Cigarette smoking effect on human cochlea responses.

BACKGROUND: Smoking is one of the most important risk factor in increasing of non-communicable disorders, especially chronic diseases such as cancer, stroke, heart and respiratory diseases. Cigarette smoking could damage the cochlea and causing hearing loss. The otoacoustic emission (OAE) is a source of information for determining cochlear responses to sound stimuli and how to change the response of the auditory system in some diseases. OAE test was sensitive to outer hair cells (OHCs) activity.
MATERIALS AND METHODS: In this study, tried to evaluate a hearing threshold of the smoker group versus non-smoker ones through pure tone audiometery, transient evoked otoacoustic emission (TEOAE) and distortion product otoacoustic emission (DPOAE) tests.
RESULTS: The results indicated that significant decrement of 8000 Hz threshold, reduced DPOAE/TEOAE amplitude in the smoker group than non-smoker one (P < 0.05). DPOAE amplitudes decline reflects the cochlear damage caused by smoking.
CONCLUSION: OAEs test was clinically non-invasive, accurate, and objective evaluation of the performance of cochlear OHCs.

INTRODUCTION

Smoking is a noticeable public health problem. A number of studies have shown its many damaging effects. Smoking is one of the most important risk factor in increasing of non-communicable disorders, especially chronic diseases such as cancer, stroke, heart, and respiratory diseases.[123] In according to the studies, smoking is considered to be a threat for the progress of hearing loss, with widely diverse outcomes. Cigarette smoking could increase the blood viscosity and decrease oxygen causing disruption and damage to hair cells of the cochlea.[456] Researches show smoking, whatever directly or indirectly secondhand exposure have been associated with hearing loss.[789] Pure tone audiometry (PTA) was conducted via audiometer to measure auditory system function. The result is recorded as an audiogram which has to main character (intensity and frequency in terms of dB and Hz, respectively) in the range of human hearing (from 250 to 8000 Hz).[10] The otoacoustic emission (OAE) is a source of information for determining cochlear responses to sound stimuli and how to change the response of the auditory system during any cochlear malfunction.[11] OAE as the main component of auditory tests differentiates the auditory sensory disorder from neurological ones and also used to monitoring cochlear status health changes.[12] OAEs indicate middle ear function and normal cochlear biologic action. Such these studies provide better understanding into outer hair cell (OHC) cochlear mechanism.[13] In this study, tried to investigate the effect of smoking on hearing via PTA and OAEs tests.

MATERIALS AND METHODS

The study sample consists of 32 patients and divided in two groups of men with aged ranges between 20 and 50 years which one group with a history of cigarette smoking and the other one normal non-smoking persons as a control group of the same age and gender. Both groups were given in to history taking either with inclusion criteria such as no history of ototoxic drug intake, diabetes, hypertension, hyperlipidemia, hypothyroid, ear infection/surgeries, and no occupational noise exposure. Exclusion criteria include any outer and middle ear disease, unilateral or conductive hearing loss. The written informed consent form was taken from all patients. Demographic data were collected. Some factors like duration of smoking, smoking intensity (number of cigarettes smoked per day) which categorized in three level: mild (<10 cigarette smoked per day), moderate (10-20 cigarette smoked per day) and severe (>20 cigarette smoked per day) and use of other tobacco products and drugs are being questioned and recorded. Then PTA with AC 40 clinical audiometer, HDA 200 audiometric headphone and tympanometry via, AD235 tympanometer and finally OAEs tests were performed by Eclipse EP25 device for whom are eligible. Any case would be excluded from this study if any evidence of middle ear disease was found during tests. Abnormal results for distortion product otoacoustic emission/transient evoked otoacoustic emission (DPOAE/TEOAE) test were considered less than 6 dB at 1000 2000, 4000 and 6000 Hz. The statistical data obtained from the two groups are analyzed via t-test and ANOVA test.

RESULTS

Mean threshold values observed in high frequency audiometry for smokers and non-smokers are presented in Figure 1. In this study, hearing threshold deterioration at frequencies between 2000 and 10000 Hz was significant particularly at 8000 Hz in smoker group compared with non-smoker one (P < 0.001).

Figure 1

Audiometry results in smoker and non-smoker groups

Decreasing of DPOAE amplitude at 1000, 2000, 4000, and 6000 Hz in smokers group rather than non-smokers was statistically significant (P < 0.001) as seen in Table 1. In spite of TEOAE amplitude declining at all frequencies in smoker than non-smoker, results are not statistically significant (P > 0.05) as shown in Table 2. Results indicated significant differences between smoking intensity (mild and severe level) and hearing threshold at 2000, 4000 and 8000 Hz in, but no statistically significant difference between smoking intensity and DPOAE/TEOAE test results was found.

Table 1

DPOAE test results in smoker and non-smoker group (dB)

Table 2

TEOAE test results in smoker and non-smoker group (dB)

DISCUSSION

OAEs test is a fast technique to evaluate OHCs motility. Cigarette smoking interfere the OHCs tasks leading to cochlear dysfunction and then hearing threshold levels were possibly affected throughout the frequency range 250-16000 Hz especially at higher frequencies. Fabry et al. study indicated that the second hand smoke exposure shown hearing loss in all frequencies.[14] In Ohgami et al. study, the effect of smoking on extra high frequency hearing thresholds of smoker group was significantly worse than non-smokers specifically at 12 kHz.[15] A study by Gopal et al. found that ABR and DPOAE could be appropriate tests for evaluating youth smokers who had normal hearing with high sensitivity[16] and in this study DPOAE was conducted because of OAE and OHC correlation. Subsequently, metabolic disruption of cochlea impacts DPOAE and then poorer thresholds point out the cochlear dysfunction. Nomura et al. study, the effects of smoking on DPOAE was a significant decrement in the DPOAE amplitude in smokers than non-smokers[17] that was consistent with the results of this study.

CONCLUSION

This study shows that smoking has destructive effects on hearing, as the smoker group had not as good as auditory thresholds in high frequencies (above 8000 Hz), poorer response levels to OAEs when compared to non-smokers. In according to the results of human studies, OAEs test was clinically non-invasive, accurate, and objective evaluation of the performance of cochlear OHCs. This can be important for the design of experiments applied and can easily be observed amplitude changes in response to a specific stimulus. Cochlear nonlinearity and frequency selectivity could be changed because of noise exposure, infection and drugs. DPOAE amplitudes reduction reflects the cochlear damage caused by smoking. Another finding of this study is the ability to record more sensitive at high frequency via DPOAE rather than TEOAE test. Finally it is recommended to follow-up the study group after 5 years to recheck the effect of extended duration on their hearing thresholds.