Hearing Thresholds of Nigerian Navy Personnel Serving in Lagos Area

Introduction: Navy personnel are exposed to several risk factors that increase hearing thresholds and impair hearing. The aim of the study was to determine the prevalence of hearing impairment among Nigerian Navy personnel. Materials and Method: This was a cross sectional study of Nigerian Navy personnel working in various ships. Structured, self-administered questionnaire was used to obtain information on hearing. All the subjects had ear examination and pure tone audiometry of both ears. Results: The prevalence of hearing impairment was 22(14.7%) in the right and 17(11.3%) in the left ears. The Pure Tone Average for the right and left ear were 19.0±8.6dB and 17.4±6.7dB respectively. There was a statistically signicant association between noisy work place and total hearing loss (p<.01), pure tone average in the left ear (p<.05) and frequencies of 3000 Hz (p<.05), 2000 Hz (p<.001) and 1000Hz (p<.05) in the left ear. The association between past history of ear infections and pure tone average in the left ear (p<.05) and the frequency 2000 Hz (p<.005) in the left ear were signicant. The length of service years, occupational group, age, sex report of change in hearing, non-use of hearing protection and presence of tinnitus did not show signicant association with pure tone averages nor on the frequencies tested. Conclusion: The signicant unilateral hearing impairment on the right ear suggests the need for more research on hearing impairment in the Nigerian Navy as well as development of a comprehensive hearing conservation programme.


Introduction
Adult-onset hearing loss is regarded by the World Health Organization as the 15th most serious health problem. [1] About 16% of disabling hearing impairment worldwide has been attributable to exposure to loud noise in the work-place. [2]In a population based study that investigated hearing threshold among various occupations in Norway, Engdahl et al reported pure tone average prevalence ratios of 3.8, 3.3 and 5.8 for senior military o cers, non-commissioned o cers and seamen respectively at high frequencies of 3, 4 and 6kHz. This prevalence ratio placed military o cers at the 16th position among the 62 occupation groups studied. [3] This high prevalence ratio of occupational hearing impairment among the military compared to other occupation populations were also reported in other studies. [4,5] Barney et al on the other hand had reported a lesser prevalence of hearing impairment among military o cers compared to non-commissioned o cers. [6] Higher ranking and older o cers were found to have a higher prevalence of hearing impairment compared to the younger, lower ranking o cers. [7] This was attributed to the in uence of age as a risk factor in the development of hearing impairment. Helfer et al also acknowledged the in uence of ageing on a higher incidence of occupational hearing loss. [4,8] Potentially hazardous noise sources in the military are the weapon systems, jet engines and other aircraft, generators of ships and communication systems. Military weapons are characterized by an almost instantaneous rise in the pressure in the ear. [8] 155-160dB peak for small calibre weapons and 178-190dB for large calibre weapons. [8] The small calibre weapons mostly affect the 1000Hz frequency range while the large calibre weapons such as the mortars, howitzers and explosions affect the lower frequencies. [10] In sea-going ships, sound levels as high as 106dB have been measured. [8]Radio operators and sonar technicians are found to be exposed to high sound levels.
[8]Navy personnel serving on-board ships at sea are also exposed to continuous noise level for a long duration of time making recovery of the auditory system for acoustic injury di cult. [8] The Nigerian Navy personnel, like other military personnel, are also exposed to impact noise from military weapons, continuous noise from the machines and communication equipment onboard the ships. The Naval helicopter pilots and air-crew are also vulnerable. Noise levels in the cockpits of civil and military aircrafts range from 95 to 105dB [11]with noise levels at the entrance of the ear canal of the pilots documented to be between 90dB and 100dB. [12]In the literature, there is a paucity of accurate population based data on the prevalence of acoustic or noise induced hearing loss among military personnel. [13,14]The diversity of the various areas and duties within the Navy makes it imperative to study the risk of hearing loss among the personnel of the Nigerian Navy.
It would be appropriate to look for studies in Africa, possibly West Africa, then Nigeria.

Materials And Method Study Design
This was a cross sectional study of Nigerian Navy personnel serving onboard various ships at the Naval Base Apapa, Lagos

Study Setting
The study was performed on Nigerian Navy personnel serving in Lagos area. The study was carried out at the Sick Bay of the Nigerian Navy Ship BEECROFT, Apapa. Ethical approval was obtained from the University of Ibadan and University College Hospital Ethical Review Board for the conduct of the study. Written permission was obtained from the Nigerian Navy for the conduct of the study and informed consent was obtained from the participants with the assurance that any information obtained in the course of the study will be kept con dential.

Study Population and Recruitment
Participants included 150 Nigerian Navy personnel serving in Lagos Areas between 22 and 59 years across all occupational groups. The Inclusion Criteria included consented Nigerian Navy personnel serving in Lagos who had no history of ear pathology while subjects with perforated tympanic membrane or discharging ear, subjects on ototoxic drugs such as aminoglycosides, quinine, vincristine etc and those with congenital ear abnormality were excluded.
The participants had a structured, self-administered questionnaire which elicited data on biologic and demographic characteristics, length of service years, job description, length of time in the present job description, history relating to hearing loss, past medical, drug and exposure to noise in work place, ear symptoms like ear pain, ear discharge tinnitus, hearing loss and vertigo. This was followed by an examination of the ears and otoscopy and Pure Tone Audiometry. The pinnae were examined for deformities.
Otoscopy was performed to assess the external auditory canal for stenosis, presence of wax, in ammation or ear discharge. The tympanic membrane was visualized for the presence of perforation.
Pure tone audiometry was carried out on all the participants using a standard, calibrated clinical audiometer (Amplivox model 260, Amplivox Ltd, Oxford, United Kingdom) with TDH 39 headphones tted to audio-cups to further attenuate ambient environmental sound to determine their hearing thresholds. Calibration of the audiometer was done according to the system adopted by International Standard Organisation 389 of 1991. [15] The tests were performed in a quiet room with maximum permissible ambient sound pressure level according to the recommendations of the International Standard Organisation 8253-1 (2010). [16] The sound level before each audiometry was determined using a pulsar model 14 sound level meter. Each ear was screened separately from the level of 0dB HL at frequencies 0.25, 0.5, 1, 2, 3, 4, 6, 8 KHz. Air conduction was tested using well-tting TDH 39 headphones tted to audiocups to further attenuated ambient environmental sounds. Bone conduction was tested using a bone vibrator applied to the mastoid bone. [17,18] The participants were informed to raise their hands to indicate the perception of the sound clicks.
The hearing thresholds were determined using the modi ed Hughson Westlake procedure. This entailed initially presenting the rst tone at 30dB at 1kHz. If the subject responds, the signal was reduced in 10dB steps until the subject no longer responded. The test tone was then increased in 5dB steps until a response was obtained. Where the subject did not respond to the initial rst tone, the signal was increased in 5dB steps until the subject responded. Thereafter the signal was decreased at 10dB steps until there was no response.
The steps were repeated at least 3 times. The signal level at which a subject responded 2 out of 3 times was recorded as the threshold at that particular test frequency. The steps were thereafter repeated for all the frequencies tested.
Pure Tone Averages was calculated for frequencies 500Hz, 1kHz, 2 kHz and 4 kHz, and values above 25dB HL in 1 or both ears are reported as impaired hearing. Hearing impairment above 25dB HL affecting frequency range of 3 -6 KHz are reported as high frequency hearing loss while that affecting frequency range 0.5 -2 KHz are reported as low or mid frequency hearing loss.

Variables
The independent variables included sex, age, occupational group, work place noise level and years of service while the dependent variables were the hearing threshold level at various frequencies with pure tone average.

Statistics
Data were analysed with the Statistical Package for Social Sciences version 16 (SPSS window Version 16 Inc, Chicago, Ilinois, USA) software. The categorical variables including sex, length of period in service strati ed into a range of 10 years (0-10, 11-20, 21-30, >30), work place noise level graded (no, mild, moderate and severe noise level) and occupation groups were summarized in frequency and proportions. The age and hearing thresholds were summarized with mean (standard deviation). Association of gender, period in service(categorized) and occupational groups with the prevalence of hearing loss were evaluated with Chisquare while independent t-test was used to assess the mean hearing thresholds level differences at various frequencies between the left and right ear. A p-value of 0.05 or less was taken as statistically signi cant.

Demographic Characteristics
The study included 150 respondents, made up of 135 males and 15 females (M: F = 9:1) The mean ±SD of the participants was 32± 9.1years.
Among the ve main occupational groups encountered, seaman branch was the predominant accounting for 92 (61.3%). However, the engineers were the oldest among the occupation groups while the information were the youngest (Table 1). More than half of all the participants 87(58%) reported working in a noisy environment with 8(5.3%) of the total saying the noise is constantly present in their work environment. Presence of noise in the work environment was particularly prominent among the participants in the engineering group (Figure 1). A previous history of ear infection was reported by 14 (9.3%) participants, of which 12(8%) had been in the Navy for less than 10years.

Audiometric Findings
The Pure Tone Average for the right and left ear were 19.0±8.6dB and 17.4±6.7dB ( Table 2)

Mean Threshold Values and Occupational Groups
The mean threshold levels among the various occupational groups were seamen19.8 ± 9.6, Engineering 19.2±7.3dB, logistic15.8±6.5 dB, medical18.0±5.9 dB and information18.0±4.9 dB. The differences in the mean threshold values across all the frequencies as well as the pure tone averages were however not statistically signi cant (state the p vale or con dence interval).

Mean Threshold Values and Duration in Service
The mean threshold values across all frequencies in both ears were mainly elevated in personnel who had spent 21-30years in service, however, the difference was statistically signi cant for the left ear (Tables 3).

Grading of Hearing Impairment
There was a statistically casual relationships between noisy work place with total hearing loss (p<.01), pure tone average in the left ear (p<.05) as well as frequencies of 3000 Hz (p<.05), 2000 Hz (p<.001) and 1000Hz (p<.05) in the left ear. There was also a statistically signi cant association between past history of ear infections and pure tone average in the left ear (p<.05) and the frequency 2000 Hz (p<.005) in the left ear. The other independent variables such as length of service years, occupational group, age, sex report of change in hearing, non-use of hearing protection and presence of tinnitus did not have any statistically signi cant relationship with pure tone averages nor on the frequencies tested.

Discussion
The main ndings in this study were that hearing thresholds were worse in the right ear compared to the left ears. Increasing age, duration of years spent in service, working in a noisy environment and past history of ear infections affected hearing thresholds in this study. There was no statistical difference in the hearing thresholds across the occupational groups studied.
This study revealed higher threshold values of frequencies in the right ears compared to the left ears of the participants. Our nding is similar ndings of Ologe et al [18] in a study on Nigerian workers who reported the right ear being worse than the left ears. They suggested it might be due to environmental factors. However, Abel [9] in Canada reported worse hearing in the left ear. Another explanation for the interaural difference could be that louder noise may have been reaching the right ear more than the left ear in a majority of the participants as explained by the Stenger effect. [20] The relevance of this interaural difference might be important during auditory rehabilitation purposes where the decision of monoaural or binaural hearing aids or cochlear implantation and hearing aids may be required to be tted.
The prevalence of hearing impairment (Pure Tone Average > 25dB HL) was 14.7% in the right ear and 11.3% in the left ear. The criteria for making a diagnosis of unilateral, asymmetrical hearing impairment [20,21] was met in the audiograms of 10 (6.7%) of the participants. The signi cance of unilateral asymmetrical hearing impairment is that these participants might need to be further screened for other causes of asymmetrical hearing impairment apart from noise [21] and more importantly for medicolegal reasons. [20] Our nding leads to an inference that age was a major determinant of hearing thresholds and in agreement with other studies. [19,22] Previous studies had reported the effect of past ear infections on impaired threshold levels. [23,24] The impact of ear infections on hearing impairment was signi cant in this study unlike the ndings in a study carried out in Canada. [19] This nding of elevated hearing thresholds among relatively young personnel may make it pertinent to enforce ear screening processes during the recruitment of young adults into the Nigerian Navy. The nding of an association of noisy work place and impaired hearing thresholds was also reported by Abel. [19] It may thus be deduced that working in a noisy environment has a bearing with hearing impairment. An attempt at determining whether the hearing impairment seen in the participants were noise induced was di cult as none of the audiograms was classical as in published works. [25,26] A limitation of this study was that the audiometric measurements were done only once, hence it might be di cult to determine if the measurements were not as a result of temporary threshold shifts. Attempts were made at overcoming this limitation by performing the measurements on the rst working day of the week after a likely 48 hours free from work. The response by participants of working in a noisy environment may require a further comprehensive survey of the Nigerian Navy work environment. It can also form a basis for the development of a hearing conservative programme.

Conclusion
The right ear showed worse hearing thresholds across all the frequencies. The prevalence of hearing impairment in the right ear was 14.7% and 11.3% in the left ear. The hearing threshold of Nigerian Navy personnel is signi cantly affected by their age, noisy work environment as well as past ear infections.    Figure 1 Distribution of noisy work place among the various occupational groups