Noise-Induced Hearing Loss

Noise-Induced Hearing Loss

High sound levels can produce both temporary and permanent hearing losses due to over stimulation and/or mechanical trauma.  A sensorineural hearing loss produced by the damaging effects of over stimulation by high sound levels, usually over a long period of time, is called a noise induced hearing loss. In contrast, the term acoustic trauma usually refers to the hearing loss produced by extremely intense and impulsive sounds like explosions or gunshots. They can mechanically traumatize the eardrum, middle ear, and/or cochlear structures in addition to producing damage by over stimulation, and often from a single insult.

Almost everybody has experienced temporary hearing difficulty (often with tinnitus) after being exposed to high sound levels of one kind or another, such as loud music, construction noise, lawn mowers, subways, etc. This short-term decrease in hearing sensitivity is sensorineural in nature and is called a temporary threshold shift (TTS). In general, a TTS can be produced by sound levels greater than 80 dB sound pressure level (SPL). As the intensity and/or duration of the offending sound increases, the size of the TTS gets bigger and the time it takes for recovery gets longer. A permanent threshold shift (PTS) exists when the TTS does not recover completely, that is, when hearing sensitivity does not return to normal. Because PTS could refer to just about any permanent hearing loss, we generally lengthen the term to noise induced permanent threshold shift (NIPTS) for clarity. The nature and severity of a NIPTS is determined by the intensity, spectrum, duration, and time course of the offending sounds; the overall duration of the exposures over the years; and the patient’s individual susceptibility to the effects of noise. In addition, the amount of hearing loss produced by noise exposure is exacerbated if vibration is also present, and by the use of potentially ototoxic drugs.

The kinds of anatomical and physiological abnormalities caused by noise exposure range from the most subtle disruptions of hair cell metabolic activities and losses of stereocilia rigidity (leading to “floppy cilia”) to the complete degeneration of the organ of Corti and the auditory nerve supply. Both outer and inner hair cells are damaged by noise, but outer hair cells are more susceptible. Some of the abnormalities include metabolic exhaustion of the hair cells, structural changes and degeneration of structures within the hair cells, morphological changes of the cilia (so that they become fused and otherwise distorted), ruptures of cell membranes, and complete degeneration and loss of hair cells, neural cells, and supporting cells. Mild metabolic disruptions and floppy cilia can be reversible, and are thought to be related to TTS. It should be noted in this context that oxidative stress associated with accumulations of free radicals has been identified as a factor in noise-induced hearing loss Greater amounts of interference and damage are associated with permanent hearing losses.

Unfortunately, noise exposures capable of producing temporary hearing loss can also cause permanent neural degeneration. Permanent degeneration of the auditory nerve cells even though the TTS was completely resolved and there was no loss of hair cells. Noise-induced impairments are usually associated with a notch-shaped high-frequency sensorineural loss that is worst at 4000 Hz although the notch often occurs at 3000 or 6000 Hz as well. The reason for the notch in this region is not definitively established. One explanation is that this region is most susceptible to damage due to the biology and mechanics of the cochlea. The cochlea with a boost in the 2000 to 4000 Hz region because of the resonance characteristics of the outer and middle ear. Noiseinduced losses tend to be bilateral and more or less symmetrical; however, there are many exceptions, especially when one ear has been subjected to more noise than the other. Not all “noise-induced” audiograms conform to the idealized picture in. Analyses of the progression of noise-induced hearing losses across many studies have revealed that the general audiometric pattern of noise-induced hearing loss evolves as noise exposure continues over the course of many years. The hearing loss typically begins as a notch at 4000 Hz. As noise exposure continues, the notch widens to include a wider range of frequencies, but continues to progress most noticeably at 4000 Hz. After perhaps 10 to 15 years of exposure, the progression of the loss at 4000 Hz often slows down, and progression now becomes more apparent at other frequencies, such as 2000 Hz.

WHY A NOTCH AT 4,000 HZ?

In humans, the frequency of maximum cochlear damage is one-half to one octave above the frequency of maximum stimulation. This phenomenon has to do with the angle of curvature of the human cochlea as well as less blood perfusion in the basal end of the cochlea compared to the apex. The human external ear (pinna and ear canal) influences the physical properties of sound outside the head (i.e., in the diffuse field) by resonating at frequencies between 2,000 and 4,000 Hz, depending on the volume and the length of the ear canal; for larger adult ears the maximum ear canal resonance, as measured with a probe microphone, is 2,600 to 3,000 Hz. In children, with shorter ear

canals with a smaller diameter, this ear canal resonance is higher in frequency. This resonance serves to amplify sound by 15 to 25 dB relative to the diffuse field (for instance, as measured at the shoulder) at the resonant frequency. Acousticians and engineers have referred to this resonance as the transfer function of the open ear (TFOE) or the external ear transfer function and is known to audiologists as the real ear–unaided response (REUR). When fitting hearing aids, placement of an earmold results in disruption of this normal ear canal resonance, resulting in insertion loss. The real ear–aided response (REAR) must provide amplification to compensate for the insertion loss, just to get back to the sound level that would arrive at the eardrum without the earmold or hearing aid in place. For broadband sound, the result of the TFOE (REUG) is an overall level measured at the eardrum roughly 7 dB higher than measured at the shoulder. Given that most environmental sound is relatively broadband, the frequency range of maximum stimulation is roughly one-half to one octave below 4,000 Hz. This is another reason why the 4,000-Hz frequency region is the most susceptible to damage.