The impact of Tinnitus
Tinnitus impacts an estimated 15% of the population ().
Its impact is felt in a number of different ways (as described left) and financially, as the No. 1 claimed benefit at the VA and and estimated $26 Billion in societal costs (American Tinnitus Association)
What Initiates Tinnitus?
Tinnitus usually starts with damage to the Cochlea (shown here), whether that is the nerves or hair cells.
This damage creates abnormal or phantom signals to be created
Why does it become chronic?
The abnormal signals are processed by the primary auditory cortex (A1). A1 has a "tonotopic map" meaning that each frequency has a specific location in the brain responsible for processing it.
A1 can fail to adapt, or incorrectly adapt to the abnormal signals., creating a chronic condition
So what can be done?
The classical approach which relies on cognitive behavioral therapy.
The idea is to improve the individual's "Cognitive Control", or their ability to control their attention. Our brains are often really good at ignoring sounds that are unimportant, and if we train that, we can reduce the perception of the tinnitus and therefore its impact.
New therapies, such as brain stimulation, are looking to change the sensation of tinnitus.
By changing the activity of the auditory nerves, either near the cochlea or in the primary auditory cortex, you may be able to change the volume of the tinnitus, maybe even mute it.
A meta-analytic approach
We took a meta analysis approach, which means instead of collecting new data, we take gather all of the current studies on the topic and treat those as our data points.
We searched pubmed, google scholar, and the grey literature for all studies that used tDCS as an intervention for tinnitus
Included studies with and without control groups (controlled trials vs case studies). Some studies included more than 1 treatment group, which we separated
There are a number of different settings for tDCS that can affect the results. We tried to separate the studies based on the settings each used, hoping to figure out which settings led to the best outcomes.
So what did we find?
Overall both the controlled trials and the case studies showed a small but significant effect, such that the use of tDCS improved (reduced) tinnitus symptoms
Targeting the DLPFC had a much larger effect than targeting the LTA. This would suggest that tDCS is better at helping control perception, enhancing the same mechanisms as conventional therapy. tDCS is not as effective at changing sensation, as evidenced by the smaller effect size when targeting the LTA (the auditory cortex)
Interestingly, despite the difference in target mechanism, there was no difference in effect depending on the symptom.
This may be because of limitations in how these symptoms are calculated, but we don't know.
While not surprising given the last 2 results, but there is no interaction effect between target mechanism and outcome. Whether you are trying to reduce distress or loudness, targeting the DLPFC has a larger effect
So what does it all mean?
A small but significant effect
Our results show a benefit to using tDCS to treat tinnitus. Surprisingly, targeting the DLPFC outperformed targeting the LTA for both perception and sensation.
Based on these results future research should focus on trying to combine tDCS to DLPFC with current Cognitive Behavioral Therapy practices.
New Relevant Research
Since I released the preprint, a couple of large reviews, including one meta-analysis have also been published. Both agree with the overall conclusions of my analysis.