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Noise-induced hearing loss (NIHL) is a significant public health problem in industrialized nations across the world. In the United States, the Occupational Safety and Health Administration (OSHA) provides workplace guidelines for noise exposure limits whereas the National Institute for Occupational Safety and Health (NIOSH) provides more conservative recommendations on exposure limits.
Although there are differences between these organizations with respect to recommended exposure limits, both OSHA and NIOSH monitor noise-induced damage to the inner ear using audiometric permanent changes in hearing thresholds.
Exposures that damage hearing are not necessarily painful or even annoying. After overexposure, NIHL recovers with an exponential time course for 2–3 weeks, depending on initial severity. Thresholds may fully recover defined as “temporary” threshold shift or stabilize at an elevated value which is called “permanent” threshold shift. Permanent NIHL is due to destruction of cochlear hair cells or damage to their mechano-sensory hair bundles.
Recent studies in animal models suggest interruption of synaptic communication between sensory inner hair cells and subsets of cochlear nerve fibers. This occurs early before the onset of hearing loss, The affection of these neurons alters auditory information processing, whether accompanied by threshold elevations or not, and is a likely contributor to a variety of perceptual abnormalities, including speech-in-noise difficulties, tinnitus and hyperacusis. This affection was described as cochlear synaptopathy.
Consequences of cochlear hearing loss on speech perception and psychophysical central auditory abilities have not been well documented. The aim of this research was to explore central auditory processing in individuals with history of occupational noise exposure.
The present study consisted of two groups: control group (30 normal healthy subjects)and study group(30 subjects with history of occupational noise exposure) aged between 20 and 50 years. They were subjected to Basic audiological evaluation: Pure tone audiometry for frequencies 250-8000 Hz for air conduction and 500-4000 Hz for bone conduction, speech reception threshold (SRT) using Arabic spondee words list and speech discrimination scores using Arabic phonetically balanced words. And Tympanometry, ipsilateral acoustic reflex, central auditory processing questionnaire and Psychophysical central auditory tests: including Arabic Speech Intelligibility In Noise (SPIN) test for adults, Arabic Dichotic digit test (version I, II), Gap In Noise (GIN) test, Duration pattern test and Arabic Time Compressed Speech tests(TCST)(30% and 60% compression).
The results of the present study demonstrated that the study group had a lower performance in all Psychophysical central auditory tests when compared with age matched control group, with the highest percentage of abnormality in temporal resolution and temporal patterning followed by selective attention. Temporal integration and dichotic listening abilities are equally affected in about half of the study group.
As the most frequent complaints in APD questionnaire in the study group were difficulty in speech discrimination, asking a lot to repeat and difficulty in music appreciation, this complaints were compatible with temporal patterning test results. Accordingly, APD questionnaire can be applied as a screening tool for detection of central auditory processing disorders in subjects with history of noise exposure.
The present study showed affection of several auditory skills that reflected central auditory processing disorder. This has adverse effect on listening, communication and social abilities, it can be concluded that patients with occupational noise exposure had poor performance in all central auditory tests when compared with controls.
This means that occupational noise exposure can lead to impaired central auditory processing. Accordingly, it is recommended to apply remediation programs in patients with history of noise exposure.