Purpose: Auditory acclimatization refers to systematic changes in auditory performance over time, linked to the acoustic modifications introduced by hearing aids, with improvements extending beyond task-specific or training effects. This pre-registered systematic review expands on previous reviews by examining a broader range of outcomes, including auditory (e.g., speech recognition, electrophysiological responses) and selected non-auditory measures (e.g., self-reported measures). It aimed to determine the presence and magnitude of acclimatization and identify influencing factors, focusing on controlled trials comparing post-fitting changes in aided outcomes against a control group. Notably, this was the first review to comprehensively report on self-reported outcomes, marking a potentially significant contribution to the field.

Method: A systematic literature search was conducted in CINAHL, PubMed, and Web of Science in March 2024. Eligibility criteria, based on the PICOST framework, required studies to focus on new adult hearing aid users with sensorineural hearing loss using air conduction hearing aids, reporting speech recognition, self-reported, or electrophysiological outcomes, and employing a comparator in prospective designs with at least two data points measured within the same condition to observe changes over time. Exclusions applied to studies involving children, advanced feature devices, surgical implants, non-peer-reviewed work, or those with no comparator (uncontrolled studies). The review was registered on PROSPERO and followed PRISMA guidelines. A meta-analysis was attempted but excluded due to missing data.

Results: The systematic review included 25 controlled studies on auditory acclimatization. Of these, 18 examined behavioural speech recognition, with 10 reporting acclimatization, 1 mixed result, and 7 no acclimatization. Among 8 studies with self-reported outcomes, 3 supported acclimatization, 3 showed mixed results, and 2 found no evidence. For electrophysiological outcomes, 4 of 7 studies reported acclimatization, and 3 did not. Consistent hearing aid use and hearing loss severity appeared to influence acclimatization, while cognitive abilities and age showed no significant impact.  Of the 25 studies reviewed, 16% were rated as good quality, 80% as fair, and 4% as poor, with common quality issues including lack of randomization, blinding, and insufficient reporting on sample size.

Conclusion:  This review highlighted the complexity of auditory acclimatization to hearing aids, which could be influenced by various factors. Evidence suggested that acclimatization occurred in some users and for certain outcomes, while not for others, with overall modest improvements and variable results. The most consistent gains were observed in speech recognition in noise and self-reported measures, including the APHAB, HHIE, and GHABP questionnaires, although changes were generally modest. Future studies should include essential statistical data, such as correlation coefficients for within-subject designs, to enable robust meta-analyses, prioritize well-controlled studies, and ensure early baseline and key interval measurements to better isolate and quantify acclimatization effects.