The first objective of this systematic review was to evaluate the evidence that auditory training influences tinnitus-related problems. Overall, 9 out of the 10 studies reported a statistically significant change. However, our preceding narrative synthesis of the quality assessment concluded that this evidence is not of sufficient quality to be confident that auditory training represents a strategy for improving the percept and severity or handicap of tinnitus. The evidence is simply not yet robust enough to guide treatment.
Turning now to the second objective concerning whether auditory training could provide an effective clinical management strategy, we discuss key issues that are fundamental to determining the efficacy of auditory perceptual training for tinnitus and, subsequently, to realising its development into a clinically useful tool. Whilst most studies reported some significant effect of auditory training on their chosen outcome measures, none explored the implications for their finding on the desired endpoint of this line of research, i.e. tangible benefit for the person who has tinnitus. This leaves open questions about the nature of any translational benefit of auditory training for tinnitus, and whether any benefits are specific to auditory training. We suggest a number of practical recommendations for future research to be considered when designing a high-quality randomised controlled trial.
First, it is important to evaluate whether or not the appropriate outcome measure is being used. The selection of a primary outcome measure should be driven by the desired observation. McFerran and Baguley [56
] propose that, from the perspective of the audiologist, inhibiting tinnitus should be the aim of clinical intervention. Such a viewpoint would suggest a psychoacoustic measure of tinnitus (such as tinnitus loudness or minimum masking level) as the primary outcome measure rather than a self-reported measure (such as a reduction in tinnitus distress). However, based on this review, we recommend that a combination of psychoacoustic and self-reported outcome measures is implemented so that we can understand how the intervention reduces tinnitus percept and alleviates tinnitus distress.
Second, confidence in the stability of pre-training measures is crucial in order to be able to attribute patient benefit to the specific intervention. Perhaps this issue is of utmost concern for self-reported outcome measures using questionnaires about tinnitus handicap or distress, because clinical intuition suggests that the psychological response often naturally diminishes over time. For example, Dohrmann et al. [35
] provide information on the test–retest reliability of the Goebel-Hiller tinnitus questionnaire by measuring tinnitus severity twice before training. The score reduced from 34.5 to 29.5, with no further change after training. A drop of 5 points equates to a clinically meaningful shift in tinnitus severity from moderate to mild, which the authors attribute to an ‘anticipation of treatment’ effect. Hence, designs that do not confirm a stable baseline measure cannot attribute any change after auditory training to be a specific benefit of the intervention. To rule out non-specific benefits, we recommend applying a test–retest procedure during the pre-training stage or using a crossover comparison design.
Third, it is important to evaluate whether or not the outcome reflects a clinically significant change that shifts someone from one category of tinnitus severity to a less severe category. A change in Goebel-Hiller tinnitus questionnaire score of 5 points is considered clinically relevant and observable in a clinical population [54
]. Indeed, the two studies using this questionnaire reported a mean change of at least this magnitude [35
]. Herraiz et al. [32
] favoured the tinnitus handicap inventory, and a change of between 4.9 and 9.4 was reported as significant between the training and control groups. The tinnitus handicap inventory is a well-validated questionnaire for an initial assessment of tinnitus severity, but is less sensitive as a measurement tool for treatment outcome. The 95% confidence interval of the tinnitus handicap inventory score is 20 points [55
], meaning that a change of at least 20 points is required for the difference to be considered clinically significant. Hence, across the studies from Herraiz et al., auditory perceptual training would appear not to facilitate a significant clinical
benefit. Moreover, details of individual benefit were not provided, so any indication of how many participants reduced their tinnitus handicap by 20 or more cannot be assessed. In the future, we hope that the work from Henry et al. [58
] will deliver a standardised tinnitus outcome measure for clinicians and researchers. These authors are developing a tinnitus functional index questionnaire, specifically to have high discriminative and evaluative validity for the assessment of tinnitus treatment outcome. Until this becomes available, we advise against the use of the tinnitus handicap inventory as a self-reported outcome measure of change
Fourth, given the commitment of time and resources that auditory perceptual training can involve, it is important that any benefit is proven to be maintained beyond the training period. Longer-term outcomes are largely unreported in the reviewed studies and follow-up assessment should be a priority for future research.
Fifth, research to date has focused on the alleviation of tinnitus symptoms and has ignored usability issues relating to the training technologies. Whilst proof of concept is certainly an essential part of this aspect of translational research, to be broadly successful, any training regime must have intrinsic motivation [59
]. Indeed, a recent review of the efficacy of auditory training in adults highlighted that training needs to be engaging and to promote a desire to train through feedback or reward [61