Much of the existing research on this area was conducted pre-2017 when the OTC hearing aid category did not exist in the United States. As the hearing devices (i.e., PSAPs, hearables, direct-mail hearing aids) used in the research discussed below were available to consumers via DTC channel, we can call them as DTC hearing devices. Nevertheless, a few systematic reviews on consumer hearing devices suggested that the available literature can be grouped into three key themes focusing on 1) acoustic quality of hearing devices, 2) consumer surveys, and 3) clinical trials as discussed below (Manchaiah et al., 2017; Maidment et al., 2018; Tran & Manchaiah, 2018; Chen et al., 2022). First, a series of studies examined electroacoustic characteristics in the test box (e.g., frequency response, distortion, equivalent input noise) of DTC hearing devices such as PSAPs and hearables which showed mixed results. Some studies showed that these devices were of very poor acoustic quality (Callaway & Punch, 2008; Chan and McPherson, 2015), whereas other studies concluded that some of these devices have appropriate acoustic characteristics for people with hearing loss (Reed et al., 2017). These results highlight the importance of quality of acoustic output in device selection. It is noteworthy that some of these studies point to the fact that higher priced devices generally have better acoustic quality (Almufarrij et al., 2019). It is also important to note that most of the evaluated devices are not currently offered as FDA-regulated OTC devices. Second, a few large-scale consumer surveys on DTC hearing aid users in the United States (Kochkin, 2010) and Japan (EHIMA, 2022) show that the benefit and satisfaction reported by users of devices such as PSAPs and direct-mail hearing aids is generally much lower when compared to users of prescription hearing aids fitted by HCPs. The reason for this can be attributed to the poor quality of DTC devices available a decade ago, as well as users may not have met one of the five pre-requisites discussed above. Finally, the third group of studies included clinical trials focused on the outcomes of DTC hearing devices (Maidment et al., 2018; Tran & Manchaiah, 2018; Chen et al., 2022). These studies generally showed positive outcomes in self-reported hearing aid benefit and satisfaction measures [e.g., Abbreviated Profile of Hearing Aid Benefit (APHAB)] as well as in behavioral measures (e.g., speech in quiet, speech in noise). However, the main criticism of these studies is that they generally used single-group pretest-posttest study designs without a control group (e.g., Sacco et al., 2016; Mamo et al., 2017).

In another study, Humes et al. (2017) performed a three-arm double-blind placebo-controlled trial comparing a gold standard audiologist fitted group with a consumer decides self-fit group and a placebo group (n = 154 across all groups). Participants from all three groups used prescription hearing aids, although the self-fit group used a device with pre-set programs and the placebo group used a device with no functional gain. The study results showed that the self-fit group presented with only slightly poorer outcomes in self-reported and behavioral measures when compared to an audiologist-fitted group demonstrating the efficacy of the OTC service delivery model. In a follow-up study, Humes et al. (2019) further examined the consumer decides self-fit model with less front-end screening in a double-blind clinical trial (n = 40). Participants were asked to choose one of the pre-programed hearing aids (like a pre-set OTC hearing aid) one of which included a placebo device with functional gain. The outcomes of the two groups with pre-programmed hearing aids with gain were comparable and were superior to the placebo group. The study also highlighted that the presence of red-flag conditions (e.g., cerumen) did not impact the purchase decision of the users raising some concerns about consumers ability to self-identify their candidacy for OTC hearing aids.

All the studies discussed above used either early generation DTC hearing devices (Manchaiah et al., 2017; Tran & Manchaiah, 2018) or prescription hearing aids with limited features to simulate an OTC hearing aid (Humes et al., 2017; Humes et al., 2019). Moreover, they focused on either the device or the service delivery model which limits the ecological validity and generalizability. Overall, the key takeaway from these studies is that if users choose an appropriate device, then they are likely to have some measurable benefit from using them.

Table 1 presents a summary of completed as well as on-going RCTs studies examining OTC hearing aids and/or the service delivery models. Of these, only two studies have been published in a peer reviewed journal (Sabin et al., 2020; De Sousa et al., 2023) and two other studies are marked as complete in the clinical trials registry. It appears that most of these RCTs (4 of the 8 listed in Table 1) are industry sponsored studies examining self-fitting algorithms or process when compared to audiologist-fitted devices for regulatory approval from the FDA. Two of the ongoing studies that are funded by a federal agency (National Institute on Deafness and Other Communication Disorders; NIDCD) and a non-profit organization (i.e., Patient-Centered Outcomes Research Institute; PCORI) seem to have a large sample size (n = 240–591) and aim to investigate service delivery models. In most of the studies, OTC hearing aids that are currently on the market or likely to come to the market in the near future are being investigated which increases their ecological validity. All of the studies include self-reported and/or behavioral measures as primary and secondary outcome measures. Unlike blinded RCTs (e.g., Humes et al., 2017, 2019), these studies use devices that are currently on the market with the existing branding information. There may be some placebo effects that could potentially impact outcomes of both self-reported and behavioral outcomes (Dawes et al., 2013). This highlights a need for studies that also include more objective outcomes like electrophysiological markers to examine non-subjective benefits of these devices and associated service delivery models (Glick & Sharma, 2020).

There is an immediate need for research on all aspects of OTC hearing aids including hearing device characteristics, service delivery models, the user and the complex interaction between these three domains. The following are some aspects that we think are highly relevant and timely to inform hearing healthcare policy, clinical care, industry decisions and increase much-needed hearing aid uptake. The key question during the last decade was whether OTC hearing aids provide measurable benefit to users. This question continues to be important but since devices are already available on the marketplace through a regulatory framework (requiring FDA non-inferiority trials for self-fitting OTC hearing aids), other pressing questions should be prioritized. Important questions should consider for whom OTC devices and respective service delivery models work and what the predictors of success are. We outline some specific questions below:

Hearing devices related:

⁃ What is the range in electroacoustic characteristics of OTC hearing aids that are currently on the market and how many of them are appropriate for individuals with mild-to-moderate hearing loss?

Service delivery model related:

⁃ What are the contextual facilitators and barriers to implementation of OTC service delivery models in different settings according to stakeholders such as users, HCPs, patient organizations, managers of the health systems, companies manufacturing and distributing OTC hearing aids as well as potential payers such as insurance companies?

User related:

⁃ Are there specific users based on their biographical, demographic, and audiological variables who are more likely to seek OTC hearing aids and successfully navigate the OTC service delivery model in terms of key prerequisites?