| Auditory Restoration |
 |
| 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 [1]. 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 [2]. Regardless of the age at implantation however , there is a rapid improvement in auditory skills during the first year of device use [3]. 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.
|
Table of Contents
|
1.0 Assessment of Candidacy
1.1 Age
In order to obtain the best possible outcome, cochlear implantation should occur at the earliest age possible. Early implantation allows for several long-term advantages. It allows for earlier and improved auditory experience, which leads to language development during the appropriate sensitive window[4]. Research shows that the age at which the cochlear implantation occurred had an effect on the long-term morphological changes of cortical evoked potential to speech[5]. Thus more effective language learning could result from supporting neurological systems that might have been formed due to the early auditory information that is provided by the cochlear implant[4]. According to Barker and colleagues the earlier the implantation (at 12 months), the earlier the patients would develop expressive language at a rate that approximated those of their normal-hearing peers[4]. In spite of those advantages one has to take into consideration the risks associated with performing such an intrusive procedure and anesthesia in such young children[6]. Nevertheless, with the help of a pediatric anesthesiology team, an experienced surgeon can safely perform such an operation, providing the child (12 months or younger) with the advantages of early auditory stimulation[6].
 |
| This graph demonstrates the findings of Holman et al.'s study (2013) that show the audiologic skills development from the age of implantation (9 or 15 months). It is shown that children who receive the implantation earlier in life have access to auditory information for a longer amount of time thus allowing for better speech and language development [6] |
1.2 Cost
The total average costs of cochlear implantation in North America is reported to be around $50,000-$100,000 or more. This sum is affected by several factors such as the pre-operative evaluations, the choice of device, the hospital fees, physicians’ fees, the needed medication and the rehabilitation following the surgery. Cochlear implantation, however, is a highly cost-effective medical procedure that increases a person’s quality of life while also providing long-term net savings to the affected families and to society[7].
2.0 Patient Evaluation
In order for the patient to be a possible candidate for the procedure, clinicians have to ensure that the patient meets the standardized selection criteria[8]. A multidimensional approach is needed to determine candidacy for cochlear implant as it has an impact on a wide range of life experiences. In the USA, clinicians follow the guidelines of theFood and Drug Administration (FDA) when determining whether they should suggest cochlear implantation to their patients[9]. Since the first use of cochlear implant, the FDA has increasingly relaxed their guidelines, due to the continuously increasing advantages of implantation, including individuals with lesser levels of hearing impairment and children of a younger age[8]. However it is important to ensure that even when the patient meets all the required criteria, he/she should only receive the surgery if his/her hearing ability is greater with an implant rather than with a hearing aid.
40% of people being referred for implant candidacy evaluation from non-specialists decide not to undergo the procedure[10]. Reasons for not having the implantation include the patient changing their minds once further educated, having greater benefit with amplifications with revised fittings, and health related complications.
2.1 Initial Consultation
The initial consultation is the first step in the implant evaluation process. During this time the patient, or family, is asked to bring all the required medical documents needed by the doctor and to complete certain clinician-administered questionnaires that would establish the extent to which the patient uses hearing in his/her daily life[9]. Examples of such tests are the Infant-Toddler Meaningful Auditory Integration Scale (IT-MAIS)[11] and the Meaningful Auditory Integration Scale (MAIS)[12]. IT-MAIS is given to parents of children younger than 2 years while MAIS is given to parents of children older than 2 years.
This time is also used to discuss all matters related to the implantation such as device options, cochlear implant technology, rehabilitation programs, financial obligations, expectations and risks[9]. Once all the patients’ or family’s questions have been answered and they are aware of step-by-step procedure, the audiological, medical and psychological assessment can be done.
2.2 Audiological Evaluation
A core component of the candidacy evaluation is determining the type and severity of the patient’s hearing impairment. This assessment is done using objective measures such as the Immittance testing, Otoacoustic Emissions (OAEs), Auditory Brainstem Response (ABR), and Electric Auditory Brainstem Response(EABR)[9]. Some of these tests are used in combination with each other, or to verify information received by behavioral testing. When having concerns about the electric stimulability of the ear to be operated on or the presence/absence of the VIIIth nerve, the EABR is used in clinics[13]. Cochlear implantation is only performed if a positive EABR is acquired[9].
As previously mentioned it is important to ensure that there is a significant benefit of having an implant when compared to having a hearing aid. Aiding testing should thus also be performed for each ear individually and for both with aided soundfield thresholds 250-4000 Hz. The difference in the comparison between the thresholds obtained from the aiding testing and from the standard cochlear implant (10-40 db HL across the frequency range of 250-4000 Hz) should determine whether the implant would increase the patient’s quality of hearing[9].
2.3 Medical Evaluation
Imaging evaluation is only performed in preselected candidates. Nowadays Computed Tomatography (CT) and Magnetic Resonance Imaging (MRI) are used to evaluate the whole auditory pathway and any abnormalities therein, allowing clinicians to determine those best suited for implantation and the type of cochlear device needed[14]. CT is important in visualizing bone anatomical details that are required for surgical planning while the MRI is important for correctly predicting cochlear patency by detecting intracochlear fluid among other factors[14]. These two techniques in combination with fMRI are used in order to detect any pathologies such as labyrinthine ossification, cochlear otosclerosis, post-traumatic hearing loss, cochlear malformations, cochlear dysplasia and anomalies of the cochlear nerve[15].
2.4 Psychological Evaluation
Cochlear implantation causes a radical change in communication and lifestyle, which could have profound psychological consequences[8]. An implant patient requires an entourage that will provide sufficient motivation and support throughout the whole process in order to complete the program of rehabilitation and achieve the best possible results[8]. Pre-operative psychological assessment should therefore be done to ensure the patient’s and family’s emotional and psychological preparedness. It has been shown that the patient’s or family’s frustrations due to irrational expectations have hindered the very best attempts of an implant team[16]. The attitudes and expectations of the patient/family should therefore be analyzed. Furthermore the patient/family should be informed of the length and limitations of the rehabilitation.
The successful accomplishment of all these different assessments increases the chances of obtaining an effective implantation, a significant increase in the patient’s hearing ability and, hopefully, providing him/her with a better lifestyle.
3.0 Bilateral vs Unilateral Implantation for Bilateral Deafness
There have been a lot of debates about performing bilateral versus unilateral implantation on bilaterally deaf children. Unilateral implanted patients are able to accurately understand speech in quiet conditions, however when exposed to noise many will find it difficult[17]. Arguments against bilateral implantation focus on the risk/benefit ratio, the additional costs and the risks to the contralateral ear. The risk/benefit ratio is in favor for the unilateral implantation as although the risk factor of the second implantation is 100% of the first surgery, the gain is only 20%[17]. It should be noted however that these statistics are based on test efficiency results rather than the patient’s satisfaction. Furthermore, the cost of bilateral implantation is also twice as much as only one-sided operation. Both risk/benefit and cost/benefit would thus favor simultaneous rather than sequential bilateral implantation, as that would require a single surgery and hospitalization[17]. Another consideration is whether it is more advantageous for the patient to use the contralateral ear or preserve it for future possible technologies[18]. Some parents prefer to wait for better treatments while others prefer to give their child every possible advantage during their early years of development[17].
Even though all these considerations have valuable input, bilateral implantation has been increasingly favored as research exposes the experiential advantage that binaural processing provides for speech intelligibility and sound localization.
 |
| Binaural Processing |
3.1 Binaural Processing
Binaural processing includes primarily the head shadow effect, the binaural summation effect and the binaural squelch effect[19].
The head shadow effect has the largest effect on hearing with binaural cochlear implantation and does not need central auditory processing[17]. This effect occurs when listening to speech while being surrounded by noise. If for example the auditory stimuli comes from the right side of the head, the left side is ‘protected’ from the interfering noise which results in a better signal to noise ratio (SNR). This then allows the individual to selectively pay attention to the ear with a better SNR and improve understanding[17].
The binaural summation effect occurs when the central auditory system processes the same input from both ears resulting in a perceptually louder signal, which allows increased sensitivity to the differences in frequency and intensity.
The binaural squelch effect occurs when different inputs from both ears are processed in the central auditory system resulting in a better signal reaching the auditory cortex[17]. This allows for better separation of speech from the surrounding noise.
3.2 Speech Intelligibility
The main goal of bilateral implantation is to increase the patient’s hearing quality and experience. In their paper, Asp et al. showed that even in the presence of noise, bilateral implanted patients performed better at speech recognition than unilateral implanted patients[20]. Parental reports also indicated that children performed better at daily situations when using bilateral cochlear implants[20].
3.3 Sound Localization
Sound localization is important for the normal functioning in everyday life such as recognizing from where a car horn sound is coming from. Laszig et al. demonstrated that accuracy at sound localization for unilateral implanted patients was at about 90° while for bilateral implanted patients it much better, at about 50° [21]. Other researches also support the advantage of having bilateral implants in localizing sound[17].
4.0 Possible Risks
Cochlear implantation has come to be accepted as a safe and efficient procedure for auditory rehabilitation. As in any other surgery however there are certain risks and possible complications that the patient and family should know about before taking the decision. Postelmans et al. conducted a study in which major complications were defined as those requiring further surgery or device removal, while minor complications were treatable by medical or audiological management[22]. Major complications such as device failure and wound infections mediated by the middle –ear pathology occurred in 3.6 % of the patients while minor complications occurred in 32% but were all treated successfully[22]. Minor complications included fever, pain, electrode malfunction and vertigo. The incidence rate of pneumococcal meningitis in the children having received cochlear implants under the age of 6 was 30 times that of children of the same age group in the general US population in 2000 covered by the Active Bacterial Core Surveillance program of the CDC[23]. Reefhuis et al. recommended that parents and health care providers attend to the possible signs of meningitis post-surgery[24]. Furthermore surgeons should inform the family of any malformations to the inner-ear or other discoveries found during the operation. It is hoped that with the use of pneumococcal vaccines the incidence of bacterial meningitis in cochlear-implanted children will decrease[24].
This video was made by the FDA in order to remind healthcare practitioners about the importance of pneumococcal vaccination in children receiving cochlear implants.
5.0 Brain plasticity involving the ‘unused’ auditory cortex
During the period of deafness multiple changes that depend on the age of onset of deafness, the cause and type of deafness and the amount of time the immature auditory pathways are left without notable sensory input[25]. In deaf animal models and humans it has been found that structural changes evolve in the auditory nerve, the brainstem and the cortex. Although research shows that the auditory nerve and brainstem changes can be stimulated by the cochlear implant, cortical changes however are limited at least in part by the amount of reorganization that happened during the deaf period[25]. As Gordon et al. termed it, the auditory thalamo-cortical areas follow a “Use it or lose it” notion[25]. During the period of deafness auditory nerve and brainstem function development are stopped while the thalamo-cortical areas undergo significant changes. These changes could be driven by other sensory inputs such as visual signals. This process is known as cross-modal plasticity[26].
Some brain plasticity changes found in deaf infants show decreased white matter and increase in grey matter in the anterior portion of Heschl’s gyrus[27]. There was also a change in the uniformity of the two hemispheres in that area. Other studies demonstrated Positron emission tomography (PET) data that showed a change in the underused auditory cortex of deaf individuals from a level of hypometabolism to a normal or even hyperactive level of metabolism over time[28], [26].
It is logical to suspect that the reorganization and development of the auditory cortex of a deaf individual is influenced by the visual system[25]. Deaf people rely heavily on visual cues in their surrounding for communication (sign language) and for everyday life activity. Interestingly deaf individuals demonstrate no improved visual acuity than normal[29]. However they do have better peripheral vision and more accurately perceive moving targets than do normal-hearing people[30]. Fascinatingly, the cross-modal plasticity occurring in the ‘unused’ auditory cortex allows areas, such as temporal cortices that would have normally responded to sound, process visual information[31], [29].
| Cross-modal Plasticity |
 |
| Finney and colleagues (2001) used fMRI to measure the activity in the auditory cortex (Brodmann's area 41) caused by a visual stimuli, in deaf people.[31] |
Hey Vie,
Great page, very organized with lots of pictures. like the linkings to the outside websites.
I would recommend you one thing: to move your reference years from the middle (in bibliography) to the end of your sentences.
thank you:)
Actually I am not sure if we should use APA or Nature format …
Aw yea Vie, this is looking awesome! I used the nature format from the individual assignment assuming it was meant to be carried straight through until the end. Wikipedia uses some weird format with quotation marks, I just stuck with Nature (hoping I did it right, LOL).
thxx:) Yeah I'm gonna change it to nature
Legit. I couldn't help but smile at the thought of a deaf baby undergoing extensive psychological evaluation (I'm assuming the parents are more the focus of this process?). Super thorough though I'm impressed. Is the cost of a bilateral implant really double? Honest to gracious berries you'd think you'd get some sort of two for one deal (surely it's not double when man (orrrr womannnn) hours are taken into consideration). I read that bit on cross modal effects (which is cool no doubt) thinking you were talking about activity being seen in the temporal lobe opposite to the one which should have been affected by the implant. Do you see increased bilateral lobe activity with a unilateral implant? Cuz that would be COOL!!