is recommended (> 80 dB) or necessary (> 85 dB) are not sufficient at high altitudes. We suggest providing protective devices above an altitude of 1500 m (“ear threshold altitude”) when noise level is higher than 75 dB and using them definitively above 80 dB. This takes the individual reaction on hypobaric hypoxia at high altitude into account.

A clinical trial of Nicergoline to prevent temporary threshold shift

Background: In Thailand, military personnel attending the annual firing practice are at risk for noise-induced hearing loss (NIHL). Nowadays, hearing protection devices have been approved to prevent NIHL. Furthermore, N-acetyl-cysteine, Vitamin B12 or Magnesium were proved to be effective against temporary threshold shift (TTS). However, limited research regarding the effectiveness of nicergoline on preventing TTS has been reported. Methods: A randomized controlled trial was conducted. Two hundred twenty-four participants were evaluated for general physical status, hearing threshold levels and blood chemistry. After the inform consent form was explained and signed, the participants were divided into 2 groups. Nicergoline 30 mg twice daily intake was prescribed to the study group (n=119) for 3 weeks. The placebo was prescribed to the control group (n=105) for 3 weeks, as well. At the end of the second week, they had to attend the firing practice. All participants had to wear the silicone ear plugs. In addition, the audiometry was measured within 48 hours after the end of the firing practice. Moreover, aural symptoms (tinnitus and aural fullness) and the side effects of the medication were recorded. Results: TTS was detected in both groups 14 ears – 10 ears from the control group and 4 ears from the study group (p=0.075). The post-firing practice audiometry showed that the average hearing threshold levels of the study group significantly improved than that of the control group across all frequencies (p<0.05). Moreover, the audiometry from the control group worsened at 250 and 6K Hz (p<0.05). The duration of tinnitus and aural fullness in the study group occurred over a shorter period than that in the control group. Conclusion: The TTS was found in both groups without statistical significant differences. The effectiveness of nicergoline from the study demonstrated tinnitus reduction after the firing practice. Furthermore, the hearing threshold significantly improved in the study group.

Noise

Noise can affect hearing in the occupational setting but can have other effects where exposures are non-occupational. For clinical purposes, noise is measured in decibels weighted according to the sensitivity of the human ear (dB(A)). Regardless of source, the effects of overexposure to noise are similar. Initially there is a temporary threshold shift, where reversibility of hearing loss is possible with removal away from further noise. Noise-induced hearing loss occurs following prolonged or intense exposure, with poor prospects for improvement of hearing. The classical audiogram for noise-induced hearing loss shows a 4 kHz dip. Non-auditory effects of prolonged noise exposure include annoyance, sleep disturbance, hypertension, and cardiovascular disease, stress, and impaired cognitive performance. Prevention of noise-induced hearing loss is by reducing exposure to noise at source minimizing exposure time, using hearing protection, and participating in surveillance.

Sound levels in movie theaters: is there a potential for hearing loss?

Purpose: The goal of this study was to determine the frequency of attendance at movie theaters, the most popular genres of movies, the sound levels inside movie theaters, and if, based on frequency of attendance, a relationship could be established between temporary threshold shift and noise levels inside movie theaters. Method: A survey was first distributed to moviegoers at several different venues. Movies were selected based on the most popular genres. A noise logging dosimeter was utilized during the viewing of 16 movies in two different multiplex theaters to determine if sound levels exceed those deemed hazardous by the Occupational Safety and Health Administration. Results: Results indicated that most surveyants go to the movie theater about once a week. The noise levels obtained for 16 movies did not exceed those established by OSHA as hazardous in nature. Conclusion: Given that the maximum dose recorded by any movie viewed was 7.6% (out of 100%), it is highly unlikely that the average person would sustain a temporary threshold shift from movie viewing alone. Even if a person attended the movie theater on a daily basis, there is no evidence to suggest that movie viewing alone could cause a temporary shift in hearing.