Hybrid Electroacoustic Devices:
Many patients have sloping hearing loss, with good hearing preservation at low frequencies and profound loss at high frequencies.
Insertion of an intracochlear electrode usually results in loss of residual natural hearing, due to trauma to intracochlear structures, inflammation and loss of the normal environment of the inner ear hair cells during surgery. Even with conventional cochlear implant electrodes, however, it was noted that occasionally the residual natural hearing was preserved after surgery.
This led to efforts to design slimmer, less traumatic electrodes which would preserve residual hearing on a more consistent and regular basis. The high frequencies could then be stimulated electrically while the low frequencies could be stimulated acoustically with a Hybrid ElectroAcoustic Device. This should in theory allow for better hearing than with a cochlear implant alone, especially in the presence of noise.
Early research focussed on short straight electrodes which would be partly inserted into the cochlea to stimulate the high frequency neurones (which are at the base of the cochlea), while leaving the distal cochlea and low frequency hearing untouched. However it is not possible to guarantee preservation of hearing after inserting an electrode, and even when hearing is preserved it may deteriorate later due to intracochlear fibrosis or the underlying disease causing the hearing loss in the first place. If a short electrode is used and the low frequency hearing drops, the short electrode will not provide electrical stimulation of the whole cochlea. However, insertion of a long electrode, even a thin atraumatic design, carries an increased risk of trauma to the cochlear structures and loss of residual hearing. Determining the optimum depth of insertion to allow full electrical stimulation if required while maximizing the chances of preserving residual hearing is the current focus of electrode design and development.
Newer electrodes have been designed to be slimmer, softer and with soft tips to minimize trauma during insertion.The Nucleus straight array is a thin atraumatic electrode that is optimally inserted to a depth of 20mm and minimizes trauma and hearing loss. MedEl’s EAS electrode is also designed for an atraumatic insertion depth of 20mm, while some surgeons prefer the Flex28 electrode which allows insertion up to 28mm, although as stated above deeper insertion may be associated with more loss of residual hearing. The Advanced Bionics mid-scalar electrode is also thin and avoids trauma to the cochlear structures, and has potential for hearing preservation.
In addition to electrode design changes, a range of “Soft Surgery” techniques are used to maximize the chances of preserving hearing. Low speed drills, minimal drilling, and a round window approach to electrode insertion all reduce cochlear trauma. Slow careful insertion of the electrode reduces hydrostatic trauma and trauma to the basilar membrane. Patients are given steroids during surgery, and often for 1 week after surgery to reduce inflammation and fibrosis within the cochlea. The role of intracochlear steroids and steroid eluting electrodes is still being determined. Antibiotics are routinely given during cochlear implant surgery, but a postoperative 1 week course of antibiotics may be indicated to maximize the chances of hearing preservation.
Various studies report a range of hearing preservation results. If the low frequency hearing remains above 65dB after surgery, acoustic stimulation is worthwhile, and the high frequencies are stimulated electrically. If the low frequency hearing drops below this level, immediately or delayed, electrical stimulation is the only option, so adequate coverage of the cochlea for electrical stimulation is required. This may involve revision surgery to reposition the electrode, or replacement with a device with a longer electrode.
Indications: EAS devices are indicated for patients with significant residual low frequency hearing as shown below within the shaded area), and monosyllable word scores of <60% at 65dB SPL in the best aided conditions.
When successful, the results from EAS hybrid devices exceed those from cochlear implants alone, as shown.