DEFINITION OF TEMPORARY AND PERMANENT THRESHOLD SHIFTS
Hearing loss because of noise has been recognized in humans for centuries (1). However, it was only in the 20th century that the phenomenon of noise-induced hearing loss (NIHL) was rigorously investigated in animals, allowing a more accurate determination and definition of the disease. Such studies have demonstrated that exposure to excessive sound produces hearing loss (threshold sensitivity loss), with the magnitude of the initial shift and the degree of recovery depending on characteristics of the exposure in the level, time, and frequency domains, and on characteristics of the individual, as noted later. Threshold shifts that recover to baseline levels in the hours, days, or weeks after exposure are termed temporary threshold shifts (TTS). More injurious exposures can produce threshold sensitivity losses containing both temporary and permanent components, in which the majority of the TTS resolves but a measurable permanent threshold shift (PTS) has evolved (e.g. [2,3]). Threshold shifts of up to ∼50 dB immediately after a single-noise exposure may recover completely, whereas more extensive immediate hearing losses are likely to result in permanent losses of hearing sensitivity (e.g. [3,4]). Continuous or repeated exposures to noise that only induce a TTS may evolve to a PTS if repeated (5), as occurs in occupational noise exposure. Therefore, PTS can be defined as noise-induced threshold shift that persists after a period of recovery subsequent to the exposure. In animal models, recovery has been reported for periods extending up to 3 weeks; therefore, it may be premature to define a threshold shift as temporary until at least 3 weeks postexposure, when a permanent threshold shift arises.
Although the smallest level of TTS or PTS that can be reliably measured in an individual has not been well defined given test-to-test variability in individuals, several standards have been set for what is considered a significant hearing loss or “standard threshold shift” (STS). The Occupational Safety and Health Administration states that an STS is a 10 decibel (dB) increase in hearing threshold averaged across 2000, 3000, and 4000 Hz in the same ear from an individual's baseline or recent annual audiogram (29 CFR 1910.95). An STS is a reportable work-related injury once it has been reconfirmed with a retest within 30 days of the initial test and results in a hearing threshold of at least 25 dB in the affected ear. Therefore, most occupational hearing loss or PTS is under reported since Occupational Safety and Health Administration only requires an STS to be reported.
The Department of Defense (DoD) policy for the military's Hearing Conservation Program and the American Speech-Language-Hearing Association (ASHA) similarly define STS by a 10 dB shift average using the same frequencies, “in either ear without age corrections” (DoD I 6055.12, 2010 [currently under revision]; ASHA, http://www.asha.org/public/hearing/Degree-of-Hearing-Loss/, 2015). In contrast, the National Institute of Occupational Safety and Health recommended definition of an STS is “an increase of 15 dB in hearing threshold level at 500, 1000, 2000, 3000, 4000, or 6000 Hz in either ear, as determined by two consecutive audiometric tests,” with the second test required to reduce false-positive findings (6). A significant negative STS (improved hearing) is further defined by the DoD as a decrease of 10 dB or greater change (improvement in hearing) for the average of 2, 3, and 4 kHz in either ear. An early warning shift STS (decrease in hearing) is defined as a 10 dB or greater change at 1, 2, 3, or 4 kHz in either ear. Therefore, a consistent measure between TTS and PTS involves a 10 dB shift from baseline hearing involving one or more frequencies in the same ear.
CHARACTERISTICS OF PTS
PTS is sensorineural and varies across frequencies, depending on characteristics of the exposure, the transmission characteristics of the external and middle ears, and the innate sensitivity of different regions of the cochlea to damage.
Noise damage is typically most extensive at frequencies above those of the exposure (7), a phenomenon well explained by nonlinearities in the cochlear mechanical response to sound (8). This is most apparent for TTS and for low levels of PTS. However, noises to which human ears are exposed often are broadband in frequency composition. These signals are shaped (some frequencies amplified, others reduced by filtering) by passage through the external and middle ears (9). Resonance in the ear canal produces amplification of acoustic frequencies whose wavelengths are approximately four times the length of the canal, which for humans results in enhancement of frequencies around 4 kHz. This contributes to an enhanced “notch” of PTS at 4 to 6 kHz for exposure to broad-band stimuli. Finally, as with many other forms of damage, the basal cochlea seems to be most vulnerable to noise. Although the reason for this is not entirely clear, it may be related to higher levels of antioxidants in apical hair cells as well as higher rates of metabolic activity in basal hair cells (10). This basal sensitivity results in a tendency for TTS and PTS to be more extensive at high frequencies.
CHARACTERISTICS OF TTS
TTS is a change in hearing threshold that recovers to preexposure levels (baseline) over time. The amount of time to recover to baseline may be relatively fast (minutes to hours) or slow (days to weeks). The severity of the initial insult, as well as the time course of the recovery, is dependent on a number of factors including: the type of insult or trauma, the intensity and duration of the insult (single versus repeated, short versus long exposures), and the stimulus type (impulse/impact sound or continuous noise including wide or narrow-band noise). Individual susceptibility is dependent on the use of hearing protective devices, the quiet time or rest between exposures, and the level of hearing loss before exposure. Individual susceptibility to TTS may also be influenced by age, sex, previous history of noise exposure, diabetes, genotype, and other personal or environmental factors such as smoking and diet. Although these factors are at play for PTS as well, unlike PTS, TTS is a change in hearing sensitivity that recovers to baseline or within test/retest criteria in minutes, hours, days, or weeks with the upper limit being 30 days postexposure. TTS and PTS outcomes will vary as a function of the insult and individual factors.
MECHANISMS OF TTS
Historically, TTS was largely thought to be a mechanical process that involved structures within the outer and middle ears including the ear drum, ossicular chain, and middle ear muscles through the acoustic reflex. Extremely intense noise exposure is also known to mechanically damage the cochlea, disrupting the connections between the tectorial membrane and outer hair cell stereocilia, damaging the stereocilia themselves, breaching the integrity of the reticular lamina, or even disrupting the basilar membrane.
However, recent work in several preclinical studies has demonstrated a significant involvement of several sensorineural inner ear structures including hair cells and their stereocilia, supporting cells within the organ of Corti, endothelial cells and fibrocytes within the stria vascularis and spiral ligament, and dendritic processes of the auditory nerve (11,12). Molecular and biochemical changes have been identified which include proinflammatory and proapoptotic processes (13). These changes have been shown to alter the normal function of several critical processes within the cochlea including the endolymphatic potential that drives hair cell depolarization (14), cellular membranes, and mitochondria responsible for hair cell and supporting cell activity, and neural innervation of the inner hair cell that conduct impulses to the auditory brainstem. In addition, changes in the activity or metabolism of neurons in the cochlear nucleus, superior-olivary complex, and inferior colliculus have been observed (15). In support of this noise-induced change in inner ear biology and pharmacology and its relevance in establishing the TTS, several preclinical studies have demonstrated a significant reduction in TTS when the animals were administered otoprotective compounds or drugs immediately before noise exposure (16–20).
MECHANISMS OF PTS
Although intense sounds such as blast can damage the conductive apparatus of the outer and middle ears, producing permanent hearing loss through tympanic membrane rupture or ossicular dislocation, PTS is generally considered to be a sensorineural phenomenon restricted to the cells of the cochlea. The most recognized cause of PTS is damage to and loss of cochlear hair cells. The mechanisms by which this damage can occur are not known with certainty. However, there is extensive evidence implicating the generation of reactive oxygen species within hair cells during and after overexposure (12). This leads to the activation of stress signaling pathways such as the JNK MAP kinase cascade (21), which can in turn lead to cell damage, apoptosis, and/or necrosis (22). The biochemical pathways leading to hair cell damage/death are undoubtedly complex, and also seem to include competing survival pathways that attempt to rescue hair cells and restore their function. It is the balance of these competing pathways that determine the fate of the cell. The outer hair cells, responsible for the exquisite sensitivity and frequency and selectivity of the cochlea, are the most sensitive to damage (2,3).
Noise also can target hair cell synapses and neurons directly, even when the hair cells themselves remain and recover normal function. The insult is observed acutely as a glutamate-like “excitotoxicity” that includes swelling and retraction of afferent terminals from beneath inner hair cells (23). Recent work in animal models shows that noise-induced loss of synapses and afferent terminals is rapid and permanent (24,25). Loss of spiral ganglion neurons is comparatively slow, and can be “primary,” that is, occurring without noise-induced hair cell loss (24,25) or “secondary” to the loss of their inner hair-cell targets (26,27). Such synaptic and neural loss can exacerbate the functional consequences of noise exposure by reducing the ability of the VIIIth nerve to encode auditory signals with fidelity, with or without loss of threshold sensitivity (28). Thus, lack of PTS does not imply that auditory function is normal.
It should be noted that our understanding of the mechanisms of NIHL remains incomplete. For example, many of the processes that have been proposed to mediate hearing loss would take considerable time to develop. However, impulse exposures, even those that do not result in PTS, produce hearing loss essentially instantaneously, without immediate loss of cells. Presumably, this represents a disruption of cochlear cells at the microstructural and protein levels. In another example, it has recently been suggested that the initial 10 to 15 dB of TTS may serve as a mechanism to extend the dynamic range of hearing, rather than representing a damage mechanism (29). Finally, noise exposure that accumulates over a lifetime of occupational exposure may well involve different processes than more acute damage (30). Further studies of NIHL mechanisms are clearly warranted.
CONSEQUENCES OF PTS
ASHA uses the following threshold-based definitions of hearing loss: none (normal hearing) (−10–15 dB), slight (16–24 dB), mild (25–40 dB), moderate (41–55 dB), moderately severe (56–70 dB), severe (71–90 dB), or profound (>91 dB). Thus, 10 dB of PTS would have different consequences depending upon the initial level of hearing, for example leaving one individual with normal hearing (by definition) while increasing hearing loss from mild to moderate in another. One reasonable strategy may be to calculate the resulting hearing handicap as per the American Academy of Otolaryngology (AAO) 1979 criteria, or using the ASHA criteria, both of which incorporate a low fence of 25-dB HL. Key differences include the frequencies included in the calculation (AAO-1979: 0.5, 1, 2, and 3 kHz; ASHA: 1, 2, 3, and 4 kHz) and the growth rate for impairment for PTA thresholds above the low fence value (AAO-1979: 1.5% per dB; ASHA: 2% per dB) (31,32). However, if PTA thresholds were below 25 dB HL after the exposure, the PTS would be “missed” using this scheme, as there is deemed to be no handicap below the low fence value. Moreover, functional losses that have no threshold change correlate would not be recognized using these strategies.
The consequences of threshold sensitivity loss have been well documented in animal studies of auditory physiology and psychophysical studies of human auditory function. Loss of 40 dB of hearing sensitivity is associated with a loss of outer hair cells, which as noted above are responsible for the lower ranges of hearing sensitivity, and for the sharply tuned responses of the cochlea to individual frequencies. The loss of these cells leads to a degraded ability to discriminate sounds, especially in noisy environments. More severe hearing loss is associated with the loss of inner hair cells, which transmit sensory information from the cochlea to the central auditory system. In addition, a rearrangement or loss of the adjacent supporting cells including Hensen's cells, Dieter's cells, and inner and outer pillar cells may contribute to further impairment or loss of passive amplification. Loss of all inner hair cells from a cochlear region eliminates auditory response and loss from the entire cochlea results in total deafness.
Of course, the consequences of PTS are dependent upon the degree and frequency range of the loss and total loss of hearing from noise exposure is rare. However, with PTS leading to moderate and especially severe hearing loss, many facets of life become extremely challenging (33). Communication is significantly impacted. This can lead to difficulty in performing military duties or in obtaining/retaining civilian employment. Social interactions are also heavily impacted, with the result that individuals with hearing loss can become withdrawn and isolated. This can in turn lead to depression and possibly cognitive decline (33). In the case of blast injury, hearing loss can exacerbate the effects of traumatic brain injury, even when traumatic brain injury is mild (34). Another consequence of noise exposure is an increase in sensitivity to other forms of hearing loss, including ototoxicity (35) and aging (36).
There is also a strong, positive correlation between the presence of noise-induced permanent hearing loss and tinnitus (37). Although tinnitus can be a benign condition, a large fraction of individuals with tinnitus experience distress that can be extreme (e.g. ). A lesser correlation is observed for hyperacusis (e.g. ), another negative sequela of PTS.
The authors acknowledge the DOD Hearing Center of Excellence for the support of Pharmaceutical Interventions for Hearing Loss (PIHL) working group and publication of this article.
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