In 1995 Erik Borg and his colleagues published a comprehensive review of a number of studies dealing with noise exposure and its effect on both temporary threshold shift (TTS) and permanent threshold shift (PTS). In short, Borg found no relation between TTS, that typically resolves in 16-18 hours, and PTS other than to conclude that one needs to have some TTS before permanent hearing loss is noticed. In these studies, the measurement of puretone acuity was assessed.
This review of the literature echoed the results of many previous studies (e.g. Ward et al., 1976) and was a source of frustration for many in the hearing health prevention fields. Industrial hygienists, physicians, and audiologists could not use the presence or absence of TTS to predict whether an individual was more or less susceptible to future hearing loss. TTS appeared to be just a benign quirk of noise exposure that would soon disappear.
Over the last 10-15 years, however, other researchers have begun to use tools that go beyond the simplistic puretone audiogram. While cochlear deficit as a result of noise or music exposure can be assessed with some modicum of validity with puretones, more advanced and more objective testing such as otoacoustic emission testing and even wave I in ABR, have been used. These tests, combined with modern scanning techniques show that even after the measureable effects of puretone TTS have resolved, there is host of permanent neural pathologies. These include:
- rapid inner hair cell (afferent) synaptic loss
- rapid inner hair cell dendritic loss
- slow spiral ganglion cell loss, and
- persistent reductions in suprathreshold neural responses (i.e., reduced wave I ABR).
These data mostly derive from the laboratory of Dr. Sharon Kujawa, and Dr. Charlie Liberman at the Massachusetts Eye and Ear Infirmary, Harvard Medical School. (Kujawa and Liberman, 2009; Kujawa and Liberman, 2006; Gates, Schmid, Kujawa, Nam, and D’Agostino, 2000). The findings in Table 1 are adapted from Kujawa and Liberman (2006, 2009) and show that while hair cell pathology (i.e, puretones and optoacoustic emission results) does resolve as TTS resolves over time, neural pathology (e.g., wave I ABR) including spiral ganglion cell loss, and synaptic and dendritic loss, does not resolve.
| Resolved after TTS | Did not resolve after TTS | |
| Puretone thresolds | X | |
| Otoacoustic emissions | X | |
| ABR wave I | X |
[Based on the data from Kujawa and Liberman (2006, 2009)]
Of special interest is that the Kujawa and Liberman (2006) study provides data that early noise exposure may “render the inner ears significantly more vulnerable to aging” (p. 2115) and the Gates et al, (2000) study indicates that noise induced hearing loss may continue to increase after retiring from the occupational noise environments, especially if the noise induced hearing loss is significant at the time of work retirement.
TTS may leave the cochlear hair cells intact but resolvable TTS, depending on the exposure, can result in permanent neural damage in the auditory system.
Hearing loss prevention is the cornerstone to any intervention. This is true whether we are dealing with industrial noise, music, sporting events, or recreational noise sources such as hunting. TTS is no longer something that can be dismissed as a benign and interesting artifact of noise or music exposure. It is an important measureable piece of data that in some sense may be even just as important as the sound level (in dBA) or the duration of exposure. Measurement of noise or music levels is important, as is the dose, but there will be individual variations of the effect of the received dose. Ultimately the question arises whether a certain exposure to noise or music did, or did not, create a pathological result in an individual person. Individual assessment of TTS would provide information for each person and will provide invaluable long-term prevention information for that person.
It seems that two different types of researchers and two different sets of tools have been used. Before 2000, measures of pure tone acuity were used and researchers published in journals relating to noise and noise control such as the Journal of the Acoustical Society of America. After 2000, more in depth tools were used that go beyond the cochlea and assess the integrity of the VIIIth auditory nerve and other neural structures. These researchers published in the Journal of Neurology, Hearing Research, and the Journal of Neuroscience.
