Comparative analysis between effective gain and functional gain in bone-anchored hearing aid users

Abstract The cases of ear malformations, conductive, mixed, and single-sided deafness hearing loss are candidates for surgery and use of Bone-Anchored Hearing Aids (BAHA). Commonly, the literature highlights two procedures to assess the benefits and characteristics of amplification in users: functional gain (FG) and effective gain (EG). Objective Estimate and compare the EG and the FG to evaluate the benefits obtained by users of BAHA and, later, to compare tests of speech perception in silence and in noise. Methodology The sample (n=79) was divided into four groups, implanted from February 2014 to February 2021. The following tests were analyzed: pure-tone audiometry by air and bone; research of audiometric thresholds in free field; speech perception tests in silence and in noise. Results EG presented lower values than FG in all frequencies. The positive results of the speech perception tests were correlated with worse FG values. EG is the best method for evaluation, as it allows a proper comparison between devices, as well as a comparison with the prescription of validated rules. Conclusions A better evaluation of results was observed on the EG values, indicating that it is a relevant method to assess auditory performance. In addition, the FG results were incompatible with the benefits obtained in the speech perception tests, showing that it is not a reliable tool for monitoring the results with the use of BAHA.


Introduction
The rehabilitation of hearing-impaired individuals requires the adaptation of Hearing Aids (HA), which can modify, amplify, and give these individuals access to speech sounds. However, cases in which the benefit of using hearing aids is limited or even nonexistent. The advancement of technology allowed the development of surgically implantable hearing aids.
Cases of ear malformations, conductive hearing loss, mixed hearing loss, and individuals with profound unilateral hearing loss, Single-Sided Deafness (SSD) are candidates for surgery and the use of BAHA. 1 In 1960, Von Békesy described the principle of sound transmission by bone and found no difference between air and bone sound transmission for the basilar membrane. This process was named air-bone cancellation. 2,3,4 The BAHA follow this principle and were introduced in the 1980s, allowing a greater notoriety on the subject. 5  Commonly, the literature indicates two procedures to assess the benefits and characteristics of amplification in patients using BAHA: functional gain (FG) and effective gain (EG). 6 FG is defined and estimated by the difference between the threshold obtained in free field with and without the use of BAHA (obtained during audiometry in free field). The frequencies included in this estimation are 500, 1000, 2000, and 4000Hz. 4,5 EG is estimated by the difference between the cochlear thresholds obtained during bone conduction research in pure-tone audiometry comparing them to the thresholds obtained in free field audiometry with these implantable devices. 6,7 The hypothesis that guided this research was that EG is the best method to evaluate the benefits obtained by BAHA users, considering the results of speech perception tests in silence and in noise (signal/noise ratio Data were collected using a two-channel audiometer,  this analysis, statistically significant values were considered at the 5% level (p≤0.05).

Results
The results in Table 1  ANOVA with an independent factor.

Discussion
The first finding (Table 1)    The comparison between the groups regarding gender showed no statistically significant difference. Thus, the groups were similar considering gender.
These data show that the sample was homogeneous.
In a retrospective study to characterize the etiologies and audiological status of hearing impairments in Brazil, the populations studied showed that hearing loss is more frequent in males. However, we should mention that neither this study nor our research was population-based, therefore, the results suggest that the findings regarding gender vary according to the origin of the population studied. 14 Regarding the predominance of the most common types of hearing loss, and reporting the context found in the literature, epidemiological data from neonatal hearing screenings, without gender differentiation, show a predominance of sensorineural losses, accounting for 87.3% of hearing loss diagnoses, conductive hearing losses for 6.7%, and mixed losses for 6.0%. 15 Our study found a predominance of mixed hearing losses, since patients who undergo BAHA surgery have craniofacial malformations as etiologies, due to the specificity of the service.
When comparing the gains, we found a statistically significant difference between the functional and effective gains for all frequencies (Table 2), and, in all cases, EG had a lower value than FG, confirming that EG has better numerical results. The descriptive results regarding the mean by frequency among the four groups (Table 4)  function. 6 If the patient has sensorineural hearing loss, the hearing threshold with and without a free-field hearing aid deteriorates at the same rate. Thus, the difference between the threshold remains constant and, consequently, FG remains unchanged. 6 In patients with conductive hearing loss, the "unaided" free field hearing threshold is impaired, whereas the "with aid" threshold remains unchanged, this means that the FG improves when the conductive loss is lower, thus, it is dependent on the remaining air-bone gap. 6 Authors show that the EG is used as the main measure of the BAHA effectiveness, as the bone conduction thresholds obtained in pure-tone audiometry are properly masked to eliminate the participation of the best cochlea. When the individual is evaluated in free field, the response is nonspecific, with participation of the best ear, due to the low attenuation during bone conduction and vibrations in the skull bone. 6 On the other hand, the EG has the proven advantage that it can be compared to validated prescriptive rules, for example, the NAL (National Acoustic Laboratories) rule or the Desired Sensation Level (DSL) Rule. 18 Studies with bilateral BAHA users suggest that interaural attenuation is sufficient to evoke different responses of the two cochleae. Therefore, the reasoning allows us to conclude that we need to consider the VO obtained in ATL, as these are properly masked and named for compatible ears, also with the differentiation between right and left cochlea. 19 Our study also found that a higher FG correlates with a lower signal-to-noise ratio and a speech recognition threshold in silence (Table 3 and Figure   1), that is, when the performance with the prosthesis is worse, the speech test results are better. In a comparative study between two BAHA, the results showed that the speech perception scores for both devices improved in silence and noise. All users reported that their devices improved their quality of life. Overall satisfaction scores between device fitting and audiological testing were also statistically significant. This study shows that speech perception tests are linked to user satisfaction, that is, the better the speech recognition, the better the satisfaction. 20 The literature also highlights that speech tests are Comparative analysis between effective gain and functional gain in bone-anchored hearing aid users