Optimum Response of Air-conduction Induced Ocular Vestibular Evoked Myogenic Potential in Drivers.

Context: During driving, the plane of movement in drivers is horizontal. Hence, utricles and vestibulo-ocular reflex (VOR) pathway are over stimulated. The ocular vestibular evoked myogenic potential (oVEMP) is utilized to evaluate the function of utricle and VOR pathway. Aim: This study aimed to assess the function of utricle and VOR using oVEMP among the drivers and compare it with non-professional drivers. Study Design: Comparative cross-sectional study.
Methods: A total of 30 non-professional drivers and 30 professional drivers between ages of 18 and 45 years were evaluated in this study. Drivers with minimum of 5 years driving experience and minimum 3 h of driving per day were included. The oVEMPs were recorded for all the participants using alternating polarity 500 Hz tone bursts stimuli. Statistical Analysis: To calculate mean and standard deviation for all the groups, descriptive statistics was used and for group comparisons Independent t-test, Analysis of Variance, and Mann-Whitney U test were used.
Results: The oVEMP of professional drivers exhibited significant delay in peak latency of N1 and P1 than those of non-professional drivers. Significant delay in P1 latency and reduced peak amplitudes were observed in professional drivers with greater than 10 years of experience on comparison with drivers less than 10 years of experience. Conclusions: Current study opens a new research in understanding the effect of over-stimulation of vestibular system in drivers. Driving for longer period may have effect on latency and amplitude parameters of oVEMP. Copyright:

INTRODUCTION

Drivers are prone to several health problems, as they consume major time in noisy, polluted, and unsafe occupational environment in the form of poor posture, deficient diet, and irregular working hours and duration. Their health is an important issue in occupational health, public health, policy of transport, and employment conditions.[1] In vestibular system, otolith organs (saccule and utricle) help in perceiving both vertical and horizontal linear acceleration movements. During driving, utricle is sensitive as the plane of motion is horizontal.[23] On forward acceleration, inertia leads the otoliths to pause behind the portion of utricle which stimulates the sensory nerves. The brain compares the neural impulses from the otolith organ, eyes, and stretch receptors in neck and interprets the head orientation. This stimulation will lead to Vestibulo-Ocular Reflex (VOR). VOR is reflexliable for image stabilization during movement. This image stabilization is attained by generating the eye movement in the direction opposed to the movement of head and image is preserved on the middle of visual field.[34] Driving requires the interaction of cognitive, sensory, and vestibular systems for postural control, spatial orientation, and navigation.[5]

Vestibular Evoked Myogenic Potentials (VEMPs) are responses from otolith organs evoked by acoustic, vibratory, or galvanic stimuli. For extra ocular muscle response of VEMP (oVEMP) first negative peak (N1) arises at 10 min and first positive peak (P1) at 15 min.[678] The pathway for oVEMP and transverse VOR pathway are alike, they emerge from otolith organ and terminate at the contralateral inferior oblique muscle of eye crossing through superior vestibular nerve, vestibular, and ocular nuclei.[79] There are diverse studies on both normal and disordered population to guarantee that responses of VEMP are from otolith organ.[79]

In drivers, plane of movement is in horizontal plane than vertical or angular acceleration; hence, the utricles are more stimulated and in turn the vestibulo-ocular pathway. This helps the drivers to maintain balance. The oVEMP helps to assess the status of utricle and VOR. There are studies available in healthy individuals as well as several pathological conditions but limited literatures are observed in drivers. Thus, this study aims to assess the functions of utricle and VOR using oVEMP among drivers and compare with non-professional drivers.

METHOD

The study was approved by the Institutional Ethics Committee (1296/IEC/2017). The study design was comparative cross-sectional study and method of sampling was purposive sampling.

Participants

A total of 60 participants were divided into 30 non-professional drivers in Group I and 30 professional drivers in Group II. The participants age range was between 18 and 45 years (Group I: mean: 23.6, SD: 2.92; and Group II: mean: 35.3, SD: 6.22). Further Group II was divided into three subgroups equally as auto drivers, bus drivers, and car drivers (Group IIA, Group IIB, and Group IIC, respectively).

Normal hearing thresholds in frequency range of 500 Hz to 2 KHz were included. The professional drivers with minimum 5 years of experience in driving and minimum 3 h of driving/day were included in Group II, of which 9 participants had less than 10 years of experience and 21 participants had greater than 10 years of experience. Individuals with history or complaint of otological and vestibular complaints were excepted from the study.

Procedure

The purpose of the study was explained and informed consent was obtained from all participants. Normalcy of auditory system and vestibular system was assured through case history, pure tone audiometry, immittance audiometry, and behavioral balance assessment.

Intelligent Hearing System- Evoked Potential Acquisition System USB Jr. Hardware with Smart-EP software, version 5.00 was used to record oVEMP. Insert ear phones (EAR-3A) was used to deliver stimulus. The oVEMP was recorded for all the participants using standard protocol. Surface silver-plated electrodes were positioned 1 cm (active electrode) and 3 cm (reference electrode) lower in the center of each inferior eyelid contralateral to stimulus ear, and ground electrode at the forehead. Single channel recording was done with electrodes positioned on the side contralateral to stimulus side. Study participants were directed to uplift their gaze by 300 to 350 in the center line.[101112] Alternating polarity 500 Hz tone-bursts, boosted by 1 min rise/fall time and 2 min plateau time, presented through insert earphones at 125 dB Sound Pressure Level (SPL). Repetition rate of 5.1 Hz was used. Two hundred sweeps of Electromyography activity was recorded by an analysis time of 50 min.[8] The responses were band-pass filter of 0.1 Hz to 1000 Hz.[13] The responses were multiplied by factor of 50,000 irrespective of the filter. Adequate rest period between recordings was given to avoid involuntary eye blinks and muscle strain. Two replicable recorded waveforms with the above protocol were taken for analysis.

Waveform analysis

The presence of oVEMP peaks N1 and P1 were examined by two experienced audiologists. N1 and P1 peaks were analyzed for latencies, amplitude, N1-P1 complex amplitude, and Asymmetry Ratio (AR).

Statistical analysis

The tabulated values of waveform parameters were statistically analyzed using SPSS software (version 22.0). Descriptive statistics was done to estimate mean and standard deviation (SD). Independent t-test was applied for comparison of Group I and Group II. Analysis of Variance (ANOVA) was used to compare the Group II subgroups. Mann–Whitney U test was applied to compare parameters among the subgroups.

RESULTS

In specific, the study's objective was to identify and compare latency and amplitude related parameters between drivers and non-professional drivers. Within driver group, the subgroups of type of vehicle, years of experience, and duration of driving per day were compared.

Comparison of oVEMP responses between group I and group II

Latency, amplitude, and AR measures were compared between Group I and Group II. From Table 1, we can infer that right and left VEMP and mean value of P1 and N1 latency are longer for Group II. Mean amplitude of P1, N1, N1-P1 complex amplitude, and AR are comparatively large for Group I compared with Group II. Independent sample t-test shows statistically significant difference in P1 latency and N1 latency on right and left VEMP and failed to show significant difference in P1 amplitude, N1 amplitude, peak-to-peak amplitude, and AR.

Table 1

Comparison of Mean (SD) of oVEMP parameters between Group I and Group II

Parameters	Group I (n=30) Mean (SD)	Group II (n=30) Mean (SD)	P
Right VEMP
P1 Latency (ms)	15.6 (.64)	16.4 (.97)	0.00*
N1 Latency (ms)	10.7 (.55)	11.4 (.76)	0.00*
P1 Amplitude (µV)	2.92 (1.41)	2.42 (2.08)	0.28
N1 Amplitude (µV)	2.36 (1.61)	2.13 (2.19)	0.64
N1-P1 Complex amplitude (µV)	5.66 (3.08)	5.54 (4.70)	0.91
Left VEMP
P1 Latency (ms)	15.7 (.68)	16.2 (1.01)	0.04*
N1 Latency (ms)	10.6 (.55)	11.03 (.61)	0.01*
P1 Amplitude (µV)	3.06 (1.23)	2.62 (2.59)	0.41
N1 Amplitude (µV)	2.30 (1.16)	2.19 (2.57)	0.84
N1-P1 Complex amplitude (µV)	5.45 (2.51)	5.28 (4.60)	0.86
Asymmetry ratio	17.3 (7.74)	14.5 (11.04)	0.26

SD: Standard deviation; P: Probability. *Significance <0.05. Statistical Test: Independent t-test

Comparison of oVEMP responses among group II

On comparing latency and amplitude measures of right and left VEMP between the subgroups of drivers, Table 2 shows mean values of latency and amplitude parameters are almost similar for the subgroups of Group II. ANOVA failed to show statistically significant difference in amplitude and latency parameters of right and left VEMP between Group IIA, Group IIB, and Group IIC.

Table 2

Comparison of Mean (SD) of oVEMP parameters among subgroup of drivers

Parameters	Group IIA (n=10) Mean (SD)	Group IIB (n=10) Mean (SD)	Group IIC (n=10) Mean (SD)	P
Right VEMP
P1 Latency (ms)	16.6 (.96)	16.3 (.95)	16.3 (1.06)	0.63
N1 Latency (ms)	11.5 (.83)	11.3 (.70)	11.2 (.80)	0.72
P1 Amplitude (µV)	2.08 (1.92)	2.56 (1.97)	2.61 (2.48)	0.83
N1 Amplitude (µV)	1.51 (1.82)	2.47 (2.32)	2.41 (2.49)	0.56
N1-P1 Complex amplitude (µV)	5.19 (4.82)	5.98 (4.65)	5.47 (4.11)	0.94
Left VEMP
P1 Latency (ms)	16.6 (.96)	16.1 (1.14)	15.9 (.89)	0.25
N1 Latency (ms)	11.3 (.65)	11.1 (.59)	10.7 (.51)	0.13
P1 Amplitude (µV)	2.25 (2.55)	2.39 (1.94)	3.23 (2.28)	0.67
N1 Amplitude (µV)	2.06 (2.36)	1.78 (1.41)	2.74 (3.64)	0.70
N1-P1 Complex amplitude (µV)	5.33 (5.39)	5.35 (4.49)	5.19 (4.37)	0.99
Asymmetry ratio	6.70 (2.72)	16.35 (13.6)	20.3 (9.54)	0.01*

SD: Standard deviation; P: Probability. *Significance <0.05. Statistical Test: ANOVA

Comparison among Group II based on experience of driving

Group II was further divided into drivers with less than 10 years and greater than 10 years of driving experience. On comparison, Table 3 shows mean amplitudes and peak-to-peak amplitude are reduced and mean latency values are comparatively delayed in drivers with more than 10 years of experience. Mann–Whitney U test shows statistically significant difference in P1 latency, P1 amplitude, N1 amplitude, N1-P1 complex amplitude of right VEMP, and P1 amplitude of left VEMP among two groups. There was no statistically significant difference in P1 latency, N1 latency, N1 amplitude, peak-to-peak amplitude of left VEMP, and N1 latency of right VEMP.

Table 3

Comparison of Mean (SD) of oVEMP parameters among Group II based on experience of driving

Parameters	<10 years (n=9) Mean (SD)	>10 years (n=21) Mean (SD)	P
Right VEMP
P1 Latency (ms)	15.8 (1.11)	16.7 (.81)	0.03*
N1 Latency (ms)	11.03 (.85)	11.5 (.70)	0.06
P1 Amplitude (µV)	3.87 (2.71)	1.80 (1.42)	0.01*
N1 Amplitude (µV)	3.50 (3.05)	1.54 (1.44)	0.03*
N1-P1 Complex amplitude (µV)	8.45 (5.31)	4.30 (3.66)	0.02*
Left VEMP
P1 Latency (ms)	15.8 (.97)	16.3 (1.02)	0.22
N1 Latency (ms)	10.9 (.59)	11.1 (.63)	0.22
P1 Amplitude (µV)	3.98 (3.29)	2.05 (2.06)	0.04*
N1 Amplitude (µV)	3.19 (3.64)	1.77 (1.91)	0.14
N1-P1 Complex amplitude (µV)	7.10 (5.07)	5.29 (4.29)	0.12
Asymmetry ratio	14.6 (9.88)	14.4 (11.7)	0.10

SD: Standard deviation; P: Probability. *Significance <0.05. Statistical Test: Mann-Whitney U test

Comparison among Group II based on duration of driving

Group II was also compared for drivers with duration of driving/day as less than and more than 5 h of driving. Table 4 shows that mean latency values are almost similar in both groups and mean amplitude of N1, P1, and peak-to-peak amplitude are comparatively lower in drivers who drive more than 5 h/day. Mann–Whitney U test failed to display statistically significant difference among both groups.

Table 4

Comparison of Mean (SD) of oVEMP parameters among Group II based on duration of driving

Parameters	<5 h (n=12) Mean (SD)	>5 h (n=18) Mean (SD)	P
Right VEMP
P1 Latency (ms)	16.2 (1.07)	16.5 (.92)	0.67
N1 Latency (ms)	11.2 (.79)	11.2 (.66)	0.83
P1 Amplitude (µV)	3.01 (2.74)	2.03 (1.46)	0.26
N1 Amplitude (µV)	2.85 (3.01)	1.65 (1.33)	0.64
N1-P1 Complex amplitude (µV)	6.77 (5.87)	4.73 (3.70)	0.50
Left VEMP
P1 Latency (ms)	16.1 (1.04)	16.2 (1.03)	0.72
N1 Latency (ms)	11.1 (.73)	11.1 (.54)	0.73
P1 Amplitude (µV)	3.29 (3.22)	2.18 (2.06)	0.20
N1 Amplitude (µV)	2.70 (3.27)	1.86 (2.01)	0.23
N1-P1 Complex amplitude (µV)	6.19 (5.20)	4.69 (4.21)	0.61
Asymmetry ratio	13.1 (9.23)	15.3 (12.3)	0.60

SD: Standard deviation; P: Probability. *Significance <0.05. Statistical Test: Mann-Whitney U test

DISCUSSION

In this study, response of oVEMP was present in 100% of both Group I and Group II. However, on comparing responses of non-professional and professional drivers, latency of N1 and P1 occurred significantly delayed in professional drivers than non-professional drivers. These differences could be observed only in latency and not in the amplitude of the peaks. We could also observe that mean and SD value of AR were poorer in professional drivers when compared with non-professional drivers. This did not show any statistically significant difference between the groups. The value of AR for both the group are within the normal range, 14 ± 10%,[14] 19.3 ± 8.0%,[8] 18.6 ± 11.6%.[10] Our study results are similar to the previous findings where oVEMP was studied in 17 bus drivers and was reported that mean latency is longer and complex amplitude is higher in bus drivers. But there was no statistically significant difference in latency and amplitude parameters of oVEMP among bus drivers and control group, which suggested possibility of spontaneous recovery by central compensation, thus no prominent vestibular symptoms.[15] In vestibular evoked potentials, peripheral pathologies are better detected with amplitude values. In contrast, latency is related to central vestibular pathway.[3] Thus, delay in latency may be attributed to delay in vestibule ocular reflex in drivers.

On comparing the parameters between subgroups of drivers, no significant difference observed between auto, car, and bus drivers. Duration of driving also did not have effect on the parameters among professional drivers. But interestingly, on comparing the parameters based on experience of driving, delayed latency and lower amplitude was obtained in drivers with greater than 10 years of experience. Statistically significant variance in right VEMP P1 latency, amplitude and left VEMP P1 amplitude were observed. This support that when years of experience in driving are increased there might be over-stimulation of the vestibular system and which leads to decreased latencies. Also significant reduction in amplitude parameters in drivers with greater than 10 years of experience can be justified by poor spatial orientation in elderly drivers as supported by a literature where, spatial orientation is difficult to maintain for elderly drivers during a way finding task.[16] When compared with other professionals where vestibular stimulation is involved, drivers are prone to have effect of noise on their vestibular system. Literature review suggests delayed latency and lower amplitude in individuals with Noise Induced Hearing Loss (NIHL).[17] In our study, delayed latency and lower amplitude may attribute to effect of noise in the drivers. There is no difference within drivers based on the vehicle they drive as same linear acceleration is involved in all types of drivers.

CONCLUSION

Current study opens research in understanding the effect of overstimulation of vestibular system in drivers. The oVEMP may give an indication of any difference in function of utricle in drivers. The oVEMP parameters significantly differ in professional drivers compared with non-professional drivers and there is impact of experience of driving in amplitude and latency parameters. Driving for longer period has effect on oVEMP parameters and it emphasizes the importance of vestibular assessment in medical selection for driver population. Further, the study can be replicated on a larger sample size, homogenous group of drivers, and drivers with longer driving duration. The effect of noise on oVEMP in drivers is unknown, which must be ruled out in further studies.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient (s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.