Combined Electric and Acoustic Stimulation: Successful Treatment Option for Partial Deafness
Authors: Chiong Charlotte M. 
Published: April 2019

Cochlear implants are now the treatment of choice for patients with severe to profound hearing loss.  Inclusion criteria for cochlear implantation have expanded, and a whole array of implantable hearing devices have been introduced over the years.  To date, more than 250 cochlear implantations have now been performed in the Philippines (Figure 1). In 2006, the first auditory brainstem implantation, and first vibroplasty or middle ear implantation in the country were done at the Philippine General Hospital (PGH).  In 2008, the first electroacoustic stimulation or partial deafness cochlear implantation surgery in the country was performed at the Capitol Medical Center by Professor Joachim Müeller of the University of Würzburg and the author. This concept, that cochlear implantation can be performed for patients with residual hearing or only partial deafness, is quite novel. There are  patients whose low frequency hearing below 1.5 kHz is still be quite good while high frequency hearing loss above 1.5 kHz is in the severe to profound range (Figure 2).  For such patients speech discrimination scores will typically fall below 60% at 65 dB sound pressure level (SPL) in the best aided condition.

            This technological advancement, often called electroacoustic stimulation (EAS), was developed in 1999 after Christoph  Von Ilberg demonstrated preserved residual low frequency hearing in a patient who underwent cochlear implantation such that the patient wore a hearing aid in the implanted ear.1

Currently, EAS devices are available from two manufacturers. Contraindications to the use of EAS are shown in Table 1. Candidates for EAS devices should have stable low frequency hearing. There should be no progressive or autoimmune sensorineural hearing loss. Also there should be no history of meningitis, otosclerosis, or any other malformation that might cause an obstruction.  The patient’s air-bone gap should be < 15 dB. Finally, there should not be any external auditory canal problems that can impede placement of the ear mould for the acoustic component.

            There are two main components of the EAS system (Figure 3). The external component is made up of a microphone that picks up sounds and a processor that separately encodes low and high frequency energy. After processing, low frequency energy is converted into an acoustic signal via the loudspeaker located in the ear hook and delivered into the external auditory canal. This acoustic signal will vibrate the tympanic membrane and ossicles so that cochlear fluids as well as the relatively intact structures of the cochlea in the apical region are stimulated. In contrast, high frequency energy is coded into radio-wave-like signals which are transmitted transcutaneously to the internal receiver.  There, electric signals are delivered to the electrode array that has been surgically implanted into the cochlea. Thus the auditory nerve receives information using two different pathways from low and high frequency sounds, and the auditory nerve signals are then transmitted to the brain.

Our Experience:

            Of the more than 100 implantations done under the Philippine National Ear Institute “CHIP” or Cochlear and Hearing Implants Programme only one was a case of EAS implantation.  This particular case demonstrates key principles and concepts that every otolaryngologist should consider. Among these are audiological evaluation, temporal bone imaging, surgical technique for hearing preservation and some quality of life issues.

Audiological Evaluation

            A 33 year old man had been seen at the clinic for over 7 years, with serial  audiograms (Figure 4-6) illustrating the presence of good and stable low frequency hearing while high frequency hearing loss  increased somewhat. The patient had been continually advised to get the best hearing aids available. However, a series of high-end hearing aids did not solve his problem of poor hearing in noisy places nor his difficulty understanding words when watching television and movies. Figure 7A shows the speech perception scores of this patient obtained  with a Word Intelligibility by Picture Identification (WIPI) test, a “closed-set test” using isolated words  while Figure 7B  represents speech scores when “open-set” Bamford-Kowal-Bench (BKB) Sentence Lists were presented to the listener in both quiet and noise prior to the implantation.

Temporal bone imaging

            A combination of high resolution computerized tomography (HRCT) of the temporal bone with both coronal and axial cochlear views, and T2-weighted normal anatomic Fast Spin Echo (T2 FSE) or 3D Constructive Interference in Steady State (3D CISS) MRI sequences of the inner ear should be done. Results from both studies should ascertain whether the cochlear duct is patent, ruling out any cochlear fibrosis or obstructive pathology. This patient’s HRCT and 3-D CISS MRI studies showed no such cochlear obliteration that would have posed intraoperative difficulties and constituted contraindications to EAS surgery (Figure 8).

Surgical Technique for Hearing Preservation

A variety of techniques have evolved over the years into what is now commonly called minimally invasive cochlear implantation. Using minimally invasive techniques, residual hearing can indeed be preserved in over 80%-90% of patients 3,4 Initially, a “Soft Cochleostomy” technique was introduced. This entailed careful low-speed drilling of the promontory with a Skeeter® drill (Medtronic Xomed, Jacksonville FL, USA) followed by the use of a mini-lancet to make an opening in the membranous labyrinth. This method avoids direct suctioning and prevents ingress of blood and bone dust into the intracochlear compartment.   Also, for this method, the endosteum is left intact after drilling a cochleostomy antero-inferior to the round window. This allows proper placement of the electrode into the scala tympani with less chance of injury to the basilar membrane. Later, a round window approach was introduced, and it also proved to be a reliable way to preserve residual hearing during cochlear implantation. For this method, a more direct round window approach is performed after careful drilling of the round window niche. A limited incision is made just large enough to allow the electrode to be inserted. For both methods, after the endosteal or round window membrane incision is made with a micro lancet, a very flexible electrode of 20 mm length is slowly inserted. During the insertion process, the cochleostomy or round window is kept under direct vision so that insertion forces are minimized.  Topical antibiotics and steroids are applied at this time to reduce any inflammatory or apoptotic reactions related to the trauma of opening the cochlea and introducing an electrode. Finally, a soft tissue plug is placed tightly around the electrode entry point into the membranous labyrinth to prevent perilymph leakage. New electrode designs that are thinner and more flexible are important contributors to the preservation of hearing.

Postoperative Outcomes and Quality of Life

            After about 4-6 weeks from the time of surgery the EAS implant is switched on. Based on our experience and that of others,3 speech perception performance  improves  with prolonged experience with the implant. Roughly 1 ½ years post-surgery this patient has achieved dramatic improvement in hearing both in quiet and in noise using the EAS compared to using only the hearing aid component or the CI component alone. Figure 9 shows this dramatic improvement in free-field pure tone thresholds. Figure 10 demonstrates the speech perception following EAS implantation compared to pre-EAS implantation. Audiologic evaluation done at the PGH Ear Unit using  20 phonetically balanced Filipino words familiar to the patient  in quiet and with 55 dB masking noise in the side of the implanted ear  clearly showed an advantage with the EAS configuration compared to either hearing aid or CI component alone. Even with noise, this patient actually performed better presumably because he may have concentrated more with the introduction of masking noise. Another factor of course is that the words have now become familiar to the patient with the previous testing done in quiet.

Notably, he reported great subjective improvement after only 10 months post-surgery.5 Interestingly the patient’s only complaint during his last follow-up was that he had not been offered bilateral EAS implantation.

It is always important for the otolaryngologist to consider the quality of hearing and quality of life of patients with hearing loss.   Intervention should not end with a referral note to a hearing aid center or dispenser. It is important to request proof of improvement not only of hearing thresholds but of speech perception outcomes in quiet and in noise. That is, one should document actual performance with the device in place, regardless of the type of device (hearing aid, an EAS device, or a Cochlear implant).  Minimal disturbance of the remaining intact structures of the cochlea of patients with low frequency residual hearing can be achieved by employing a meticulous surgical technique, by using the advanced and flexible electrodes developed by some manufacturers, and instilling intraoperative antibiotics and steroids.  Thus when one is faced with a ski-slope type audiogram it is likely the patient with this audiogram will not benefit from hearing aids.  Such patients should be offered the option of EAS implantation which combines good acoustic stimulation with electric stimulation using a shorter (than conventional cochlear implantation) but very flexible electrode system. Counseling must also be done with a special emphasis on the risk of losing residual hearing, and noting that post-operative rehabilitation may take a long period of time. This patient now has a better quality of life than was obtainable from the most expensive and advanced hearing aids in the market, and has demonstrated a new implantable solution to partial deafness. Truly, EAS technology has opened a new era in prosthetic rehabilitation for hearing impaired adults and children.5

Acknowledgement

Dr. Maria Rina Reyes-Quintos is gratefully acknowledged for performing all the excellent audiological testing following the surgery while Susan Javier and Angie Tongko of Manila Hearing Aid Center performed all the audiological testing prior to the surgery. Ms. Celina Ann Tobias, Professional Education Manager of Med-El is also credited with thanks for preparing the figures, reviewing the manuscript and interviewing the patient regarding his hearing performance following the surgery.