Electroacoustic Stimulation for Partial Deafness

Auditory senses, like all other human senses, are subject to depreciation. Individuals suffering from mild to severe hear loss have considered cochlear implantation as a means to treat their condition. In 2008, the first cochlear implantation surgery in the Philippines was conducted. Doctors Charlotte Chiong and  Joachim Mueller performed the operation in the Capitol Medical Center. 

The cochlea is an important part of the auditory system. It is where the vibrations from the ossicles transmitted from the auditory canal is delivered. The cochlea contains the Organ of Corti, which contains the hair cells that are critical to transducting mechanical vibrations to neuronal signals. 

Often called Electroacoustic Stimulation or EAS, this procedure was first developed by Christoph Von Ilberg in 1999 as he demonstrated preserved resicual low frequency hearing by cochlear implantation where the patient wore a hearing aid in the operated ear.

While having stable low frequency hearing, a 33 year old patient was found to have degrading high frequency hearing. After performing poorly in speech perception scores; and since devices such as hearing aids did not solve his case, the patient resorted to EAS. 

The EAS system is comprised of two components. The external part is a microphone that gathers sounds and a processor that separately encodes low and high frequency energy. Afterwards, low frequency energy is transformed into an acoustic signal through the loudspeaker positioned in the ear hook and delivered into the auditory canal. This low frequency energy will reach and stimulate the apical part of the cochlea. High frequency energy on the other hand are converted to radio-wave like signals transmitted to an internal receiver. Then, electric signals are delivered to the implanted electrode array. It can be seen that the auditory system receives information via different paths but is still able to collect them and send these signals to the brain.

International development of the procedure lead to way to transform cochlear implantation to be minimally invasive, while boasting a  80-90% success rate. The operation starts via Soft Cochleoscopy where a careful drilling of the membranous labyrinth is performed. This meticulous process prevents ingress of blood and bone dust into the intracochlear compartment. After this, a small inscision is done to the oval window such that the electrode can be inserted. In this procedure some tropical antibiotics are applied to the operated area to control inflammation. Lastly, a soft tissue plug is applied to the electrode entry point to prevent perilymph or the liquid in the cochlea to spill.

                     A child who had undergone cochlear implant and uses an EAS device

After a considerable amount of time, the patient reported better overall sound perception and had performed better in speech perception tests. Patients reported improved conditions along prolonged experience with the implant.

Hearing performs a great role in people's perception. By just diminishing auditory acuity a tad bit changes lives; usually for the worse. Otolaryngologists have been performing operations like the EAS to improve hearing and the quality of life of the patients. New electrode designs that are thinner and more flexible are being developed to better accommodate patients and to preserve their hearing. The EAS technology spearheaded yet another breakthrough in prosthetic rehabilitation for hearing. 

References:

Chiong, C. (2010) Combined Electric and Acoustic Stimulation: Successful Treatment Option for            Partial Deafness. Philippine Journal Of Otolaryngology-Head And Neck Surgery. 25, (2).

Goldstein, E. (2013). Sensation and Perception. Wadsworth Cengage Learning. 9th ed.