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Introduction: Noise is a preventable occupational hazard for certain professions like automobile drivers and traffic police personnel. The harmful auditory effects of noise are well known. However, little is known about the status of the vestibular function in chronic noise exposure without noise induced hearing loss. Our objective was to assess the vestibular function in chronic noise exposure. Methodology: The study was conducted with a sample size of 242 (chronic noise exposure group − 121, group without chronic noise exposure − 121). Noise estimation was carried out across various traffic intersections to assess the noise exposure levels of the exposed group. All participants underwent a detailed vestibular evaluation in the clinical vestibulometry laboratory. Results: There was no difference in nystagmus, saccades, caloric function between the two groups. The latency and amplitude of vestibular evoked myogenic potentials (VEMP) were similar in both the groups. However, dynamic posturography showed a significant difference in the Adaptation test between the two groups (P < 0.05). We also found a statistically significant difference between the static and dynamic subjective visual vertical (SVV) and the dynamic visual acuity (DVA) between the two groups (P < 0.05). Conclusion: We did not find any clinical evidence of vestibular dysfunction in the noise exposed group. However, the statistical significance of SVV and DVA as seen in this study needs to be evaluated further as an early marker for vestibular dysfunction. It remains to be seen whether the statistically significant prolongation is reversible after the noise exposure is withdrawn.
Keywords: Caloric test, dynamic visual acuity, hearing, noise, Nystagmography, posturography, subjective visual vertical
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Noise pollution is an important health hazard and affects the hearing threshold in chronic exposure. There are various non-auditory effects of chronic noise exposure which can affect the cardiovascular system, sleep pattern, and cognitive function.[1] The balance system though functionally independent from the auditory system is intricately embedded within the bony labyrinth of the inner ear and has complex neural network that connect the peripheral receptors with the higher centers. The saccule which is a part of the balance system has been found to respond to sound and plays a key role in the equilibrium.[2] Further the common vasculature of the cochlea and the vestibular organs and similar cellular architecture of the auditory and vestibular receptors can make them susceptible to the harmful effects of chronic noise exposure.[3] Traffic policemen and automobile drivers are exposed to high noise levels. Unlike industrial workers where there are regulations on the safe noise levels and usage of noise prevention strategies, traffic policemen and automobile workers are exposed to unregulated levels of noise exposure during their daily activities. Further the effect of noise in the vestibular function on these groups has not been studied. Vestibular dysfunction in these groups can be dangerous and may lead to accidents. The present study was undertaken to assess the vestibular functions and postural control in traffic policemen and automobile drivers who are exposed to chronic noise exposure and compare them with one without chronic noise exposure.
Materials and methodsThe study design was an analytical cross-sectional with 242 participants. Of them 121 were included in the exposed group and 121 were included in the control group. The exposed group comprised of traffic police personnel and automobile drivers within 25 to 50 years age-group with a minimum of 5 years of service and a noise exposure of 5 hours or more per day. The control group belonged to the age group of 25 to 50 years and comprised of healthy volunteers without any history of chronic noise exposure. The participants were enrolled into the study and allocated into either of the groups after screening with a pretested questionnaire. Persons with a definite history of any vestibular event in the past or history of taking any vestibular suppressant medications were excluded. Chronic illness like anemia, diabetes, hypertension, hypothyroidism, chronic kidney disease, and any history of psychiatry disorders were also excluded from the study.
Noise exposure at traffic intersections was assessed with a digital sound level meter (Lutron SL-4033D model, Lutron Electronic Enterprise Ltd, Taiwan) attached to a tripod and placed 120 cm from the ground level. Measurement was carried out for a period of 1 hour at three different timings viz. 9 am, 2 pm, and 6 pm and the maximum and minimum exposure levels were calculated.
The vestibular evaluation was performed with the integrated vestibular evaluation system consisting of Videonystagmography, Computerised Dynamic Posturography (CDP), Subjective Visual Vertical, and the Dynamic Visual Acuity (DVA) test (Neuroequilibrium Ltd, India). For every test the googles were calibrated before starting the tests. The tests for spontaneous nystagmus, saccades, smooth pursuit movements, subjective visual vertical test, and the DVA were performed in a semi dark room with the patient seated comfortably about 3 ft. from the projection wall. Bithermal caloric function test was performed in the semi-dark room with the patient lying supine and head elevated by 30°. Caloric irrigation was performed with the automated caloric irrigator in the following sequence R440, L440, R300, L300 (R = right ear, L = left ear) and the response was computed by the device software.
Dynamic posturography was conducted with the patient standing on the posturography platform with the safety harness in place. The following parameters were assessed: the sensory organization test (SOT), the motor control test (MCT), and the adaptation test (ADT).
The vestibular evoked myogenic potential (VEMP) test was performed with the VEMP device (Neuroaudio model, Neurosoft LLC, Russia). Before starting the test, a tuning fork test was carried out and the middle ear dysfunction was ruled out by performing impedance audiometry. The cervical VEMP test was performed with the patient in sitting position and neck rotated and flexed to opposite side to get the adequate sternocleidomastoid contraction of around 50 μV. VEMP threshold was calculated by the minimum sound intensity level at which a clear, reproducible waveform was detected. The latency and the rectified amplitude as well as asymmetry ratio were calculated by the device software. The impedance of the electrodes was less than 5 kOhm and a rarefaction stimulus with intensity of 100 dB and sampling rate of 5000 Hz was used for latency and amplitude studies. The following electrode montage was used for cervical VEMP studies.
Non inverting (+ve) electrode: Upper 1/3rd of Sternocleidomastoid (approximately 10 cm below the mastoid process).Inverting (−ve) electrode: Ipsilateral sternoclavicular junction.Ground electrode: Forehead.Statistical analysis for difference between the groups was done with two tailed unpaired t test and a P < 0.05 was considered as significant.
Results and observationsThe exposure group comprised of 66 traffic policemen and 55 automobile drivers (total = 121). The average traffic noise levels exceeded the safe permissible limits in almost all intersections with equivalent sound pressure level ranging from 82.3 to 89.1 dB. Almost 99% of the exposed group had history of working in noisy environment for more than 8 hours per day regularly. There was no complaint of hearing loss, tinnitus, or any symptomatic vestibular dysfunction among the exposed or the control group.
Spontaneous nystagmus and smooth pursuit movement were normal in both the groups. Saccades (random and fixed) did not show any statistical significance between the exposed and the control group in terms of latency, velocity, and precision. Bi-thermal caloric irrigation elicited an abnormal response in any one ear in almost 78.2% exposed and 71.7% control group without any statistical significance (P < 0.05) [Figure 1].
Figure 1 Comparison of caloric stimulation among the exposed and the control group.The cervical VEMP threshold showed no significance among the groups. The rectified amplitude of cervical VEMP ranged from 1.59 ± 0.70 (mean ± SD) for exposed group and 1.53 ± 0.73 (control group) with P = 0.537. There was no statistically significant difference between the latency of P1 and N1 [Table 1].
Table 1 Comparison of cervical Vestibular Evoked Myogenic Potential (cVEMP) latency and amplitude among the exposed and the control groupWe found interesting observations when we measured DVA as a surrogate marker of the vestibulo-ocular reflex (VOR). Even though the DVA values were within the normal range and the patients did not have any clinical finding suggesting an abnormal VOR, yet there was a statistically higher range of DVA among the exposed when compared with the controls (P = 0.017) [Table 2].
Table 2 Comparison of Dynamic visual acuity (DVA) among the exposed and the control group.The subjective visual vertical test (SVV) also showed a statistically significant difference between the exposed and the control group [Table 3]. This difference was consistent for static as well as dynamic SVV in both clockwise and anticlockwise directions (P < 0.05).
Table 3 Comparison of Subjective visual vertical (SVV) among the exposed and the control groupThe postural control was assessed with CDP. There was no statistically significant difference in the average equilibrium score of the SOT and the average score of the MCT between the groups. However, there was a statistically significant difference between the groups in the ADTs with a relatively higher sway energy score among the exposed group (50 ± 24.46) as compared with the control (44.47 ± 14.95); P = 0.02 [Table 4].
Table 4 Comparison of Computerised dynamic posturography (CDP) among the exposed and the control group. Discussion Noise is a preventable nuisance and the risk of chronic exposure remains high in certain occupation. The auditory effects of acoustic trauma and chronic noise exposure are well documented. Various studies have documented vestibular dysfunction in noise induced hearing loss.[4],[5]
Our study did not find any significant abnormality in the spontaneous nystagmus, saccadic eye movements, or the smooth pursuit. This is in concordance with the study by Emara et al.[6] It is known that chronic exposure causes gradual damage allowing adequate compensation to take place which can explain their absence in chronic noise exposure. The caloric function test detects the abnormality of the lateral semicircular canal and the semicircular canals are less frequently affected in noise induced hearing loss (NIHL) compared to the utricle and the saccule.[7] Further, animal studies show that the semicircular canals are mostly affected by impulse noise with a peak level of 158 dB at 1.1 kHz and it is unlikely for human ears to be exposed to such high intensity impulse noise from regular vehicular traffic.[8] In our study setting, the noise levels across various traffic intersections ranged from 82.3 to 89.1 dB and study participants did not have any auditory symptoms of NIHL which can explain the absence of significant difference in caloric test between the groups.
Abnormal cervical VEMP latency was reported in NIHL by various studies.[6],[9],[10] Statistically significant reduced VEMP amplitude was noted by Viola et al.[3] in their study group consisting of hearing loss with chronic noise exposure. Their study did not find any significant prolongation of latency which was probably due to the selection of control arm comprising of hearing loss without noise exposure. There was one study conducted among chronic noise exposure without any hearing loss where cervical VEMP latency was affected.[11] This study was conducted among the workers of textile industry where noise exposure can be of very high levels reaching up to more than 100 dB in certain areas.[12] It remains to be seen whether occupational noise exposure greater than 100 dB develop any vestibular dysfunction or VEMP abnormality. Our study did not find any statistically significant difference in the amplitude and latency of cVEMP. The difference in our findings with Emara et al., Zuniga et al., and Akin et al. may be because our study population included only participants with chronic noise exposure but without any hearing loss. Animal studies by Stewart et al. show a lesser reduction in amplitudes when exposed to 110 dB noise compared with 120 dB noise for the same duration of exposure among the rats. The intensity levels of the noise effect the vestibular short latency evoked response (VsEP) in animals, with recovery at lower levels and longer lasting effects at higher levels.[13],[14] The susceptibility of the vestibular end organs to various intensity of noise is not known and there is lack of data on the threshold intensity of noise and type of noise exposure that cause irreversible damage to the vestibular apparatus.
Vestibular dysfunction is known to cause postural imbalance which can be assessed with CDP. Acute vestibular de-afferentiation causes abnormal vestibulospinal reflex which results in abnormal postural sway. However, as compensation occurs the postural sway improves which can be used to assess the response to vestibular rehabilitation. Our study did not find any statistical significance in the SOT or the MCT. However, it was interesting to note that the average sway energy score in ADT was significantly higher in the exposed group then in the control group. There have been conflicting results relating to postural stability and noise exposure. There was substantial degradation of postural balance in subjects exposed to high frequency noise.[15] Bateni et al.[16] found no changes in postural sway on exposure to 80 dB intermittent white noise. However, in the study by Bateni et al. the noise exposure was given during the measurement of postural stability unlike our study where chronic exposure to noise was existent before the measurement was undertaken. VEMP correlates with the postural stability in patients with hearing loss and presence of at least unilateral functioning saccule provides good postural stability.[17] It has been seen that auditory inputs are utilized for balance control and that in the absence of auditory inputs due to hearing loss, the postural control is impaired.[18] There was no correlation between VEMP and the sway energy score of adaptation test in the present study and this may be explained by the normal hearing of the participants.
The perception of gravitational vertical is an important function of the otolith organ and plays a key role in the maintenance of balance function. SVV is a simple, effective, and rapid test. It is commonly used to assess the otolith system particularly the utricle and can be a reliable marker of the degree of vestibular compensation after an acute vestibular deafferentiation.[19],[20] There was no correlation of the SVV with VEMP latency and amplitude as was also seen in study by Mueller et al.[21] This is possible as both these tests measure different types of hair cell functions in the utricular macula. Type I cell located in the striola responds to dynamic stimulation vibration and sound while Type II cells in the peripheral aspect of the macula are more sensitive to the static orientation of the gravitational vector relative to the head position.[22] The difference in SVV tilt among the exposed group as seen in this study may be due to an early subclinical otolith involvement in chronic noise exposure. It will be interesting to note the SVV tilt after sudden acoustic trauma and assess if any changes in SVV tilt occur after prolonged exposure to loud sounds in other occupational settings.
The DVA is an indirect indicator of the status of the VOR function and measures the deterioration of the visual acuity with head movement which occurs as a result of retinal slip. DVA may be used for screening and can indicate vestibular compensation after unilateral or bilateral vestibular impairment.[23],[24] The DVA results show a statistically significant higher values in the noise exposed group compared to the control group. The VOR gain is affected in NIHL due to chronic noise exposure.[25] The clinical importance of difference in the DVA noted in this study needs to be followed up in the long run. It also needs to be seen whether early prolongation can become clinically significant over a period of time and serve as a useful indicator of future NIHL and vestibular impairment due to chronic noise exposure.
Limitations of the studyThe study was a cross-sectional one and measured the vestibular function at a single time. However, there was an unforeseen scenario due to the COVID lockdown, which resulted in drastic reduction in noise levels in the city due to vehicular restrictions. Vestibular assessment was caried out after COVID restrictions were eased out. We cannot say if this period of relative noise abstinence affected the outcome of the tests in any way by allowing a recovery or healing time for the cochlea and vestibular receptors.
ConclusionIt is well known that noise has a detrimental effect on the hearing threshold of the people and vestibular dysfunction is often coexistent with noise induced hearing loss. Our study did not find isolated clinically manifested vestibular dysfunction in the noise exposed group. Further studies will be necessary for periodic evaluation of vestibular function in the setting of occupational noise exposure. It remains to be seen whether the statistical prolongation of SVV, DVA, and ADT as seen in this study is reversible after chronic noise exposure is withdrawn. It may be worthwhile to evaluate SVV and DVA as an early indicator of impending vestibular dysfunction in noise exposure. Till definite conclusions are made, we need to exercise caution, create awareness, and adopt effective noise prevention strategies in the individuals with occupational noise exposure.
Acknowledgement
The authors acknowledge the role of DHR, ICMR in funding the study. Mr. Bitra Venkatesh and Mr. K.N.S.V. Bharath deserve our heartfelt acknowledgement for their technical support in the present study. The authors express their gratitude to Traffic division Vijayawada, who have extended their help in this study.
Financial support and sponsorship
The study is funded by the Department of Health Research (DHR) and Indian Council of Medical Research (ICMR), Government of India, under the Grant in Aid scheme of DHR. [File no: R.11012/01/2020-HR].Conflicts of interest
There are no conflicts of interest.
References
Correspondence Address:
Soumyajit Das
Department of ENT, All India Institute of Medical Sciences, Mangalagiri, Andhra Pradesh, PIN – 522503
India
Source of Support: None, Conflict of Interest: None
CheckDOI: 10.4103/nah.nah_40_22
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