Keywords
mercury - methylmercury compounds - auditory pathways - medial olivocochlear system
- otoacoustic emissions - ototoxicity
Introduction
The health impacts due to acute and/or chronic exposure are already well defined for
most chemicals. Organizations such as the US Environmental Protection Agency and World
Health Organization established exposure limits, but many people live above this limit.
Environmental contamination with persistent pollutants showing toxic and cumulative
effects on organisms, as in the case of mercury, has reached global proportions due
to the persistence and mobility of these substances.[1] Because it is very toxic and has the tendency to bioaccumulate and because of biomagnification
along the food chain, mercury is recognized as a potentially dangerous pollutant in
the marine environment.
Historically, two great exposures to methylmercury have occurred. In Minamata (Japan),
women showed hair mercury concentrations between 10 and 100 µg/g. The other great
exposure occurred in Iraq, where women with elevated hair mercury levels (∼10 µg/g)
had children in whom neurologic alterations were observed after birth.[2] Passos and Mergler, studying mercury exposure in Amazon communities, reported that
even different bioindicators have revealed mean hair mercury levels higher than 15
µg/g, which are the highest reported mercury levels in the world.[3]
The Amazon region occupies a prominent place in this scenario,[4] and the riverside populations of that region suffer the health impacts associated
with environmental contamination. Fish, the main source of dietary protein for riverside
populations, is consumed at an average of 406 g/d and is contaminated with mercury.[5] The consumption of mercury-contaminated fish gives rise to a series of neurotoxic
effects that are observed in pregnant women, fetuses, and newborns, particularly when
they are exposed to mercury during brain development. The prenatal exposure may result
in small alterations in development that appear over time.[6]
Mercury poisoning causes hearing loss in humans and animals. In a systematic review,
the authors conclude that acute and long-term exposures produce irreversible peripheral
and central auditory system damage and that mercury in its various forms of presentation
in the environment is ototoxic.[7] The consequences of hearing loss can be irreversible, with clinical alterations
that often go undetected by neurologic examination and biological markers, thus jeopardizing
the quality of life of these individuals. Follow-up of ototoxicity is important for
detecting hearing changes, thereby suggesting new treatment strategies for the patient
and intervention when a disabling hearing impairment occurs.[8]
Mercury is a liquid that expands or shrinks very accurately according to the temperature.
These particular properties are extremely useful for various applications such as
the production of electronic measuring equipment, fluorescent lamps, batteries, dental
amalgam, and cosmetics (in small amounts). The mercury used can be released to the
environment at different stages of the production, such as industrial gas, industrial
waste, and contaminated waste. Thus, it can destroy the soil, air, and water, affecting
environmental and human health directly.[4]
Once mercury enters the aquatic ecosystems, the microorganisms transformed into a
highly toxic form of mercury called methylmercury, which is accumulated and biomagnifies
in fish and shellfish and those who consume them. The levels of methylmercury in some
fish species can reach millions of times above the levels present in water from its
surroundings.
Even at low doses of exposure, mercury, especially methylmercury, can penetrate the
placenta and appear in breast milk. This can disturb development of brain functions
and can create deficits in language skills, memory, attention, and motor and visual
skills. When exposure to mercury combines with malnutrition, the risk grows considerably.
Mercury exposure can be monitored through hair, blood, breast milk, urine, and nails.
The level of mercury in blood and urine shows a recent exposure. Levels in hair reflect
long-term exposure.[9] The Agency for Toxic Substances and Disease Registry estimated that the minimum
risk level for methylmercury intake would be 0.3 mg/kg per day.[10] This value is based on estimated level of adverse effects (NOAEL) of 1.3 mg/kg per
day calculated for methylmercury hair levels. The estimated level of adverse effects
was obtained from a study of the Seychelles population that observed the neurodevelopment
of children.[9]
Our objective was to analyze the inhibitory effect of the medial olivocochlear system
(MOCS) with transient otoacoustic emissions (TEOAE) responses in a riverside population
exposed to environmental mercury. To date,this survey is unprecedented in a population
exposed to mercury.
Methods
Background
The INPTAm (Instituto Nacional de Pesquisa Translacional da Amazônia) is formed by
a group of researchers interested in the health of the environment of the Amazon and
its implications for the health of the local population, conducting research in several
areas and expecting aggregate knowledge of suitable proposals for improving the quality
of life of riverside populations of the Amazon. The INPTAm also works to transfer
the knowledge to local professionals, recovery, and environmental protection.
The area selected for the proposed research was the community of Lago do Puruzinho,
located in Amazonas state, Brazil. This population was selected for the study because
the people are subsistence farmers who are exposed to environmental methylmercury
from fish, the primary source of animal protein in their diet.
This study was conducted according to the ethical aspects recommended by Resolution
196/96 of the National Health Council on studies involving humans, and informed consent
was obtained from the research subjects. The research was approved by the Ethical
Committee (Institutional Review Board/EC) from the Instituto de Estudos em Saúde Coletiva
(IESC), under number 79/2011.
All of the participants were born and lived in Puruzinho. None of the participants
had a history of any type of auditory problem or otologic surgery. During the study
period, the total population of Puruzinho was 102 people. Of these, 83 agreed to participate
in the study. Among those who agreed to participate, 25 did not meet the inclusion
criteria. The inclusion criteria were as follows: absence of otologic alteration,
confirmed by otolaryngologic evaluation and hearing thresholds ≤ 20 dB hearing level
determined by pure tone audiometry (0.5 to 8 kHz in octave intervals). Children who
did not undergo audiometry were included in the study because they had TEOAEs, type
A tympanograms (peak pressure between +20 and −40 daPa; admittance between 0.3 and
1.2 mL), and presence of contralateral acoustic reflexes. Individuals who showed an
absence of TEOAEs or a hearing loss (at any level, of any type, and affecting either
ear) were excluded from the study.
The study was conducted among 58 individuals (30 females and 28 males) with a mean
age of 17.3 years (range 1 to 47). The purpose of the research was to evaluate the
entire community independently of variables of sex and age. After otolaryngologic
evaluation, participants had tympanometry and investigation of contralateral acoustic
reflexes (AT-235 Interacoustics, Denmark, Middelfart), pure tone audiometry (AC-33
Interacoustics), and recording of TEOAEs (ILO v6, dual channel, Otodynamics, London,
United Kingdom) with nonlinear click stimulation (260 sweeps) presented at 80 ± 2 dB
peak sound pressure level (SPL). We considered that otoacoustic emissions were present
when a signal-to-noise ratio above 3 dB occurred in at least three frequency bands.
The tests, including tonal audiometry, were performed in a very quiet, although not
acoustically treated, room. The earphones used in pure tone audiometry were AUDIOCUP
(Amplivox Ltd, Oxfordshire, UK) (noise-reducing headset enclosures).
To evaluate the inhibitory effect of the MOCS, a test of linear click-evoked otoacoustic
emissions at 60 ± 2-dB peak SPL was performed, with and without the contralateral
presentation of white noise at 60 dB SPL (contralateral acoustic stimulation, CAS).
White noise was generated by the equipment and delivered in the contralateral ear
by a second channel. The presentation mode was alternate, with 10 seconds of TEOAE
recording collected without CAS being followed by 10 seconds collected with CAS. The
alternating pattern was continued until 260 sweeps in each condition were completed.
The final result was the sum of the alternate responses obtained in each condition.
The inhibitory effect was calculated from the response values by subtracting the value
obtained in the condition with contralateral noise from the value in the condition
without contralateral noise (inhibitory effect = TEOAE without CAS − TEOAE with CAS).
An inhibitory effect was considered to be present when the result showed a positive
value.
The collection of mercury from hair samples was conducted by professionals from the
Universidade Federal de Rondônia. Hair samples underwent digestion and chemical oxidation
with nitric acid (HNO3) and potassium permanganate (KMnO4). A total mercury assay was conducted at the Universidade Federal de Rondônia, utilizing
an atomic absorption spectrophotometry technique through generation of cold vapor
(Flow Injection Mercury System, FIMS-400 by Perkin Elmer, Waltham, Massachusetts).
The mercury assay was performed on 38 individuals who consented to participate in
the study. The mean mercury level in the hair samples was 11.26 µg/g (range 2.93 to
23.45). Participants were classified into three groups according to detected levels
of mercury: group 1, up to 6 µg/g; group 2, from 6.1 to 14.9 µg/g; and group 3, ≥15
µg/g.
For the statistical analysis, the measures of central tendency and dispersion were
calculated for all the numerical variables, and the measures of frequency and percentage
frequency were calculated for all the categorical and ordinal variables. The comparison
between the discrete variables noted in tables was calculated by the chi-square test
with Yates' correction. In the correlation analysis, the Pearson test was used to
measure the linear association between two variables. The comparison of means was
performed by unpaired Student t test. The data were analyzed using the Statistical Package for the Social Sciences
program (SPSS version 14.0, IBM, NY, USA). The level of significance was set at p < 0.05.
Results
The presence of nonlinear TEOAEs was observed in all research participants for the
right ear and in 56 participants for the left ear ([Table 1]). It is important to emphasize that, in the absence of response, the affected side
was excluded for evaluation of TEOAEs with linear click stimuli. Among 58 right ears,
47 showed linear TEOAEs for stimulations of 60 dB SPL (sound pressure level) with
and without contralateral noise, and these were considered for analysis. Among 56
left ears, 49 showed linear TEOAEs and were considered for analysis. The mean linear
TEOAEs were 9.34 dB for the right ear and 7.14 dB for the left ear. The mean value
of the inhibitory effect of the MOCS was 1.20 dB for the right ear and 1.26 dB for
the left ear.
Table 1
Measures of central tendency of otoacoustic emission results of the left and right
ears in the population of Puruzinho Community, Amazon, Brazil, 2012
|
Nonlinear transient, RE
|
Nonlinear transient, LE
|
Linear transient without noise, RE
|
Linear transient with noise, RE
|
Difference linear transient with and without noise, RE
|
Linear transient without noise, LE
|
Linear transient with noise, LE
|
Difference linear transient with and without noise, LE
|
|
N
|
58
|
56
|
47
|
47
|
47
|
49
|
49
|
49
|
|
Mean
|
14.20
|
13.00
|
9.34
|
8.13
|
1.20
|
7.14
|
5.88
|
1.26
|
|
Median
|
13.80
|
12.70
|
9.20
|
8.10
|
1.20
|
5.90
|
4.40
|
1.20
|
|
Standard deviation
|
4.87
|
4.55
|
5.57
|
5.74
|
1.03
|
4.71
|
4.77
|
1.00
|
|
Minimum
|
3.90
|
0.40
|
0.50
|
−1.90
|
−2.00
|
0.90
|
−4.00
|
−0.60
|
|
Maximum
|
29.80
|
22.80
|
22.10
|
21.90
|
5.30
|
18.30
|
17.40
|
5.40
|
Abbreviations: LE, left ear; RE, right ear; N, number of people.
There was no significant correlation between the values of the inhibitory effect for
the right and left ears and age ([Fig. 1]). Furthermore, there was no significant correlation between values of the inhibitory
effect and hair mercury levels ([Fig. 2]).
Fig. 1 Inhibition of medial olivocochlear system values of the right ear and left ear compared
with ages of the population from Puruzinho Community, Amazon, Brazil, 2012.
Fig. 2 Inhibition of medial olivocochlear system values of the right ear and left ear compared
with hair mercury levels in the population of Puruzinho Community, Amazon, Brazil,
2012.
Among the 58 participants, 7 (12%) showed an absence of inhibitory effect of the MOCS.
The mean level of mercury in the hair of these individuals was 12.78 µg/g. Five participants
had hair mercury levels above the acceptable limit according to the World Health Organization
(6.0 µg/g). Two participants were not tested ([Table 2]).
Table 2
Amplitude values of linear TEOAE, inhibitory effect, and mercury levels for participants
with altered examinations from the population of Puruzinho Community, Amazon, Brazil,
2012
|
Patient no.
|
Linear TEOAE without noise (dB), RE
|
Linear TEOAE with noise (dB), RE
|
Difference (dB)
|
Linear TEOAE without noise (dB), LE
|
Linear TEOAE with noise (dB), LE
|
Difference (dB)
|
Mercury levels (µg/g)
|
|
1
|
Excluded
|
Excluded
|
Excluded
|
3.80
|
3.80
|
0.0
|
ND
|
|
2
|
3.80
|
3.80
|
0.0
|
0.90
|
0.00
|
0.9
|
ND
|
|
3
|
8.20
|
8.10
|
0.1
|
3.10
|
3.40
|
−0.3
|
6.51
|
|
4
|
5.20
|
5.20
|
0.0
|
2.60
|
2.20
|
0.4
|
10.89
|
|
5
|
Excluded
|
Excluded
|
Excluded
|
3.30
|
3.90
|
−0.6
|
13.46
|
|
6
|
5.30
|
7.30
|
−2.00
|
2.10
|
1.50
|
0.6
|
15.53
|
|
7
|
11.40
|
9.80
|
1.60
|
7.40
|
7.40
|
0.0
|
17.53
|
Abbreviations: LE, left ear; ND, not determined; RE, right ear; TEOAE, transient otoacoustic
emissions.
[Table 3] shows participants classified into three groups according to their hair mercury
levels. Individuals who showed altered results of the inhibitory effect of the MOCS
in both right and left ears were excluded from each group (groups 1, 2, and 3). The
mean amplitude of the inhibitory effect of the MOCS was lower for participants with
higher concentrations of mercury in their hair.
Table 3
Suppression mean distribution in individuals participating in the hair mercury level
research across three groups
|
Average
|
G1 0–6 µg/g Hg (n = 10)
|
G2 6.1–14.9 µg/g Hg (n = 13)
|
G3 ≥ 15 µg/g Hg (n = 10)
|
|
Hg
|
4.39
|
9.97
|
19.03
|
|
Suppression of RE
|
1.17
|
1.22
|
0.84
|
|
Suppression of LE
|
1.05
|
1.28
|
0.95
|
Abbreviations: LE, left ear; RE, right ear.
Note: G1 had exposure within allowable limit according to the World Health Organization,
G2 had exposure between 6.10 to 14.90 µg/g, and G3 had exposure ≥ 15 µg/g.
Discussion
The same team that analyzed the mercury levels for this project also analyzed hair
mercury levels of the Puruzinho population in 2005 to 2006.[5] The median values in the female and male populations were 12.93 µg/g (range 2.48
to 57.04) and 18.41 µg/g (range 2.28 to 70.56), respectively. In the present study,
the median value was 10.91 µg/g (range 2.93 to 23.45). The population remains exposed
to high levels of methylmercury that are comparable to the historical great exposures
of populations in other parts of the world.
Several studies concerning the inhibitory effect of the efferent pathway demonstrated
an asymmetry between the right and left ears, suggesting an advantage of effectiveness
of the right ear and dominance of the left hemisphere.[11] The inhibitory effect of the MOCS and the measure of the TEOAE amplitude in children
and adults are valid indicators of peripheral auditory lateralization; however, they
are independent.[12] Such asymmetry may be related to the effectiveness of the efferent system in processing
acoustic signals and, consequently, to its role in auditory performance with background
noise. The Puruzinho population showed higher amplitude of responses evaluated by
the TEOAEs for the right ear; however, the inhibitory effect of the efferent pathway
was equivalent for both sides, and an advantage of the right ear was not seen, contrary
to other studies.
We observed that the amplitude of responses of the right and left ears showed a tendency
to decrease as the mercury levels increased significantly. A previous study reported
that higher biological marker values are associated with increased presence of signals
and symptoms of exposure.[13]
The inhibitory effect values of the MOCS in the Puruzinho population did not present
a significant correlation with age. This may be because the Puruzinho population is
a young population, with a mean age of 17.3 years and a maximum age of 47 years.
It was not possible to correlate mercury levels with the inhibition of otoacoustic
emissions; similarly, it was not possible to conclude that the 12% (7 participants)
of the population who showed absence of the inhibitory effect of the MOCS had alterations
arising from exposure to methylmercury. Population studies concerning the inhibition
of otoacoustic emissions cannot be found in the literature.
The population presented high levels of long-term exposure to methylmercury. The lack
of a significant relationship between the results of the auditory evaluation and hair
mercury levels at the time the hair samples were collected should not be interpreted
as implying that mercury does not alter the efferent auditory system. For this analysis,
a longitudinal monitoring of the population and a higher number of individuals participating
in the research would be necessary.
The pathophysiological effects of chronic exposure may be subtle and nonspecific and
have a long period of latency[14]; therefore, it will be important to monitor the effects of mercury exposure in the
central auditory system of the Puruzinho population over time.
Limitations can be pointed out for this study: the number of residents that agreed
to participate in the study; the young population that prevented comparisons between
age groups; and the difficulty in finding a control group that was not exposed to
mercury in a riverside population.
Conclusions
The Amazon population, and in particular the riverside population in which fish is
a main source of dietary protein, should be clinically monitored. Some studies, taking
into account utilization of different methodologies, point toward the possibility
that the Amazon region is seeing the significant silent action of methylmercury in
exposed riverside populations,[14] with the presumable appearance of the clinical expression of mercury poisoning in
the Amazon.
Although the literature refers to mercury as ototoxic, damaging the afferent auditory
system, this study cannot claim that methylmercury alters the function of the MOCS.
Existing studies indicate that the inhibitory effect of the MOCS is a promising tool
for evaluation of the efferent auditory pathways, and its specificity is very important
for diagnosing central auditory disorders.
The present study proposes that longitudinal monitoring studies of the riverside population
should be performed to learn whether analysis of the inhibitory effect of the MOCS
by measuring otoacoustic emissions may be used as an evaluation method and diagnostic
tool in populations exposed to mercury.
It is important to emphasize that this study is an initial and exploratory study;
therefore, the results may be used as a reference for new studies of other populations
exposed to methylmercury and may help to open new frontiers for multidisciplinary
research.
