Abstract
Cochlear synaptopathy refers to a subclinical hearing pathology which could potentially explain hearing difficulties within the normal hearing threshold; it is also called “hidden hearing loss”. We hypothesized that a temporary threshold shift in sudden sensorineural hearing loss (ISSNHL) also affects the function in the synapse. The aim of the study was to evaluate the presence of cochlear synaptopathy in patients who had completely recovered from unilateral SSNHL Nineteen patients who had completely recovered from ISSNHL from January 2018 to June 2021 were assessed. Complete recovery was established by pure tone audiometry (PTA) 3 months after treatment, according to the American Academy of Otolaryngology–Head and Neck Surgery criteria. Subjects completed the pure tone audiometry, speech audiometry and auditory brain stem response (ABR) test, and completed a questionnaire regarding hearing loss after hearing recovery. The ABR amplitudes of wave I and wave V, and the ratio of wave I/V of both ears (recovered side and healthy side) were assessed. A visual analog scale (VAS) and a hidden hearing loss questionnaire were used to evaluate subjective hearing difficulty. The ABR waves I of the recovered ears had a significantly lower amplitude (p = 0.002) than those of the healthy side, whereas there was no difference in wave V (p = 0.985) or in the ratio of wave I/V (p = 0.107). Some patients still felt mild hearing difficulty although their PTA results were normal, but there was no clear relationship between the VAS score, wave I amplitude and speech recognition scores. The present findings point to the possible existence of cochlear synaptopathy in ears that have completely recovered from unilateral sudden sensorineural hearing loss. We suggest that the causes of cochlear synaptopathy and of idiopathic sudden hearing loss may have something in common.
Highlights
Cochlear synaptopathy refers to a subclinical hearing pathology which could potentially explain hearing difficulties within the normal hearing threshold, and is called “hidden hearing loss” [1]
The term hidden hearing loss was introduced by Schaette and McAlpine when they found that the amplitude of auditory brainstem response (ABR) wave I was reduced in tinnitus patients with normal audiograms [3]
Animal studies have revealed a decrease in the number of ribbon synapses between inner hair cells and auditory nerve fibers as a result of repetitive exposure to noise, or aging, and such cochlear synaptopathy is considered a main cause of hidden hearing loss [5,6]
Summary
Cochlear synaptopathy refers to a subclinical hearing pathology which could potentially explain hearing difficulties within the normal hearing threshold, and is called “hidden hearing loss” [1]. Patients with cochlear synaptopathy often complain of tinnitus or difficulty in recognizing speech in a noisy environment even though they have normal hearing [2]. The term hidden hearing loss was introduced by Schaette and McAlpine when they found that the amplitude of auditory brainstem response (ABR) wave I was reduced in tinnitus patients with normal audiograms [3]. But noise exposure, aging and ototoxic drugs are considered risk factors [2,4]. Animal studies have revealed a decrease in the number of ribbon synapses between inner hair cells and auditory nerve fibers as a result of repetitive exposure to noise, or aging, and such cochlear synaptopathy is considered a main cause of hidden hearing loss [5,6].
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