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Year : 2005  |  Volume : 7  |  Issue : 29  |  Page : 31-39
Effect of exposure to a mixture of solvents and noise on hearing and balance in aircraft maintenance workers

1 Ear Institute, University College, London, United Kingdom
2 Roehampton University, London, United Kingdom

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  Abstract 

Aircraft maintenance workers are exposed to a mixture of solvents in the presence of intermittent noise. For this study these workers exposed to solvent mix and noise, were compared with mill workers exposed to noise alone, printed circuit board operatives exposed to solvents alone and those exposed to none who acted as controls. Tympanometry, acoustic reflex thresholds, transient and distortion product otoacoustic emissions, auditory brainstem potentials, nystagmography, and posturography were examined. There was a significant effect on pure tone thresholds for both noise and solvents+noise. The distortion product otoacoustic emissions declined with frequency and exhibited lower DP amplitude with noise compared to solvents and noise group. The transient emissions showed a similar effect. Over 32% of subjects with solvent and noise exposure had abnormalities of the auditory brainstem responses in terms of interwave interval prolongation. The mean acoustic reflex thresholds showed a pattern of differences which differentiate noise from solvent and noise groups. The contralateral pathway appears to be differentially affected by solvent exposure. 32% of subjects in the solvents and noise group had an abnormal posturographic finding. In the solvents and noise group 74% had abnormalities of saccades, 56% of pursuit and 45% of optokinetic nystagmus.

Keywords: Noise, solvents, aircraft maintenance workers, balance, hearing

How to cite this article:
Prasher D, Al-Hajjaj H, Aylott S, Aksentijevic A. Effect of exposure to a mixture of solvents and noise on hearing and balance in aircraft maintenance workers . Noise Health 2005;7:31-9

How to cite this URL:
Prasher D, Al-Hajjaj H, Aylott S, Aksentijevic A. Effect of exposure to a mixture of solvents and noise on hearing and balance in aircraft maintenance workers . Noise Health [serial online] 2005 [cited 2020 May 26];7:31-9. Available from: http://www.noiseandhealth.org/text.asp?2005/7/29/31/31876

  Introduction Top


Toxic nature of solvents is well recognised and in particular their acute and chronic effects on the central nervous system. Dizziness is a commonly reported feature of the effects but has not been extensively studied. Furthermore the effect of solvent exposure on hearing has, for some time been masked by the concomitant presence of noise in the workplace where solvent exposure occurred. It is beginning to emerge that not only are solvents ototoxic but in the presence of noise can compound the effect on hearing. Recent studies examining the effect of solvent exposure alone and in combination with noise have begun to show synergistic effects on hearing.

Makitie et al [1] have shown in rats that exposure to styrene at 600 ppm (for 12 h/ day 5 days/ week for 4 weeks) caused a 3dB hearing loss at 8kHz and exposure to industrial noise at 100-105 dB caused a loss between 2-9 dB but an exposure to a combination of styrene and noise caused a flat loss between 23-27 dB. Furthermore the lower concentrations at 300 and 100 ppm only induced a hearing loss when combined with noise. Clear syngersitic effect was observed above a critical level.

Sliwinska-Kowalska et al [2] showed that the relative risk for hearing loss was increased to 4.4 in workers exposed to a mixture of solvents within the exposure limits. Sliwinska-Kowalska et al [3] examined styrene exposed workers and showed that there was almost a 4-fold increase in odds of developing a hearing loss related to styrene exposure and that the odds ratios were 2-3 times higher when styrene and noise co-exposure existed compared to each acting alone.

Aircraft maintenance workers are exposed to a mixture of solvents, which include stripping agents to remove polymer based coatings and residues which contain dimethylacetamide whilst some other cleaning agents include trichloroethane which has been phased out of use by greenhouse emission protocols but has been used as a degreaser and cleaner of metals and plastics. They are also exposed to adhesives, sealants, adhesion promoters, isocyanate, zinc chromate and glycol ester paints in addition to exposure to hydrocarbon fuels such as jet fuel, and diesel which are a complex mixture including benzene, n-hexane, toluene, xylenes, naphthalene. It has been pointed out that while hydrocarbon fuel exposures occur typically below permissible exposure limits for their constituent chemicals, it is unknown whether additive or synergistic interactions may result in unpredicted neurotoxicity. [4]

During occupational use of solvents, absorption into the body is through breathing in solvent vapour into the lungs and via contact with skin. It is estimated that breathing occurs around 12 times per minute with five to eight litres of air exchanged every minute. During exercise or heavy manual work with deep inspiration, the number of litres exchanged may increase to over a 100 per minute, thereby increasing the solvent absorption through this route. Once absorbed the body metabolism transforms the solvent into water soluble components that are excreted in urine. The clinical effects depend on the amount absorbed and the duration of exposure. For many chemicals an exposure limit is given for an average eight-hour industrial exposure to which workers may be repeatedly exposed without adverse effects. The key elements necessary for exposure/effect relationship evaluation are the absorption, biotransformation and toxicity. But if a combination of exposures occurs then the interaction between the toxic elements also have to be considered.

A summary of the observed effects of some specific solvents are given in [Table - 1].

It has been shown that low-level chronic exposure to jet fuel vapour in aircraft maintenance personnel can lead to increased postural sway. [5] A significant association between solvents (benzene, toluene, xylene) and increased postural sway was observed. Hearing loss and brainstem response latency prolongation were reported by Chen et al [6] in aircraft maintenance workers and firemen working at airport facilities. The fact that both peripheral and central auditory pathway damage was reported by the authors implies that the effect was not only due to aircraft noise but additional exposures to solvents which may be responsible for the more central effects observed. Exposure to noise alone rarely affects the central conduction time in the auditory pathway as indicated by the prolongation of the brainstem response latencies.


  Materials and Methods Top


Four groups of people were examined, aircraft maintenance workers exposed to solvents and noise, and mill workers exposed to noise alone, printed circuit board operatives exposed to solvents only and those exposed to neither acted as controls. The number of subjects tested in each group with their mean age are shown in [Table - 2].

Procedure

The subjects were interviewed to complete a questionnaire relating to their personal health and work history and exposure. Each subject had height, weight (to determine body mass index) and blood pressure measurements taken. Prior to audiometric testing, an otoscopic examination of the ears was undertaken to rule out any presence of excessive wax or any perforation or other abnormalities. The following tests were conducted on each subject.

Tympanometry and acoustic reflex measurements

Tympanometry which measures the compliance of the tympanic membrane with change in pressure was conducted to exclude any middle ear dysfunction, prior to the pure tone audiometry. Subjects with conductive hearing loss were excluded from the analysis. An automatic GSI 38 tympanometer was used for the purpose. Ipsilateral and contralateral reflex thresholds were also recorded at 500Hz, 1kHz and 2kHz. Acoustic reflexes at 4kHz were not considered as they are very frequently absent.

Pure tone audiometry

Air conduction audiometry was conducted in a sound proof booth using the Hughson-Westlake automatic assessment with a Castle Excalibur GA1001 PC based audiometer. The patient was clearly instructed to respond to the lowest audible sound with a button press and the threshold recorded automatically by the computerised system. Any significant departures from the pattern expected over the frequency range were re-examined manually to correct for any discrepancies. The results were stored on computer for further analysis.

Transient otoacoustic emissions

Transient emissions were evoked by 80dBSPL click stimuli using the Biologic Scout Sport OAE system with a test protocol covering 1.kHz to 6kHz range. The subject had the probe placed in the ear canal with a disposable ear tip of appropriate size to secure a good seal. The programme performs an in-the-ear calibration and adjusts the stimulus intensity level to the protocol target value. If the calibration procedure is successful, the programme automatically moves to the averaging phase. The TOAE reproducibility and response amplitude with respect to the noise floor value (TE-NF) were analysed for each frequency band namely 1, 1.5, 2, 3, 4kHz and across the whole range1.2-3.4kHz.

Both ears were tested and the results analysed separately and summed across the two ears.

Distortion product otoacoustic emissions

The distortion product emissions were recorded using the standard ototoxic protocol covering the 1.5kHz to 10kHz range. The lower frequency primary tone intensity was set at 65dB and higher frequency intensity level was set at 55dB. The ratio of F1 to F2 was set at 1.22. The reproducibility and the response level with respect to the noise floor (DP-NF) were analysed by frequency band across the range. There were four points per octave. The test was repeated for each ear to check for reliability.

Auditory evoked potentials

The auditory brainstem response was recorded using the Biologic Navigator system. Disposable electrodes were attached to the mastoids, chin and the forehead at the hairline after cleansing the skin and slight abrasion to reduce contact impedance of the electrodes.

Ipsilateral and contra-lateral responses to stimulation of each ear separately were recorded on two occasions with an alternating click stimulus presented at 80dB. The responses were recorded over a time window of 10ms and analysed for Waves I, III and V latencies. Responses were considered outside the normal range if the inter-wave interval between I-V exceeded 4.4 ms or if either wave I or III were present in the absence of Wave V.

Nystagmography

Video-nystagmography was performed using the Chartr VNG system. The subject was seated 4 feet away from the light bar, the range sensor provided an indication of the distance between the light bar and the subject. The video goggles were positioned on the subject's eyes whilst the subject looked straight at the centre of the light bar. The video image was viewed and the subject's pupils were aligned prior to the test session by adjusting the camera and the mirrors. The brightness and contrast adjustments were made to acquire a satisfactory image. The saccades were recorded to horizontal random position of a small circular light on the bar. Light target moved randomly every 1.25 seconds over a 34 degree arc. Horizontal eye position and target position were recorded. The gaze test consisted of the measurement of horizontal eye position with light target centred, 30 degrees to right and left in the light and without vision (cover on goggles). The tracking or pursuit test examined horizontal eye position with the light target moving sinusoidally at frequencies of 0.2,0.3,0.4,0.5,0.6 and 0.7 Hz over a 34 degree arc. The optokinetic test consisted of a multiple light target moving right/leftward at 20 to 60 degrees per second.

Posturography

The posturographic measurements were made using the NeuroCom system. The subject stood on a platform initially with eyes open then closed, first on a firm surface then on a 4 inch foam. Each test had three trials and the mean sway velocity as well as the centre of gravity alignment were recorded and compared with control data for an age range within the decade of the individual under test.

Noise measurements

Measurements of the noise level were taken from aircraft maintenance workers over short durations were taken on 9 occasions over three days over a period of a year. Both LAeq and LApeak levels including levels over the spectral range were recorded.[Table - 9]

As can be seen from the table above, the range of levels varies on different occasions depending on whether the aircraft engines are running at the time. The relatively quieter periods occur when the engine is turned off.

The major difference in the noise exposure between the two groups is the fact that the aircraft maintenance workers are exposed to a wide range of levels with relative quiet periods whereas the noise alone group are in constant noise during their work time.

Solvent exposure

Aircraft maintenance workers were exposed to a complex mixture of solvents which occur in stripping agents, cleaning fluids, paints, and jet fuel. These are a mixture of benzene, n-hexane, toluene, xylenes, naphthalene, trichloroethane, dimethylacetamide, etc. The exposure was estimated from years of work and the type of work undertaken in that time. It was not possible to take urine or blood measurements of the workers.


  Results Top


Pure tone audiometry

Predictably, the main effect of frequency was highly significant [*F (3.2, 1039.8) = 377.5, P <0.001; MSE = 148.1]. [Figure - 1] below shows that a highly significant main effect of exposure group [F (1, 320) = 77.4, P <0.001; MSE = 1516.5] was due to the fact that the noise group's thresholds were higher at all frequencies. The divergence of the two groups' thresholds at the upper end of the frequency range was reflected in a significant interaction [F (6, 1920) = 21.1, P <0.001; MSE = 80.2].

The pure tone audiometric thresholds for the groups tested are shown in [Table - 3].

The number of subjects in the controls and solvents only group were relatively small compared to the other groups which meant that meaningful threshold comparisons across all groups were not possible. In addition the mean threshold (across frequencies and ears) for the noise group was significantly higher at 35.3 dB compared with the solvent and noise combined group with a mean threshold of 20.8dB. This is largely due to the difference in the level and duration of noise exposure across these two groups.

It is worth noting that if the mean PTA across the frequency range is considered and a cut-off of 20dB is applied, 5.6% of the controls fall outside compared with 33.3% of the solvent + noise group although both groups have very similar mean PTA thresholds.

Unfortunately it was not possible to match the noise exposure levels in the two groups.

PTA: Ear differences

There were significant differences [Table - 4] between ears at 500Hz and 1kHz for the controls but not at any other frequency, for noise 500Hz, 2kHz, 3kHz, 4kHz, and 8kHz showed significant differences but none were observed at 1kHz or 6kHz. For solvents and noise group, significant differences were observed at 1kHz, 4kHz and 6kHz but not at 500Hz, 2kHz, 3kHz and at 8kHz.

Distortion product emissions

The distortion amplitude [Figure - 2] declined with frequency, as confirmed by a highly significant main effect of frequency [*F (6.5, 1244.1) = 22.8, P <0.001; MSE = 50]. In addition, the noise group exhibited lower DP amplitude compared to solvent + noise group [F (1, 191) = 28.7, P <0.001; MSE = 301]. However, the downward trend was reversed at the high end of the frequency range. Although, both groups showed recovery from 6 kHz upwards, the reversal was particularly prominent for the noise group at 7.7 kHz. This was reflected in a highly significant interaction [F (11, 2101) = 12.6, P < 0.001; MSE = 29.6].

Transient otoacoustic emissions (TOAE) S/N ratio

A mixed-design ANOVA was used to examine the between-group difference on the TOAE amplitude shown in

[Figure - 3]. As in the previous analyses, the noise group was more affected [F (1, 191) = 24, p < .001; MSE = 44.3]. The highly significant main effect of frequency [*F (2.9, 555) = 18.4, P <0.001; MSE = 157.3] reflected the peak at 1.5 kHz. The interaction failed to reach significance.

TOAE reproducibility

The main effect of frequency was highly significant [*F (2.8, 526.9) = 27.7, P <0.001; MSE = 460.7], as was the main effect of exposure group [F (1, 190) = 21.4, P <0.001; MSE = 2256.7]. The reproducibility for the noise group was lower by approximately 20% as shown in [Figure - 4].

Auditory brainstem responses

A significant number of subjects (32.4%) had abnormally prolonged inter-wave interval (Wave I-V) in the solvent and noise group. However, a comparison of the mean latencies for all waves across the groups failed to reveal any differences.

Acoustic reflex thresholds

Mean Ipsilateral and contralateral reflex thresholds for the right and left ears for Noise and Solvents and Noise Groups are shown in [Table - 5] below. The only significant differences in the mean reflex thresholds were observed in the contralateral recordings from the right ear at 500Hz, 1kHz and 2kHz.

No significant difference was observed between right ipsilateral and left ipsilateral or between right contralateral and left contralateral reflex thresholds in any group. But as shown in [Table - 6] below the correlation between ipsilateral and contralateral reflex thresholds for the right ear and left ear were significant for almost all frequencies for the control and noise groups but not significant for the left ear for the solvent and noise group. Furthermore the threshold difference between ipsilateral and contralateral reflex thresholds

[Table - 7] is only found to be significant for the control and noise groups and not for the solvent+noise group. These findings indicate a pattern of differences in reflex measurements which differentiate between noise and solvent+noise group. The contralateral pathway appears to be differentially affected by solvent exposure. The number of subjects in the solvent +noise group that had an absent reflex ipsilaterally was 25.1% compared with 41.2% contralaterally.

Posturography

Subjects were examined under four conditions namely on firm surface with eyes open and closed and on foam with eyes open and closed. Mean centre of gravity sway velocity outside the age-adjusted normal values constituted an abnormality. 32.3% of subjects in the solvents and noise group had an abnormal posturographic finding.

Video-nystagmography

Saccades

Three aspects of saccadic activity were analysed in the solvent and noise group. These were the accuracy of reaching target position, latency to reach target and the velocity of movement to target. Abnormality in any one area or a combination was considered abnormal if outside the age adjusted normal control values. It can be seen from [Table - 8] that 74% of subjects in the solvents and noise group had abnormal saccadic activity.

Gaze

Any nystagmic activity in the centre, left or right gaze were recorded and analysed. Only 6% of subjects in the solvents and noise group showed any significant nystagmus in any position of gaze.

Pursuit

Eye movements to following of a target stimulus were recorded and analysed with respect to velocity, gain and any asymmetry in the response. Abnormality in any aspect constituted a response abnormality. 56% of subjects in the solvents and noise group showed an abnormality.

OKN

Optokinetic nystagmus was recorded at three speeds of target rotation. The velocity and any asymmetry beyond age-adjusted normal values were considered abnormal under any speed of target. 45% of the subjects in the solvents and noise group showed some abnormal OKN function.


  Discussion Top


There have been a number of limitations to this study, the four groups originally envisaged of the same size proved to be an impossible task due to the great reluctance of the industrial concerns to take part in this study. Much time was wasted in many meetings trying to persuade the printing, manufacturing and other industries to take part in the study. Both management and unions as well as the European associations of major industrial groups were approached and several presentations were made by the NoiseChem group to encourage these groups to participate but none were willing for fear of litigation from their members.

Thus it proved that we were left with unmatched groups at the end of the study as the time for the study was limited and no extensions were permitted. Within the constraints of the groups and the absence of a match for noise exposure level between the noise alone and solvent+noise group only limited conclusions may be drawn from this study. However many interesting and valid observations have been possible and with grouping of data across labs within the NoiseChem group further conclusions may be drawn from the studies conducted by the group.

It is clear from the data that despite the control and solvents group having a similar mean age and mean PTA thresholds, 33.3% of the solvent group had a hearing loss (mean PTA threshold across the frequency range greater than 20dB) compared to 5.6% of the controls. The noise alone group had a greater exposure to noise than the solvent and noise, and this was confirmed by the greater mean loss of 35.3dB compared to solvents+noise group of 20.8 dB. The distribution of the loss across the frequency range was very similar for the two groups showing a significant shift in threshold from 2kHz with a maximum at 6kHz in both groups. As it was not possible to match the noise exposure in these two groups, it is not possible to comment further on the combined exposure although the solvents group without noise exposure did show a greater mean loss than the controls and solvents +Noise groups. However the significant difference in the number of subjects across groups requires caution in further interpretation.

Further analysis of the comparison across the ears revealed interesting differences between groups. There were no significant differences between the right and left ears for the noise alone group at 1kHz and 6kHz but the differences were significant for the Solvents+Noise group at these frequencies. The significance of the asymmetry due to the combined exposure is not clear but may be associated with more central effects on the auditory pathway.

The comparison of the noise alone and solvents +noise groups for distortion product emissions reveals that both groups show a decline in DP amplitude with frequency as would be expected from the hearing threshold deterioration in the high frequencies in both groups. The mean level of DP amplitude was significantly worse for the noise alone group particularly at around 4kHz. It is interesting to note that although both groups show a significant recovery of emission amplitude from 6kHz, this was much more pronounced for the noise than with solvents+noise group.

The transient emissions amplitude and reproducibility showed a similar pattern of decline with frequency and being worse for the noise alone group.

It is clear that the emissions reflect the relative noise exposure in the two groups. The solvent +noise group with periods of relative quiet fared better than those with continuous high noise exposure levels.

Ipsilateral and contralateral acoustic reflex thresholds for both the right and left ears were compared across the two groups. A significant difference in the mean acoustic reflex threshold between noise alone and solvents+noise group was only observed in the contralateral reflex thresholds in the right ear. This asymmetric threshold difference also adds to the asymmetry observed in the pure tone audiometric thresholds at certain frequencies. The ipsilateral/contralateral difference in reflex threshold was significant at most frequencies for both ears for the control and noise alone group but not at any frequency for either ear for the solvents+noise group. This significant observation shows an absence of any increase in reflex thresholds contralaterally for the solvents+noise group. Normally there is 5-7 dB increase in contralateral reflex threshold which is absent in the solvents+noise group. As there is no difference in the ipsilateral reflex thresholds across groups, the lower contralateral reflex thresholds in the solvents+noise groups is the key difference between groups.

Another interesting finding is that the correlation between ipsi and contralateral reflexes in the control and noise groups is highly significant especially for the left ear but is not at all significant for the solvents+noise group especially for the left ear.

These findings of altered contralateral reflexes imply an effect of solvents on the central crossed auditory pathway as no deficiencies were observed in the noise alone group or the controls.

The effect of solvent exposure on the central auditory nervous system is further supported by abnormalities of the auditory brainstem response in the solvent+noise group. In this group prolongation of central conduction time for the auditory brainstem response wave I-V interval, presence of Waves I and III in the absence of Wave V, and unrepeatable responses were observed although no significant differences in the mean latencies of the Waves was noted across groups.

Furthermore, effects on the balance system were observed with posturographic recordings showing an abnormality of postural sway in 32.3% of subjects in the solvents+noise group.

These investigations have shown that there are clear observable effects on the audio-vestibular system of solvent+noise exposure. The hearing impairment shows a similar pattern to that observed with noise alone and is dependent on the level and duration of noise exposure. It is clear that the relative quiet periods in the exposure for the solvent+noise group compared with the continuous high level of noise in the noise alone group provided some protection in that the mean loss in the noise alone group was significantly worse than that for the solvent+noise group. However, in the solvent group changes to the central auditory pathway were observed with contralateral reflex thresholds showing group differences. Auditory brainstem response abnormalities in the solvent+noise group also indicated that the central auditory pathway was affected in this group. This is in agreement with a study of airport employees by Chen et al [6] who reported prolongation in central conduction time in intervals I-V and III-V.

Postural sway abnormalities were detected in about a third of the aircraft maintenance workers. This is in agreement with Smith et al [5] who showed a significant association between solvent exposure and increased postural sway response. Clearly, the solvents have a subtle influence on the vestibular proprioceptive interaction.

Occupational exposures to solvents are common particularly mixtures of solvents. Odour detection for some solvents can occur at very low concentrations. The threshold limit value or TLV is an average eight-hour exposure to which workers may be repeatedly exposed without harmful effects. These values are defined for specific solvents but not for any combinations as found in work environments. The determination of limit values considers effects on animals and where possible humans and are based on adverse effects in terms of carcinogenicity or toxic effects on the central nervous system. The effect on hearing or balance systems is rarely considered in the setting of limit values. Solvent exposure has been implicated in specific sensory impairment as for example colour perception or hearing damage but again there is very little research which has examined damage of the senses in the same individual worker. Styrene, toluene, n-hexane and carbon disulfide have been shown to affect both colour vision and hearing.

The effect of a mixture of solvents on the auditory system appears to occur both at the end organ level as well as in the nervous pathway.


  Acknowledgements Top


This study was conducted under NoiseChem QLRT-2000-00293 project funded by the EC.

 
  References Top

1.Mδkitie AA, Pirlova U, Pyykk φ I, Sakakibara H, Riihimδki V, Ylikoski J. The ototoxic interaction of styrene and noise. Hear Res 2003;179:9-20.  Back to cited text no. 1    
2.Sliwinska-Kowalska M, Zamyslowska-Szmytke E, Szymczak W, Kotylo P, Fiszer M, Dudarewicz A, et al. Hearing Loss among workers exposed to moderate concentrations of solvents. Scand J Work Environ Health 2001;27:335-42.  Back to cited text no. 2    
3.Sliwinska-Kowalska M, Zamyslowska-Szmytke E, Szymczak W, Kotylo P, Fiszer M, Wesolowski W, et al. Ototoxic effects of occupational exposure to styrene and co-exposure to styrene and noise. J Ocupp Environ Med 2003;45:15-24.  Back to cited text no. 3    
4.Ritchie GD, Still KR, Alexander WK, Nordholm AF, Wilson CL, Rossi J 3rd, et al. A review of the neurotoxicity risk of selected hydrocarbon fuels. J Toxicol Environ Health B Crit Rev 2001;4:223-312.  Back to cited text no. 4    
5.Smith LB, Bhattacharya A, Lemasters G, Succop P, Puhala E 2nd, Medvedovic M, et al. Effect of Chronic low-level exposure to jet fuel on postural balance of US Air Force personnel. J Occup Environ Med 1997;39:623-32.  Back to cited text no. 5    
6.Chen TJ, Chiang HC, Chen SS. Effects of Aircraft noise on hearing and auditory pathway function of airport employees. J Occup Med 1992;34:613-9.  Back to cited text no. 6    

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Correspondence Address:
Deepak Prasher
Professor of Audiology, Ear Institute, University College London, 330 Gray's Inn Road, London WC1X 8EE
United Kingdom
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1463-1741.31876

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    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]
 
 
    Tables

  [Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5], [Table - 6], [Table - 7], [Table - 8], [Table - 9]

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