Characteristics of the Unilateral Hearing Loss Group

Presbycusis can reduce the hearing of a person over 50 years old by 0.5 to 1dB. In the study of the group > 50 years old, there were 2 cases accounting for 0.63%, so the effect of presbycusis on the rate of hearing loss is not much.

4.1.2.2. Prevalence of hearing loss

a, General hearing loss rate: The research group consisted of 315 people, 56 people had hearing loss in one ear, accounting for 17.78%, and 142 people had hearing loss in both ears, accounting for 45.08%. This result shows that a large proportion of the Armored Corps soldiers exposed to harmful noise have hearing loss. If we only consider the number of hearing loss in both ears, we see that this figure is significantly higher than the rate of 12.5% reported in Ho Xuan An's study when investigating the hearing of 240 crew members of an armored unit in 2003 [5]. If we compare it with the statistics of some other reports, for example, 20% of naval sailors had hearing loss in Khuong Van Chu's study in 2015 [6]. Our results are similar to Nguyen Hoang Luyen's when studying submarine sailors, the rate of hearing loss was 45.2% [9]. However, it is lower than the rate of hearing loss of 48.5% of workers at the Viet-Uc steel factory in Hai Phong according to Nguyen Thanh Hai's study in 2016 [11]. Toh's study in 2002 [102] with 818 Singaporean conscripts had a hearing loss rate of 3.67% (95% CI 2.48-5.19). Joseph's study in 2016 [62] with 16,500 navy and marine soldiers had a hearing loss rate of 39%. The rate of hearing loss in our study was more different. Explaining the difference in the results of the reports, we believe that because our study surveyed all positions of three armored units, the reality of hearing loss can be caused by many different factors, not just noise, such as ear, nose and throat diseases; acute or chronic otitis media; having had ear surgery due to infection... In addition, the difference in statistical rates can be due to the selection of subjects. Specifically, in the study

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In our study, all soldiers exposed to harmful noise were given complete monotone audiometry in the field in a standard portable soundproof room, measuring hearing thresholds at all fundamental frequencies for both air and bone conduction. After the measurement, any soldier with an average hearing threshold >20dB was classified as having SGTL. In Ho Xuan An's study, in the initial screening step in the field, soldiers were only given preliminary audiometry at 30dB and 60dB at two frequencies, 1000Hz and 4000Hz. If SGTL was above this level, the soldier was taken to the hospital for complete monotone audiometry and confirmed whether SGTL existed or not based on the results of this audiometry. Thus, in theory, during the first screening process in Ho Xuan An's study, there may have been some subjects with SGTL but at a mild level (from 21-30dB) that would have been missed. On the other hand, Ho Xuan An's research subjects were limited to members of the driving crew, while our study studied all subjects exposed to harmful noise (> 85dB) in many working positions in the Armored Corps such as teachers, repairmen, crew members: crew chiefs, drivers, gunners, loaders, etc. On the other hand, regarding the characteristics of the armored corps, in addition to high-intensity noise, there are also adverse factors to health in general and hearing in particular such as vibration, temperature, dust, etc. Comparing the survey methods, it can be explained that the rate of SGTL in our study is higher than in other studies.

b, Rate of hearing loss by age: Rate of hearing loss by age in the study group 21-30 years old: 3.17%; group 31-40 years old: 27.31%; group > 41 years old: 32.37%. Our results are different when compared with the study of Ho Xuan An 2003 [5], specifically the age group > 41 years old is 22.45% and 21-30 years old is 29.25%. Author Helfer 2010 [49] epidemiological study on hearing loss and noise-induced hearing damage of the US military from 2003-2005 showed that the rate of hearing loss is highest in the group ≥ 40 years old. The explanation for this is as mentioned in the example of subject selection of

Author Ho Xuan An only selected tank drivers as the subject, while our study selected a broader subject including people working in positions exposed to harmful noise. However, there is a similarity that the older the age group, the higher the rate of hearing loss. Our study on the difference in the rate of hearing loss between the age groups over 40 and under 40 was significant with OR: 2.49 (95% CI: 1.53-4.04), similar to Helfer's finding that the group over 40 years old had the highest rate of hearing loss [49]. c, Rate of hearing loss by military age: Rate of hearing loss by military age in the study ≤ 10 years: 5.4%; 11 - 20 years: 29.2%; ≥ 21 years: 32.07%. The rate in our study is similar to that of Ho Xuan An's study, the military age group < 5 years has a rate of SGTL: 3.85%, with the military age group > 20 years: 21.57%. It can be said that the harmful effects of noise increase with exposure time, manifested in military age.

However, compared with the hearing loss rate in the population of over 5.5% (WHO 2020 [112]), the hearing loss rate in this study is many times higher. This is also a point that requires further in-depth analysis.

4.1.2.3. Characteristics of the group with unilateral hearing loss

This group had 56 soldiers with unilateral syphilis, the proportion of syphilis in each ear was almost equal, right ear 46.43% and left ear 53.57%.

To better understand the cause of SGTL in the above soldiers, we analyzed the morphology of the tympanic membrane of SGTL ears. Through clinical examination, we found that most of these SGTL ears had normal tympanic membrane morphology, shiny and mobile tympanic membrane (accounting for 78.57%), other abnormalities of the tympanic membrane such as fibroadhesive lesions, tympanic membrane perforation or opacity accounted for only a very small percentage.

In addition, analyzing the tympanometry results, most of the above SGTL ears have tympanometry type A (accounting for 93.16%), only a few ears have abnormal tympanometry type (B and C). Thus, when combining the tympanic membrane morphology factor

and tympanometry type, it can be inferred that the majority of SGTL ears are caused by the inner ear because the tympanic membrane and middle ear function are normal. This inference is also confirmed when 53/56 ears are of the type of sensorineural deafness, there are only two cases of mixed deafness and one case of conductive deafness.

Unilateral sensorineural deafness is a disease caused by damage to the cochlear hair cells in the inner ear or by damage behind the cochlea on one side (usually rare clinically and difficult to distinguish from simple cochlear damage). In order to diagnose unilateral sensorineural deafness, it is often necessary to use many clinical and paraclinical tests such as measuring the stapedial reflex, blood tests, imaging or measuring the auditory brainstem potential. The most common causes of unilateral sensorineural deafness are sudden deafness, Meniere's disease or cerebellopontine angle tumors. In the case of 56 people with unilateral sensorineural deafness, we rarely think of the above causes because the soldiers all had a history of good health, no other pathological manifestations and had annual health check-ups that did not detect any abnormalities.

Toh 2002 [102] study on hearing disorders and the role of regular hearing screening. Descriptive analysis based on the results of self-administered questionnaires of 818 Singaporean conscripts, pure tone audiometry, demographic factors, noise exposure. The study results showed that unilateral hearing loss occurred in 17 cases (56.7%) and bilateral hearing loss occurred in 13 cases (43.3%). The characteristics of noise-induced hearing loss are often quite diverse, and can occur in one or both ears. The most common form is sensorineural hearing loss in both ears. However, noise-induced SGTL in one ear is not uncommon, especially common in cases of noise exposure from one side such as SGTL after exposure to loud explosions, using musical instruments in the dominant hand, using weapons in the dominant hand... Noise-induced SGTL occurring in one ear can also be explained based on the different sound sensitivity levels of each ear in each person. Besides, most of SGTL are in

mild level, meaning only 10-20 dB reduction compared to the healthy ear. The morphology of the audiogram at this stage is also not typical of an SGTL due to sound trauma (nearly half of SGTL ears have horizontal audiograms). Thus, to confirm whether these SGTL ears are due to noise or other causes, more in-depth examination and longer follow-up time are needed.

4.1.2.4. Characteristics of the bilateral hearing loss group

The study group of 315 people had 142 people with SGTL in both ears, accounting for 45.08%. There was no difference in the classification of deafness as well as the level of SGTL between the two ears with p>0.05. Due to the nature of the work of armored personnel who are often exposed to noise, to analyze the characteristics of the SGTL group in both ears, we paid attention to the classification of deafness, the level of SGTL and the frequency of SGTL. Regarding the classification, the number of receptive deaf ears accounted for 91.9% of the total number of SGTL ears, very few cases of conductive or mixed deafness. When examined endoscopically, the morphology of the eardrum of each SGTL ear, the image of a normal, shiny eardrum also accounted for the majority of 77.46%. In addition, when measuring the tympanometry for SGTL ears, only 7.04% of the ears belonged to type B and C, meaning that very few ears had middle ear pathology. Thus, combining the results of otoscopy, tympanometry and complete monotone audiometry, we found that most of the subjects with SGTL in both ears were caused by the inner ear, very few SGTL cases occurred due to middle and outer ear diseases. Compared with the study by Ho Xuan An in 2003, the rate of receptive deafness in the group of 240 soldiers operating armored vehicles was 12.5% [5], much lower than ours. Explaining the difference in the above rate, we found that the above study was conducted quite a long time ago. At that time, the author only conducted a preliminary hearing survey in the field at two frequencies of 1000Hz and 4000Hz with an intensity level of 30dB, which may have missed many soldiers with SGTL levels less than 30dB. In addition, to survey the status of middle ear damage, the author also only made a preliminary assessment based on ear examination with a headlamp and regular examination and evaluation.

The Valsalva maneuver was subjectively evaluated for each ear without objective tests such as tympanometry to confirm the diagnosis. The limitations of the aforementioned equipment have significantly affected the research results, making the actual number possibly higher than the published number.

For frequencies with SGTL showing mild hearing threshold increase (from 21-40dB), the ears had relatively uniform hearing threshold increase between frequencies from 500Hz to 8000Hz. However, for cases with moderate to severe hearing threshold increase (from 41-80dB), most ears had hearing threshold increase at high frequencies. This high-frequency hearing loss can again be directly observed through the audiogram morphology when the audiogram has a high-frequency defect (102/284=35.92%) and a horizontal high-frequency SGTL pattern (103/284=36.27%) as the two predominant types of morphology.

Toh 2002 [102] study with 818 Singaporean conscripts on hearing disorders, of the 30 subjects with hearing loss, 19 (63.3%) had hearing loss at high frequencies, 7 (23.3%) at low frequencies and 4 (13.4%) at all frequencies. The risk of hearing loss was higher in those who frequently went to nightclubs than in the other group (RR: 2.72, 95% CI 1.09-6.76). Early hearing loss at high frequencies due to noise has long been demonstrated in the world literature. In the early stages, hearing loss is only very limited at 4000Hz to 30 - 40 dB in both ears, occurring when the subject is first exposed to noise for several weeks to several months. In the second stage after many years of exposure to loud sounds (5-7 years), the audiogram has a clear V-shaped defect, the peak can reach 50-60dB at 4000Hz and spread to 3000-6000Hz. Later, along with the progression of the disease, lower frequencies are also gradually affected, leading to a steepening pattern of high-frequency deafness and finally a horizontal audiogram. Compared with the above description, up to 35.92% of subjects with SGTL in both ears in this study were in the second stage of cochlear damage due to noise exposure. In this stage

At this stage, the survey to detect hearing damage and intervention to stop noise exposure is very important in stopping the progression of the disease. Thus, the survey results of the study show that there are quite a few officers and soldiers who need intervention to reduce noise exposure to prevent permanent severe SGTL sequelae due to noise. The study results also show that the majority of new subjects appear at the level of mild deafness (90.8%), meaning that if intervened early, this SGTL level will not have too great an impact on the general hearing of officers and soldiers and often do not need rehabilitation intervention measures.

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When analyzing the relationship between service time and the rate of hearing loss, we found that the longer the service time, the higher the rate of hearing loss, the difference was statistically significant with p<0.001. This is also the finding of Kyaw N. Win et al. in 2015 when studying the rate of hearing loss in the armed police force of Brunei [68]. Ho Xuan An's 2003 study on domestic armored personnel carriers also gave similar results on the relationship between service time and the rate of hearing loss [5]. However, the military age factor often goes hand in hand with the age of the research subjects, so the increase in the rate of hearing loss in both ears may be related to age as well as military age. To be more specific, the confounding factors of age and military age were not mentioned in the univariate analyses.

Noise-induced SGTL is characterized by high-frequency hearing loss that may show up as a high-frequency defect in the audiogram [47], [69], [89]. A study conducted in Taiwan [117] investigated nearly 10,000 workers exposed to noise >85 dB from various sectors. They found that 34% of the workers had hearing thresholds at 4000 Hz higher than 40 dB in one or both ears.

Arve et al found minimal noise-induced SGTL deficits at 3–6 kHz in about one-third (37%) of the workers in their study despite them being asymptomatic [73].

Oleru's results on hearing thresholds in a Nigerian automobile assembly plant showed that noise-induced SGTL in workers was more common at higher frequencies. Similarly, noise-induced SGTL was more common at 4000 Hz than at 1000 and 2000 Hz. Hearing loss in the workplace began at 4000 Hz and then spread to higher and lower frequencies [83].

Several phenomena associated with loud noise have been studied. Intracochlear damage tends to occur early and to a maximum extent in the 4000 Hz frequency range. This progresses steadily over several years of initial exposure and then tends to plateau. Typically, the next affected area is in the 6000 Hz region, followed by the 8000 and 2000 Hz regions, where damage occurs more slowly [75].

The group of SGTL in both ears had 91.9% of audiograms with receptive deafness, 35.92% of audiograms had high-frequency defects and 90.8 cases were mild deaf. This result is consistent with the cited studies because with an average military age of 18.94, it shows that SGTL progresses over many years to form high-frequency defects on the audiogram. However, most of the hearing is mild deafness, if these SGTL soldiers are not properly cared for in terms of their hearing status, the SGTL condition will continue to progress over time, affecting work and daily life.

4.1.3. Some factors related to hearing loss

Age, military age, and history of noise exposure were associated with the prevalence of hearing loss in the Armored Corps as discussed above. However, when analyzing univariate regression with age groups, military age groups, and history of noise exposure, we did not find any association.

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