Sept 2017 Hearing Journal
Poor performance after cochlear implantation is uncommon. When it does occur, the problem is often related to the device. Overall, about five to seven percent of children and one to three percent of adults will experience device failure, which can be categorised as a hard or soft failure. Since the U.S. Food and Drug Administration (FDA) considers cochlear implants class III high-risk devices, all failed devices are returned. In fact, manufacturers need to track every device from the time it is shipped out until it is returned due to a device failure or when the patient passes away. For an explanted device, the manufacturer needs to perform an analysis, then submit a report to the FDA and the cochlear implant team that removed the device.
A hard failure occurs when the device completely fails to function. This can be an electronic malfunction resulting from a blow to the device or from a loss of hermiticity, particularly when body liquids enter the device, causing a short circuit. Hard failures can also occur when electrodes are shaved off due to the device's micro-motions. This can also happen when there is significant kinking of the electrode during the insertion process, which can eventually lead to serious electrode damage. Prior to explantation, an integrity test is performed by the manufacturer representative. If a hard failure is confirmed, explantation and re-implantation are planned.
A soft failure is when a device does not fail the manufacturer's integrity test but exhibits some problems. When a user feels that the device is not functioning as well as it used to, a thorough investigation of the implant and external components are performed by the audiology team. This includes switching the cables, using different programs on the processor, and having the patient demonstrate how to use the device. Once a technical issue has been ruled out, the device manufacturer is contacted to perform an integrity test on the device. Integrity tests include a thorough assessment of the internal receiver and stimulator and an evaluation of the stimulation rate and intra-cochlear electrodes using common ground testing. Once these processes are found to be normal, imaging is obtained to evaluate the position of the electrode. In our clinic, we prefer the use of a CT scan, which can determine the exact location of the electrode and the number of contact points inside and outside of the cochlea. Some clinics use x-rays to evaluate the electrode's position. While x-rays show if the electrode is curled in the cochlea, they do not show if some of the electrodes are outside of the cochlea. In addition, the electrode can rarely be curled in the superior semicircular canal, which can be misinterpreted as a correct placement. The primary advantage of the x-ray over the CT is the lower radiation exposure for the patient. If the imaging shows that the electrode is fully within the cochlea, a final check is done with the manufacturer representative to confirm if the device is a soft failure. After this confirmation, the revision process can begin.
In surgery, great care should be taken to protect the explanted device. If the device is damaged, detailed notes of the intra-operative damage should be made to inform the manufacturer of the damages before and during the surgery. The explanted device, along with the notes, are placed in a special container and sent to the manufacturer for testing. In our practice, most of the surgical process is generally performed under a microscope to prevent any inadvertent damage to the cochlear implant. After an initial incision, the wound is dissected slowly under the microscope until the electrode is found. In children, this process is more difficult because of the regrowth of the mastoid cortex. Sometimes the implant electrode may weave in and out of the regrown mastoid cortex in a child. The electrode and device body should be dissected gently. The device body and ground electrode (if present) are removed, and a new device body is placed in the subcutaneous pocket. The electrode is then removed from the cochlea and immediately replaced with the electrode of the new device. This process must be done quickly to make sure that new electrode is placed into the cochlea's fibrous pocket, which contained the previous electrode, before it collapses. Thus, the new electrode must be placed as soon as the explanted electrode is removed. The surgery is followed by intraoperative testing with impedance and neural response telemetry.