9     Electroacoustic Stimulation

9.1 Hearing Preservation and Electroacoustic Stimulation

Several investigators have reported the unintended presence of residual hearing after cochlear implantation surgery, showing that hearing preservation is indeed possible while at the same time opening the cochlea and inserting an electrode. As early as 1993 reports on soft surgery techniques were presented in the search for the most effective surgical principles of hearing preservation.1 Prevention of blood and bone dust entering the cochlea, as little drilling on the cochlea as possible, very slow and atraumatic insertion of the cochlear electrode in the midportion of the scala tympani, and use of atraumatic, tapered, and flexible implants were some of the measures discussed.

With the introduction of electroacoustic stimulation (EAS) in 1999, hearing preservation became even more important because now this intended residual hearing was in fact being used.²

In EAS the low, apical frequencies are preserved and acoustically stimulated, while the high, basal frequencies are electrically stimulated with a cochlear electrode. For this technique a combined electric and acoustic speech processor had to be developed (Fig. 9.1) and the length of the cochlear electrode array was shortened in an attempt to preserve the residual hearing in the low frequencies.³ The audiometric results showed an increase in speech understanding in quiet and in noisy situations in comparison with only cochlear implantation,^4,5 making this a very beneficial option for patients with a ski-slope audiogram, who before this development found themselves caught between hearing aids that gave inadequate amplification and standard cochlear implantation with the attendant risk of losing their residual hearing.⁶

With the special surgical techniques necessary for this approach and the development of a combined electroacoustic speech processor, high success rates can now be achieved.^7,8

Nevertheless, loss of hearing after EAS still occurs either in the early or in the late phase, without the reasons being completely known. This puts the surgeon’s aims in jeopardy, because the success of EAS cannot be predicted preoperatively, while at the same time the decision for the shorter electrode used in EAS has already been made with the intention of not affecting the cochlear apical hair cells. Also, a shorter electrode is not favorable with future progression of the underlying pathology. Current research therefore focuses on hearing preservation using electrodes that are of nearly standard length but thinner and more flexible, to provide better pitch matching and possibilities for use as a “normal” cochlear electrode when residual hearing fades.

9.2 Indication for EAS

9.3 Influential Factors in Hearing Preservation

9.3.1 Atraumatic Surgical Technique

The round window insertion is used in this type of surgery to achieve adequate positioning of the electrode in the midportion of the scala tympani, without damage to the modiolus or the spiral ganglia.9,10 As explained in Chapter 2, the round window margin cochleostomy (RWMC) is often used, in which the crista semi-lunaris and crista fenestrae are removed by drilling for easier introduction (see Fig. 5.1.14). The amount of bone removal in this procedure is minimal, ~1 mm.¹¹ However, a normal round window cochleostomy (RWC) without drilling is even better if anatomy allows. While performing this, care must be taken not to damage the cochlear aqueduct, the basilar membrane, and the osseous spiral lamina, which are all closely related to the bony overhang above the round window.⁹ Also the common modiolar vein, which provides drainage of the entire cochlea and is situated closely to the cochlear aqueduct, should be preserved because once it is compromised this will lead to deterioration of cochlear function.

The long axis of the scala tympani is entered from superior-posterior to anterior-inferior (the right angle of insertion), in the middle of the scala tympani, below the basilar membrane and spiral ligament. These techniques will minimize insertion trauma.

Mechanical Trauma Due to Insertion

Fractures of the osseous spiral lamina, disruption of the basilar membrane, tearing of the lateral spiral ligament or of the endothelial layer of the scala tympani, and leakage of traumatized blood vessels can all lead to trauma and subsequently to hearing deterioration in the cochlea.¹² Disruption of the inner ear homeostasis with perilymph and endolymph mixing due to trauma to the separating membranes also leads to toxic effects on the hair cells and loss of the residual hearing. For these reasons, in addition to the RWC or RWMC technique as described, whenever resistance is encountered during introduction of a cochlear electrode, the electrode should not be inserted with force: after pulling it back slightly, a second gentle attempt should be made after rotating the electrode slightly.

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