INTRODUCTION
Noise is an unpleasant sound which has many detrimental effects on physical and mental well-being. Noise-induced hearing loss (NIHL) and acoustic trauma are one of the common effects of long-standing noise exposure or sudden acoustic trauma. The effect of noise exposure on the auditory pathway is well established, and there are clear guidelines on safe exposure levels. Approximately 12% of the global population are at risk from NIHL.[1] The risk of NIHL increases in the setting of occupational noise exposure, and approximately 68% of traffic policemen are affected by some degree of hearing loss due to prolonged exposure to traffic noise.[2]
NOISE AND THE VESTIBULAR SYSTEM
Noise can affect the vestibular system. Various hypotheses have been made regarding the mechanism of noise-induced vestibular dysfunction. Acoustic trauma can cause contusion of the labyrinth and the acoustic energy delivered although the round window membrane can cause direct injury to the vestibular epithelium. Animal model studies have shown damage to the saccule and the utricle on sudden noise exposure.[3] The histological changes after continuous noise exposure revealed changes in the macula with separation of the sensory epithelium along with significant stromal cell apoptosis.[4] There is also a possibility of metabolic degeneration of the sensory elements of the vestibular apparatus in response to loud sound.[5] Animal studies have demonstrated that oxidative stress due to low-frequency noise impairs the calcium homeostasis in the endolymphatic system which is crucial for balance physiology.[6] The vestibular apparatus has also demonstrated a capacity to recover following damage due to acute exposure. Although there are enough evidence from animal studies on the harmful effect of noise on balance physiology, there is a paucity of literature on the outcome of repeated noise exposures on the balance physiology of humans. Such chronic noise exposure is a very common scenario in certain occupational settings and is an area that demands further research. Noise-induced vestibular dysfunction has not received as much attention as the NIHL probably because of lack of acute clinical manifestations. Sudden asymmetric vestibular dysfunction usually presents clinically as unilateral vestibular de-afferentiation. However, chronic noise exposure in occupational settings can affect symmetrically and is of gradual onset. This can allow vestibular compensation to take place which may mask vestibular symptoms. Further, there are no diagnostic parameters or guidelines at present to attribute isolated vestibular dysfunction to chronic noise exposure. Therefore, there is a high probability of the condition remaining undiagnosed until there is an onset of NIHL. The balance function is of paramount importance in daily life, and the implications of balance dysfunction in certain professions such as traffic policemen and automobile drivers cannot be overemphasized. Studies have demonstrated levels of noise above the permissible limits at traffic intersections in various cities across India.[7,8] As such traffic police personnel are one of the most vulnerable to chronic noise exposure considering the amount of time spent in noisy intersections.
IMPACT OF VESTIBULAR DYSFUNCTION IN DAILY LIFE
Vestibular dysfunction adversely affects the quality of life. Acute vestibular dysfunction may be incapacitating but usually recovers within a period of 2–3 weeks once adequate compensation takes place. Chronic vestibular dysfunction is not as dramatic at presentation but significantly affects the quality of life. The patient is at a higher risk of fall in such situations, and it affects the person’s ability to perform the daily tasks independently and thus also affects the economic, social, and mental stability of the individual. Further vestibular dysfunction in certain professionals such as traffic policemen, automobile divers, and air force pilots can be dangerous to self and others.
VESTIBULAR DYSFUNCTION IN NOISE: THE EVIDENCE SO FAR
Various studies have been undertaken on noise-induced vestibular dysfunction. Many such studies have evaluated vestibular function in the presence of NIHL. There has been evidence of decrease in vestibular end-organ response in the noise-exposed group as measured by electrophysiological tests.[9,10] Isolated noise-induced vestibular dysfunction in the absence of NIHL remains an unexplored entity and probably an underdiagnosed one also. The presentation of chronic vestibular dysfunction is often vague, and in the absence of NIHL, the exposure history to noise is rarely probed.
It is known that the saccule and the utricle are susceptible to noise-induced damage and so it may be worthwhile to study the effect of the latency and the amplitude of vestibular evoked myogenic potential (VEMP) in such chronic noise exposure. Emara and Gabr found an increase in the latency of P13 and N23 in cervical VEMP and an absence of ocular VEMP responses in chronic noise exposure but with normal hearing threshold.[11] This can be a potential indicator of subclinical vestibular abnormality and it needs to be seen if similar findings are replicated in the setting of various occupational noise exposures. The lateral semicircular canal is also affected in low-frequency noise exposure and as such it will be interesting to study the effects of the bithermal caloric tests in such patients. Few studies have demonstrated a decrease in the gain in the vestibulo-ocular reflex from the lateral semicircular canals with the video head impulse test.[3]
Postural stability is a complex phenomenon which depends on the functional integrity of the vestibular system, the ocular system, and the proprioception and is intricately coordinated by the higher functions mediated by the cerebellum and the cortex. Noise is known to increase annoyance and impair cognitive and psychomotor function.[12,13] As such its effect on postural stability in long run needs to be seen, especially in the setting of chronic occupational exposure.
THE KNOWLEDGE GAP AND SCOPE FOR FURTHER RESEARCH
The rapid pace of urbanization has led to an increase in noise levels in cities across the world. The effects of noise pollution on general health are well known and frequently encountered. NIHL is also commonly encountered in routine otolaryngology practice. Human balance physiology is an interesting area, and there has been a gradual increase in the number of patients attending with disorders of equilibrium and balance. However, noise is usually not counted among the common etiologies causing balance dysfunction.[11] Our knowledge is limited at present regarding safe sound exposure for the balance function. We do not have a clear-cut definition or criteria for the diagnosis of noise-induced vestibular dysfunction. We do not know as of now whether chronic noise exposure predisposes to the early onset of age-related vestibulopathy. There are no definite markers to identify vestibular dysfunction at an early stage. The literature has highlighted the role of oxidative stress secondary to noise exposure in animal studies. It remains to be seen whether similar mechanism exists in the human inner ear and whether antioxidant plays any role in the reversal or progression of the disease process. The temporal association of noise and vestibular function needs to be studied so that it can be assessed whether the dysfunction if any is temporary or permanent.
CONCLUSION
Vestibular physiology has been an evolving area over the past few decades, and the concept of management of various vestibular disorders has undergone significant changes over time. There has been a paradigm shift in management protocols from that of vestibular suppression to early vestibular rehabilitation in various vestibular disorders. The effect of long-term noise exposure on balance function needs to be explored in setting of occupational noise exposure as it can have an immense effect on the safety and efficacy of various professionals exposed to such noisy environment. It is imperative to understand the response of the vestibular system to noise at the molecular levels so that therapeutic measures can be adopted. At the same time, it is important to identify vestibular dysfunction at an early stage clinically so that further exposure can be prevented and permanent damage is averted.
Financial support and sponsorship
Dr. Soumyajit Das has received an extramural grant for a Research Project from the DHR, ICMR, Government of India, under the Grant in Aid Scheme of DHR (File no: R.11012/01/2020-HR).
Conflicts of interest
There are no conflicts of interest.
Acknowledgment
We acknowledge the role of the Department of Health Research (DHR), Indian Council of Medical Research (ICMR), in funding the project.
REFERENCES
1. Le TN, Straatman LV, Lea J, Westerberg B. Current insights in
noise-induced
hearing loss:A literature review of the underlying mechanism, pathophysiology, asymmetry, and management options. J Otolaryngol Head Neck Surg 2017;46:41.
2. Tandel BN, Macwan JE. Road traffic
noise exposure and hearing impairment among traffic policemen in Surat, Western India. J Inst Eng India Ser A 2017;98:101–5.
3. Stewart CE, Holt AG, Altschuler RA, Cacace AT, Hall CD, Murnane OD, et al. Effects of
noise exposure on the vestibular system:A systematic review. Front Neurol 2020;11:593919.
4. Akdogan O, Selcuk A, Take G, Erdoğan D, Dere H. Continuous or intermittent
noise exposure, does it cause vestibular damage?An experimental study. Auris Nasus Larynx 2009;36:2–6.
5. Raghunath G, Suting LB, Maruthy S. Vestibular symptoms in factory workers subjected to
noise for a long period. Int J Occup Environ Med 2012;3:136–44.
6. Tamura H, Ohgami N, Yajima I, Iida M, Ohgami K, Fujii N, et al. Chronic exposure to low frequency
noise at moderate levels causes impaired balance in mice. PLoS One 2012;7:e39807.
7. Bhosale BJ, Late A, Nalawade PM, Chavan SP, Mule MB. Studies on assessment of traffic
noise level in Aurangabad city, India.
Noise Health 2010;12:195–8.
8. Kundu Chowdhury A, Debsarkar A, Chakrabarty S. Critical assessment of day time traffic
noise level at curbside open-air microenvironment of Kolkata City, India. J Environ Health Sci Eng 2015;13:65.
9. Shupak A, Bar-El E, Podoshin L, Spitzer O, Gordon CR, Ben-David J. Vestibular findings associated with chronic
noise induced hearing impairment. Acta Otolaryngol 1994;114:579–85.
10. Tseng CC, Young YH. Sequence of vestibular deficits in patients with
noise-induced
hearing loss. Eur Arch Otorhinolaryngol 2013;270:2021–6.
11. Emara AA, Gabr TA. Chronic
noise exposure:Impact on the vestibular function. Adv Arab Acad Audio Vestib J 2014;1:71–9.
12. Themann CL, Masterson EA.
Occupational noise exposure:A review of its effects, epidemiology, and impact with recommendations for reducing its burden. J Acoust Soc Am 2019;146:3879.
13. Zeydabadi A, Askari J, Vakili M, Mirmohammadi SJ, Ghovveh MA, Mehrparvar AH. The effect of industrial
noise exposure on attention, reaction time, and memory. Int Arch Occup Environ Health 2019;92:111–6.
