Manabi Hyperacusis
Scientific information on sound therapy for hyperacusis: rationale, mechanisms and literature.
Manabi Hyperacusis
Sound therapy treats hyperacusis primarily through the reversal of the pathologically increased central auditory gain – the maladaptive neuroplastic amplification that arises when the brain compensates for reduced cochlear input. Sustained, controlled exposure to low-level sound reduces this excess gain and – together with habituation of the limbic-autonomic response to sound – raises the loudness discomfort levels (LDLs) and the dynamic range, so that moderately loud sounds are no longer perceived as unbearably loud. [1–2]
Rationale
Hyperacusis is thought to arise when reduced afferent input from the cochlea (due to hearing loss, noise or ototoxicity) triggers a homeostatic increase in the excitability of central auditory neurons. Animal models show that despite reduced neural output from the cochlea, sound-evoked responses in the inferior colliculus, the medial geniculate body, the auditory cortex and the amygdala are proportionally enhanced – the neuronal correlate of an increased central gain that makes ordinary sounds appear excessively loud. [3–4] Human MRI studies confirm this, showing hyperactivity in Heschl's gyrus, the superior temporal gyrus and parahippocampal regions of affected patients. [5]
The mechanistic premise of sound therapy is that this gain is neuroplastic and thus reversible: supplying a sustained, tolerable acoustic stimulus to the deprived auditory system removes the "stimulus deprivation" that drove the compensatory upregulation, allowing the central gain to renormalize. [1]
Mechanism of Action
Evidence
The central gain model is strongly supported by animal and imaging data, but the mechanism is not fully understood: Radziwon et al. found that even with central amplification, the responses in cortex and amygdala were insufficient to fully explain behavioural hyperacusis, pointing to further, as yet unidentified mechanisms. [4] Clinically, the evidence for the efficacy of sound therapy is limited by a lack of well-controlled trials, and treatment effects are often moderate and confounded by the counselling and amplification components used at the same time. [1][8] Improperly aggressive exposure can also worsen symptoms, and excessive protection with earplugs (silence) is counterproductive, as it further increases the central gain. [2]
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