The reduction in sensitivity level of ear on exposure to noise, described as shift in threshold of hearing can be temporary or permanent. This reduction shows high individual variation and occurs more on exposure to sounds in the range of 2-6KHz. Temporary threshold shift (TTS) and Permanent threshold shift (PTS) represent the most common hearing effect of acute and chronic high level acoustic stimulation (Quaranta et al, 1998).
The interesting aspects concern the definition of duration of hearing loss (for TTS) and whether there is any relationship between the two entities.
TTS is commonly defined as "rise in auditory threshold during short exposure to noise which normally disappears in 24 hours but may last as long as a week"
The fact is post exposure improvement of threshold following such exposure may continue for 30 days or longer and there may not be return to preexposure threshold level (I.e PTS may occur).
The exact issue is there is no consensus as to how short or long one waits before pronouncing TTS a PTS. The exact point at which a TTS becomes a PTS is very difficult to identify. Age, exposure duration, exposure to other ototoxicity factors, intensity of the impact noise are other factors determine PTS.
The second issue is whether outcome of TTS studies can be grafted wholesale into prevention of PTS.
For ethical reasons, studies on PTS focused on animals, while human studies concentrate on creating temporary hearing losses (TTS). Typically the latter assumes that PTS must always be preceded by TTS. This has been shown not be strictly true. TTS is simply not a predictor of PTS, and we do not currently have good predictor of TTS.
The underlying molecular mechanisms are different for TTS (glutamate accumulation and tectorial membrane tip disconnection) and PTS (cell death related to apoptosis or necrosis). So TTS may not tell us much about PTS.
Further Reading
- Marshall Chasin. What does TTS tell us about PTS? Available at hearing health matters.org/hear the music/2012/what-does-tts-tell-us-about-pts/
- Scand Audiol Suppl 1998;48:75-86
- Otol Neurotol. 2016 Sep;37(8):e263-70.
