Neurophysiological changes in sudden deafness and in progressive sensorineural hearing loss patients studied by electrocochleography

Sudden deafness (SD) and progressive sensorineural hearing loss (PSNHL) are two different disease entities. SD is an obscure entity with diverse causes, complex pathophysiology and course. Progressive SNHL has often clear aetiology but both diseases require investigation. The aim of this study was to investigate the cochlear neurophysiological changes occurring in SD and in PSNHL by the use of electrocochleography (ECoG). 11 patients with SD related to cochlear failure (mean age 58.6 years) presented with either unilateral (n = 10) or bilateral (n = 1) SD and 31 patients (mean age 54.6 years) with progressive SNHL were studied. Transtympanic ECoG and conventional audiometry were performed. Action potential (AP), summating potential to action potential (SP/AP) ratio, and cochlear microphonic (CM) output were evaluated and compared with pure tone average air conduction thresholds (PTA). Four SD patients (36.3%) with severe hearing loss had SP/AP > 30% indicating the presence of endolymphatic hydrops (EH), whereas four patients with PSNHL (12.5%) had EH. AP amplitudes correlated in both diseases (SD (r = −0.7, p < 0.05), SNHL (r = 0.4, p < 0.05)) with hearing threshold level (AP decreased when hearing threshold decreased). There was no significant difference between the amplitudes of the CM output, which reflects the functional integrity of OHCs, and the levels of hearing threshold within the disease groups, nor between the two disease groups. ECoG was successful in determining the presence of EH in SD and in some patients with progressive SNHL. CM output showed no specific pathophysiological difference in the function of outer hair cells between SD and PSNHL groups. ECoG was a limited investigative tool to provide all information about the functional status of the cochlea in patients with sudden deafness and in patients with progressive SNHL.