Effect of personal music system use on sacculocollic reflex assessed by cervical vestibular-evoked myogenic potential: A preliminary investigation.

Listening to music through a portable personal music system (PMS) is a growing trend, especially among the youth. The preferred listening level in such kinds of PMS has been reported to cross the safe levels and its impact on the auditory system was demonstrated in several previous investigations. Owing to the commonality in several aspects between the auditory and the vestibular systems, it appears likely that the deleterious effects of PMS use could also be impinging on the vestibular system, which has never been investigated. The present study therefore, aimed at evaluating the effects of PMS use on the sacculocollic reflex assessed by the cervical vestibular-evoked myogenic potential (cVEMP) technique. Thirty-two regular PMS users and 32 nonregular PMS users underwent cVEMP testing using alternating polarity 500 Hz tone bursts. The results revealed no significant group difference in latencies and interaural asymmetry ratio. However, the cVEMP was significantly reduced in the group of individuals in whom the diffused field equivalent sound pressure levels (SPLs) were above the damage risk criteria (DRC) compared to those with diffused field equivalent SPLs below it (P< 0.01). Therefore, the use of PMS at high levels of volume controls could be deleterious to the vestibular well-being of an individual.

Introduction

Listening to music has always been one of the most effective ways of recreation irrespective of age, gender, or ethnicity. Not long ago, people flocked to concerts or gathered around the television or radio sets to listen to music. However, after the advent of the portable personal music system (PMS) that combines headphones or earphones with the music players, the exposure to music seems to be rising rapidly. A large majority of today's youth can be seen using the portable PMS. The most common forms of portable music players these days are digital audio players such as the MP3 players or iPoDs.

The scientific community is abuzz with the publications regarding the measured output from these digital audio players when combined with earphones or headphones and the consequences that could follow. In one such report published online, the free field equivalent sound pressure level (SPL) at maximum volume control setting of a PMS, when clubbed with the insert earphones, was reported to be in the range of 91-121 dBA.[1] Another study reported the output levels vary across manufacturers and the style of headphones although smaller headphones (ear bud or insert type) were shown to generally produce higher SPLs compared to larger ones (supraaural or circumaural type) for the same volume control setting.[123] They also reported peak SPLs to be in excess of 130 dB SPL in a few earphone and personal music player combinations.

The maximum permissible SPL that is believed to spare the ear of the deleterious effects of loud sound exposure has been fixed at 85 dBA for exposure duration of 8 h per day.[456] From this baseline, a time-intensity trade of 3 dB per doubling of duration is permissible without any damaging consequences (i.e., if the exposure intensity increases by 3 dB, the exposure duration must be halved to ensure against noise-induced damage to the ears).[46] Exposure levels beyond these values have been reported to produce damaging consequences to hearing health.[789] Elevated workplace noise or other noise has been shown to not only cause hearing impairment, hypertension, ischemic heart disease, annoyance, and/or sleep disturbances but also changes in the immune system and occasionally birth defects.[10] Music, being a specific variety of noise, could be thought to produce similar effects as other noises.

The trend in concurrent research is inclined toward demonstrating the adverse effects of PMS use on hearing function. Though overexposure to loud sound through PMS is also under the purview of noise-induced hearing loss (NIHL), the current research trend is to use the term “music-induced hearing loss.”[11] The cochlear function was shown to be compromised irrespective of the cause being noise overexposure or abusive use of the PMS.[78912131415] This was proved not only through the incidence of 4-6 kHz notch in the audiogram but also through reports of decreased amplitudes of otoacoustic emissions (OAEs).[12131415]

The above studies have proved that prolonged noise or music exposure can have an adverse effect on auditory function. Owing to common genesis from the otic placodes of ectoderm, anatomical proximity, similar cellular ultrastructures, common arterial supply, similar principles of mechanical transduction, and sharing of a continuous membranous labyrinth with the cochlea, the vestibular end organs could be assumed to be adversely affected by the vibrations produced by noise.[1617181920] Therefore, it is likely that the high levels of noise that cause NIHL will also damage the vestibular end organs,[21] the saccule being one of them. This possibility is further enhanced by the inherent capability of the saccule to withstand much lesser force than the Reissner's membrane in a number of lower animals (guinea pig, frog, and fish) and also in human beings.[22] While the tensile strength of 36.33 gf/mm2 and breakage pressure of 64 mmHg were reported for the saccular membrane in human beings, the tensile strength and breakage pressure for Reissner's membrane were observed to be 49.75 gf/mm2 and 106.7-142.2 mmHg, respectively.[22] This indicates that a similar level of stimulation by sound is likely to be far more damaging to the saccule than the cochlea.

The animal studies show remarkable similitude between the damage patterns observed within the cochlea and the saccule after exposure to intense noise.[2123] The illustrative human studies have also reported increased body sway,[2425] higher prevalence of nystagmus,[26] existence of higher percentage of complaints related to the vestibular system,[2728] and reduced responses of vestibular-evoked myogenic potential (VEMP)[29] in individuals exposed to high levels of noise. In fact, one of the studies has shown the onset of vestibular symptoms well before the evidence of clinically detectable hearing loss in individuals exposed to industrial noise.[28] Direct mechanical destruction and metabolic decomposition with subsequent degeneration of sensory elements are believed to be the main mechanisms producing destruction of the vestibular end organs by continuous exposure to loud sounds.[30]

Balance is a function maintained by three systems, namely, the vestibular, somatosensory, and visual systems. Even within the vestibular system, there is a further subdivision of work. The otolith organs are responsible for maintaining balance during linear acceleration and tilt movement with respect to gravity, whereas semicircular canals aid in the sustenance of balance during angular acceleration.[3132] Several tests have been found useful in assessment of the functionality of these specific vestibular units. These include caloric irrigations and rotatory chair tests by means of electronystagmography (ENG) or videonystagmography (VNG), posturography, and VEMP.

VEMPs are important clinical tools which are used to determine the function of the otolith organs and vestibular nerves. They can be recorded from several muscles of the body. When recorded from the sternocleidomastoid muscle, these potentials are termed as cervical vestibular-evoked myogenic potential (cVEMP). Cervical VEMPs assess the saccular function, which is mediated by the sacculocollic pathway.[3334] Studies on cVEMP have shown the stimulabilty of the saccule by sound levels exceeding 90 dB SPL.[353637] In people exposed to loud noises, the exposure levels constantly exceed 90 dB SPL. This would indicate constant saccular stimulation in these individuals, which could render the saccule susceptible to the deleterious effects of noise. Several investigators have explored the effects of occupational noise on cVEMP responses and found reduced response amplitudes and/or prolonged latencies in the individuals exposed to occupational noises compared to the controls.[383940] The constant noise over stimulation is likely to cause damage to the saccular membrane even before affecting the Reissner's membrane because the tensile strength as well as breakage point of the former are lesser than that of the latter, which predisposes the former to noise-induced damage even before high levels of noise can cause a rupture of the later.[22]

Similar to noise, loud music produced from the PMS could be also thought to create saccular overstimulation and result in saccular damage. While there is sizeable research evidence showing the effect of music overexposure from the PMS on the auditory system,[12131415] there is paucity of research data supporting a similar impact on the saccule and sacculocollic pathway. So there is a need to investigate the effects of music overexposure, especially from the PMS on the vestibular system. Therefore, the present study aimed at investigating the effects of PMS use on the functioning of the saccule and sacculocollic pathway by means of recording cVEMP from these individuals.

Methods

Participants

The study involved two groups of subjects in the age range of 15-30 years who were the participants. Hereafter, the group of regular music listeners will be called the RML group and the group of participants who are not using the PMS regularly as the non-RML group. Each group consisted of 32 participants, each of whom were nominated to one of the two groups based on the fulfillment of certain criteria (mentioned below in subsequent paragraphs) after obtaining informed written consent for their participation in the study on a nonpayment basis. The method involved in the study was approved as a part of the postgraduate dissertation.

The RML group consisted of regular users of PMS for more than 1 h per day for a period of at least 2 years at a volume control setting in excess of 60% of the maximum possible in a PMS. This criteria was similar to the one adopted in the previous studies on the effect of PMS use on the auditory system.[21541] The degree of hearing loss was not considered for exclusion as the degree of sensorineural hearing impairment has been demonstrated to produce no impact on cVEMP.[33344243] Further, the presence of retrocochlear pathologies, neuromuscular problems, and autoimmune diseases were ruled out by a qualified and experienced medical practitioner.

The selection of the participants in the non-RML group was based on the fulfillment of the criteria of PMS use for less than 1 h per day with volume control setting at or below 60% of the maximum possible for a music system. A similar criterion was employed for selection of the participants in the control group by some of the previous studies, which checked the effect of PMS on the auditory system.[21541] In addition to the above, the participants had their auditory thresholds within 15 dB HL across the audiometric range of frequencies and uncomfortable loudness level (UCL) in excess of 100 dB HL to allow discomfort-free cVEMP recordings. Lack of positive history of conductive hearing loss, retrocochlear pathology, vestibular pathologies, neuromuscular problems, and autoimmune diseases was ensured through evaluations by a qualified and experienced medical practitioner. The presence of transient-evoked otoacoustic emission (TEOAE) responses with signal to noise ratio (SNR) of +6 dB and response reproducibility beyond 80% were also considered for the selection criteria.

The participants of both groups were instructed to avoid the use of vestibulotoxic drugs and muscle relaxants at least 48 h before cVEMP acquisition. In case stopping these drugs was not feasible, they were requested to inform about the intake of the same. Such participants were not considered for the study.

Procedure

The participants were evaluated in two stages after the fulfillment of the subject selection criteria. The first stage included the probe microphone measurement of the output SPL of the PMSs. The second stage accommodated recording of the cVEMP. Both these stages were performed inside well-illuminated, air-conditioned, and sound-treated rooms with ambient noise levels well within the permissible limits.[44]

Measurement of output sound pressure level

The measurements in this stage were obtained only from the participants of the RML group owing to nonuse of the PMS by several of the non-RML group participants. The output SPL of PMS was measured using probe microphone system FONIX 7000 (Frye Electronics, Tigard, USA). The loudspeaker was placed at an angle of 45° and at a distance of 30 cm from the participant. The standard reference microphone (default with the FONIX 7000 system) was placed above the pinna before inserting the probe tube into the ear canal at an insertion depth of 28 mm from the tip of the tube to the intertragal notch. After leveling the system, the real-ear unaided response was measured for an output of a composite signal at 65 dB SPL across the octave and midoctave frequencies from 200 Hz to 8,000 Hz. Further, without changing the probe tube position, the earphones of the PMS were placed in the ear. The volume control of the PMS was adjusted to the level that the participants had set when using the system in the outside environment. Upon obtaining the curve stabilization, the system was stopped and SPLs were measured across the above mentioned frequencies.

The measured output SPLs were converted to equivalent diffused field SPL by subtracting the transfer function of the open ear. The transfer function for the open ear was obtained by calculating the difference between reference location at the opening of the ear canal and probe microphone SPL near the eardrum for sweep frequency tone presented at 65 dB SPL. The output SPLs were converted into dBA values by adding A-weighted adjustment values.[45] The overall dBA was then calculated by logarithmically adding the dBA values at each frequency. This transformation was done to compare the output of the PMS to the damage risk criteria (DRC) proposed for occupational noise exposure owing to the lack of existing standards for hazardous music level exposure. For these calculations, the same procedure was followed as described previously.[15] The 8-h equivalent A-weighted noise exposure level was calculated using the formula given below:

Leq8h = LT + 10 log10 (T/8)

where Leq8h is the 8-h equivalent continuous noise exposure, “LT” is the level of exposure for the period “T,” and “T” is the exposure time in hours (average music listening duration in hours per day).

Recording of cervical vestibular-evoked myogenic potentials

The Bio-logic Navigator Pro Auditory-Evoked Potential System (Natus Medical Incorporated, Illinois, USA) version 7.2.1 was used to acquire cVEMP from all the participants. The participants were seated in a comfortable chair in an upright position. The recording sites were cleaned with a commercially available abrasive gel Nuprep (Weaver and Company, Colrado, USA) to obtain acceptable electrode impedances. The gold plated electrodes were placed using adequate amount of commercially available conductive paste Ten20 (Weaver and Company, Colrado, USA) and secured in place with surgical tape. The inverting (negative) electrode was placed at the sternoclavicular junction, the noninverting (positive) electrode at the upper one-third of the sternocleidomastoid muscle and the ground electrode on the forehead. This electrode placement was similar to previous studies.[4647484950] Absolute impedance and interelectrode impedance were maintained below 5 kΩ and 2 kΩ, respectively. The participants were instructed to turn their heads away from the side of stimulation in order to tense the sternocleidomastoid muscle (SCM). cVEMP normalization was applied on the recorded responses in order to control the effect of variable muscle tension on the cVEMP responses, as this was found to be an efficient technique for controlling the variability in cVEMP amplitude.[5152] The VEMP module of the Bio-logic Navigator Pro Auditory-Evoked Potential System has a prestimulus rectification function as a default. The system averages the prestimulus electromyography (EMG) level for a duration of 20 ms. The obtained value is used to divide the poststimulus EMG at each of the 256 recorded points along the cVEMP waveform. This results in another waveform, which is called the EMG normalized waveform. Alternating polarity tone bursts of 500 Hz were ramped with 2-ms rise/fall time and 1-ms plateau time, as found most appropriate for recording cVEMP.[50] The stimuli were presented at 125 dB SPL at a repetition rate of 5.1/s. Analysis window was set to 74 ms, which included 20 ms prestimulus (base line) recording. The responses were averaged across 200 sweeps after being band-pass filtered between 10 Hz and 1,500 Hz and multiplied by a factor of 5,000. The artifact rejection was switched off in order to avoid unnecessary rejection of the inherently large muscle potentials produced for the task involved in recording cVEMP.

Analysis

The EMG-normalized waveforms were analyzed by two independent experienced audiologists to check for interjudge reliability and to arrive at the appropriate marking of peaks. The first positive (P1) and first negative (N1) peaks were marked and the individual latencies and peak-to-peak amplitude were obtained. In addition, the interaural difference ratio (IAD), also called asymmetry ratio (AR) or interaural amplitude ratio (IAAR), was calculated using the standard formula given by Li, Howlden, and Tomlinson (1999).[53] As per this, the IAAR is calculated by dividing the absolute difference in the peak-to-peak amplitude between the two-side cVEMP by their sum and multiplying the obtained value by 100 to obtain the percentage IAAR.

The interjudge reliability for peak identification and marking was evaluated using Chronbach's alpha test and Pearson's correlation analysis, which revealed excellent reliability and correlation between the judges (α ≥ 0.9; r ≥ 0.9). Following this, the markings of only one of the judges was used for further statistical analysis using Statistical Package for the Social Sciences (SPSS) software version 17 (International Business Machines Corporation, New York, USA). In addition to the descriptive statistics, one-way repeated measures analysis of variance (ANOVA) with group as between subject factor was done to compare the ears and the groups on each of these parameters except IAAR. For the comparison of IAAR between the groups, one-way ANOVA was used. Pearson's correlation analysis was used to examine the correlation between the field equivalent dBA and the peak-to-peak amplitude.

Results

cVEMPs were recorded from all the subjects in both the groups. Figure 1 shows the individual and grand averaged cVEMP waveforms recorded from non-RML and RML groups. The responses were present bilaterally in all the individuals of both the groups, which resulted in a response rate of 100% in the present study.

Figure 1

The individual and grand averaged cervical vestibular evoked myogenic potential responses obtained from the RML group (top panels) and non-RML group (bottom panels)

The cervical VEMP waveforms were analyzed for individual peak latencies, peak-to-peak amplitude, and interaural asymmetry ratio. The obtained values from all the individuals of both the groups were subjected to descriptive statistics in order to obtain mean and standard deviation for the above mentioned parameters. Table 1 shows the mean and standard deviation of various cVEMP parameters obtained from the participants of both the groups of the present study.

Table 1

The mean and standard deviation of cervical vestibular-evoked myogenic potential response parameters for regular music listeners and nonregular music listeners

Parameters	Non-RML group		RML group
	Right	Left	Right	Left
P1 latency (in ms)	13.52(1.27)	13.75(1.22)	13.63(1.41)	13.87(1.63)
N1 latency (in ms)	22.55(1.92)	22.80(1.80)	22.84(2.45)	22.67(2.50)
Peak-to-peak amplitude (in μV)	29.78(14.41)	29.12(10.32)	29.66(16.22)	30.58(11.53)
IAAR (in %)	12.59 (9.74)		16.35 (11.72)

Non-RML = Nonregular music listeners, RML= Regular music listeners, IAAR = Interaural asymmetry ratio, The numerical values within brackets represent standard deviations

The results of one-way repeated measures ANOVA with group as a between-subjects factor revealed no significant main effect of ear [F (1, 62) = 3. 10, P > 0.05] and group [F (1, 62) = 0. 11, P > 0.05] on P1 latency. There was also no significant interaction between ear and group for P1 latency [F (1, 62) = 0.002, P > 0.05]. Similarly, there was neither a significant main effect of ear [F (1, 62) = 0.05, P > 0.05] and group [F (1, 62) = 0.87, P > 0.05] nor a significant interaction between ear and group [F (1, 62) = 0.02, P > 0.05] for N1 latency. The comparison for peak-to-peak amplitudes also revealed a similar trend toward the latencies with no significant main effect of ear [F (1, 62) = 0.01, P > 0.05] and group [F (1, 62) = 0.04, P > 0.05] and no significant interaction between ear and group [F (1, 62) = 0.40, P > 0.05]. The comparison between the groups using one-way ANOVA also revealed no significant main effect of group on IAAR [F (1, 62) = 1.94, P > 0.05]. This prompted a further division of the subjects in the RML group into two subgroups based on the DRC (above and below the DRC), similar to the one used in one of the previous studies.[15] This was done in order to examine whether the subjects who crossed the DRC produced similar/dissimilar results on cVEMP response parameters compared to those below DRC. The Leq dBA for 8 h of each participant in the RML group has been shown in Figure 2.

Figure 2

The free field equivalent sound pressure levels (LeqdBA for 8 h) of the subjects in regular music listeners group. The horizontal dotted line represent the DRC level recommended by the National Institute for Occupational Safety and Health (1998), Indian Ministry of Environments and Forest (2000), and International Standard Organization (1999)[456]

Out of the 32 subjects in the RML group, 12 subjects were found to exceed the DRC of 85 dBA for 8 h per day as per the national and international standards in this respect.[456] These 12 participants were subcategorized as the ARML group. The subjects exposed to music levels below the DRC in the RML group were subcategorized as the BRML group. This caused unequal group sizes with the BRML group having 18 subjects and the non-RML having 32 subjects. In order to equate the groups regarding size, 12 subjects were randomly selected from the BRML and the non-RML groups each. Further group comparisons were done between non-RML, ARML, and BRML groups using 12 subjects in each group. Figure 3 shows the cVEMP responses from the non-RML, ARML, and BRML groups.

Figure 3

The individual and grand averaged cervical vestibularevoked myogenic potential responses from all the three groups. The top most panels show responses from the nonregular music listener group, middle panels from the subgroup within the regular music listeners group who had exposure levels exceeding the DRC (ARML group), and the bottommost panels represent recordings from the regular music listeners who were exposed to sound levels below the damage risk criteria (BRML group)

The cervical VEMP waveforms were analyzed for individual peak latencies, peak-to-peak amplitude, and interaural asymmetry ratio across the three groups. The obtained values from all the individuals of non-RML, ARML, and BRML were subjected to descriptive statistics in order to obtain mean and standard deviation for the abovementioned parameters. Table 2 shows the mean and standard deviation of various cVEMP parameters obtained from the participants of non-RML, ARML, and BRML group.

Table 2

The mean and standard deviation of cervical vestibular-evoked myogenic potential response parameters for nonregular music listeners, regular music listeners with Leq 8 h above damage risk criteria, and regular music listeners Leq 8 h below damage disk criteria

Parameters	Non-RML group		BRML group		ARML group
	Left	Right	Left	Right	Left	Right
P1 latency (in ms)	14.16(1.52)	13.98(1.09)	13.69(1.42)	13.44(0.94)	14.22(1.53)	14.03(1.43)
N1 latency (in ms)	23.22(2.11)	22.87(2.39)	22.01(2.18)	22.47(1.83)	23.23(2.05)	23.63(2.58)
Peak-to-peak amplitude (in μV)	29.46(8.60)	32.80(13.50)	35.61(12.90)	34.99(8.19)	23.28(12.11)	18.39(9.23)
IAAR (in %)	12.30 (8.74)		11.13 (8.47)		21.71 (15.09)

Non-RML = Nonregular music listeners, ARML = Regular music listeners with free field equivalent sound pressure level (LeqdBA for 8 h) above damage risk criteria, BRML = Regular music listeners with LeqdBA for 8 h below damage disk criteria, IAAR = Interaural asymmetry ratio, The numerical values within the brackets represent standard deviation

One-way repeated measures ANOVA with group as a between-subjects factor was administered in order to examine the within-group and between-groups differences for latencies and peak-to-peak amplitude of cVEMP. The results revealed no significant main effect of ear [F (1, 33) = 1.19, P > 0.05] and group [F (1, 33) = 0.77, P > 0.05] on P1 latency. There was also no significant interaction between ear and group for P1 latency [F (2, 33) = 0.01, P > 0.05]. Similarly, there was neither a significant main effect of ear [F (1, 33) = 0.58, P > 0.05] and group [F (1, 33) = 0.99, P > 0.05] nor a significant interaction between ear and group [F (2, 33) = 1.34, P > 0.05] for N1 latency. The comparison for peak-to-peak amplitudes however, revealed a significant main effect of group [F (1, 33) = 6.67, P < 0.05] but not of ear [F (1, 33) = 0.04, P > 0.05]. There was also no significant interaction between ear and group [F (2, 33) = 2.54, P > 0.05]. The Bonferroni adjusted multiple comparisons for pairwise comparisons between the groups revealed significantly lower peak-to-peak amplitude in the ARML group compared to the BRML (P < 0.01) and non-RML (P < 0.05) groups. In terms of the IAAR, the results of one-way ANOVA revealed no significant main effect of group on IAAR [F (2, 33) = 3.21, P > 0.05].

Since peak-to-peak amplitude was the only parameter of cVEMP that was significantly different between the groups, the relationship between peak-to-peak amplitude of cVEMP and the free field equivalent dBA levels for 8 h was investigated using Pearson's correlation analysis in the RML group. The results revealed a significant negative correlation between the dBA values and peak-to-peak amplitude [r = − 0.293, P < 0.05]. Figure 4 shows the scatter plot between Leq8h dBA and peak-to-peak cVEMP amplitude in the RML group.

Figure 4

Scatter plot depicting the relationship between the free field equivalent dBA values and peak-to-peak amplitude of cVEMP. The diagonal solid line represents the regression curve

Discussion

In the present study, cVEMPs were recorded from 64 participants (32 in the RML group and 32 in the non-RML group). Responses were present bilaterally in all the subjects of both the groups, which meant 100% response prevalence. The response rate of the non-RML group, which comprised healthy participants with no vestibular pathology was similar to the response rate for healthy individual in the literature (88-100%).[3454] In the present study, the response rate for the RML group was also 100%. Although the response rate was 100% in the RML group, it may not necessarily indicate a normal sacculocollic pathway. It can only indicate that the sacculocollic pathway is functional. The 100% response rate bilaterally might have been due to the bilateral nature of the effect of loud music exposure as most of the people listen to music through binaural earphones.

The results of the present study showed no difference in latency of P1 and N1 between the two groups. Even upon further subdivision of the RML group into ARML and BRML on the basis of DRC, the results revealed a lack of significant difference in latencies between the groups. The present study is the first to have looked at the effects of listening to music through PMS on the functioning of the sacculocollic pathway and hence, there are no studies with which to compare the findings of present study. However, excessively loud music is also a kind of noise and the effects of noise on the sacculocollic pathway functioning has been well-explored.[383940]

Kumar et al. reported significantly prolonged P1 and N1 latencies in individuals exposed to occupational noise at levels exceeding the DRC (>90 dBA used in their study) compared to the group of healthy individuals.[39] Other studies also reported similar prolongation of absolute peak latencies of cVEMP in individuals exposed to hazardous levels of occupational noise.[3840] Thus, the findings of the present study are in dissonance with those reported previously on the effects of noise on the sacculocollic pathway functioning. The differences between the existing studies mentioned above and the present study could be attributed to the differences in the extent and duration of noise exposure. Kumar et al. reported a significant positive correlation between the degree of hearing loss and cVEMP latencies in the group of individuals who had NIHL after being exposed to occupational noise for 10 years.[39] This shows the existence of a positive relationship between the cochlear and the sacculocollic pathway. In the present study, none of the participants in the RML group exhibited a significant hearing loss at any of the frequencies. This indicates a smaller extent of damage in the audiovestibular periphery.

The damage due to noise exposure has been reported to generally start from the inner ear and progress to the neurons at a much later stage, as has been shown for the auditory system.[5556] In terms of cVEMP, the latencies have only been reported to be affected (prolonged) for disorders involving the nervous system such as multiple sclerosis and vestibular schwannoma.[575859] The pathologies that affect only the saccule but not the nerve, such as Meniere's disease and BPPV, have been shown to produce normal-like latencies.[4960] Since the present study has shown normal-like latencies in both the subgroups (ARML and BRML), the inference from the above studies would suggest the lack of neuronal involvement in the subjects used in the present study. Therefore, it appears that damage caused by music overexposure in the present study was limited to the saccule and there was no impact of it on the inferior vestibular nerve fibers that supply the saccule.

The peak-to-peak amplitude in the ARML was significantly reduced compared to non-RML group. Although there are no studies on the effect of PMS use on the cVEMP, there seems to be an agreement between the findings of the present study and those reported on the effect of occupational noise (music also being a type of noise) exposure on the cVEMP.[383940] Evidences in guinea pigs after long-term or continuous noise exposure also showed detrimental effects on the cervical VEMP.[61] However, these results were in disagreement with those reported in a study on the effect of short-term disco music exposure (128 dB for 3 h) on the amplitude of the cVEMP.[25] They proposed that the possible reason behind such a finding was the irritative involvement of the macular receptor following the acoustical stress. The irritative phase of any pathology usually has been shown to last for a few hours to a few days.[6263] However, the recruits of the ARML group in the present study had minimum exposure duration of 2 years and the irritative phase could not have been present in them.

The results of the IAAR of the present study revealed no significant difference between the ARML, BRML, and non-RML groups although IAAR is an amplitude related parameter and the ARML group demonstrated significantly smaller amplitude of cVEMP than the other two groups. There are no studies exploring the effect of regular music listening through the PMS on the cVEMP. However, as stated earlier music is also a form of noise and hence, the findings could be compared against the studies on the effect of occupational noise exposure on cVEMP. While Kumar et al. did not explore the effects of occupational noise exposure on IAAR, Akin et al. reported normal IAAR (<40%) in about 70% of the participants with NIHL, even though the hearing loss was asymmetrical in several of these participants.[3940] Thus, the findings of the present study are in consonance with those of Akin et al.[40] The reason behind the lack of significant asymmetry in cVEMP between the ears could be hidden in the kind of independent variable (which was music in the present study) being looked into. Listening to music through earphones/headphones mostly involves the use of both ears at the same level. Since the exposure to the causative factor is bilaterally symmetrical, its impact is likely to be symmetrical as well. Further support for this comes from studies on the effect of PMS use on the cochlear function that reported bilaterally symmetrical elevations of threshold and reduction of the amplitude of OAEs.[6465]

The findings of correlation analysis showed a significant negative correlation between dBA and peak-to-peak amplitude. The studies on the effects of PMS use on the auditory system have shown a positive correlation with thresholds and negative correlation with the amplitude of OAEs.[15] The findings of the present study are therefore, in agreement with the findings related to the auditory system. This indicates that there is a negative impact of the PMS use on the saccule's functioning and the impact is exacerbated as the exposure level of music from the PMS increases.

In terms of the complaint of vestibular disturbances, none of the participants of the RML (ARML and BRML) as well as the non-RML groups revealed any problem related to vestibular deficits. They also did not report any difficulty in the activities of daily living, which were informally assessed. This was in contrast to the results of cVEMP, which showed the presence saccular deficits (reduced amplitude) in the ARML group. This might be attributed to the bilateral nature of the deficit as well as gradual nature of the damage, which would have allowed the central compensation to occur. It might also be caused by the more subtle nature of the pathology, which is evidenced by diminished cVEMP responses rather than complete absence of cVEMP. It means that though the saccular function was compromised, it was probably not sufficient to have produced the symptoms. In fact, a combination of the above three factors might have caused lack of vestibular symptoms in the participants of the ARML group despite significantly reduced amplitudes of the saccular response (cVEMP).

The literature on the anatomical predisposition of the saccular membrane to the damaging effects of noise has shown that this (saccular membrane) is likely to be damaged even before the high levels of noise can create a Reissner's membrane rupture owing to lesser tensile strength and breakage pressure of the formal than the later.[22] This would make for an interesting comparison between the starting point of damage to the saccule and that of Reissner's membrane. However, since OAEs were not acquired from the RML group in the present study, such a comparison could not be done. Future studies may look to explore this aspect.

Conclusion

The results of the present study demonstrated the deleterious effects of PMS use at higher volume controls on the sacculocollic reflex. While the damage was found to reflect a nonneural involvement in these individuals, prolonged exposure to loud music could produce results similar to that of occupational noise exposure. Hence, the younger generation, which was the age group involved in the present study need to resort to listening to music through PMS at levels below 60% of the maximum allowable volume control in their system in order to prevent damage to the saccule and the sacculocollic reflex pathway.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.