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Sound Distortion in Cochlear Implant

Djalilian, Hamid R. MD

doi: 10.1097/01.HJ.0000602924.22061.51
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Dr. Djalilian is the director of neurotology and skull base surgery and a professor of otolaryngology and biomedical engineering at the University of California, Irvine.

An 87-year-old man with a history of bilateral cochlear implantation developed arrhythmia and valvular heart disease. His first cochlear implant (CI) surgery 12 years prior to his presentation was uneventful. The second CI surgery performed 18 months before presentation was also uneventful. However, the patient reported that the left CI sounded too low in frequency and was more “base than treble.” Multiple adjustments to the implants were performed by an audiologist; however, the sound quality did not improve at all. A CT scan of the patient's temporal bone is on the right.

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Diagnosis: Deep Electrode Insertion

At its surface, cochlear implantation may appear to be a simple operation. It is often said that the more difficult part of the cochlear implantation process is not the surgery but the mapping process performed by the implant audiologist. In the same way that an audiologist's experience is important in determining the long-term outcome of the patient, the nuances of the surgery are also critical in determining the CI recipient's quality of life and performance outcome.

Placing an electrode into the cochlea may not seem too complicated. However, performing any surgical step incorrectly or suboptimally can affect the patient's long-term quality of life and CI performance. From the initial incision to the closure process, perfection is required in each step—as in all surgical procedures. The incision must be made with a right angle between the blade and the skin so that the two skin edges are perpendicular, thereby preventing the skin edges from beveling, which could cause edema of the flap that overlaps the other. Edema causes protrusions of the overlapping flap, and in the long run, could rub against the behind-the-ear component of the CI processor.

When preparing the implant body pocket, the angle of the skull needs to be considered. Ideally, the implant body should be placed posterior to the coronal (vertical) equator of the skull and superior to the axial (horizontal) equator of the skull. This ideally positions the implant body such that the magnet of the external device is not visible from the frontal view of the patient. In addition, placing the implant body inferior to the horizontal equator of the skull tends to cause movement and even displacement of the device if it's not secured. Placing the device too far in front positions it under the temporalis muscle, which in time can cause the implant body to move due to the anterior contractions of the temporalis muscle.

The soft tissues need to be handled gently by minimizing the use of electrocautery (usually performed to prevent/stop the bleeding). The more destruction to the tissue with electrocautery, the more edema after surgery. This will result in a longer healing time and more pain after the surgery. Wound edges should not be cauterized because this can lead to wound breakdown or infection and the healing of that area will be delayed. Bleeding should be controlled to prevent the accumulation of blood under the skin, which can lead to device movement and potential wound infection.

During mastoidectomy, unlike a traditional surgery for infections, the edges of the mastoid cavity are not saucerized (tapering of the edges to create a more shallow cavity). Sharp edges with a small notch that secures the electrode help reduce the likelihood of migration or motion of the electrode.

One of the most critical areas of cochlear implantation is the facial recess, which is a 3 mm space between the facial and chorda tympani nerves (taste nerve). Many surgeons afraid of damaging the facial nerve will err on the side of caution and drill too close to the chorda tympani nerve, which is approximately 0.3 mm in diameter and can easily get injured during surgery. While damage to the chorda tympani nerve does not universally cause a loss of taste in the front and side of the tongue, about 60 percent of patients experience a change in or loss of taste. This is one of the most common reasons why many patients hesitate when presenting for cochlear implantation. Using a small (1 mm) drill bit inside the facial recess and having thorough knowledge of the anatomy of the facial nerve will enable the surgeon to stay in the middle of the recess and leave at least 0.5 mm of bone on both the facial and chorda tympani nerves. If the distance between a patient's chorda tympani and facial nerves is very small, we use a 0.5 mm drill bit to allow us to work more precisely and prevent any chorda tympani injury.

Facial nerve paralysis is a dreaded complication that can only be prevented with the clinician's thorough knowledge of the anatomy and strategies like keeping the posterior ear canal wall thin. It can be augmented by using facial nerve monitoring during surgery. Fortunately, facial nerve injury is very rare, unlike the chorda tympani nerve injury. Nevertheless, many patients mention it as one of the reasons they are hesitant to undergo cochlear implantation.

The electrode's point of entry into the cochlea has evolved over time. Initially, electrodes were placed through the round window, which has changed to cochleostomy. With the newer electrode designs that are softer and thinner, placing the electrode into the cochlea can be easily performed through the round window, which guarantees placement into the scala tympani. Compared with scala vestibuli placement, scala tympani placement will likely lead to better patient outcomes (Otol Neurotol. 2008 Oct;29(7):920-8.). Hearing preservation can be best achieved via atraumatic electrode insertion and slow insertion, as well as by preventing the intrusion of the blood and bone into the cochlea and reducing the manipulation of the cochleae.

This patient's CT scan showed that the electrode on the left was inserted too deeply. The basal turn on the left did not show any active electrode contact points (appears smooth; Fig. 1). In comparison, the right side showed a bumpy-appearing electrode (contact points) in the basal turn (Fig. 2). The apical turn had electrodes on the left, which should not occur in a 20 mm electrode. However, the right side had no electrodes in the cochlear apex (Figs. 3, 4, 5, 6), which means the electrode was pushed in approximately 8 mm deeper than usual. The patient was given the option to have the electrode pulled out slightly or the apical electrodes turned off. The patient opted to have the apical electrodes turned off and undergo remapping.

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BONUS ONLINE VIDEOS: VISUAL DIAGNOSIS

Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.

Video 1. Axial (horizontal) CT of the left temporal bone showing the deeply inserted electrode.

Video 2. Axial (horizontal) CT of the right temporal bone showing the normally inserted electrode.

Video 3. Coronal (parallel to the face) CT of the left temporal bone showing the deeply inserted electrode position in the coronal plane.

Video 4. Coronal (parallel to the face) CT of the right temporal bone demonstrating a normal electrode position.

Video 5. Sagittal (parallel to auricle) images of the left temporal bone showing electrodes in the apical turn of the cochlea, where a 20 mm electrode should not be present.

Video 6. Sagittal (parallel to auricle) images of the right temporal bone demonstrating the normal appearance of electrodes in the cochlea on sagittal imaging.

Watch the patient videos online at thehearingjournal.com https://journals.lww.com/thehearingjournal/Pages/collectiondetails.aspx?TopicalCollectionId=23.

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