2.2 Design & Setting
This retrospective observational study recruited Korean adults with AHL who purchased a hearing aid at one secondary-referral, university-based hospital. The inclusion criteria were as follows: (1) 19 years old or above; (2) ear anatomy compatible with wearing a conventional hearing aid; (3) AHL, as defined above; (4) hearing aid fitted to the worse ear; and (5) sensorineural or mixed type HL. The exclusion criteria were as follows: (1) conductive hearing loss; (2) observed neurological or psychiatric disorder; (3) fluctuating hearing loss; (4) self-reported physical or mental health unsuitable for hearing aid fitting and use; and (5) inadequate cognitive competence for responding to questionnaires.
At 1, 2, 3, 6 and 12 months after purchase, hearing aid was fitted using the National Acoustic Laboratories (NAL) formula. The degree and type of hearing loss were classified using PTA4 and the criteria set by the World Health Organization.10 AHL was defined as hearing loss with an interaural threshold difference of 15 dB or greater in the PTA4.9
Cases of AHL were classified into four subgroups according to the criteria defined in Table ​1. Our classification modified previous studies of Van de Heyning, et al.9 Because single sided deafness (SSD) subgroup is a poor candidate for hearing aid, we excluded SSD cases and analyzed cases of three subgroups (unilateral hearing loss (UHL), and asymmetrical hearing loss type 1 (AHL1) and type 2 (AHL2)), which were classified based on the poorer ear PTA4 of 70 dB HL and the better ear PTA4of 30 dB HL.
Twelve months post-fitting, performance of hearing-aid use was evaluated and classified into three groups: (1) successful users, (2) intermittent users, and (3) failed users. Successful users met all of the following criteria: (1) wearing the hearing aid for more than four hours per day; (2) wearing the hearing aid for twelve months or longer after purchase; (3) showing an improved speech discrimination score more than 2% in the HINT; (4) showing improved sound localization; and (5) scoring 21 points or more in the IOI-HA.11 Patients who stopped using within the first twelve months after purchase were classified as failed users. Cases that did not meet the criteria for either successful and failed users were considered intermittent users.