Operational Variables

For purposes of this article “Occupational noise” is the acoustic energy received by workers in their auditory system in certain industries. It is mainly an unpleasant noise for her/his hearing. The sustained exposure can cause permanent hearing damage. There are four types of occupational noise: continuous, intermittent, impulsive and low frequency noise.⁽⁵⁾

On the other hand, “Music” is an intentionally organized acoustic art, whose medium is sound and silence, with core elements of pitch (melody and harmony), rhythm (meter, tempo, and articulation), dynamics, and the qualities of timbre and texture”.⁽⁶⁾

Objective

The goal of this communication was comment the usefulness of early identification of hearing loss of CSNE based on audiological markers and discuss the changes in the used clinical methodology for its searching.

Noise in Working Centers

The Center for Diseases Control in the United States of America (USA) reported that 22 million workers were exposed to hearing damage by high levels of work noise each year.⁽⁵⁾ In other example, the Mexican Institute of Social Security (IMSS, for the abbreviation in Spanish) registered hearing damage by noise exposure at working places as the second cause of a typified work disease susceptible of indemnization.⁽⁷⁾

The working environment is considered harmful for purposes of clinic audiological evaluation based on what the worker reports. This includes: full time work in a noisy environment, in which loud intensity talking is needed for the worker to be heard by her/his work peers,⁽⁸⁾ (see Table 1), or whether tinnitus or hearing loss are present at the end of the working day. This last manifestation is known as “transient hearing loss”, in opposite to a “permanent hearing loss”.⁽⁹⁾

Preventive Measures Against Noise Exposure in Work Environment

The most common way to prevent hearing loss by noise exposure is occlusion of the external auditory meatus with plugs or headphones, even though it is not the most effective one. Greater effectiveness is reported through reduction of sound emissions from industrial machinery, and through changes in workplaces provided with environmental features that are able to reduce vibrational energy around workers, but these changes are the most expensive. One study of Costa-Meira et al. in 18-65 year-old workers highlighted the disadvantage position of females because of poor use of auditory protection. On the other hand, males use more protection especially either because their socio-economic level is better or because they have had a previous audiological test.⁽¹⁰⁾

Detection and Risk Factors

In the Beaver Dam Offspring Study, Tremblay et al. selected a group of people excluding Hispanics.⁽⁸⁾ (see Table 1). Although exclusion of Hispanics was later considered as a limitation by the authors themselves in their paper conclusions, they used a four questions scale selected from the Hearing Handicap Inventory for Adults Screening and the Hearing Handicap Inventory for Elderly, and detected as main risk factors: exposure to solvents, metals, and noise, low economic income, less physical activity, substance abuse, several audiological examinations, tinnitus and dizziness. Other study such as the carried-out by Kwon and Lee described also as main risk factors: poor dietary habits, smoking, and co-morbidity with chronic degenerative diseases.⁽⁹⁾

Some authors suggest a better tolerance for hearing damage in young subjects because of greater elasticity of the basilar membrane. Although Barrero et al. stated that the opinion is not conclusive,⁽¹¹⁾ these authors observed that the first symptoms of hearing loss appeared in middle-aged workers, and that there may be related with being close to a noise sources at her/his working place.⁽¹¹⁾ There are other reports of age related synaptopathy so there may be a possible overlapping of some risk factors proposed in different studies.

Prendergast et al. asked themselves: “is the cochlear synaptopathy by noise exposure present in youths?”.⁽¹²⁾ To answer the question they performed a study in a group of young subjects aged >18 years, with a mean of 23.3 year old, analyzing the possible influence of age in noise exposure. These authors used psychoacoustics tests and evaluated the amplitude of wave I in ABR, in youths with poor evidence of CSNE (Table I). The authors excluded maturational aspects through intersubject variability, and found low frequency of CSNE and stated as a possible explanation the greater vulnerability of some subjects to CSNE. Researchers added a possible recovery factors hypothesis in the ribbon type preganglionic synapsis damage that could be observed in animal models, such as chinchillas exposed to noise.

Among work environment with dangerous activities for hearing are those carried out in factories with noisy machines and heavy equipment as sawmills. Other examples are building construction, mining, and army industries with activities with gun shoots, moreover leisure activities, hunting, and shooting, highlight by their increased risk for hearing damage.

After the youth, some authors identified ageing as a risk factor for hearing injury independent of the type of noise exposure. It has been observed that there is an increased frequency of hearing loss in both genders in older people. Women may show best hearing than men despite environmental noise exposure. Inheritance seems to be related with hearing loss after noise exposure in older age.⁽¹¹⁾ (see Table 1). At the same time, other authors mentioned that inner ear damage by noise exposure affects the left ear more frequently than the right one.⁽⁹⁾ An a possible explanation of this fact was given by Kwon and Lee,⁽⁹⁾ who speculate that predominance of hearing loss at the left side was gave by the protective effect of the left olivo-cochlear fasiculi to the right auditory pathway, which may help to explain why in some audiologic tests, such as the middle ear muscle reflex show asymmetric alterations.⁽¹¹⁾

Prendergast et al. measured the interaction among intensity and duration of noise in working environment as the main variables of their research.⁽¹²⁾ Summation of time exposed to noise by the worker was the more important variable in the risk of hearing damage (see Table 1).

Besides physiologic factors, the fine architecture of the structures of the cochlea is damaged by noise by means of metabolic mechanisms which may persist for a long period. The factors that cause damage are: an increase of free radicals, reactive mechanisms to oxygen and nitrogen in the outer hair cells followed by production of cytokines and activation of apoptotic mechanisms. High level of noise produce mechanical adverse effects to injuring the hair cells, the pillar cells, and the tectorial membrane.⁽⁹⁾

General Considerations

CSNE with HHL may be a subtle pathology hard to identify in its beginnings but with important consequences in the hearing health of workers in noisy industries and in individuals exposed to high level of noise in leisure activities such as it will be commented as follows.

The hearing impairment was observed in individuals that even have suspected normal hearing, and was experimented as suboptimal function in their daily hearing. This is why CSNE they belong to the HHL category, although in their paper Tremblay et al. considered better the term: “hearing difficulty” and also found that 12% (n = 682 with age groups between 21 to 67 year-old) had this HHL.⁽⁸⁾ (see Table 1).

Besides clinical aspects, there are several research unknown fields involved in CSNE research, such as: anatomical, physiological, molecular, biochemical and genetics topics. I.e. Lobarinas et al.,⁽¹³⁾ emphasized the implications of the ribbon type synapsis in CSNE and its possible role when is damaged, as a predictor of the temporary threshold shifts of more than 30 dB in the first 24 hours after noise exposure in rats that were exposed to noise between 106 to 108 dB between 8 to 16 KHz. The same authors referred to the limited relationship between recreational noise and the decrease of amplitude of wave I of the ABR, they also found inconsistence in this decrease except when there is a significant level of hearing loss or a permanent threshold shift.⁽¹³⁾

Audiological Tests and Groups of Study

Tonal audiogram is the first test of an audiological examination. Cochlear damage by noise exposure causes an increased threshold in high frequencies, firstly in 3, 4 and 6 KHz and a recovery at 8 KHz. Low frequencies can also be altered with the time. Abnormalities in ABR and P300 wave of the event related potentials can also be present.⁽⁹⁾ Vigilance of risk factors and a sinergy with ototoxic drugs and solvents exposure most be performed.⁽⁸⁾ We join to the proposal of other authors of performing high frequency audiometry and not to limit the study of patients to pure tone audiometry and non-distorted speech audiometry.⁽¹⁴⁾ It is also necessary a more precise analysis of audiograms obtained with half an octave in high frequency tones.

It is important to take into account that the same pathophysiology of the cochlear synaptopathy is present even when hearing loss is not hidden.⁽¹⁴⁾ Moreover, there can be thresholds over the normal range in few frequencies in superficial loss levels.⁽¹⁵⁾

In function of the intensity, the study of phonetically balanced words of Jerger and Hayes described the “roll over” phenomenon identifying a greater neural damage in individuals with tumors such as vestibular Schwannomas.⁽¹⁶⁾ In CSNE, mistakes in tests with words could be less dramatic than in tumors at supra threshold levels.

Middle ear muscles reflexes (MEMR) are supra-threshold audiological tests used in the identification of hearing loss. Guest et al. proposed the use of MEMR, being the most sensitive indicators of CSNE and suggest that are related with the amplitude of wave I of ABR in mice.⁽¹⁷⁾However, in human beings, contrary of what was expected, the authors did not find an association between MEMR and other tests for cochlear synaptopathy or with tinnitus (see Table 1).

Additionally the papers by Prendergrast et al.,⁽¹²⁾ Lobarinas et al.,⁽¹³⁾ and Le Prell,⁽¹⁴⁾, they state that it is possible, that in order to present CSNE, human beings that have been exposed to high noise, possibly have had previous hearing loss in high frequencies.

Laboral Legislation

The medico-legal aspects in relation to the compensation to which the worker is entitled for the hearing damage caused by exposure to noise in the work environment, specifically assess permanent hearing disorders, that are established based on tonal audiometry in conversational frequencies. The remuneration of the percentage of hearing impairment is made based on binaural loss by averaging the hearing thresholds in the frequencies 500, 1000, 2000, 3000 and/or 4000 Hertz, which are those used in human communication. It shall not be considered hearing loss when the hearing threshold is 25 dB. Practice that is carried out both in Europe and in many countries of America and that does not include any compensation for patients with normal or near-normal audiograms.⁽¹⁸⁾ In agreement, the Mexican legislation for noise related hearing loss due to exposure in working environments does not imply indemnization for patients with normal audiograms or near normality. Therefore, this point deserves more attention in the future because it will be the subject of discussion for future legislative policies, which should include personalized prevention and specific treatment strategies based on a comprehensive view of the worker, occupation, genetics and pathology of an individual.

Noise and Music

Musical experience at high sound levels has less frequency of CSNE with respect to workers exposed to industrial environment with noisy machines. This because music in college context can result in a favorable effect for brain plasticity and psychoacoustics performance also. It is believed that industry environment is more aggressive to the inner ear compared with environmental music of several types and with other loud sounds at leisure time. Furthermore, cultural patterns between university people and industry workers could be very different, especially in developing countries. Studies of Skoe and Kraus have considered the possibility that music exerts a protector effect due to educative musical training in early life.⁽¹⁸⁾ In this study, one evident benefit was observed in the register of significant improvement of ABR in the studied subjects.⁽¹⁹⁾