According to the World Federation of the Deaf there are over 70 million deaf individuals worldwide, most of whom have sensorineural hearing loss. Sensorineural hearing loss occurs when auditory structures such as the hair cells in the cochlea, the cochlea itself, or the vestibulocochlear nerve (CN VIII) are damaged. People suffering damage to these auditory structures can be aided by a variety of techniques. One such technique is cochlear implantation, requiring surgery that bears extensive and crucial implications in the patient’s emotional, social and cultural spheres. Before undergoing the invasive procedure, patients and their family should be informed of the differences between surgical approaches and their respective risks. Successful cochlear implants, of which over 120,000  have been implemented, have allowed patients to regain hearing by directly stimulating the spiral ganglion cells, located between damaged inner hair cells and the central nervous system. If the patient cannot receive a cochlear implant due to conditions such as cochlear malformations, hearing can be restored using an alternative approach - the auditory brainstem implant. Over 500 successful surgeries  have been reported using this technology, which applies direct stimulation to the cochlear nuclei to provide patients with the sensation of hearing. Alternatively, research pertaining to auditory restoration via regeneration of lost or damaged hair cells shows promise in restoring hearing without the need of an implant. Recent work by Mizutari et al  showing partial hearing restoration in rodent models suggests that hair cell regeneration therapy may be a reality in the not so distant future.
main article: 1. Pre-operative Patient Information for cochlear implantation
author: Virginie NKL
|Improving lifestyle through the use of cochlear implant|
Cochlear implantation is a life changing surgery that bears extensive and crucial implications in the patient's emotional, social and cultural spheres. In order to acquire the best possible language and hearing skills the implantation should be done during early childhood . This is due to a sensitive period for cochlear implantation that will allow for the optimal development of auditory processing due to early on brain plasticity . Regardless of the age at implantation however , there is a rapid improvement in auditory skills during the first year of device use . Medical , acoustic and vestibular evaluations are necessary to determine whether the patient is suitable for the implantation. Before undergoing such an invasive procedure, patients should undergo substantial psychological evaluation in order to ensure the patient's or parents' complete understanding of the benefits and limitations of the device. For bilateral deaf patients the differences between a unilateral or bilateral implantation and their respective disadvantages need to be considered. Risks associated to cochlear implantation include device failure, pneumococcal meningitis and vestibular malfunction. The patient, and family, should be aware of these pre-operative factors before undergoing cochlear implantation.
Keywords: Patient evaluation, Brain plasticity, Bilateral cochlear implantation, Unilateral cochlear implantation, Meningitis, Thalamo-cortical, Metabolism & Cross-modal plasticity.
|Fig1. Adapted from KidsHealth.Org..An overall model of a CI inserted in the cochlea|
Since the discovery of cochlear implantation (CI) in1957 by Djourno and Eyriès, CI has become a very common yet invasive procedure for people with severe to profound sensorineural hearing loss (having a threshold of hearing between 71 and 90 dB HL). This type of deafness is due to a congenital or trauma induced inner hair cells damage. Since these hair cells are missing/damaged and interfering with the connections of ear to the central nervous system (both afferent and efferent inputs), CI is used with general anesthesia to bypass these hair cells and directly stimulate the ganglion cells of auditory nerve and thus, reactivating inputs to the CNS. Current research on optimized multichannel 18mm electrodes and safe surgical approaches for the insertion of implant has evolved from the successful insertion of 220,000 cochlear implants worldwide. It has been found that the scala tympani, a fluid filled chamber running along the long axis of cochlea, is the best place to insert the prosthesis while insertion in scala vestibular (SV), another tube in the cochlea, creates more trauma induced hair cell apoptosis. Every CI consists of the following parts (Fig.1): 1) a microphone 2) a speech processor 3) a transmitter magnet located on the skin and a receiver implanted on the skull 4) an electrode array that is inserted into the cochlear by two different surgical approaches (the traditional cochleostomy approach and a newer round window approach). Along with the major surgery there are also two minor procedures before the insertion of implants: mastoidectomy (removal of mastoid cells) and tympanatomy (opening of the eardrum). Overall, CI has been very effective in recovering hearing loss and improving the quality of life of patients.
|An Auditory Brainstem Implant|
|An example of the path from microphone to electrode in an ABI. Bottom right - an example of
the electrode panel stimulating the cochlear nuclei
Potthoff, M. (2013). ABI how it works image [image format].
retrieved from http://newsroom.hei.org/news/fda-approves-clinical-trial-of-242830
Recently, researchers have been investigating the possibility of implanting electrodes into patients with complete sensorineural hearing loss in order to restore their hearing. The most common approach used is the cochlear implant (CI), although there are some cases where cochlear implantation is not possible. In cases where cochlear implantation would provide no hearing improvement, an auditory brainstem implant (ABI) is the alternative method of hearing restoration and over 500 have been implanted with success since 1979 . An ABI is most commonly inserted in patients whom have neurofibromatosis type 2 (NF 2), an autosomal dominant disease causing tumors of the glial cells in the spinal cord and brain. NF 2 normally causes hearing loss due to bilateral vestibular schwannomas; tumors of the schwann cells supplying the vestibulocochlear nerve. Once inserted, an auditory brainstem implant utilizes an external microphone to pick up sounds from the environment which are then processed into a series of electrical pulses that are directly delivered to the cochlear nuclei by electrodes. Pulses stimulate many different cell types of the cochlear nuclei, and although some cell types are thought to provide upper brain regions with information on pitch or vowel identification, the mechanism behind why ABI provides a degree of auditory restoration in patients is largely unknown. With continuing research the ABI is a growing possibility for hearing restoration in those unable to receive a cochlear implant.
Image Source: Gizmodo
According to the World Federation of the Deaf there are 70 million deaf individuals around the world. Life for the hearing impaired has greatly improved over the decades with great headway made in the advances of cochlear implants (CI). At this current time, the success of cochlear implants is variable and despite best efforts they are not perfect. One of the biggest debates regarding cochlear implants is confirming whether or not the procedure is worthwhile to specific populations. Various studies conclude the success of the implants is largely age-dependant. Closely linked with age is a candidate’s level of language acquisition prior to hearing loss. Those afflicted with deafness prior to language acquisition are significantly hindered compared to those who have learned language. Deprivation of verbal language alters neurological structures which consequently result in prelingual cochlear implant users having a harder time rehabilitating. This prelingual caveat can be bypassed to some degree if the cochlear implant is implanted before the critical window of development closes.
Partaking in preoperative research and committing to the surgery is only half the journey for cochlear implant candidates. Once the implant has been surgically placed into the body, patients must to adjust to their new device and way of life. Children tend to benefit more from the implants than adults do, and postlingual recipients learn faster than prelingual recipients.  Despite its limitations, most recipients of cochlear implants report satisfaction with their devices. Apart from the restorative ability of cochlear implants, implants have provided its users with psychological benefits such as autonomy and social inclusivity.
|Hair cells, found within the cochlea of the inner ear,
transduce soundwaves into nerve impulses and
send them to the auditory centers of the brain .
An alternative approach to surgery, regeneration therapy, is quickly becoming the focus of research attention. The field has been active since the discovery in the 1980’s that hair cells in neonatal chicks were able to regenerate after acoustic/ototoxic insult, a scenario which was once thought impossible. Since then, research has made use of the naturally regenerative qualities of hair cells in bird models, the easy access of the auditory system in zebrafish for in vivo studies, and mammalian cochlear explants as the foci for potential models of treatment. Regeneration of hair cells is thought to rely on the supporting cells which underlie the auditory epithelia. These cells, either through mitosis and subsequent differentiation, or through direct transdifferentiation, have been shown to act as a reservoir for the growth of new hair cells. Genetic assays have been used to identify genes being up- and down-regulated following hair cell injury. Of particular interest are those involved in the Notch signalling pathway – a pathway which has been shown to inhibit hair cell regeneration in rodent models. Inhibition of this pathway via pharmacological administration has been shown to restore hearing in mice following acoustic insult. These findings suggest that hair cell regeneration therapy may be a realistic alternative to surgery in the not so distant future.