Impaired auditory neural performance, another dimension of hearing loss in type-2 diabetes mellitus

To evaluate auditory performance in subjects with poorly controlled type-2 diabetes mellitus, using a simple test battery assessing sensitivity to pure tones and neuronal function.

Enrolled subjects, aged between 23 and 79 years, reported several auditory dysfunctions. They were tested using pure-tone audiometry, otoacoustic emissions for cochlear-function evaluation, and measurement of middle-ear muscle-reflex thresholds in search of an auditory neuropathy.

Compared to the standard established for an age-matched normative population, the distribution of averaged pure-tone thresholds in enrolled subjects shifted by about one standard deviation with respect to the normal distribution, in line with past reports of mild sensorineural hearing impairment in diabetes, even though many diabetic subjects fell well within the normative interval of audiometric thresholds. Otoacoustic emissions showed that pure-tone thresholds correctly predicted the status of cochlear sensory cells that, by amplifying sound, ensure normal hearing sensitivity. Whereas the observed hearing losses should not have affected the acoustic levels above which the protective middle-ear muscle reflex is triggered by intense sounds, this reflex was undetectable in around 40% enrolled subjects, a marker of auditory neuropathy.

Auditory-neural function should be evaluated to identify diabetic subjects whose hearing is impaired. Simple automatic tests are available for this purpose, for example middle-ear muscle reflex detection, which turns out to be more sensitive than the standard audiogram.