Noise-Induced Hearing Loss Follow-up

Updated: Aug 01, 2022
  • Author: Neeraj N Mathur, MBBS, MS, DNB, MAMS, FAMS; Chief Editor: Arlen D Meyers, MD, MBA  more...
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  • Oxidative stress plays a substantial role in the genesis of noise-induced cochlear injury that causes permanent hearing loss.

  • In adults with healthy audiograms, ear vulnerability to noise can be elicited and predicted by the use of objective DPOAE measurements. DPOAEs rely on the contractile properties of the outer hair cells. In response to tonal sound stimuli, these cells can generate retrograde wave sounds, the intensity of which can be captured and recorded by a very sensitive probe microphone in the external auditory canal. Any subclinical dysfunction of the middle ear or cochlea outer hair cells could slightly reduce DPOAEs and could influence the hearing performance in individuals exposed to noise and thus finding reduced DPOAEs in individuals with otherwise healthy pure-tone audiograms could be used in identifying them for their increased vulnerability to noise-induced hearing loss. [25]

  • In chinchillas, noise-induced cochlear oxidative stress (NICOS) can be reduced by the following means:

    • Acetyl-L-carnitine (ALCAR) is an endogenous mitochondrial membrane compound that helps maintain mitochondrial bioenergetics and biogenesis in the face of oxidative stress.

    • Carbamathione is a glutamate antagonist.

    • D-methionine (MET) can enhance cellular reduced glutathione (GSH) levels (ie, improve cochlear GSH deficiency state).

  • MET’s otoprotective action has a documented role, in a variety of species, against a variety of ototoxic insults including cisplatin-, carboplatin-, aminoglycoside- and noise-induced auditory threshold elevations and cochlear hair cell loss. Also MET can rescue individuals from permanent noise-induced hearing loss when MET is initiated 1 h after noise exposure. [26]

  • The studies reveal the potential for the use of NAC in a clinical population exposed to noise. [27] However, as seen experimentally in chinchillas, l-NAC treatments (N -acetyl-l-cysteine) designed to reduce or prevent NIHL has a limited effectiveness. Long-duration exposures at levels that lead to PTS in excess of 50 dB or more with severe loss of OHCs and IHCs in the basal half of the cochlea may lead to the chronic production of excessively high levels of ROS or other free radicals that not only overwhelm the cells endogenous defense mechanisms but also the ability of antioxidant drugs to combat the ROS assault. [28]

  • In an animal study on guinea pigs, alpha tocopherol has been found to significantly decrease noise-induced auditory brainstem response threshold shifts and attenuate noise-induced outer hair cell stereocilia loss. This also supports the notion that reactive oxygen species (ROS) is involved in metabolic damage of the organ of Corti.

  • In a study of guinea pigs, direct infusion of dexamethasone into the perilymphatic space was observed to have a protective effect against noise-induced trauma.

  • Deterrence is the only accepted management method for NIHL. OSHA requires hearing conservation programs if noise exposures in the workplace exceed 85 dBA. The Walsh-Haley noise standard requires that engineering or administrative controls be used to ensure that noise levels do not exceed the PEL. If noise levels cannot be brought down to the PEL, hearing protection must be provided. Its use must be enforced.

  • Noise levels must be monitored, either with area monitoring or personal dosimetry. Personal dosimetry is required if workers are exposed to variable noise levels.

  • Noise levels must be posted in work areas.

  • The following features are essential for an adequate hearing conservation program:

    • Hearing conservation programs must include a baseline audiometry performed within 6 months of onset of exposure for all employees. The audiogram must be obtained when the employee has not been exposed to hazardous noise for at least 14 hours.

    • Annual audiometric testing should be performed for workers whose TWAs equal or exceed 85 dBA.

    • Workers exposed to TWA of 85 dBA or higher are required to have annual training about the effects of noise on hearing and the purpose of audiometric testing and hearing protective devices (HPDs).

    • A large number of additional recording and reporting requirements are included in OSHA regulations.

  • If exposure to loud environmental noises cannot be avoided, hearing protection should be used. Unfortunately, enforcement has been sporadic. HPDs vary considerably in their effectiveness, comfort, and cost. The following information should be taken into consideration when considering HPDs:

  • Only devices that are designed for hearing protection and tested for efficacy should be used. Items such as cotton, tissue paper, and expended cartridge casings provide no meaningful noise attenuation.

  • Earplugs are available with attenuation levels from as low as 10 dB to as high as 32 dB. They can be purchased over the counter or custom made. Earplugs can be as effective as earmuffs. However, earplugs are effective only when properly inserted. When earplugs are improperly inserted, noise attenuation may be eliminated or greatly reduced. Earplugs are especially useful when noise exposure is continuously sustained.

  • Earmuffs can provide as much attenuation as can earplugs. An advantage of earmuffs is that they are easy to correctly place: whether they are properly inserted or installed is not an issue. Earmuffs are especially useful when exposure to noise is relatively intermittent. Runway workers may need to put on and take off earmuffs many dozens of times a shift. These workers would not likely put earplugs in and out that frequently; if they did, many of those installations would probably be imperfect.

  • Earmuffs that permit normal hearing in the absence of a loud noise are now available. The muffs are able to detect the presence of a loud noise and attenuate it before it reaches the human ear. These devices have achieved a much higher level of acceptability among sports shooters because they permit normal hearing except when a firearm is discharged. Sports shooters often worry that their efficiency is impaired if they cannot hear environmental sounds and that their safety is imperiled if they cannot understand what others around them are saying.

  • The most effective ear protection is the ear protection the person is willing to wear.



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  • TTS are reversible, but PTSs are not. No method of treatment is available, and no recovery is expected once a PTS has occurred.

  • However, hearing loss should not progress if exposure to the injurious noise is eliminated. Moreover, as the severity of the hearing loss increases, the rate of progression decreases, provided the injurious stimulus remains constant.


Patient Education

For excellent patient education resources, visit eMedicineHealth's Ear, Nose, and Throat Center. Also, see eMedicineHealth's patient education article Hearing Loss and Tinnitus.