Surgery is performed under a general anesthetic without muscle relaxation to allow for facial nerve monitoring. The device location is marked out with the use of templates. It is important to place the internal device far enough posteriorly so that the processor that is placed behind the ear does not lie against it and render the underlying skin at risk. In an effort to avoid the complications associated with a skin flap breakdown, it is imperative to plan the skin incision in order to provide adequate exposure while both preserving tissue viability and avoiding the placement of a suture line directly over implanted hardware. The location of prior incisions should be taken into account. Most surgeons use a postauricular incision that may be extended slightly more superiorly than what might be typically used in a routine ear surgery. The current devices can be placed through a 3–4 cm cosmetically acceptable incision. In children, the scar is less likely to widen with head growth if it is located only in the postauricular area and does not extend up into the scalp. Due diligence should be rendered in manipulating the tissues of the skin flap to assure minimal trauma.
Mastoidectomy & Cochleostomy
After the initial incision, the periosteum is elevated from the mastoid and a mastoidectomy is performed. The facial recess is opened to gain access to the middle ear—specifically to the promontory, the round window niche, and the stapes (Figure 68–7). The chorda tympani nerve is preserved and the incus buttress can be left in place. According to the shape and size of the particular device chosen, a well may be drilled posterior to the mastoid cavity in the cortex to harbor the receiver–stimulator package. In children, this dissection is often carried down to the dura so that the device can be recessed. This better protects the device from trauma and is more cosmetically appealing. In adults, because of thicker bone, the device can be adequately recessed by removing bone to the inner table of the skull. The device can be placed in a tight subperiosteal pocket or secured by various means such as suture.
After the device is seated, a cochleostomy is then made inferior to the round window membrane with a goal of affording access to the scala tympani (Figure 68–9). Principles of minimally traumatic surgery should be employed in all cases in an effort to preserve structure and function. These atraumatic techniques include elements such as a more inferiorly based cochleostomy site to assure wide clearance of the osseous spiral lamina, minimizing access of bone dust or blood into the cochlea, avoiding suctioning of perilymph, slowing methodical electrode array insertion, and utilizing adjuvants such as corticosteroids and/or lubricants. Following successful insertion, small pieces of fascia or periosteum are used to carefully seal around the cochleostomy.
The basal turn of the cochlea is approached via drilling out the facial recess. The coiled electrode array is inserted into a cochleostomy that is fashioned 1 mm anterior and inferior to the round window.
Preservation of residual hearing is possible with a full insertion of a standard electrode array when minimally traumatic surgical technique is utilized, but with current techniques it cannot be reliably achieved. Hearing preservation, and thus electroacoustic stimulation are best obtained by implanting a shortened electrode that does not extend into and disturb the apical cochlear neural elements. Because these devices are quite small and flexible, residual hearing can be preserved in more than 90% of patients at the time of surgery. Unfortunately, there is the potential for delayed hearing loss, typically in the first 3–6 months, the etiology of this remains unknown and is actively being investigated. A direct round window membrane incision may be favored over cochleostomy by some surgeons for its potential to be less traumatic.
Intraoperative Electrical Tests
Depending on the device and the availability of audiology support, intraoperative electrical tests can be performed to confirm the proper functioning of the device. Evoked potentials and stapedial reflexes can be measured, which may be particularly helpful in programming with young children. Finally, if cochlear nerve action potentials can be recorded and a stapedial reflex is elicited, the surgeon can be quite confident that the device is indeed in the cochlea. After wound closure, a Stenver's view x-ray can be obtained for further confirmation of the device location and for reference in the case of future trauma or device migration (Figure 68–10). Patients may be discharged from the hospital on the same day, or they may spend one night in the hospital.
Postoperative x-ray image of a cochlear implant. Note the spiral coil within the cochlea.