By Xiying Guan, Ph.D
A typical inner ear has two mobile windows: the oval and round window (RW). The flexible, membrane-covered RW allows fluid in the cochlea to move as the oval window vibrates in response to movement from the stapes bone during sound stimulation.
Superior canal dehiscence (SCD), a pathological opening in the bony wall of the superior semicircular canal, forms a third window of the inner ear. This structural anomaly results in various auditory and vestibular symptoms. One common symptom is increased sensitivity to self-generated sounds or external vibrations, such as hearing one’s own pulse, neck and joint movement, and even eye movement. This hypersensitive hearing associated with SCD has been termed conductive hyperacusis.
Recently, surgically stiffening the RW is emerging as a treatment for hyperacusis in patients with and without SCD. However, the postsurgical results are mixed: Some patients experience improvement while others complain of worsening symptoms and have asked to reverse the RW treatment. Although this “experimental” surgical treatment for hyperacusis is increasingly reported, its efficacy has not been studied scientifically.
In the present study, we experimentally tested how RW reinforcement affects air-conduction sound transmission in the typical ear (that is, without a SCD). We measured the sound pressures in two cochlear fluid-filled cavities—the scala vestibuli (assigned the value “Psv”) and the scala tympani (“Pst”)—together with the stapes velocity in response to sound at the ear canal. We estimated hearing ability based on a formula for the “cochlear input drive” (Pdiff = Psv – Pst) before and after RW reinforcement in a human cadaveric ear.
We found that RW reinforcement can affect the cochlear input drive in unexpected ways. At very low frequencies, below 200 Hz, it resulted in a reduced stapes motion but an increase in the cochlear input drive that would be consistent with improved hearing. At 200 to 1,000 Hz, the stapes motion and input drive both were slightly decreased. Above 1,000 Hz stiffening the RW had no effect.
The results suggest that RW reinforcement has the potential to worsen low-frequency hyperacusis while causing some hearing loss in the mid-frequencies. Although this preliminary study shows that the RW treatment does not have much effect on air-conduction hearing, the effect on bone-conduction hearing is unknown and is one of our future areas for experimentation.
A 2017 ERG scientist funded by Hyperacusis Research Ltd., Xiying Guan, Ph.D., is a postdoctoral fellow at Massachusetts Eye and Ear, Harvard Medical School, in Boston.