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Year : 2014  |  Volume : 16  |  Issue : 69  |  Page : 89--94

Raynaud's phenomenon among men and women with noise-induced hearing loss in relation to vibration exposure

Hans Pettersson1, Lage Burstr÷m1, Tohr Nilsson2,  
1 Umeň University, Department of Public Health and Clinical Medicine, Occupational and Environmental Medicine, Umeň, Sweden
2 Umeň University, Department of Public Health and Clinical Medicine, Occupational and Environmental Medicine, Umeň; Sundsvall Hospital, Department of Occupational and Environmental Medicine, Sundsvall, Sweden

Correspondence Address:
Hans Pettersson
Umeň University, Occupational and Environmental Medicine, Department of Public Health & Clinical Medicine, SE-901 87 Umeň
Sweden

Abstract

Raynaud's phenomenon is characterized by constriction in blood supply to the fingers causing finger blanching, of white fingers (WF) and is triggered by cold. Earlier studies found that workers using vibrating hand-held tools and who had vibration-induced white fingers (VWF) had an increased risk for hearing loss compared with workers without VWF. This study examined the occurrence of Raynaud's phenomenon among men and women with noise-induced hearing loss in relation to vibration exposure. All 342 participants had a confirmed noise-induced hearing loss medico legally accepted as work-related by AFA Insurance. Each subject answered a questionnaire concerning their health status and the kinds of exposures they had at the time when their hearing loss was first discovered. The questionnaire covered types of exposures, discomforts in the hands or fingers, diseases and medications affecting the blood circulation, the use of alcohol and tobacco and for women, the use of hormones and whether they had been pregnant. The participation rate was 41% (n = 133) with 38% (n = 94) for men and 50% (n = 39) for women. 84 men and 36 women specified if they had Raynaud's phenomenon and also if they had used hand-held vibrating machines. Nearly 41% of them had used hand-held vibrating machines and 18% had used vibrating machines at least 2 h each workday. There were 23 men/6 women with Raynaud's phenomenon. 37% reported WF among those participants who were exposed to hand-arm vibration (HAV) and 15% among those not exposed to HAV. Among the participants with hearing loss with daily use of vibrating hand-held tools more than twice as many reports WF compared with participants that did not use vibrating hand-held tools. This could be interpreted as Raynaud's phenomenon could be associated with an increased risk for noise-induced hearing loss. However, the low participation rate limits the generalization of the results from this study.



How to cite this article:
Pettersson H, Burstr÷m L, Nilsson T. Raynaud's phenomenon among men and women with noise-induced hearing loss in relation to vibration exposure.Noise Health 2014;16:89-94


How to cite this URL:
Pettersson H, Burstr÷m L, Nilsson T. Raynaud's phenomenon among men and women with noise-induced hearing loss in relation to vibration exposure. Noise Health [serial online] 2014 [cited 2020 Nov 26 ];16:89-94
Available from: https://www.noiseandhealth.org/text.asp?2014/16/69/89/132087


Full Text

 Introduction



Hearing loss relates to age and the rate at which hearing is lost increases with exposure to noise. The risk of noise-induced hearing loss can be modified by genetic, chemical, or medical factors. [1],[4] Another interacting factor for the risk of noise-induced hearing loss might be exposure to vibrations. Workers using vibrating hand-held tools are exposed to hazardous levels of noise and to hand-arm vibrations (HAV). Long-term exposure to HAV could cause workers to develop white fingers (WF). WF or Raynaud's phenomenon is characterized by episodic constriction in the blood supply to the fingers causing finger blanching. When the vasospasm is believed to be secondary to vibration it is called vibrations-induced white fingers (VWF). Workers using vibrating hand-held tools who have VWF have an increased risk for noise-induced hearing loss compared with workers without VWF but of similar age and with similar noise exposures. [1],[5],[6],[7],[8],[9]

Episodes of "WF" among those with WF or Raynaud's phenomenon are primarily triggered by cold or emotional stress. [10] Primary Raynaud's phenomenon has no related disease of external identified cause but could have a genetic component. Secondary Raynaud's phenomenon comprises cases of WF that is linked to an underlying disease or is caused by an external factor. VWF is one example of secondary Raynaud's a phenomenon that causes finger blanching as a vasospastic overreaction to cold.

A cross-sectional study by Palmer et al. [11] found an association between finger blanching and hearing loss in people who had never worked with hand-held vibrating tools or in noisy environments. Both men and women in the study had an increased risk of hearing loss if they had finger blanching.

A study based on a postal questionnaire was used to further explore the occurrence of Raynaud's phenomenon among men and women with noise-induced hearing loss in relation to vibration exposure.

 Methods



Participants

The study sample consisted of men and women who had a confirmed noise-induced hearing loss accepted by the insurance company AFA Insurance as work-related and as such had received financial benefits from AFA Insurance in Sweden between 1995 and 2004. AFA Insurance is owned by Swedens labour market parties and they insure employees within the private sector, municipalities and county councils in Sweden. The participants noise-induced hearing loss had been graded from 1% to 15% disability depending on the severity of the noise-induced hearing loss. The definition of 1% disability is a combined mean hearing threshold level at 2000 and 3000 Hz that is equal to or more than 35 dB and also that the combined mean hearing level at 4000 and 6000 Hz is equal to or more than 45 dB. The worker must also have been exposed to noise for at least 10 years at noise exposure levels of more than 85-90 dB (A). For shorter noise exposure durations the noise exposure must have been above 90 dB (A). Hearing loss from impulse noise exposures must have been from noise exposures above 135 dB (C) or 115 dB (A). For 15% disability the hearing threshold is more than 40 dB at 1000 Hz and above 60 dB at 2000 Hz. Also, the combined mean hearing threshold level at 3000, 4000 and 5000 must be above 50 dB. The noise exposure criteria are the same as for 1% disability.

The men and women had to be between the ages of 18 and 55 when their noise-induced hearing loss was confirmed to be included in the study. The men were randomly chosen and 86 men were from the northern region of Sweden, 85 men were from the middle region and 90 men were from the southern region. Because there were only a few women who had a confirmed work-related noise-induced hearing loss, we chose to invite all such women to participate in the study. The study sample consisted of 261 men and 81 women. After excluding participants for whom a current address was unavailable, the final sample consisted of 246 men and 78 women who received the questionnaire. A reminder was sent if the participants had not responded the 1 st time. The questionnaire study was approved by the Regional Board of Ethics for Medical Research in Umeå, Sweden (Dnr 08-151 M).

Questionnaire

The men and women in the study sample were invited to answer a questionnaire covering their health status and the kinds of exposures they had at the time when their noise-induced hearing loss was first discovered. The questionnaire included types of exposures, discomforts in the hands or fingers, illnesses and medications affecting the blood circulation, if they used tobacco (smoked or used snuff) daily or used alcohol weekly and for women, the use of hormones and whether they had been pregnant.

Exposures

If the participants stated that they had used hand-held vibrating machines when they first discovered their noise-induced hearing loss, they were classified as being exposed to HAV. Participants who did not use any hand-held vibrating machines were classified as not exposed to HAV. The participants were then asked to subjectively estimate how many minutes per working day and for how many years they had been working with vibrating machines until they first discovered their hearing impairment. Noise exposure was addressed in the questionnaire by asking the participants to subjectively estimate their daily noise exposure. They were also asked to estimate how many minutes per working day they had a noise level where the participant could not talk to another person 1 m away without raising their voice. The participants were also asked for how many years they had worked in their occupation at the time they discovered they had noise-induced hearing loss. Their cold exposure was estimated by questions on how much of a working day they were outside.

Classification of Raynaud's phenomenon

Participants were classified as having WF based on their response to the question "Do you have white (pale) fingers of the type that appear when exposed to damp and cold weather (see picture)?" If they answered "yes" to this question they were classified as having WF and were asked for how many years they had WF symptoms.

The subsequent classification of WF as primary Raynaud's phenomenon, or secondary Raynaud's phenomenon was based on information on the subject's exposures, injuries and diseases.

Participants were classified as having possible primary Raynaud's phenomenon if they had no HAV exposure and if they had WF with no other apparent cause (WF PR ). Such causes were frostbite on any hand, surgery for carpal tunnel syndrome, hospitalization due to injury, disease or illness with influence on the vascular circulation such as diabetes, hypertension, heart diseases, or rheumatic diseases such as scleroderma, joint or muscle disease, thyroid disease, asthma, bronchitis, migraine, or arm or wrist fractures. Use of medications for migraines, vascular spasms, cardio-vascular disease or hypertension was also categorized as other cause.

For a participant to be classified with possible secondary Raynaud's phenomenon, due to vibration (VWF SR ), the participant had to be exposed to HAV and not fulfilling the criteria of other causes of WF.

Statistics

All statistical analysis was performed with IBM SPSS Statistics version 20 (IBM Corporation. Software Group. Somer. NY. USA). The prevalence of use of hand-held vibrating machines, WF and WF PR and VWF SR were calculated. The noise exposure duration in hours was calculated by multiplying the number of years at the current occupation multiplied by 220 workdays/year and then by the number of minutes per day in a noisy environment. The HAV exposure was calculated by multiplying the number of minutes per day working with hand-held vibrating machines by 220 workdays/year and then by the total number of years working with hand-held vibrating machines. The mean (standard deviation [SD]) ages for all participants who answered the questionnaire were calculated at the time when the participants' noise-induced hearing loss was confirmed. The relative risk (RR) and 95% confidence intervals (95% CI) was calculated for WF among participants who were exposed to HAV were compared with a reference group of participants who were not exposed to HAV. Also, the RR with 95% CI of WF was calculated as the prevalence of WF among those exposed to HAV divided by the prevalence of WF among those not exposed to HAV. The RR of WF among participants in the Northern Region compared with WF in the southern region.

 Results



The participation rate was 41% (n = 133) with 38% (n = 94) for men and 50% (n = 39) for women with noise-induced hearing loss. In the northern, middle and southern regions of Sweden, the participation rates for men were 42%, 39% and 35%, respectively. The mean (SD) age of the participants who answered the questionnaire was 41 (9) years [39 (10) for men and 46 (8) for women] and for those who did not answer the mean age (SD) was 41 (9) years [40 (9) for men and 44 (7) for women]. The average age was almost the same for those men and women who answered or did not answer the questionnaire. The most common occupations among the participants were teachers (n = 15), military personnel (n = 13) and welders (n = 4).

Totally 84 men and 36 women reported information on WF and also specified if they had used hand-held vibrating machines. The most commonly used hand-held vibrating machines were grinders, drillers and screwdrivers. Nearly 41% (n = 49) of the 84 men and 36 women had used hand-held vibrating machines and 18% (n = 21) had used vibrating machines for at least 2 h each workday. Nearly 37% had reported WF among participants exposed to HAV compared with 15% among those who were not exposed to HAV [Table 1].{Table 1}

In our study, there were a total of 23 men and six women who had cases of WF. The mean age, average noise exposure duration and the use of tobacco and alcohol were about the same for those with WF compared with those without WF [Table 2]. There was a RR of 2.4 (95% CI: 1.2-4.6) for WF among participants who were exposed to HAV compared with participants not exposed to HAV.{Table 2}

Among the 29 participants with WF, there were 8 (20%) men with WF who were not exposed to HAV and five had WF PR . Of the men exposed to HAV, there were 15 with WF (34%) and eight of them had VWF SR [Table 3]. Among the women exposed to HAV, three women had WF and among women who were not exposed to HAV two women had WF PR and one had VWF SR [Table 3]. The prevalence of WF among men was 27% compared with 17% among women.{Table 3}

There were four participants not classified as having WF PR but who had WF and were not exposed to HAV. These four participants had either frostbite in the hand (two men), took medication for cardiac or hypertension problems (one man) or had an operation for carpal tunnel syndrome (one woman). Among the participants not classified as having VWF SR , nine had either taken medication for cardiac or hypertension problems or vascular spasms (three men and two women), been hospitalized following an accident (one man), or had frostbite in their hands (two men) and one man had both frostbite in his hands and had been operated on for carpal tunnel syndrome.

The average noise exposure durations until the discovery of noise-induced hearing loss were shorter among men with WF PR and for men with WF and not exposed to HAV compared with men without WF [Table 4]. Participants with or without WF and who were exposed to HAV had about the same noise exposure duration except for women [Table 4]. The average exposure to HAV was about the same for all participants with WF compared to those without WF [Table 4].{Table 4}

There was, on average, less use of hearing protection among participants who were not exposed to HAV compared to those participants who were exposed to HAV when the participants discovered they had hearing loss [Table 4].

 Discussion



The low participant rate limits the possibilities for generalization of the results from this study. Furthermore, all the participants answered a questionnaire about their working conditions and health status. However, the classification of suffering from WF or not was based on information from the questionnaire and this self-reported symptoms has not been verified by medical investigations and could, therefore, include both primary and secondary Raynaud's phenomenon. Moreover, the used question is not specifically related to primary or secondary Raynaud's phenomenon and could have generated false classification of primary Raynaud's phenomenon being a secondary Raynaud's phenomenon.

All men and women in this study had a medico-legally confirmed work-related, noise-induced hearing loss. The participants hearing impairment had been graded from 1% to 15% disability depending on the severity of noise-induced hearing loss. The mean level of disability was about the same for men and women with or without WF who used or did not use hand-held vibrating machines.

Among the participants, 41% had used hand-held vibrating tools and 18% used these types of tools for at least 2 h and each workday. This is higher numbers than the corresponding numbers among all Swedish workers using hand-held were 9% use hand-held vibrating tools for at least 2 h and each workday. [12] The noise exposure from these hand-held vibrating tools could be one explanation for the higher prevalence of noise-induced hearing loss among the participants. One could also speculate that the higher prevalence is due to simulations exposure to HAV. One earlier longitudinal study suggested an increased risk of hearing loss from HAV in a noisy environment, [9] but this conclusion is not supported by other longitudinal studies. [1],[13]

There were 29 cases of WF among the participants in this study. For male construction workers in Sweden exposed to HAV, prevalence of WF of 13.4% has been reported. [14] In the present study there was prevalence of WF of 34% among men with noise-induced hearing loss who were exposed to HAV and the prevalence of VWF SR was 20%. There was a RR of 2.4 (95% CI; 1.2-4.6) for WF among participants who were exposed to HAV compared with participants not exposed to HAV. Earlier longitudinal and cross-sectional studies have found an increased risk of hearing loss among workers with VWF compared to workers without VWF who have similar noise exposure and are of similar age. [1],[5],[6],[7],[8],[9] Therefore, WF from exposure to HAV might be a factor that increases the risk of noise-induced hearing loss.

In this study the prevalence of WF among men with noise-induced hearing loss who were not exposed to HAV was 20% and 13% of the men had WF PR . Among Swedish male office workers not exposed to HAV prevalence of 8.4% for WF has been reported. [14] An earlier cross-sectional study on finger blanching found that there was an increased risk of hearing loss for men and women with finger blanching who were not extensively exposed to HAV or noise. [11] One could therefore speculate if WF is a trigger for noise-induced hearing loss. In the present study men with WF and no exposure to HAV had shorter noise exposure durations until the discovery of hearing loss compared to men without WF and no HAV exposure and to men with or without WF who were exposed to HAV.

Participating women who were both exposed and not exposed to HAV had prevalence of WF of 17%. An earlier cross-sectional study among women in Sweden reported a similar prevalence of Raynaud's phenomenon of 16%. [15]

Pyykkö et al. suggested that the same mechanism that causes restriction of the blood supply to the fingers could also restrict blood supply to the cochlea. HAV could possibly trigger an over-activation of the sympathetic nervous system (SNS) causing a restriction in blood supply to the fingers and cochlea that would lead to finger blanching and ischemia in the cochlea. [5],[16] HAV might reduce the blood supply in the fingers by stimulating the SNS in the fingers causing VWF. [17],[18],[19] Noise exposure might reduce the blood supply to the cochlea. [20],[21],[22],[23] If HAV also reduce the blood circulation in the cochlea then combined noise and HAV exposure might increase the blood circulation in the cochlea and increase the risk for hair cell loss and hearing loss. The function of the SNS in the cochlea is not fully understood. It is believed that the SNS controlled the blood supply to the cochlea. [20],[22],[24] During noise exposure the systolic blood pressure will rise and also the cochlear blood flow, but when the blood pressure is too high then the autonomic nervous system could reduce the cochlear blood flow. [24] The SNS might have a protective effect on the cochlea, but there are studies that also suggest a harmful effect. [25],[26],[27],[28] Palmer et al., [11] found an increased risk of hearing loss among men and women not extensively exposed to HAV or noise and further studies on the mechanism causing a possible increased risk of hearing loss for men and women with finger blanching are recommended.

Among those participant who were exposed to HAV there were 37% who had WF and 63% stated that they used tobacco. Smoking may increase the risk of noise-induced hearing loss and of VWF. [29]

Earlier cross-sectional studies have found that workers using hand-held vibrating tools in tropical regions have a lower prevalence of Raynaud's phenomenon than those in colder regions. [30],[31] A cohort study found that HAV-exposed workers in the northern region of Sweden were at a higher risk of WF than workers in the southern region. [14] The effect of HAV on WF might be increased in cold environments. [14] Among participants in our study there was no increased risk of WF in the northern region compared with the southern region of Sweden.

Other diseases and injuries that could cause peripheral circulation disturbances are frostbite in the hands operations for carpal tunnel syndrome, hospitalization following an accident and taking medication for cardiac or hypertension problems or vascular spasms. These factors could disturb the blood circulation and be a factor for VWF SR that is not from HAV exposure.

There are some possible limitations in this study. The main limitation was that the participant rate was low at 41% with less men (38%) answering than women (50%). The selection of participants for this study could be biased because all participants were selected due to confirmed noise-induced hearing loss. This selection makes it difficult to compare the prevalence of WF in other groups of workers. The participants in the present study stated when they first discovered that they had hearing loss. A problem with discovering hearing loss is that it comes gradually over time. It can be difficult to exactly know when the hearing loss occur. Some of the different groups of workers in the present study might have a clear understanding when the hearing loss occurred. Military personal might have notice at once if they lost hearing when firing a gun. Also, teachers might be more aware of their hearing since it is so important to use when teaching. The participation rate was highest in the northern and lowest in the southern region but still very low. There were 13 military personnel in this study and six had WF. Military personnel often have impulse noise exposure from firing weapons during their work and this could cause hearing problems. Impulse noise exposure causes noise-induced hearing loss from mechanical injury to the hair cells in the cochlea while continuous noise exposure causes metabolic changes that cause hair cell death and noise-induced hearing loss. [23] The suggested possible increased risk of noise-induced hearing loss from HAV or VWF comes from studies on workers exposed to continuous and not impulse noise, thus the military personnel could have affected the results if they had acquired noise-induced hearing loss from impulse noise and not from continuous noise exposure. There could be some participants who developed WF PR who then began working with HAV and were, therefore, classified with VWF SR . It is possible that this study has some misclassification of WF PR and VWF SR even after excluding other causes of WF such as frostbites, carpal tunnel syndrome, hospitalization following an accident and medication for cardiac and hypertension problems or vascular spasms. This study has no information as to when the participants developed frostbite or carpal tunnel syndrome or were hospitalized for an accident. It could have been before or after they developed WF. The noise exposure at the participants' current work was used in this study and we have no information on the level of noise exposure or HAV exposure before the current job. Noise exposure and HAV exposure as minutes per day may vary over time and among machines. The usage of hearing protection is the percentage of noise exposure time during the current job at the time when they discovered noise-induced hearing loss and there is lack of information on earlier use of hearing protectors.

 Conclusion



Among the participants with hearing loss with daily use of vibrating hand-held tools more than twice as many reports WF compared with participants that did not use vibrating hand-held tools. This could be interpreted as Raynaud's phenomenon could be associated with an increased risk for noise-induced hearing loss. However, the low participation rate limits the generalization of the results from this study.

 Acknowledgments



The authors would like to acknowledge the financial support of AFA Insurance (Project 2007-0104). We also thank Michel Normark, Elisabeth Molander and Tezic Kerem for their support in selecting participants and administering the questionnaire.

References

1Pyykkö I, Pekkarinen J, Starck J. Sensory-neural hearing loss during combined noise and vibration exposure. An analysis of risk factors. Int Arch Occup Environ Health 1987;59:439-54.
2Quaranta A, Portalatini P, Henderson D. Temporary and permanent threshold shift: An overview. Scand Audiol Suppl 1998;48:75-86.
3Hodgkinson L, Prasher D. Effects of industrial solvents on hearing and balance: A review. Noise Health 2006;8:114-33.
4Lee CA, Mistry D, Uppal S, Coatesworth AP. Otologic side effects of drugs. J Laryngol Otol 2005;119:267-71.
5Pyykkö I, Starck J, Färkkilä M, Hoikkala M, Korhonen O, Nurminen M. Hand-arm vibration in the aetiology of hearing loss in lumberjacks. Br J Ind Med 1981;38:281-9.
6House RA, Sauvé JT, Jiang D. Noise-induced hearing loss in construction workers being assessed for hand-arm vibration syndrome. Can J Public Health 2010;101:226-9.
7Iki M, Kurumatani N, Satoh M, Matsuura F, Arai T, Ogata A, et al. Hearing of forest workers with vibration-induced white finger: A five-year follow-up. Int Arch Occup Environ Health 1989;61:437-42.
8Miyakita T, Miura H, Futatsuka M. Noise-induced hearing loss in relation to vibration-induced white finger in chain-saw workers. Br J Ind Med 1987;13:32-6.
9Pettersson H, Burström L, Hagberg M, Lundström R, Nilsson T. Noise and hand-arm vibration exposure in relation to the risk of hearing loss. Noise Health 2012;14:159-65.
10Lawson IJ, Burke F, McGeoch K, Nilsson T, Proud G. Part three-Diseases associated with physical agents. Vibrations. In: Baxter PJ, Adams PH, Aw TC, Cockcroft A, Harrington JM, editors. Hunter's Diseases of Occupations. 10 th ed. Sect. 2. Oxford: Oxford University Press; 2010. p. 489-512.
11Palmer KT, Griffin MJ, Syddall HE, Pannett B, Cooper C, Coggon D. Raynaud's phenomenon, vibration induced white finger, and difficulties in hearing. Occup Environ Med 2002;59:640-2.
12SWEA. The Work Environment 2011. Solna: Swedish Work Environment Authority; 2012.
13Starck J, Pekkarinen J, Pyykkö I. Impulse noise and hand-arm vibration in relation to sensory neural hearing loss. Scand J Work Environ Health 1988;14:265-71.
14Burström L, Järvholm B, Nilsson T, Wahlström J. White fingers, cold environment, and vibration - Exposure among Swedish construction workers. Scand J Work Environ Health 2010;36:509-13.
15Leppert J, Aberg H, Ringqvist I, Sörensson S. Raynaud's phenomenon in a female population: Prevalence and association with other conditions. Angiology 1987;38:871-7.
16Pyykko I, Starck J. Vibration syndrome in the etiology of occupational hearing loss. Acta Otolaryngol 1982;386 Suppl: 296-300.
17Stoyneva Z, Lyapina M, Tzvetkov D, Vodenicharov E. Current pathophysiological views on vibration-induced Raynaud's phenomenon. Cardiovasc Res 2003;57:615-24.
18Egan CE, Espie BH, McGrann S, McKenna KM, Allen JA. Acute effects of vibration on peripheral blood flow in healthy subjects. Occup Environ Med 1996;53:663-9.
19Sakakibara H, Yamada S. Vibration syndrome and autonomic nervous system. Cent Eur J Public Health 1995;3 Suppl: 11-4.
20Shi X. Physiopathology of the cochlear microcirculation. Hear Res 2011;282:10-24.
21Quirk WS, Seidman MD. Cochlear vascular changes in response to loud noise. Am J Otol 1995;16:322-5.
22Nakashima T, Naganawa S, Sone M, Tominaga M, Hayashi H, Yamamoto H, et al. Disorders of cochlear blood flow. Brain Res Brain Res Rev 2003;43:17-28.
23Ramsden RT, Saeed SR. Part three-Diseases associated with physical agents. Section one: Noise. In: Baxter PJ, Adams PH, Aw TC, Cockcroft A, Harrington JM, editors. Hunter's Diseases of Occupations. 10 th ed. Oxford: Oxford University Press; 2010. p. 459-86.
24Degoute CS, Preckel MP, Dubreuil C, Banssillon V, Duclaux R. Sympathetic nerve regulation of cochlear blood flow during increases in blood pressure in humans. Eur J Appl Physiol Occup Physiol 1997;75:326-32.
25Hildesheimer M, Henkin Y, Pye A, Heled S, Sahartov E, Shabtai EL, et al. Bilateral superior cervical sympathectomy and noise-induced, permanent threshold shift in guinea pigs. Hear Res 2002;163:46-52.
26Wada T, Takahashi K, Ito Z, Hara A, Takahashi H, Kasakari J. The protective effect of the sympathetic nervous system against acoustic trauma. Auris Nasus Larynx 1999;26:375-82.
27Horner KC, Giraudet F, Lucciano M, Cazals Y. Sympathectomy improves the ear's resistance to acoustic trauma-Could stress render the ear more sensitive? Eur J Neurosci 2001;13:405-8.
28Borg E. Protective value of sympathectomy of the ear in noise. Acta Physiol Scand 1982;115:281-2.
29Starck J, Toppila E, Pyykkö I. Smoking as a risk factor in sensory neural hearing loss among workers exposed to occupational noise. Acta Otolaryngol 1999;119:302-5.
30Futatsuka M, Inaoka T, Ohtsuka R, Sakurai T, Moji K, Igarashi T. Hand-arm vibration in tropical rain forestry workers. Cent Eur J Public Health 1995;3 Suppl: 90-2.
31Yamamoto H, Zheng KC, Ariizumi M. A study of the hand-arm vibration syndrome in Okinawa, a subtropical area of Japan. Ind Health 2002;40:59-62.