- Email: [email protected]
Cochlear implants: Referral, selection and rehabilitation
ARTICLE IN PRESS Current Paediatrics (2006) 16, 360–365
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cupe
Cochlear implants: Referral, selection and rehabilitation D. Owensa, A. Espesob, J. Hayesc, R.G. Williamsd a
SpR in Otolaryngology, University Hospital of Wales, Heath Park, Cardiff CF4 4XN, UK Senior House Officer in Otolaryngology, University Hospital of Wales, Heath Park, Cardiff CF4 4XN, UK c Teacher of the Deaf, University Hospital of Wales, Heath Park, Cardiff CF4 4XN, UK d Consultant Otolaryngologist, University Hospital of Wales, Heath Park, Cardiff CF4 4XN, UK b
KEYWORDS Cochlea; Implantation; Children; Deafness
Summary Profound bilateral hearing loss is an uncommon but significant cause of morbidity in the paediatric community. Without adequate treatment, children fail to develop the ability to develop linguistic and communicative skills, with a significant impact on education, socioemotional development and future professional prospects. Cochlear implants have dramatically changed the prospects for profoundly deaf children. This article reviews cochlear implantation in respect to candidate selection, the procedure, its complications and rehabilitation. & 2006 Elsevier Ltd. All rights reserved.
Introduction Hearing loss is the most common sensory deficit in children in the UK. Loss can be broadly split into those that have lost hearing before speech development (pre-lingual) and those after it (post-lingual). Hearing loss occurs in 1–3 in 1000 live births per year, half of these are considered as profound with a deficit 490 dBHL, these children are all pre-lingually deaf.1 Acquired profound hearing loss is less common, but occurs in approximately 80 children in the UK per year.2 These children can be either pre- or post-lingually deaf depending on their age at onset of deafness. The overwhelming majority of hearing impairments are as a direct result of loss or developmental failure of the hair cells of the cochlea. With absent or dysfunctional hair cells, sound is not transformed into neural stimuli for transmission E-mail address: [email protected] (D. Owens).
to the higher auditory centres for processing. Hair cell loss occurs because of a number of conditions and insults (Table 1). In most children with sensory neural hearing loss (SNHL), some residual hair cell function is present and hearing loss is not defined as profound. For these children hearing augmentation with simple hearing aids is successful with appropriate rehabilitation. For most children with profound hearing loss however, conventional aids are not sufficient to gain adequate hearing for speech discrimination. Until the advent of cochlear implants, little could be done for these children other than development of communication skills with sign-language and lip-reading. Profoundly deafened children often failed to develop intelligible speech, with subsequent reduction in educational and professional prospects. With the development of cochlear implantation prospects for these children have greatly improved. Now most implanted children can expect similar audiological outcomes to those with moderate to severe hearing loss with
0957-5839/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cupe.2006.07.003 Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
ARTICLE IN PRESS Cochlear implants: Referral, selection and rehabilitation
Table 1
361
Common causes of cochlea hair-cell dysfunction in children.
Causes of hair-cell dysfunction in children Congenital Genetic
Non-genetic
Examples
Autosomal dominant: Stickler, Melnick Fraser Autosomal recessive: Ushers, Pendreds, Jervell, Lange-Nielsen X-linked: Alports, otopalatodigital Maternal factors: diabetes, toxaemia, cytomegalovirus infection Delivery complications: Rh disease, prematurity
Acquired Local infection: otitis media, suppurative labyrinthitis General infection: meningitis, measles, encephalitis, chicken pox, mumps Trauma: head injury Noise exposure
aids depending on time of loss and cognitive function. For a successful outcome in cochlear implantation the onset of hearing loss is important; in early life the brain retains the ability to produce neural connections in response to specific stimuli, this is known as neural plasticity. In the absence of sound stimulation the brain auditory neural pathways will not develop. As age increases, the ability to produce neural connections diminishes until at approximately the age of 10 this is largely lost. Pre-lingually deafened children with profound loss should undergo cochlear implantation before 3 years of age to gain the best results, although significant benefit is gained if implanted before 5 years of age. For post-lingual children the pathways have generally developed therefore treatment will produce good results.
Cochlear implants A cochlear implant is an electronic prosthetic device that acts to convert external physical sound to electrical impulses in place of the deficient hair cells. Cochlear implantation was first attempted in 1957 by Djourno et al.3 who described the insertion of single electrode implants in two adults for the treatment of deafness. Technological advances including an increase in the number of electrodes within the device have and varied electrode lengths placed at different levels within the cochlea making use of its tonotopical arrangement have made implantation a therapeutic rather than research procedure. These advances allowed frequency differentiation and the possibility of sound and speech recognition. Initially the procedure was only recommended in post-lingually deafened adults. In the 1980s and 1990s the age criteria fell, in 1995 children younger than 2 were being considered.4 The basic implant consists of three basic parts: (1) Microphone: situated externally usually on the ear but in younger children potentially body worn. Sound is converted to an electrical signal that is transmitted to the external processor. (2) External processor and transmitter coil: the external processor converts the analogue impulse into a defined code depending on strategy. The code is transmitted via radiofrequency through the skin by a transmitter coil
that is held externally over the receiver–stimulator by a magnet. The external processor typically sits behind the ear but in younger children who do not tolerate this position again body worn processors are available. (3) Receiver–stimulator: the received digital code is translated into rapid electrical impulses distributed to the cochlea by the multi-electrode array. The standard electrode array now has between 12 and 24 separate electrodes which are placed at different areas of the cochlea in situ. At the time of writing a typical cochlear implant costs approximately £35,000.
Coding strategies Sound has two main components: frequency and loudness; the normal ear uses these to evaluate noise. Without differentiation sound perception is possible but speech comprehension is not. The cochlear implant is able to produce variations in frequency and loudness. Frequency is produced by the use of band filters that separate sound into different pitch groups specific to a single electrode. Although effective this process does reduce the number of frequency variations that can be experienced. Banding also produces loudness, the greater the number of signals within a frequency group the greater the impulse produced. By alterations in the band settings the cochlear implant can be tailored to the individuals needs.
Candidate selection Referral from generalist Referral for consideration of cochlear implant has evolved over the years and is summarized in Table 2. In general hearing loss should be profound 490 dBhl at the 2 and 4 kHz frequencies, in the neonate this may be detected on evoked response audiometry. It should be ensured that the loss is sensori-neural and that all other possibilities have been excluded. Concurrent conductive losses (including otitis media with effusion) should be investigated and ameliorated if possible. Hearing aids can be trialled but should not slow referral for specialist opinion. Children who fail to benefit
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
ARTICLE IN PRESS 362
Table 2
D. Owens et al.
Referral criteria for cochlear implants.
Referral criteria for children under 5 years Bilateral sensori-neural hearing loss of 490 dBhl at 2 kHz+4 kHz Click ABR thresholds at 4 90 dBNhl No minimum age of referral Children with additional needs will always be considered Parental/Guardian consent for referral obtained Referral criteria for children 5 years and over Children with sudden onset or progressive hearing loss Bilateral sensori-neural hearing loss of 490 dBhl at 2 KHz+4 KHz Children whose primary form of communication is speech Children with additional needs will always be considered Parental/guardian consent for referral obtained
from aiding if fulfilling these criteria all children should be referred for consideration.
Cochlear implant clinic The decision on whether cochlear implant is indicated is made in a multidisciplinary setting and relies on clinical, audiological and radiological evaluation. Clinical evaluation The clinical evaluation begins with a detailed history followed by physical examination. Otological history includes age of onset of hearing loss, bilaterality of the loss and continued risks for further hearing loss (exposure to toxins, etc). Information on previous or concurrent ear problems is also necessary, children with SNHL are at least as likely to develop otitis media with effusion (OME) as the normal hearing age group,5 subsets such as syndromic SNHL have other problems including craniofacial abnormalities, which can make OME more likely. Previous otological surgery is also important as it may cause significant problems with implant placement. Examination involves a general review of the child fitness for surgery in syndromic children cardiovascular (Jervell and Lange-Nielsen) and respiratory abnormalities must be investigated and treated to reduce risk during anaesthesia. Otological examination is important to assess for evidence of conductive loss, which can be potentially ameliorated and to examine the tympanic membrane. Tympanic membrane perforation is often suggested as a contra-indication to cochlear implantation, however, in our experience implantation and myringoplasty may occur under the same anaesthetic without increase in complications. Audiologic evaluation The criteria, as previously discussed, are regularly evaluated and altered but the goal remains the same, to identify those children in whom an implant will produce functional hearing. In children undergoing evaluation it is necessary to establish a baseline hearing threshold to monitor outcome. A hearing aid trial is undertaken irrespective of the level of hearing loss to establish if hearing augmentation is
sufficient. Following a failed hearing aid trial, a battery of audiological tests are undertaken depending on the child’s age, to assess the benefits of cochlear implantation. During this phase input is elicited from audiologists, parents, teachers and speech and language therapists to decide whether further investigation and consideration is warranted.
Radiological evaluation Currently radiological evaluation for cochlear implantation uses fine cut computed tomography (CT) and magnetic resonance imaging (MRI) of the temporal bones. CT is often enough to delineate the bony anatomy of the cochlea providing evidence of congenital abnormalities and ossification, which can follow deafness due to meningitis. Common congenital abnormalities of the cochlea include: Mondini’s dysplasia and widened vestibular aqueduct syndrome, which are present in syndromic and non-syndromic SNHL. Complete absence of a cochlea prevents implantation but even with partial absence implantation may be possible. MRI reviews the soft tissues; this is required for all cases of congenital deafness and may be indicated when central pathology needs to be excluded, including absence of the vestibulo-cochlea nerve. High-resolution T2-weighted imaging with or without gadolinium is often used. Two contraindications from radiological review are Michels deformity where there is absence of the cochlea, and second, absence of the central pathway especially the vestibulo-cochlea nerve. Actual guidelines for treatment vary depending on the institute involved. Criteria for selection are similar to the referral guidelines but as technique and implant technology improve it is likely that those with less profound hearing loss will eventually be considered. It is uncommon for implantation to occur at less than 12 months, due to a number of factors including the difficulty in assessing residual functional aidable hearing, the risk of anaesthetic and the thickness of the skull. Pre-lingual children require treatment as soon as possible due to reducing neural plasticity, many groups would suggest implantation no later than 5 years of age, some institutes will implant pre-lingual children up to the age of 10. The results of this latter group are not as
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
ARTICLE IN PRESS Cochlear implants: Referral, selection and rehabilitation
Figure 1
363
Intraorbital X-ray of bilateral cochlear implants in a child.
favourable for the development of functional hearing and other methods of communication do need to be established. For post-lingually deafened children the urgency is not as great as the auditory neural pathway has developed.
Parental commitment and understanding Before implantation can be undertaken it is vital to make sure parents understand and support the long-term commitment involved in the cochlear programme. They need to be aware that the procedure is not a one off cure, and that a great deal of time and effort is required for the best results. Without parental support the best functional outcome is unlikely to be achievable.
Surgical procedure The surgical procedure lasts about 2 h in duration requiring general anaesthesia and a post-operative overnight stay. The approach is via a post-auricular incision. The mastoid cavity is opened and all air cells removed. At this point the surgeon identifies the facial nerve and short process of the incus to prevent injury to these structures. A posterior tympanotomy is performed and the basal turn of the cochlea is identified. A 1.5 mm hole is drilled through the cochlea in to the scala tympani. The afferent electrode is placed through the cochleostomy and advanced as far as possible into the cochlea to reach the lower-frequency areas. The efferent electrode is placed deep to the temporalis muscle and a separate well is prepared for the internal part of the
implant. The skin is then closed in layers and a firm dressing and head bandage applied for 48 h. Post-operative transorbital radiographs are taken to ensure positioning (Fig. 1). Post-operatively the child will experience some discomfort, which can be controlled adequately with simple analgesia. Some experience vertigo from vestibular irritation, this is commonly transient and resolves within the first 48 h. The surgical site is allowed to heal for around 1 month followed by ‘switch on’ where they receive the external part of the device that is programmed for their particular hearing requirements.
Complications Complications from surgery have been reported to be as high as 5–10% although not in our own departments’ experience. Common problems include:
Wound infections The most common complication, and usually treated with local wound care and oral antibiotics. In severe infection surgical explantation may be required to control spread, reimplantation can occur following resolution of infection. Skin infection may be caused by erosion due to the magnetic attachment of the external device. If the magnet is too powerful pressure necrosis of the skin followed by infection may occur. Once noted, adjustments to magnet strength can be made which will prevent the condition worsening.
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
ARTICLE IN PRESS 364
D. Owens et al.
Facial nerve injury A rare complication but reported due to the position of the nerve within the operative field. With transection complete facial weakness occurs with little chance of recovery. By monitoring throughout the procedure and by anticipation of the possibility of finding abnormal anatomy during surgery very few cases should occur. Tinnitus A common problem post-operatively due to the loss of residual hearing on implant placement. When encountered in this situation it is likely to resolve in time, often substantially following ‘switch on’. Vestibular dysfunction A risk as the inner ear has been breached. Temporary dysfunction is often experienced but permanent dysfunction has been reported. Providing there is no history of bilateral dysfunction with rehabilitation and time vertiginous symptoms should resolve. If a history of balance problems is elicited at the candidacy interview it is prudent to arrange pre-operative investigation. Meningitis Currently a hot topic in relation to cochlear implantation as some suggests an increased risk following implantation.6 The risk is limited but maybe higher than the normal population up to 2 years post-implantation. Due to these potential risks, pre-operative vaccination against Haemophilus and Pneumococcus should be given as part of the general treatment.
Rehabilitation Programmes of auditory training in children with implants are highly individualised depending on age, cognition and level of hearing gain. Post ‘switch on’ the child must demonstrate awareness/detection of sound (detection). Table 3
Rehabilitation then commences, the child is taught to determine whether sounds are different (discrimination), to recognise sounds, and to associate meaning with them (identification), and to respond appropriately to verbal commands (comprehension). Language development is undertaken, the child is taught to imitate phonemes and syllables, with progression to co-articulation, production of more complex words and eventually sentences. In rehabilitation, competence in hearing and speech should be developed together to produce total communicative function. The first step is ‘switch on’, which occurs 1-month postimplantation when wound healing is complete. For the next few months intensive auditory and speech training is undertaken by a team consisting of a teacher of the deaf and a speech and language therapist. During these sessions the individual is assessed in a variety of ways, specific help can be arranged and tuning/coding requirements are undertaken. In children full rehabilitation takes many years and it often takes 6–18 months to establish a clear initial benefit when the learning the learning curve starts to drop.
Assessment and outcome Assessment takes place at approximately 3, 6 and 12 months post-implantation and annually thereafter. In children, assessment is influenced by the child’s language status at the time the hearing was lost, their age at implantation and to a degree their cognition. Numerous tests have been developed to assess the outcome of implantation at varying stages post-operatively. Two main assessment tools exist, the Category of Auditory Performance score (CAP) and the Speech Intelligibility Rating (SIR; Table 3). These examples are only two of many assessment tools used in evaluation and are only part of the total assessment. Outcome measurements are difficult in children but two main goals are set:
Assessment tools used in rehabilitation following cochlear implantation. Score
Category of performance Use of telephone with loud speaker Understanding conversation without lip reading Understanding common phrases without lip reading Discrimination of speech sounds without lip reading Identification of environmental sounds Response to speech sounds Awareness of environmental sounds No awareness of environmental sounds Speech intelligibility rating Connected speech to all intelligible listeners. Child is understood in everyday contexts Connected speech is intelligible to a listener who has had experience of a deaf persons speech Connected speech is intelligible to a listener who concentrates and lip reads Connected speech is unintelligible. Intelligible speech is developing in single words when context and lip reading cues available Connected speech is unintelligible. Pre-recognisable words in spoken language, primary mode of communication may be manual
7 6 5 4 3 2 1 0 5 4 3 2 1
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
ARTICLE IN PRESS Cochlear implants: Referral, selection and rehabilitation (1) Word understanding. The objective markers vary depending on whether the child is pre- or post-lingually deafened. Post-lingual children often have word understanding as high as 100% that of the normal hearing population at 3 year follow-up. For pre-lingual children who represent the majority of the paediatric implantation population results are not as good and are variable depending on the aetiology of the hearing loss. Progress tends to be slower and improvements level off with time. (2) Mainstream schooling. Implantation in the very young has a goal of communication sufficient to allow mainstream schooling. Many children do achieve age-appropriate speech recognition and production for this purpose. It is not uncommon for children implanted at 2–3 years old to be able to attend mainstream schooling by 5–6 years of age with continued support from the cochlear implant team.
365
References 1. Copeland BJ, Pillsbury HC. Cochlear implantation for the treatment of deafness. Ann Rev Med 2004;55:1557–67. 2. O’Donoghue GM. Hearing without ears: do cochlear implants work in children? Br Med J 1999;318:72–3. 3. Eisen MD. Djourno, Eyries, and the first implanted neural stimulator to restore hearing. Otol Neurotol 2003;24: 500–6. 4. NIH Consensus Development statement: Cochlear implants in adults and children. J Am Med Assoc 1995;274:1955–61. 5. Brookhouser PE, Beauchaine KL, Osberger MJ. Management of the child with sensorineural hearing loss. Pediatr Clin North Am 1999;46(1):121–40. 6. Biernath KR, et al. Bacterial Meningitis among children with cochlear implants beyond 24 months after implantation. Pediatrics 2006;117:284–9. 7. Gantz BJ, Turner C, Gfeller KE, Lowder MW. Preservation of hearing in cochlear implant surgery: advantages of combined electrical and acoustical speech processing. Laryngoscope 2005;115:796–802.
Future developments The world of cochlear implants is constantly changing as experience in the techniques develop and technological advances take place. The ultimate goal is the totally implantable device but this is still some way off. At present the development of hybrid devices, which maintain residual hearing function but stimulate the regions that have been lost7 is of great interest requiring further research.
Further Reading 8. Gibson WPR. Cochlear implants. In: Booth JB, editor. Scott Brown’s Otolaryngology. Oxford: Butterworth-Heinemann; 1997. p. 1–25.
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cupe
Cochlear implants: Referral, selection and rehabilitation D. Owensa, A. Espesob, J. Hayesc, R.G. Williamsd a
SpR in Otolaryngology, University Hospital of Wales, Heath Park, Cardiff CF4 4XN, UK Senior House Officer in Otolaryngology, University Hospital of Wales, Heath Park, Cardiff CF4 4XN, UK c Teacher of the Deaf, University Hospital of Wales, Heath Park, Cardiff CF4 4XN, UK d Consultant Otolaryngologist, University Hospital of Wales, Heath Park, Cardiff CF4 4XN, UK b
KEYWORDS Cochlea; Implantation; Children; Deafness
Summary Profound bilateral hearing loss is an uncommon but significant cause of morbidity in the paediatric community. Without adequate treatment, children fail to develop the ability to develop linguistic and communicative skills, with a significant impact on education, socioemotional development and future professional prospects. Cochlear implants have dramatically changed the prospects for profoundly deaf children. This article reviews cochlear implantation in respect to candidate selection, the procedure, its complications and rehabilitation. & 2006 Elsevier Ltd. All rights reserved.
Introduction Hearing loss is the most common sensory deficit in children in the UK. Loss can be broadly split into those that have lost hearing before speech development (pre-lingual) and those after it (post-lingual). Hearing loss occurs in 1–3 in 1000 live births per year, half of these are considered as profound with a deficit 490 dBHL, these children are all pre-lingually deaf.1 Acquired profound hearing loss is less common, but occurs in approximately 80 children in the UK per year.2 These children can be either pre- or post-lingually deaf depending on their age at onset of deafness. The overwhelming majority of hearing impairments are as a direct result of loss or developmental failure of the hair cells of the cochlea. With absent or dysfunctional hair cells, sound is not transformed into neural stimuli for transmission E-mail address: [email protected] (D. Owens).
to the higher auditory centres for processing. Hair cell loss occurs because of a number of conditions and insults (Table 1). In most children with sensory neural hearing loss (SNHL), some residual hair cell function is present and hearing loss is not defined as profound. For these children hearing augmentation with simple hearing aids is successful with appropriate rehabilitation. For most children with profound hearing loss however, conventional aids are not sufficient to gain adequate hearing for speech discrimination. Until the advent of cochlear implants, little could be done for these children other than development of communication skills with sign-language and lip-reading. Profoundly deafened children often failed to develop intelligible speech, with subsequent reduction in educational and professional prospects. With the development of cochlear implantation prospects for these children have greatly improved. Now most implanted children can expect similar audiological outcomes to those with moderate to severe hearing loss with
0957-5839/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cupe.2006.07.003 Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
ARTICLE IN PRESS Cochlear implants: Referral, selection and rehabilitation
Table 1
361
Common causes of cochlea hair-cell dysfunction in children.
Causes of hair-cell dysfunction in children Congenital Genetic
Non-genetic
Examples
Autosomal dominant: Stickler, Melnick Fraser Autosomal recessive: Ushers, Pendreds, Jervell, Lange-Nielsen X-linked: Alports, otopalatodigital Maternal factors: diabetes, toxaemia, cytomegalovirus infection Delivery complications: Rh disease, prematurity
Acquired Local infection: otitis media, suppurative labyrinthitis General infection: meningitis, measles, encephalitis, chicken pox, mumps Trauma: head injury Noise exposure
aids depending on time of loss and cognitive function. For a successful outcome in cochlear implantation the onset of hearing loss is important; in early life the brain retains the ability to produce neural connections in response to specific stimuli, this is known as neural plasticity. In the absence of sound stimulation the brain auditory neural pathways will not develop. As age increases, the ability to produce neural connections diminishes until at approximately the age of 10 this is largely lost. Pre-lingually deafened children with profound loss should undergo cochlear implantation before 3 years of age to gain the best results, although significant benefit is gained if implanted before 5 years of age. For post-lingual children the pathways have generally developed therefore treatment will produce good results.
Cochlear implants A cochlear implant is an electronic prosthetic device that acts to convert external physical sound to electrical impulses in place of the deficient hair cells. Cochlear implantation was first attempted in 1957 by Djourno et al.3 who described the insertion of single electrode implants in two adults for the treatment of deafness. Technological advances including an increase in the number of electrodes within the device have and varied electrode lengths placed at different levels within the cochlea making use of its tonotopical arrangement have made implantation a therapeutic rather than research procedure. These advances allowed frequency differentiation and the possibility of sound and speech recognition. Initially the procedure was only recommended in post-lingually deafened adults. In the 1980s and 1990s the age criteria fell, in 1995 children younger than 2 were being considered.4 The basic implant consists of three basic parts: (1) Microphone: situated externally usually on the ear but in younger children potentially body worn. Sound is converted to an electrical signal that is transmitted to the external processor. (2) External processor and transmitter coil: the external processor converts the analogue impulse into a defined code depending on strategy. The code is transmitted via radiofrequency through the skin by a transmitter coil
that is held externally over the receiver–stimulator by a magnet. The external processor typically sits behind the ear but in younger children who do not tolerate this position again body worn processors are available. (3) Receiver–stimulator: the received digital code is translated into rapid electrical impulses distributed to the cochlea by the multi-electrode array. The standard electrode array now has between 12 and 24 separate electrodes which are placed at different areas of the cochlea in situ. At the time of writing a typical cochlear implant costs approximately £35,000.
Coding strategies Sound has two main components: frequency and loudness; the normal ear uses these to evaluate noise. Without differentiation sound perception is possible but speech comprehension is not. The cochlear implant is able to produce variations in frequency and loudness. Frequency is produced by the use of band filters that separate sound into different pitch groups specific to a single electrode. Although effective this process does reduce the number of frequency variations that can be experienced. Banding also produces loudness, the greater the number of signals within a frequency group the greater the impulse produced. By alterations in the band settings the cochlear implant can be tailored to the individuals needs.
Candidate selection Referral from generalist Referral for consideration of cochlear implant has evolved over the years and is summarized in Table 2. In general hearing loss should be profound 490 dBhl at the 2 and 4 kHz frequencies, in the neonate this may be detected on evoked response audiometry. It should be ensured that the loss is sensori-neural and that all other possibilities have been excluded. Concurrent conductive losses (including otitis media with effusion) should be investigated and ameliorated if possible. Hearing aids can be trialled but should not slow referral for specialist opinion. Children who fail to benefit
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
ARTICLE IN PRESS 362
Table 2
D. Owens et al.
Referral criteria for cochlear implants.
Referral criteria for children under 5 years Bilateral sensori-neural hearing loss of 490 dBhl at 2 kHz+4 kHz Click ABR thresholds at 4 90 dBNhl No minimum age of referral Children with additional needs will always be considered Parental/Guardian consent for referral obtained Referral criteria for children 5 years and over Children with sudden onset or progressive hearing loss Bilateral sensori-neural hearing loss of 490 dBhl at 2 KHz+4 KHz Children whose primary form of communication is speech Children with additional needs will always be considered Parental/guardian consent for referral obtained
from aiding if fulfilling these criteria all children should be referred for consideration.
Cochlear implant clinic The decision on whether cochlear implant is indicated is made in a multidisciplinary setting and relies on clinical, audiological and radiological evaluation. Clinical evaluation The clinical evaluation begins with a detailed history followed by physical examination. Otological history includes age of onset of hearing loss, bilaterality of the loss and continued risks for further hearing loss (exposure to toxins, etc). Information on previous or concurrent ear problems is also necessary, children with SNHL are at least as likely to develop otitis media with effusion (OME) as the normal hearing age group,5 subsets such as syndromic SNHL have other problems including craniofacial abnormalities, which can make OME more likely. Previous otological surgery is also important as it may cause significant problems with implant placement. Examination involves a general review of the child fitness for surgery in syndromic children cardiovascular (Jervell and Lange-Nielsen) and respiratory abnormalities must be investigated and treated to reduce risk during anaesthesia. Otological examination is important to assess for evidence of conductive loss, which can be potentially ameliorated and to examine the tympanic membrane. Tympanic membrane perforation is often suggested as a contra-indication to cochlear implantation, however, in our experience implantation and myringoplasty may occur under the same anaesthetic without increase in complications. Audiologic evaluation The criteria, as previously discussed, are regularly evaluated and altered but the goal remains the same, to identify those children in whom an implant will produce functional hearing. In children undergoing evaluation it is necessary to establish a baseline hearing threshold to monitor outcome. A hearing aid trial is undertaken irrespective of the level of hearing loss to establish if hearing augmentation is
sufficient. Following a failed hearing aid trial, a battery of audiological tests are undertaken depending on the child’s age, to assess the benefits of cochlear implantation. During this phase input is elicited from audiologists, parents, teachers and speech and language therapists to decide whether further investigation and consideration is warranted.
Radiological evaluation Currently radiological evaluation for cochlear implantation uses fine cut computed tomography (CT) and magnetic resonance imaging (MRI) of the temporal bones. CT is often enough to delineate the bony anatomy of the cochlea providing evidence of congenital abnormalities and ossification, which can follow deafness due to meningitis. Common congenital abnormalities of the cochlea include: Mondini’s dysplasia and widened vestibular aqueduct syndrome, which are present in syndromic and non-syndromic SNHL. Complete absence of a cochlea prevents implantation but even with partial absence implantation may be possible. MRI reviews the soft tissues; this is required for all cases of congenital deafness and may be indicated when central pathology needs to be excluded, including absence of the vestibulo-cochlea nerve. High-resolution T2-weighted imaging with or without gadolinium is often used. Two contraindications from radiological review are Michels deformity where there is absence of the cochlea, and second, absence of the central pathway especially the vestibulo-cochlea nerve. Actual guidelines for treatment vary depending on the institute involved. Criteria for selection are similar to the referral guidelines but as technique and implant technology improve it is likely that those with less profound hearing loss will eventually be considered. It is uncommon for implantation to occur at less than 12 months, due to a number of factors including the difficulty in assessing residual functional aidable hearing, the risk of anaesthetic and the thickness of the skull. Pre-lingual children require treatment as soon as possible due to reducing neural plasticity, many groups would suggest implantation no later than 5 years of age, some institutes will implant pre-lingual children up to the age of 10. The results of this latter group are not as
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
ARTICLE IN PRESS Cochlear implants: Referral, selection and rehabilitation
Figure 1
363
Intraorbital X-ray of bilateral cochlear implants in a child.
favourable for the development of functional hearing and other methods of communication do need to be established. For post-lingually deafened children the urgency is not as great as the auditory neural pathway has developed.
Parental commitment and understanding Before implantation can be undertaken it is vital to make sure parents understand and support the long-term commitment involved in the cochlear programme. They need to be aware that the procedure is not a one off cure, and that a great deal of time and effort is required for the best results. Without parental support the best functional outcome is unlikely to be achievable.
Surgical procedure The surgical procedure lasts about 2 h in duration requiring general anaesthesia and a post-operative overnight stay. The approach is via a post-auricular incision. The mastoid cavity is opened and all air cells removed. At this point the surgeon identifies the facial nerve and short process of the incus to prevent injury to these structures. A posterior tympanotomy is performed and the basal turn of the cochlea is identified. A 1.5 mm hole is drilled through the cochlea in to the scala tympani. The afferent electrode is placed through the cochleostomy and advanced as far as possible into the cochlea to reach the lower-frequency areas. The efferent electrode is placed deep to the temporalis muscle and a separate well is prepared for the internal part of the
implant. The skin is then closed in layers and a firm dressing and head bandage applied for 48 h. Post-operative transorbital radiographs are taken to ensure positioning (Fig. 1). Post-operatively the child will experience some discomfort, which can be controlled adequately with simple analgesia. Some experience vertigo from vestibular irritation, this is commonly transient and resolves within the first 48 h. The surgical site is allowed to heal for around 1 month followed by ‘switch on’ where they receive the external part of the device that is programmed for their particular hearing requirements.
Complications Complications from surgery have been reported to be as high as 5–10% although not in our own departments’ experience. Common problems include:
Wound infections The most common complication, and usually treated with local wound care and oral antibiotics. In severe infection surgical explantation may be required to control spread, reimplantation can occur following resolution of infection. Skin infection may be caused by erosion due to the magnetic attachment of the external device. If the magnet is too powerful pressure necrosis of the skin followed by infection may occur. Once noted, adjustments to magnet strength can be made which will prevent the condition worsening.
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
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Facial nerve injury A rare complication but reported due to the position of the nerve within the operative field. With transection complete facial weakness occurs with little chance of recovery. By monitoring throughout the procedure and by anticipation of the possibility of finding abnormal anatomy during surgery very few cases should occur. Tinnitus A common problem post-operatively due to the loss of residual hearing on implant placement. When encountered in this situation it is likely to resolve in time, often substantially following ‘switch on’. Vestibular dysfunction A risk as the inner ear has been breached. Temporary dysfunction is often experienced but permanent dysfunction has been reported. Providing there is no history of bilateral dysfunction with rehabilitation and time vertiginous symptoms should resolve. If a history of balance problems is elicited at the candidacy interview it is prudent to arrange pre-operative investigation. Meningitis Currently a hot topic in relation to cochlear implantation as some suggests an increased risk following implantation.6 The risk is limited but maybe higher than the normal population up to 2 years post-implantation. Due to these potential risks, pre-operative vaccination against Haemophilus and Pneumococcus should be given as part of the general treatment.
Rehabilitation Programmes of auditory training in children with implants are highly individualised depending on age, cognition and level of hearing gain. Post ‘switch on’ the child must demonstrate awareness/detection of sound (detection). Table 3
Rehabilitation then commences, the child is taught to determine whether sounds are different (discrimination), to recognise sounds, and to associate meaning with them (identification), and to respond appropriately to verbal commands (comprehension). Language development is undertaken, the child is taught to imitate phonemes and syllables, with progression to co-articulation, production of more complex words and eventually sentences. In rehabilitation, competence in hearing and speech should be developed together to produce total communicative function. The first step is ‘switch on’, which occurs 1-month postimplantation when wound healing is complete. For the next few months intensive auditory and speech training is undertaken by a team consisting of a teacher of the deaf and a speech and language therapist. During these sessions the individual is assessed in a variety of ways, specific help can be arranged and tuning/coding requirements are undertaken. In children full rehabilitation takes many years and it often takes 6–18 months to establish a clear initial benefit when the learning the learning curve starts to drop.
Assessment and outcome Assessment takes place at approximately 3, 6 and 12 months post-implantation and annually thereafter. In children, assessment is influenced by the child’s language status at the time the hearing was lost, their age at implantation and to a degree their cognition. Numerous tests have been developed to assess the outcome of implantation at varying stages post-operatively. Two main assessment tools exist, the Category of Auditory Performance score (CAP) and the Speech Intelligibility Rating (SIR; Table 3). These examples are only two of many assessment tools used in evaluation and are only part of the total assessment. Outcome measurements are difficult in children but two main goals are set:
Assessment tools used in rehabilitation following cochlear implantation. Score
Category of performance Use of telephone with loud speaker Understanding conversation without lip reading Understanding common phrases without lip reading Discrimination of speech sounds without lip reading Identification of environmental sounds Response to speech sounds Awareness of environmental sounds No awareness of environmental sounds Speech intelligibility rating Connected speech to all intelligible listeners. Child is understood in everyday contexts Connected speech is intelligible to a listener who has had experience of a deaf persons speech Connected speech is intelligible to a listener who concentrates and lip reads Connected speech is unintelligible. Intelligible speech is developing in single words when context and lip reading cues available Connected speech is unintelligible. Pre-recognisable words in spoken language, primary mode of communication may be manual
7 6 5 4 3 2 1 0 5 4 3 2 1
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.
ARTICLE IN PRESS Cochlear implants: Referral, selection and rehabilitation (1) Word understanding. The objective markers vary depending on whether the child is pre- or post-lingually deafened. Post-lingual children often have word understanding as high as 100% that of the normal hearing population at 3 year follow-up. For pre-lingual children who represent the majority of the paediatric implantation population results are not as good and are variable depending on the aetiology of the hearing loss. Progress tends to be slower and improvements level off with time. (2) Mainstream schooling. Implantation in the very young has a goal of communication sufficient to allow mainstream schooling. Many children do achieve age-appropriate speech recognition and production for this purpose. It is not uncommon for children implanted at 2–3 years old to be able to attend mainstream schooling by 5–6 years of age with continued support from the cochlear implant team.
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References 1. Copeland BJ, Pillsbury HC. Cochlear implantation for the treatment of deafness. Ann Rev Med 2004;55:1557–67. 2. O’Donoghue GM. Hearing without ears: do cochlear implants work in children? Br Med J 1999;318:72–3. 3. Eisen MD. Djourno, Eyries, and the first implanted neural stimulator to restore hearing. Otol Neurotol 2003;24: 500–6. 4. NIH Consensus Development statement: Cochlear implants in adults and children. J Am Med Assoc 1995;274:1955–61. 5. Brookhouser PE, Beauchaine KL, Osberger MJ. Management of the child with sensorineural hearing loss. Pediatr Clin North Am 1999;46(1):121–40. 6. Biernath KR, et al. Bacterial Meningitis among children with cochlear implants beyond 24 months after implantation. Pediatrics 2006;117:284–9. 7. Gantz BJ, Turner C, Gfeller KE, Lowder MW. Preservation of hearing in cochlear implant surgery: advantages of combined electrical and acoustical speech processing. Laryngoscope 2005;115:796–802.
Future developments The world of cochlear implants is constantly changing as experience in the techniques develop and technological advances take place. The ultimate goal is the totally implantable device but this is still some way off. At present the development of hybrid devices, which maintain residual hearing function but stimulate the regions that have been lost7 is of great interest requiring further research.
Further Reading 8. Gibson WPR. Cochlear implants. In: Booth JB, editor. Scott Brown’s Otolaryngology. Oxford: Butterworth-Heinemann; 1997. p. 1–25.
Please cite this article as: D. Owens et al., Cochlear implants: Referral, selection and rehabilitation, Current Paediatrics (2006), doi:10.1016/j.cupe.2006.07.003.