An absence or disturbance of cochlear hair cells causes most cases of deafness. This defect in normal cochlear function, specifically, in the transduction of a mechanical acoustic signal into auditory nerve synaptic activity, represents a broken link in the delicate chain that constitutes the human sense of hearing. Cochlear implants afford an artificial means to bypass this disrupted link via direct electric stimulation of auditory nerve fibers.
Although current technological and scientific boundaries preclude the artificial transduction of sound by using the exact native cochlear patterns of synaptic activity at the level of each individual residual auditory nerve fiber, knowledge of these native patterns has aided the development of cochlear implants by allowing the processing of speech into novel synthetic electronic codes that contain the key features of spoken sound. By using these codes to systematically regulate the firing of intracochlear electrodes, it is possible to convey the timing, frequency, and intensity of sound. Cochlear implants have progressively evolved with increasing complexity and elegance from an experimental concept to a proven tool used in the management of patients with sensorineural hearing loss (SNHL). Worldwide, the number of implants is rapidly increasing. As with many other technology-driven medical treatment modalities, recent innovations in microcircuitry and computer science are continuing to drive the performance profiles of cochlear implants to new heights.
Currently, three separate corporations manufacture multichannel implant systems that are commercially available and approved by the FDA for use in both adults and children. Although expensive, multiple studies have demonstrated that the cost-utility of cochlear implantation is excellent and that it compares well with other common medical interventions.
All modern implant systems function by the use of the same basic components including a microphone, a speech processor, and an implanted receiver–stimulator (Figure 68–1).
Schematic depiction of how cochlear implant systems operate. 1. Sound is detected by an external microphone. 2. This signal is directed to an external sound processor. 3. Once processed, a digital electronic code is sent by a transmitting coil situated over the receiver–stimulator via radiofrequency through the skin. 4. The receiver–stimulator delivers electronic impulses to electrodes on a coil located within the cochlea according to whichever strategy is being used by the processor. 5. Electrodes electrically stimulate spiral ganglion cells and auditory nerve axons.
Microphone & Receiver-Stimulator
Sound is first detected by a microphone (usually worn on the ear) and converted into an analog electrical signal. This signal is then sent to an external processor where, according to one of a number of different processing strategies, it is transformed into an electronic code. This code, a digital signal at this point, is transmitted via radiofrequency through the skin by a transmitting coil that is held externally over the receiver–stimulator by a magnet. Ultimately, this code is translated by ...