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3.5.5.Clinical evaluation of noise damage
3.5.5.1. Hearing loss
NIHL refers mostly to deafness, the inability to hear certain sounds, but this hearing loss is most often associated with considerable difficulties in auditory discrimination of simultaneous sounds, such as speech understanding in a noisy environment, which affects notably social interactions. In addition NIHL is often associated with tinnitus which may also be very damaging to a person’s living, and sometimes also with hypersensitivity to loud sounds. Overall NIHL has a noxious socio-economic impact for both the affected person and the society in which he lives. The noise-induced auditory impairments are most often progressive and insidious, they begin at high frequencies with only slight disturbances hardly perceptible which usually disappear within some time after noise exposure and so they are almost always neglected. However, over time and repeated exposures these troubles progressively increase to become a patent nuisance, but then physiological damages to the ear are almost always irreversible and at present quasi incurable.
Subjective audiometry
NIHL is evident on the audiogram as mild or moderate bilateral sensory (cochlear) hearing loss, predominantly at high frequencies. The greatest hearing loss is commonly at 4 kHz, giving rise to the typical 4 kHz notch in the audiogram pattern (Alberti 1977). Although the notched audiogram is the most specific audiometric feature of NIHL, recent studies re-emphasized that caution should taken to this feature as it can be seen in ear pathologies of other causes and that a significant number of NIHL do not show a notched audiogram (Murai 1997, McBride and Williams 2001, Schmuzigert et al. 2006).
In line with previous studies several articles confirmed that very often in NIHL acoustic sensitivity at the very high frequencies (which are not measured in usual audiometry) shows deficiencies starting earlier than at other classical frequencies (Wang et al. 2000, Ahmed et al. 2001, Schmuzigert et al. 2006).
It was also confirmed that NIHL due to impulse sounds such as firearms shots produce on average more loss at high frequencies and may have longer-lasting effects (Schmuzigert et al. 2006, Tambs et al. 2006).
Although hearing losses appear stabilized over years it may continue to progress and affect lower frequencies (Gates et al. 2000, Brickner et al. 2005). The loss of sensitivity for quiet sounds is accompanied by a loss of frequency resolution which then affects speech recognition. Typically, people with NIHL complain of loss of perceived clarity of speech and greater difficulty than normal following speech in a background of noise.
Objective audiometry
Auditory-evoked potentials can be useful to monitor and/or ascertain NIHL. Past studies using late cortical potentials and brainstem potentials were forsaken in recent years (only the new technique of steady state evoked potentials was tried (Hsu et al. 2003)) while all attention was given to early sensory cochlear responses known as otoacoustic emissions (OAEs).
Transient-evoked otoacoustic emissions (TEOAEs) allow a quick check of cochlear sensitivity and responsiveness to sound however because it uses a transient sound it lacks frequency specificity. TEOAEs were found to provide a coarse but reasonably good indication of NIHL quite often permitting to detect alterations occurring earlier than classical puretone subjective audiometry (Avan et al. 2000, Attias et al. 2001, Wang H, et al. 2004, Lapsley Miller et al. 2004, Konopka et al. 2005a, Konopka et al. 2005b, Jedrzejczak et al. 2005, Nottet et al. 2006, Job et al. 2002; Job et al. 2007).
Distortion-product otoacoustic emissions (DPOAEs) allow a frequency specific testing at least at middle and high frequencies at the price of being more time-consuming. Their frequency selectivity often but not always provide a good correspondence with the pure tone audiogram and may detect earlier alterations (Morant Ventura et al. 2000, Zhang et al. 2000, Sliwinska-Kowalska and Kotylo 2001, Han et al. 2003, Namysłowski et al. 2004, Balatsouras 2004, Zhang et al. 2004, Seixas et al. 2004, Avan and Bonfils 2005, Konopka et al. 2006, Sisto et al. 2007, Shupak et al. 2007).
3.5.5.2. Vestibular effect
It has been repeatedly observed that alterations of the vestibule (the other mechanoreceptor of the inner ear besides the cochlea participating in balance and posture) could show signs of dysfunction in several cases of NIHL. These vestibular responses to sound are also known as the Tullio phenomenon and can be objectively studied using the vestibule-collic reflex. This was confirmed by recent studies which also showed that there was no clear relation between the degrees of vestibular and of cochlear dysfunctions (Teszler et al. 2000, Golz et al. 2001, Wang et al. 2001, van der Laan 2001).
3.5.5.3. Noise-induced tinnitus
The prevalence of tinnitus (or ringing in the ear) in noise-exposed populations seems to be much higher than in general populations. It has been estimated at prevalence 37% for less than 10 years of exposure and 50% for 11-30 years of exposure to noise. Noise-induced tinnitus may be temporary or permanent. This can be the only indication of hearing damage in the early stage, which may then be accompanied by hearing loss with continued exposure. Recent studies confirmed that when associated with NIHL it is almost invariably of high pitch, with a tonal or narrow frequency-band timbre. It has been reported that the duration of tinnitus is not related to the amount of acoustic trauma (Nottet et al 2006). Tinnitus appears very early after an impulse sound trauma, as well as other very loud sound exposures, then it is often temporary. In opposition, in several continuous long-term noise exposures scenarios it often appears after years, but remains permanent.
Trials to alleviate tinnitus through physiological means re-emphasized the efficiency of electric stimulation of the cochlea (although potentially hazardous in the long term) and explored with uncertain success several drug treatments. Psychological therapies remain the most common option (Axelsson and Prasher 2000, Konopka et al. 2001, Kowalska and Sulkowski 2001, Markou et al. 2001, Mrena et al. 2002, Attias et al. 2003, Emmerich et al. 2002, Nicolas-Puel et al. 2002, Rosenhall 2003, Mrena et al. 2004, Bauer 2006, Holgers 2006, Nottet et al. 2006, König et al. 2006, Nicolas-Puel et al. 2006, Mrena et al. 2007). When tinnitus becomes permanent, wearing a hearing aid may also provide help.
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The SCENIHR opinion states:
3.5.6.Vulnerability factors
It remains a puzzle to observe a very large interindividual variability in susceptibility to NIHL. Whether and how much individual vulnerability is dependent upon external conditions occurring at time of acoustic trauma or internal conditions linked to the genetics and physiological condition of the subject remains unknown. Significant progresses have been performed recently on these issues.
3.5.6.1. Environmental factors
Noise exposures in combination with several chemical and physical hazards, as well as ototoxic drugs may produce more hearing impairment than could be expected from noise-only exposure.
Chemicals
Chemicals are frequent contaminants in industry, some of them might be also common in general environment (heavy metals) or are used in everyday life (paints and lacquers). They are classified into three major groups: organic solvents, heavy metals, and asphyxiants.
Almost all studies about association of solvent fumes respiration with traumatizing sound exposure confirm their clear potentiation of NIHL, (Campo et al. 2001, Morata et al. 2002, Morata et al. 2003, Sliwinska-Kowalska 2003, Sliwinska-Kowalska et al. 2005, El-Shazly 2006).
Ototoxic effects of organic aromatic solvents, such as toluene, styrene, xylene, trichloroethylene, benzene, n-hexane and their mixtures are well recognized. These chemicals are frequent air contaminants in industry, such as in paint and lacquer factories, dockyards, printing industry, yacht manufacturing, furniture making, plastics and fibers processing, rubber tires production and many other industrial activities. Exposure may also occur in domestic settings through processed wood products, plastics furnishing, paints and lacquers. Animal studies have shown that several organic solvents, as has been exemplified by styrene and toluene, damage the cochlea (predominantly the supporting and outer hair cells) in rats and the exposure produces mid-frequency hearing loss (Sliwinska-Kowalska et al. 2007). Alcohol exposure, although alone it does not produce hearing loss, increases significantly the degree of hearing impairment caused by styrene or toluene (Campo et al. 1998, Campo et al. 2000). Synergistic effects occur in rats exposed to both noise and solvents (Campo et al. 2001, Sliwinska-Kowalska et al. 2007)). It means that hearing impairment is higher than the sum of hearing loss produced by solvent exposure and noise exposure alone. In combined exposures, the most important factor for inducing hearing impairment is potency of noise exposure (level, impulsiveness); concomitant exposure to organic solvents may induce impairment where the exposure to noise alone may have little effect.
The ototoxicity of organic solvents in occupationally exposed human individuals is more difficult to elucidate. This is because the concentration of chemicals is much lower than that used in animal studies, and the workers are usually exposed to a mixture of solvents at widely varying compositions and concentrations, disabling the assessment of the effect of a single substance (Sliwinska-Kowalska et al. 2001). However, investigations on humans confirm the findings in animals. It has been shown that organic solvents have detrimental effects not only on peripheral, but also on central part of the auditory pathway (Johnson et al. 2006, Fuente and McPherson B, 2007). Thus, pure-tone audiogram might be insufficient to monitor this effect, and central auditory tests must be implemented. An additive or synergistic effect occurs in case of the combined exposure to noise and solvents, significantly increasing the odds ratio of developing hearing loss (Sliwinska-Kowalska et al. 2003, Sliwinska-Kowalska et al. 2004). The risk for hearing loss increases with the growing number of solvents in a mixture.
Extensive use of heavy metals in industry adds to the environmental exposures to these substances. Heavy metals are not metabolised by the body and accumulate in the soft tissues or in the bones, causing toxic effects. They may enter the human body through food, water, air, or absorption through the skin when they come in contact with humans in residential and occupational settings as well as in the general environment. Commonly encountered toxic heavy metals include lead, mercury, cadmium and arsenic.
Lead
Most of the lead is used for batteries. The remainder is used for cable coverings, plumbing, ammunition, and fuel additives. It has been shown that the exposure to lead results in delayed wave I latency of ABR, implying cochlear dysfunction (Osman et al. 1999). But the findings on lead-induced hearing loss are inconsistent (Farahat et al. 1997, Forst et al. 1997, Baloh et al. 1979, Counter et al. 1997, Otto et al. 1985, Buchanan et al. 1999).
There are very few studies exploring the effects of combined lead and noise exposure. Elevated hearing thresholds have not been reported for lead and noise combined exposure (Wu et al. 2000).
Mercury
Mercury is found in dental amalgams, aquatic sediments, thermometers, vaccine preservatives, to quote a few examples. It is present in the atmosphere, and also in shark-, sword-, tuna-fish and other fish species. First, mercury intoxication was reported in 1953 among persons living in the vicinity of Minamata, Japan, where mercury-containing effluent flowing from a chemical manufacturing plant into the local bay contaminated shellfish. Hearing impairment and deafness were reported among other neurological symptoms of the “Minamata disease”.
Mercury affects hearing, with central conduction time delay (ABR I-V, III-V), but cochlear function may be unaffected (Counter et al. 1998a and b, Rice and Gilbert 1992; Murata et al. 1999).
Cadmium
Cadmium is used e.g. in nickel-cadmium batteries, PVC plastics, and paint pigments. Cadmium causes dose-dependent hearing loss in rats; wave I was delayed, implying cochlear dysfunction. Zinc-enriched diet reduced the ototoxic effect of cadmium, while noise exposure shows a synergistic effect at 4 and 6kHz (De Abreu and Suzuki 2002).
Arsenic
Arsenic is released into the environment by the smelting process of copper, zinc, and lead, as well as in the manufacture of chemicals and glass. Arsenic overexposure results in disorders in the Organ of Corti beginning at the apex with the greatest hearing losses in the lower frequencies (at 125, 250, and 500 Hz). Arsenic produces also balance disturbances.
Asphyxiants
Carbon monoxide (CO) and hydrogen cyanide (H2S) bind hemoglobin heme, thereby preventing oxygen transportation. The CO intoxication (e.g. in gas stove accidents) results in hearing impairment, dizziness and headache. Dizziness and headache were also noted in the prolonged intoxication with HS2 and SO2. These gases are common air pollutants; thus, H2S and SO2 exposures affect the majority of individuals. CO and H2S potentiate damaging effect of noise to hearing in animals. The effects of combined exposure to noise and asphyxiants in human are not fully recognized.
Vibration
Vibration-induced hearing loss may be developed in patients after temporal bone surgery or in subjects working with vibrating tools. In such cases, co-exposure to noise and vibration can increase hearing threshold shift compared to noise-only exposure.
Recent studies concerning association of body vibration with sound trauma brought contradictory and inconclusive results (Palmer et al. 2002a, Silva et al. 2005).
3.5.6.2. Ototoxic drugs
Several drugs used in contemporary medicine can damage hearing. Ototoxic effect depends on the dose, way of application and the type of medicine. Although these drugs can damage hearing at different levels of the auditory pathway, majority of them exert mainly cochlear ototoxic effect and they are competitive with noise in damaging hair cells.
The main groups of drugs that can cause hearing loss are:
- antibiotics (aminoglycosides, macrolides)
- antineoplastic drugs (cisplatinum, carboplatinum)
- loop diuretics (furosemide, ethacrinic acid)
- non-steroid anti-inflammatory drugs (acetyl salicylate acid)
- antimalaric drugs.
The most commonly used drugs that have been reported in the literature to result in hearing damage are aminoglycosides and anti-neoplasmatic drugs. Aminoglycosides are used parenterally in treating severe bacterial infections. After prolonged treatment with such aminoglycosides like gentymycin, kanamycin, amikacin, hearing loss at high frequencies, tinnitus and vestibular disorders were noted. The changes in hearing are irreversible. Prior exposure to noise (and vibration) increases the risk of hearing impairment due to aminoglycosides. The ototoxic effect depends on genetically determined susceptibility; it increases with high concentration of ferrum ions in the blood, and low protein diet. Anti-oxidant substances (like Vitamins A, C and E) have been shown to be protective.
It has been shown that cancer chemotherapy with cis-platinum produces hearing loss in up to 31% of patients. As in noise-induced hearing loss and aminoglycoside-induced hearing loss, these chemotherapeutics affect mainly hair cells of the basic turn in the cochlea and result in high-frequency (above 2 kHz) hearing impairment. Noise exposure at the time of chemotherapy significantly increases the risk of hearing damage.
3.5.6.3. Genetics
The advance of genetic research and associated tools triggered a series of explorations of the human genes possibly involved in NIHL, first evidences point to some candidate genes and seem to exclude other genes (Fortunato et al. 2004, Heinonen-Guzejev et al. 2005, Yang et al. 2005, Yang et al. 2006, Van Laer et al. 2006, Yang et al. 2006, Sliwińska-Kowalska et al. 2006, Konings et al. 2007, Van Eyken et al. 2007). The mechanisms of acoustic trauma involve both metabolic stress and micromechanical damage to the outer hair cells, predominantly to their stereocilia. Thus, good candidate genes are those encoding oxidative stress enzymes, mitochondrial proteins, and proteins involved in K+ recycling pathway. The importance of oxidative stress genes has been shown in knockout mice, including SOD1-/- (Ohlemiller et al. 1999), GPX1-/- (Ohlemiller et al. 2000), and PMCA2-/- mice (Kozel et al. 2002), all of which were more sensitive to noise than their wild-type littermates. However, these results have not been confirmed in humans (Carlsson et al. 2005). A more recent study suggests a possible role of potassium recycling pathway genes in the susceptibility to NIHL in human workers (Van Laer et al. 2006).
Some of the differences in susceptibility to NIHL have been attributed to various other genetically dependent factors, like eye colour (blue-eyed more susceptible), and pigmentation (African-Americans showed a somewhat better average in hearing threshold levels than Caucasians), gender (women more susceptible than men), age, etc (Henderson et al. 1993, Pyykkö et al. 2007). Also short stature has been recently recognized as a risk factor for developing sensorineural hearing impairment (Barrenäs et al. 2005).
3.5.6.4. Other factors
The gender of an individual has been often considered as a possible influencing factor with men appearing somewhat more affected than women. The difference however seems minimal if present (Müller 1989).
A predominance of left ear vulnerability as compared with right ear has been confirmed (Nageris et al. 2007) but the difference is small and shows mostly on average data.
Cardio-vascular alteration was often studied as a possible factor influencing NIHL but data are contradictory and the subject remains a matter of debate. Recent studies tend to confirm that alterations of blood pressure can be related with NIHL but it remains unknown whether it might be a cause or a simultaneous effect (Souto Souza et al. 2001, Toppila et al. 2001, Narlawar et al. 2006, Ni et al. 2007). A more detailed presentation is provided further in this report.
Evidence that smoking increases the risk of NIHL were provided long ago, all recent studies on this matter confirmed this assertion (Mizoue et al. 2003, Ferrite and Santana 2005, Uchida et al. 2005, Burr et al. 2005, Wild et al. 2005, García Callejo et al. 2006, Pouryaghoub et al. 2007).
Vitamin deficiencies were previously suspected to influence NIHL. Two recent studies brought evidence for the involvement of vitamin B12 (Quaranta et al. 2004, Gok et al. 2004).
The cochlear efferent innervation has long been known to be involved in NIHL. Recent studies further showed that assessment of cochlear efferent functioning did not clearly relate with NIHL (Veuillet et al. 2001, Shupak et al. 2007, Wagner et al. 2005).
The production of heat shock proteins constitute a physiological response to stress, first evidence for their implication in NIHL was recently provided (Yang et al. 2004, Yuan et al. 2005).
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The SCENIHR opinion states:
3.5.7.Therapies
Many therapies have been tried in the past with at best limited positive results. Recent progress in cell biology has provided a wealth of new molecules with possible therapeutic potential and several animal experiments have provided positive and promising results (as presented in another section of this report). Clinical trials over the last years have followed these progresses and brought preliminary results.
Magnesium treatment, repeatedly found beneficial in the past, has been confirmed as efficient (Attias et al. 2004). Hyperbaric oxygenation was also confirmed as having protective effects (Winiarski et al. 2005) although some adverse effects have also been reported depending on conditions of administration. Steroid administration a classical clinical treatment for NIHL was again recently reported as beneficial (Nakaya et al. 2002, Winiarski et al. 2005).
Many new drugs with anti-oxidant properties were found protective in animal experiments, the published clinical trials however do not yet provide fully convincing evidence (Kaygusuz et al. 2001, Gok et al. 2004, Kramer et al. 2006). New drugs with anti-apoptotic properties were also shown beneficial in animal experiments, NIHL clinical therapy is limited at present to one positive report (Suckfuell et al. 2007).
3.5.8. Conclusions
Exposure to excessive noise is one major cause of hearing disorders worldwide. Some data suggest an increased risk due to increase in use of audio leisure activities. There seems to be a trend for increased risk due to PMPs, as their qualities improved and they have become used by a largely increasing proportion of the population. The noise-induced hearing impairments have received much attention in the past decades mostly because of hazards of industrial noise exposures. Based upon many scientific studies the International Standard Organization has published recommendations for health safety. A most used ISO reference for risk assessment is that an exposure to a sound level of 85 dB (A) for 8 hours a day, 5 days per week will induce in 5% of the exposed population a hearing loss of about 4 dB after 3 years and 9 dB after 45 years. These losses being considered as quasi negligible this sound level-duration was considered as a safe limit above which preventive actions should be taken. The ISO recommendations also express that, as shown by many studies, the noise-induced hearing loss is the product of sound level by duration of exposure, and follows the equal energy principle stating that a decrease of sound level if associated with a proportional increase in duration (for example a halving in sound level associated with a doubling of exposure duration) induce similar risks. All data indicate a large inter-individual variability in vulnerability to excessive sound exposures, some subjects being affected while others are not; up to now the factors underlying this variability are very poorly known.
In the last decade many new and promising data were obtained concerning the biology of pathological processes responsible for hearing impairments due to excessive sound exposures. Excessive noise can induce damage to most cell types in the inner ear, but presently the sequence of these pathological events and their cause/effect relationships remain poorly known. Several environmental factors can have detrimental effects, such as exposure to several chemicals and lowered levels of breathed oxygen which were found to increase NIHL. The study on involvement of genetic factors has only recently started and first evidences point to some possible genes and seem to exclude others. Following the development of molecular biology many new drugs were found to have protective effects against NIHL in fundamental experiments, these constitute new and very promising perspectives to prevent and cure NIHL in the future in humans. Although some few studies have started to assess some new drug treatments in humans much further research is needed over coming years before definite clinical applications can be considered.
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