8.17\ Cochlear Implants

8.17.1\ Discussion

Cochlear implants are electronic devices that can provide the sense of sound to patients with severe sensorineural hearing loss by direct stimulation of the cochlear nerve. Cochlear implants consist of an external microphone, external speech processor, external transducer coil, internal receiver-­ stimulator, and electrode array within the cochlea. The components, particularly the position of the electrodes, are well seen on the modified Stenver or Arcelin view (Fig. 8.77). The removable external component of a cochlear implant houses a microphone, a speech microprocessor, and a radio-emitting coil that not only sends a signal to the implanted receiver-stimulator but also powers the device transcutaneously. The external coil is held in place magnetically. The implanted receiver-stimulator contains a magnet, a computer chip, and an electrode array, which is surgically threaded into the cochlea. The electrodes of this array contain anywhere from 8 to 24 metallic leads that vary depending on the actual device in place. These electrodes are visible on CT as a string of metallic beads.

Typical surgery for cochlear implantation consists of performing a canal-wall-up mastoidectomy and posterior tympanotomy drilled through the facial recess, which is defined as the space between the facial nerve and the chorda tympani. This allows visualization of the round window niche where the electrode is introduced into the scala tympani of the basal turn of the cochlea either via the round window membrane itself or a small cochleostomy drilled immediately adjacent to the round window (Fig. 8.78). The degree of electrode extension into the cochlea will vary

depending on the surgeon’s wishes based on the length of the particular electrode array chosen and the portion of the cochlea that one wishes to stimulate. For routine cases, it is typical for the electrode to complete at least a 270 degree turn within the cochlea, such that all the electrodes are seated within the cochlea. In patients with ­labyrinthitis ossificans, a cochleostomy with drill out of the ossified basal turn may be required (Fig. 8.79). A recess is sometimes drilled into the bone posterior to the mastoid cavity where the body of the receiver-stimulator is to be set within a periosteal pocket in order to allow the device to sit flush along the surface of the skull.

Several imaging techniques have been used to depict cochlear implants, including conventional radiography, phase-­contrast radiography, cone beam CT, fusion of conventional radiographs and CT images, and spiral CT. Most cochlear implants are only compatible with MRI if the internal magnet is removed through a minor procedure, but at some centers 1.5 T MRI images have been safely obtained by tightly wrapping the head overlying the implant and closely observing the patient during the scan. Alternatively, the electrodes can be left in position, while the rest of the cochlear implant device is removed for the MRI. The electrodes produce negligible artifacts on MRI at 1.5 T (Fig. 8.80). It is undesirable to remove the electrodes for prolonged periods of time because it can be difficult to reinsert the device. Newer cochlear implants may be MRI compatible ­without removing the internal magnet, even up to 3 T; however, there can be artifact from the indwelling magnet that can limit the usefulness of the MRI if obtained to view structures of the head and neck. The specific model MRI guidelines should nevertheless be verified prior to scanning with MRI.

Fig. 8.78  Cochlear implant insertion through the round window. Axial CT image shows that the electrode courses through the mastoid bowl, across the facial recess, and into the cochlea via a widened round window (arrow)

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Fig. 8.79  Cochlear implant insertion via cochlear drill out. The patient had a diminutive basal turn of the cochlea. Axial (a) and coronal (b) CT images show the electrode of the cochlear implant passing through a drilled-out cochleostomy rather than through the round window and hypoplastic basal turn. The mastoid has been obliterated for perilymphatic fistula