Subjective audiometric measures in individuals with repeated acoustic trauma in the combat zone

Intense sound exposure that exceeds the pain threshold of human auditory sensitivity, known as acoustic trauma, causes significant and extensive changes in the auditory system. Throughout combat operations, military personnel often receive repeated (two or more) blast injuries. Every new case of acoustic trauma can contribute to increased severity of damage to the auditory system, causing an ‘accumulating’ effect from each subsequent trauma. In situations of intense combat operations with a high frequency of explosive weapon use, the issue of repeated combat acoustic trauma becomes particularly acute. The purpose of this work is to investigate the most informative indicators of subjective audiometry in individuals who have experienced repeated acoustic trauma in combat zones with varying degrees of auditory function impairment Materials and methods: To achieve the stated aim, we conducted an analysis of the subjective audiometry results of 52 patients with sensorineural hearing impairments who experienced repeated acoustic trauma in combat zones. The audiometric examination was conducted in a soundproof chamber where the background noise level did not exceed 30 dB, using a clinical audiometer AC-40 from Interacoustics (Denmark). Results and discussion: All selected patients with repeated combat acoustic trauma underwent a comprehensive examination, including instrumental assessments. The evaluation also included the circumstances under which medical assistance was provided to the patients, as well as any comorbidities, injuries, and other factors that could have influenced the development of sensorineural hearing loss in repeated cases of combat acoustic trauma. 44.2% of the patients experienced a single instance of repeated acoustic trauma, while 55.8% sustained more than two injuries to their auditory system. Among the military personnel we examined with combat acoustic trauma, 100% reported hearing loss, 100% experienced tinnitus, 100% had headaches, 96.2% reported dizziness, 94.2% general weakness, 98.1% fatigue, 88.4% unsteadiness while walking, poor tolerance to loud noises, auditory discomfort (94.2%), and 38.1% had difficulties with speech comprehension. The prevalence of irritability among the individuals mentioned was 96.2%, while 92.3% of the examined subjects reported changes in blood pressure. According to the results of subjective audiometry, the examined soldiers with repeated acoustic trauma frequently display bilateral asymmetric sensorineural hearing impairments. The tonal audiometric curves in the patients we examined with repeated acoustic trauma, who were in combat zones, primarily demonstrate a descending pattern, often with fragmentation and affecting the basal and medial basal regions of the cochlea. The degree of auditory function impairment in the patients we examined varied, but was mainly of moderate severity according to the International Classification. When comparing the tonal threshold audiometry results in the conventional frequency range (0.125-8 kHz) of patients with repeated acoustic trauma to the audiometric indicators of the control group, a significant difference in hearing thresholds was revealed across almost the entire tonal scale. According to tonal threshold subjective audiometry, the most pronounced significant increase in hearing thresholds among the soldiers we examined with repeated acoustic trauma is observed in the frequency range of 4, 6, and 8 kHz in the conventional range, as well as at all frequencies in the studied high-frequency range (9-16 kHz). The hearing thresholds measured at frequencies of 9, 10, 11.2, 12.5, 14, and 16 kHz were (50.28±2.33 dB), (60.23±2.66 dB), (70.27±2.96 dB), (72.56±2.99 dB), (79.92±3.72 dB), and (56.16±2.02 dB), respectively. It is important to mention that among soldiers with repeated combat acoustic trauma, there was often an increase in the thresholds for speech discrimination tests. Moreover, some of the soldiers showed a delayed increase in speech test intelligibility with rising sound pressure levels (23.1%), as well as discomfort during the perception of the speech test (17.3%). The average thresholds for numeral recognition and verbal tests in the patients we examined with repeated combat acoustic trauma were also significantly higher. Among the examined soldiers with repeated combat acoustic trauma, the differential sound pressure thresholds (DST) according to Lyusher at 2 kHz and particularly at 4 kHz were lowered to 1.3±0.05 and 0.8±0.03, respectively, indicating impairment of the receptor area of the auditory system in these patients. Conclusions: In cases of repeated combat acoustic trauma, sensorineural hearing impairments frequently develop, featuring a tonal curve typical of acoustic trauma damage, which is associated with alterations in speech audiometry, particularly speech intelligibility disorders, delayed improvement in the speech test intelligibility curve, and discomfort in perceiving the speech test. Damage to the auditory system in individuals with repeated acoustic trauma primarily impacts all areas of the auditory system, including both receptor and central components, and is also associated with signs of involvement from the central nervous and vestibular systems. The presence of a delayed increase in the speech test intelligibility curve, discomfort during speech test perception, and reduced Lyusher differential sound pressure thresholds at 2 kHz and 4 kHz in servicemen with multiple acoustic traumas indicates damage to the receptor component of the auditory system. Key words: auditory system, acoustic trauma, sensorineural hearing loss, subjective audiometry, blast injury.