Keywords audiometry - hearing loss - high-frequency - hearing loss - noise-induced - dentistry
Palavras-chave audiometria - perda auditiva de alta frequência - ruído - odontologia geral
Introduction
Noise is one of the most harmful agents for health, primarily for hearing, and it
is often unavoidable at workplaces and entertainment venues. Noise-induced hearing
loss (NIHL), which is hearing loss that is induced by high levels of sound pressure,
is one of the most common occupational diseases.
Currently, noise is a constant part of people's daily activities, as it is present
in traffic, leisure, and work; therefore, NIHL may become one of the main chronic
diseases in the future (Fiorini , 2000).
NIHL is defined as sensorineural hearing loss that occurs due to systematic occupational
exposure to high levels of sound pressure, thus causing damage to the hair cells of
the organ of Corti. Generally, this hearing loss is bilateral and symmetrical, insidious
and irreversible, and is directly related to the period of exposure and individual
susceptibility (Brazilian National Noise and Hearing Preservation Committee, 1999; Rabinowitz , 2000). This hearing disturbance is initially manifested at the frequencies of 6,000 Hz,
4,000 Hz, and 3,000 Hz, and broadens progressively to the frequencies of 8,000 Hz,
2,000 Hz, 1,000 Hz, 500 Hz, and 250 Hz. Noise rarely leads to profound hearing loss,
which generally does not exceed 75 dB for high frequencies and 40 dB for low frequencies,
and reaching its upper limit in the first 10 to 15 years of exposure (Luxon , 1998; Hanger , Barbosa -Branco , 2004; Gatto et al. , 2005).
Generally, professionals only notice a hearing difficulty when the lesion is at an
advanced stage, as the hearing symptoms are insidious and manifest late (Sava , 2005). Continuous exposure to high levels of sounds may not only result in hearing
damage, but also in a few secondary alterations, such as tinnitus, stress, physiological
alterations to the heart rhythm and to blood pressure, as well as difficulty in discriminating
speech sounds, especially in noisy environments. Noise causes physical exhaustion,
chemical, metabolic, and mechanic disturbances to the sensory hearing organ, thus
resulting in partial or total hearing loss of the organ of Corti, which is located
in the internal part of the ear. (Otoni A, Boger ME, Barbosa -Branco A, Shimizu HE, Maftum MA, 2008).
Dentists are typically exposed to 2 types of noise: external noise from the work environment
and noise from their own work equipment, such as the noise from high and low rotation
motors, compressors, air conditioners, amalgamators, aspirators, and others (Hinze , Deleon , and Mitchel , 1999). In addition to noise, they are exposed to other factors, including chemicals
(manipulated substances, especially mercury), biological agents (the oral cavity is
full of microorganisms and the risk of contracting diseases such as hepatitis and
aquired immune deficiency syndrome (AIDS) is high), mechanical agents (body lesions
caused by to the instruments that are used), social stresses (having occupation that
involves tension and requires mastery of the situation to facilitate the patient/professional
relationship), and ergonomic challenges (due to the body position while working, the
professional is subjected to back and arm problems and varicose veins) (Souza , 1997). According to Presta et al. (2004) Dentists have frequently presented with discomforts that are related to the
nature of their profession, which may progress to repetitive strain injuries or work-related
musculoskeletal disorders.
In 1959, the American Dental Association (ADA) had already recommended periodic hearing
assessments for dentists, due to their prolonged exposure to high-frequency sounds,
which were caused by instruments like high speed drills, ejection systems, ultrasound
equipment, model clippers, and instruments with high suction and vibration speeds,
which can lead to hearing loss. Studies (Altinoz et al. , 2001; Fernandes et al., 2004) that measured the noise levels in dental practices observed noise levels that
were higher than 80 dB Sound Pressure Level (SPL).
In Brazil, Law number 6.514 of the Brazilian Labor Code (Consolidação da Leis do trabalho ; CLT), relates the parameters that provide acoustic comfort for practitioners with
the norms of the Brazilian Association of Technical Norms (ABNT). The Brazilian employment
legislation considers the maximum tolerable level of noise to be 85 dB SPL in an 8-hour
shift (Segurança e Medicina do Trabalho, 1991); The Brazilian Standard (NBR) 10.152
indicates that this level should be between 35 and 45 dB (A) for a dental office.
Paraguay (1999) verified that dentists with 5 or more years of practice presented hearing
disturbances by tonal threshold audiometry.
Leggat (2000) stated that the reason that more experienced dentists suffer from hearing
alterations may be due to previous exposure to older equipment. However, this possibility
was not considered in this study, as the study population was relatively young.
Previous studies (Oliveira et al., 2007; Torrês et al., 2007; Melo et al., 2008) have shown that dental professionals should be aware of occupational noise,
as well as of the harmful consequences that it can have on their health. This understanding
should begin early, during the undergraduate course, when the professionals are being
educated, so that, aware of the risks that they may be exposed to, they will be able
to prevent them, instead of attempting to lessen or treat the problems caused by them.
In Brazil, the prevention of occupational diseases and occupational accidents began
with the CLT in 1943, and ever since, the attention to hearing in noisy work environments
has intensified. With the promulgation of Ordinance number 3.214/78 there was an important
improvement in the scope of hearing conservation efforts, by means of the Control
Program Occupational Health Medical-PCMSO (NR-7), which made the tonal threshold audiometry
mandatory.
The interest in early diagnosis has increased, and considering the progress of the
technology that is aimed at the diagnosis of hearing loss, as well as the prevalence
of hearing alterations even in the absence of complaints, which are commonly found
by tonal threshold audiometry, other methods have been used to identify hearing alterations
at an early stage.
According to previous studies, high-frequency (between 9,000 and 18,000 Hz) tonal
audiometry is an instrument that is effective for the early diagnosis of hearing problems
due to exposure to noise (Porto et al ., 2004; Lopes and Godoy , 2006; Amorin , et al. , 2008). Studies such as those conducted by Mota (2002) and Porto et al. (2004) that investigated hearing at conventional frequencies and high frequencies
in dental practices have shown a tendency toward lower thresholds, suggesting that
significant hearing problems accumulate with time. As the frequency, time of exposure,
and age increases, there is a greater decay in hearing acuity. These studies also
observed a greater incidence of hearing loss in the frequencies of 6,000 Hz and 14,000
Hz.
Lopes and Godoy (2006) compiled a literature review regarding the importance and contribution of
high-frequency audiometry for the early identification of NIHL. They demonstrated
that conventional tonal threshold audiometry alone may not be effective in the identification
or prevention of NIHL, and also described the methodological variables for its proper
execution. The authors also suggested this method should be added to the routine that
is indicated by the Occupational Hearing Loss Prevention Program. Therefore, the purpose
of this study was to investigate the hearing thresholds at conventional and high frequencies,
thus enabling early prevention of hearing loss and and improving the overall hearing
health of the population as a whole.
Method
This study began after the approval of the Ethics Committee of the University of São
Paulo under process number 043/2007. This research study was financed by the São Paulo
Research Foundation (FAPESP) funding agency under process number 2007/01074-7.
This was a cross-sectional study with 108 volunteer participants the Bauru community,
which were divided into 3 experimental groups. The groups were named I , II, and III.
Group I (GI) consisted of 44 dentists (16 males and 28 females) aged between 23 and
57 years (average of 34 years of age). Group II (GII) consisted of 36 female dental
nurses, aged between 21 and 59 years (average of 38 years of age). Lastly, Group III
(GIII) consisted of 28 prosthodontists (17 males and 11 females) aged between 17 and
53 years (average of 35 years of age).
Professionals from private dentistry offices or laboratories, universities in Bauru,
São Paulo, and hospitals that employ dentists on their staff were invited to participate.
The participants initially received clarifications regarding the purpose of this study,
which only began after the agreement to participate and the signature of the informed
consent were obtained.
For the inclusion and exclusion criteria, only professionals in the dentistry field
with at least 2 years of experience, who did not present any pre-existing diseases
such as mumps, high blood pressure, diabetes, meningitis, human immunodeficiency virus
(HIV), syphilis, and other conditions that can compromise hearing and/or pre-existing
hearing impairment were considered.
All the participants were underwent:
A specific interview and middle ear inspection: these procedures were performed to
investigate the individual features such as age, time in the profession, noisy entertainment
habits, exposure to chemical products, as well as health conditions and other diseases
that can aggravate the effect of environmental risks.
Conventional tonal threshold audiometry (250 to 8,000 Hz), high-frequency tonal threshold
audiometry (9,000 to 16,000 Hz), and speech reception threshold tests were performed
using a Siemens SD 50 audiometer. For the tests of the tonal thresholds the warble
tone was used, which was presented using aural headphones (HDA 200). The descendent
technique was used for the tests of the tonal thresholds. The hearing threshold was
established when there was a 50% positive answer for sound detection (Lopes and Godoy , 2006).
Acoustic impedance test: the acoustic immitance measures and the ipsilateral and contralateral
acoustic reflex tests of the stapedius muscle were performed with GSI Tymp Star. They
were classified as suggested by Jerger (1970).
Results
The data analysis from the specific interview revealed that 65 participants were bothered
by noise at work, 50 participants reported difficulties in speech comprehension, 8
had served in the army, 11 had acoustic trauma, 32 were exposed to chemical products,
and 35 referred to being exposed to noise during recreational activities.
[Graph 1 ] shows the mean hearing thresholds for all of the frequencies that were tested in
the 3 groups.
Graph 1. Hearing thresholds of the right ear in all of the tested groups.
Graph 2. Hearing thresholds of the left ear in all of the tested groups.
The comparison between the mean hearing thresholds of each frequency that was tested
in the 3 groups was performed using the Kruskall-Wallis test, and considered significant
by the Dunn test. We obtained a statistically significant difference in the right
ear for the frequencies of 2,000 Hz (p = .0446), 8,000 Hz (p = .0492), and 16,000 Hz
(p = .0441) when the mean of Group I (mean threshold at 2,000 Hz = 5.91 dB, at 8,000 Hz = 11.59 dB,
and at 16,000 Hz = 21.59 dB) was compared to the mean of Group II (mean threshold
at 2,000 Hz = 10.69 dB, at 8,000 Hz = 18.61 dB, and at 16,000 Hz = 32.78 dB); hence,
we verified that GII presented worse thresholds at the frequencies of 2,000 Hz, 8,000 Hz,
and 16,000 Hz in the right ear compared to GI.
It can observed that the right and left ears presented similar configurations in conventional
and high-frequency audiometry, when the mean hearing thresholds for all of the groups
is considered; however, the right ear presented worse hearing thresholds than the
left ear.
For the left ear, the frequencies of 4,000 Hz (p = .0238) and 6,000 Hz (p = .0310)
presented statistically significant differences between GI (mean threshold at 4,000 Hz = 8.41 dB
and at 6,000 Hz = 14.32 dB) and GII (threshold mean at 4,000 Hz = 14.03 dB and at
6,000 Hz = 20.69 dB) and between GI and GIII (mean threshold at 4,000 Hz = 15.36 dB
and at 6,000 Hz = 22.32 dB). At the frequency of 9,000 Hz in the left ear, there was
a statistically significant difference (p= .0397) between GI (mean theshold at 9,000 Hz = 10.91)
and GII (mean threshold at 9,000 Hz = 20.28 dB).
The comparison of the mean hearing thresholds at the frequencies of 500 Hz to 2,000 Hz,
3,000 Hz to 6,000 Hz, 9,000 Hz to 16,000 Hz, and 12,000 Hz to 16,000 Hz was performed
using the Kruskall-Wallis test, and the statistical significance was determined by
the Dunn test. For these comparisons, a statistically significant difference (p = .0147)
was obtained only when the mean thresholds between the frequencies of 3,000 Hz to
6,000 Hz in the left ear between GI (10 dB) and GII (15.93 dB) and those between GI
(10 dB) and GIII (16.25 dB) were compared.
The speech audiometry, which was performed using the speech recognition threshold,
confirmed the results of the conventional audiometry in 100% of the participants,
as did the speech recognition index, which was compatible with the hearing thresholds
found in 100% of the participants.
For the acoustic immitance and evaluation of the ipsilateral and contralateral stapedius
muscle reflexes, normal tympanograms were obtained bilaterally in 100% of the participants,
thus indicating that the middle ear did not interfere with the results that were obtained.
Discussion
This study focused on a sample of dentists, dental nurses, and Prosthodontists with
more than 2 years of experience in the dentistry field and with an average age of
35 years among the 3 groups.
In this population, hearing loss can occur due to prolonged exposure to high-frequency
sounds that are produced by the instruments that are used in their daily practice,
which results in subsequent handicaps to their communication and quality of life (Hinze , Deleon , and Mitchel , 1999).
This study showed that only 60% of the participants were bothered by noise at work
(Souza , 1997; Tôrres et al. , 2007; Melo et al. , 2008), that 43.3% of the participants reported difficulties in speech comprehension,
and that 32.4% mentioned that they were exposed to noise during recreational activities.
This study revealed that the mean hearing thresholds of all of the groups worsened
according to increases in frequency (Matthews et al. , 1997; Beltrami , 1999; Fernandes and Mota 2001; Mota , 2002; Porto et al., 2004; Silva and Feitosa, 2006; Lopes et al. , 2006; Lopes and Godoy , 2006; Lopes , Almeida , Zanconato , and Mondelli , 2007; Carvalho , Koga , Carvalho , and Ishida , 2007). For all of the groups, both conventional and high-frequency audiometry of
each ear showed similar configurations; however, by comparing both ears in the 3 groups,
we observed that the right ear showed worse mean thresholds than the left ear. These
results are in agreement with Zubick , Tolentino , and Boffa (1980), and are in disagreement with Gijbels et al. (2006), who gathered data on the effects of occupational health among dentists and
observed that hearing loss was greater for the left side for right-handed professionals,
which can be explained by the short distance between the left ear and the circular
motion/vibration equipment for right-handed professionals.
The tritonal means of 500 to 2,000 Hz and 3,000 to 6,000 Hz revealed that prosthodontists
(GII) had the worst hearing thresholds. For the high-frequency mean (9,000 to 16,000
Hz) the dental nurses (GII) had the worst hearing thresholds. These results highlight
the importance of using complementary tests in the hearing evaluation (which in this
case was high-frequency tonal threshold audiometry), and that if these tests are used
as a routine procedure in the evaluation of these professionals it would assist in
the early detection and prevention of hearing problems.
Conclusion
In this study, we concluded that the conventional hearing assessment did not identify
hearing problems in the 3 groups that were tested. However, the assessments of the
high-frequency thresholds indicated disturbances to the peripheral auditory system,
specifically on the external hair cells, thereby demonstrating greater sensitivity
for the early detection of hearing problems and favoring the use of complementary
tests as prevention tools.
