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Cochlear implants in genetic deafness
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ScienceDirect Journal of Otology 9 (2014) 156e162 www.journals.elsevier.com/journal-of-otology/
Cochlear implants in genetic deafness Xuezhong Liu Department of Otolaryngology, University of Miami Miller School of Medicine, 1120 NW 14th Ave., Miami, FL 33136, USA Received 12 August 2014; revised 17 August 2014; accepted 1 September 2014
Abstract Genetic defects are one of the most important etiologies of severe to profound sensorineural hearing loss and play an important role in determining cochlear implantation outcomes. While the pathogenic mutation types of a number of deafness genes have been cloned, the pathogenesis mechanisms and their relationship to the outcomes of cochlear implantation remain a hot research area. The auditory performance is considered to be affected by the etiology of hearing loss and the number of surviving spiral ganglion cells, as well as others. Current research advances in cochlear implantation for hereditary deafness, especially the relationship among clinic-types, genotypes and outcomes of cochlear implantation, will be discussed in this review. Copyright © 2015, PLA General Hospital Department of Otolaryngology Head and Neck Surgery. Production and hosting by Elsevier (Singapore) Pte Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Connexin Cx26/Cx30; Usher syndrome; Mitochondria genes; Waardenburg syndrome; Auditory neuropathy
Deafness is the most common birth defects in humans, with a rate of 3/1000 live births. About 1/1000 children exhibit hearing loss before school age. Over 60% of pre-lingual deafness is related to hereditary factors, while the rest is attributed to environmental or iatrogenic factors. Nowadays, newborn deafness gene screening for potential early intervention is a common practice all over the world. Conventional amplification devices may benefit some patients with hereditary deafness, but not for all types of hearing loss. Some patients with hereditary hearing loss need cochlear implant to acquire useful hearing. To examine outcomes of cochlear implant in various types of hereditary deafness, literatures on hereditary deafness are reviewed extensively, including syndromic and non-syndromic deafness. Most reports found in the relevant literatures are case reports or small cohort cochlear implant studies, with less than conclusive evaluation on the overall outcomes for hereditary deafness. The current reports
E-mail address: [email protected]. Peer review under responsibility of PLA General Hospital Department of Otolaryngology Head and Neck Surgery.
attempt to assess indications and treatment outcomes of cochlear implant in various types of hereditary deafness in relations to their clinical manifestation, genetic etiologies and temporal bone pathology. 1. Deafness related to the Cx26 and Cx30 genes Connexin is an important component of the connecting channels in cellular gap junction, facilitating intercellular electrolytes exchange. Of the known 21 types of connexins, five are expressed in the mammalian cochlea. Regulation of electrolytes across cellular membranes and balance of potassium gradient inside and outside cells in the cochlear spiral ganglion and stria vascularis cells are crucial for the generation of hair cell action potentials. Connexin dysfunction leads to potassium retention and microcirculation disorders, subsequently resulting in hair cell malfunction and degeneration (Propst et al., 2006). In addition to regulating intra- and extracellular potassium concentrations, in vitro studies have shown that channels involving Cx26 may participate in transporting secondary messenger IP3 which regulates Caþþ
http://dx.doi.org/10.1016/j.joto.2015.01.001 1672-2930/Copyright © 2015, PLA General Hospital Department of Otolaryngology Head and Neck Surgery. Production and hosting by Elsevier (Singapore) Pte Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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distribution. In cells with channels containing Cx26 and Cx30, Caþþ move faster than in cells with other types of channels. Mutation of Cx26 genes can result in disordered Caþþ movement and distribution in tissues (Beltramello et al., 2005). Cx26 gene mutations, however, do not affect vestibular functions. At this time, more than 100 types of mutation have been reported regarding Cx26, with certain racial features. For example, c.235delC (or c.233delC) is mostly seen among Asians, c.35delG (or c.30delG) among Caucasians, and c.167delG among Jews. These are all frame shift mutations, leading to premature ending of protein synthesis and hence dysfunction of connexins. Studies in China that compare cochlear implant treatment outcomes in patients with and without Cx26 gene mutations show that results in those with Cx26 are superior to those without regarding meaningful auditory integration scale, auditory behavioral grade, speech intelligibility and standardized speech learning period assessment (Yan et al., 2011). Matsushiro et al. (2002) evaluated cochlear implant outcomes in four Japanese children with homogenous Cx26 233delC mutation and profound hearing loss, and reported that speech understanding in these children were better than in deafness patients without Cx26 mutation. In addition to c.235delC mutation, multiple studies on non-syndromic deafness have also demonstrated better results in deafness patients with 35delG mutation than in those without, showing faster improvement in speech and speech understanding with better consistency. Reading capability studies have shown that patients with Cx26 mutations do not carry other complicating etiologies, such as disorders of the VIII cranial nerve, central auditory structures or cognitive centers at higher levels (Connell et al., 2007; Bauer et al., 2003). Propst et al. (2006) studied electrically evoked auditory nerve compound action potentials in patients with cochlear implant and found that intact spiral ganglion cells were present inside the cochlea in patients with Cx26 mutations. However, Cx26 mutations are not the sole accurate indicator of cochlear implant outcomes (Connell et al., 2007; Bauer et al., 2003). Compared to children with Cx26 mutations as the clear etiology for deafness, structural and molecular etiologies for deafness in nonDFNB1 children are more complex. Statistical analysis suggests that the length of implanted electrodes may be a better indicator of cochlear implant outcomes than hereditary factors (Connell et al., 2007).
2. PDS (SLC26A4) gene-related deafness The earliest records of Pendred syndrome (PDS) were two Irish sisters presenting with thyroid goiters and deafness (hence also known as goiter-deafness syndrome). The responsible gene is PDS (SLC26A4) which encodes an iodide/chloride transporter protein called pendrin in the solution carrier family. Pendrin is found in both thyroid and cochlea. Usami et al. (1999) showed that the PDS gene was also responsible for large vestibular aqueduct syndrome (LVAS). There are about 140
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types of PDS mutations discovered among patients with PDS or LAVS, with IVS7-2A > G and p.H723R being the most common among East Asia populations (Dai et al., 2008; Pare et al., 2005). Non-syndromic deafness patients carrying PDS double mutant alleles often manifest severe fluctuating hearing loss, but it is unclear if PDS mutations increase hearing loss severity or how enlarged vestibular aqueduct is related to hearing loss. Research indicates that PDS mutations lead to changes in pendrin protein structure and function, resulting in disorders in chloride transportation and inner ear fluid circulation, and subsequently enlarged vestibular aqueduct and increased endolymphatic pressure, which can cause hair cells damage and hearing loss (Kraiem et al., 1999). There are several speculations on the pathogenesis of hearing damage in LVAS. Some suggest that enlarged vestibular aqueduct disturbs endolymphatic circulation, resulting in high osmotic pressure in the endolymphatic sac and retrograde flow into the cochlea which causes damage to cochlear neuroepidermal cells and subsequently neural hearing loss. Others believe that there are weak fragile areas in the vestibule in patients with LVAS which may break when sustaining trauma or sudden CSF pressure change, leading to mix of endo- and perilymphatic fluids or perilymph fistula, and subsequently neural hearing loss (Xu and Chi, 2003). For patients with PDS mutations and severe or profound hearing loss, cochlear implant is an effective treatment to improve hearing. Cochlear implant started in 1992 and has seen significant improvement in implant devices and surgical techniques since first two successful cases in 1995, with overall improved outcomes. Since children with enlarge vestibular aqueduct tend to have gradually increasing hearing loss, they show a much broader range of age at first implant compared to those with congenital severe or profound hearing loss. Research, however, shows that postoperative hearing thresholds in these children are not significantly different from those in patients with normal inner ear anatomy (El-Amraoui and Petit, 2005).
3. Usher syndrome Usher syndrome is a recessive autosomal hereditary disease with dysfunction of both the ear and eye, manifested as congenital sensorineural hearing loss and progressive pigmentary retinitis and later retinal degeneration. At an age of 2e4 years, retinal electrophysiology test can show abnormalities with the photoreceptors, night blindness shows up at 10 years of age with loss of vision field, and finally the patient becomes blind during teen years (Rosenberg et al., 1997). Usher syndrome is the most common deafnesseblindness syndrome in human. Based on its clinical presentation, Usher syndrome can be classified in three types. Type I (UshI) is the most severe type and accounts for 30e40% of Usher syndrome cases. It is characterized by congenital severe to profound hearing loss at birth, vestibular dysfunction and impaired activity development at very young age, and progressive retinal pathological
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changes with vision deterioration e from loss of peripheral vision and central visual field sensitivity at a young age to complete blindness in early adulthood. Residual low frequency hearing in this type of Usher syndrome is usually in the range of 90e100 dB HL, unable to benefit from hearing aids and requiring cochlear implant. UshI patients demo nstrate abnormal vestibular functions and electroretinograms at an early stage (Loundon et al., 2003). However, electroretinography is a sophisticated test requiring general sedation in children and therefore not routinely used in the clinic (Liu et al., 2008). Pennings et al. (2006) conducted gene tests in 14 CI patients and found mutations in half of these patients. They concluded that gene testing could be used as one of the early diagnosis options in these patients. Molecular studies on USHI mutations show that all defective proteins appear during the time of formation of auditory hair bundles (kinocillia and sterocillia). UshI defective proteins (myosin VIIa, PDZ domain containing protein Harmonin, cadherin 23, pro-cadherin 15 and scaffolding protein Sans) are believed to be involved in the formation of hair bundle connection and adhesion. Disturbed hair bundle structures have been reported in rats lacking UshI homologous gene proteins (El-Amraoui and Petit, 2005). Wagenaar et al. (2000) examined two UshI patients and found severe degeneration in the cochlea with atrophy of the basal turn and stria vascularis and decreased number of spiral ganglion cells. In contrast to their normal counterparts, the rate of their cochlear neurons decay is as high as 68% on average, and severe degeneration of the macula sacculi can be seen in patients with genetically Ush1D or Ush1F diagnosis. Age is probably the most critical prognostic indicator in cochlear implant for Usher syndrome. Usher syndrome can cause dysfunction of both auditory and visual organs, and while sign language communication is possible at early ages, it eventually becomes impossible after loss of vision and the patient has to rely on verbal communication. Cochlear implant is therefore especially important as a way to acquire hearing and speech functions in these patients. Liu et al. (2008) compared speech rehabilitation results following cochlear implant among a group of patients with USHI syndrome aging from 2 to 15 years. Both open and closed sets word test scores showed the greatest increase in children <3 years of age when receiving cochlear implant than in those receiving implant at an age older than 6 years, indicating that early intervention before loss of vision is crucial to development of effective hearing and speech capabilities. Blanchet et al. (2007) showed that children who received cochlear implant before 3 years of age were able to benefit from regular education. Loundon et al. (2003) concluded that cochlear implant before 9 years of age yielded significant benefits, with all patients (100%) showing statistically significant improvement in open set word test scores, although only 25e75% in closed set scores. Furthermore, Young et al. (1995) concluded through their research that cochlear implant outcomes were also related to the degree of hearing loss as well as the type and intensity of rehabilitation. Clinical phenotype does not appear to be closely linked
to types of genetic mutation in UshI syndrome (Blanchet et al., 2007).
4. Mitochondria-related deafness Mitochondria DNA encodes numerous messages as well as tRNA and is at the core in coding production of ATP, which provides energy in almost all organs but especially those at high rate of metabolism. Cochlear outer hair cells and the stria vascularis are high metabolism structures. Motion of hair bundles of hair cells comes from intracochlear action potentials that are maintained by the Naþ/ Kþ-ATP pump in the stria vascularis. The basal turn of the cochlea, which responds to high frequency sounds, exhibits even higher metabolic needs than other parts of the cochlea. Mitochondrial dysfunction affects high frequency hearing first, followed by spreading to other frequencies and eventually hearing loss at all frequencies (Sinnathuray et al., 2003). Studies with temporal bone histology show that outer hair cells suffer more rapid and severe damages than inner hair cells when the number of spiral ganglion cells declines and that cochlear dysfunction progresses from the basal to apical turns (Yamasoba et al., 1999). Mitochondrial deafness can be syndromic or nonsyndromic. Those relevant to cochlear implant include “mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome”, maternally inherited diabetes and deafness (MIDD) syndrome, KearnseSayre syndrome (KSS), chronic progressive external ophthalmoplegia (CPEO) syndrome, etc. Non-syndromic mitochondrial deafness is more common than syndromic and often related to aminoglycoside ototoxicity, especially in the A1555G mutation (Sinnathuray et al., 2003; Prezant et al., 1993). Cochlear implant outcomes are generally good in cases of non-syndromic mitochondrial deafness. Tono et al. (1998) reported a case of A1555G mutation. The patient acquired sensorineural hearing loss 28 years ago following using aminoglycoside drugs and achieved 78% (54% when blinded) monosyllable test scores and 84% (76% when blinded) word test scores 10 months after cochlear implant. Ulubil et al. (2006) presented a 35 years old patient with hearing loss and A1555G mutation but no history of aminoglycoside exposure. At 1 month following cochlear implant, his average open set CUNY score was 60% (65% in HINT). Results in syndromic deafness cases are also encouraging. In a case of MELAS reported by Rosenthal et al. (1999), the 20 years old male patient achieved a 60% CID sentence test score at 6 months after cochlear implant. In a report by Yasumura et al., the 29 years old female patient demonstrated 72% and 94% scores in closed set word and sentence tests respectively, and 44% and 34% scores in open set testing, at 10 months postoperatively (Karkos et al., 2005). Cochlear implant is also effective in MIDD syndrome. A 42 years old woman in a report by Raut et al. (2002) produced a 90% BKB sentence test score. Similarly, Counter et al. (2001) reported a patient with MIDD syndrome, whose CUNY score
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increased from 5% to 84% at 3 months following cochlear implant. 5. Waardenbury syndrome (WS) WS is a relatively common autosomal hereditary syndrome, also known as auditoryepigmentary syndrome. Its clinical features include inner canthus shift, thick eye brow, heterochromia iridium, premature canities and sensorineural hearing loss of various degrees. The rate of WS among newborns is about 1/40,000 and accounts for about 2e5% of congenital deafness (Cullen et al., 2006; Nayak and Isaacson, 2003) (but only 0.44e0.46% in China (Gao et al., 2000; Yang et al., 2010)). Clinically, there are 4 subtypes within WS. Type I carries inner canthus shift, sensorineural hearing loss, heterochromia iridium, premature canities, hypopigmentation, synophridia, etc. There is no inner canthus shift in type II. Type III, also known as Klein-Waardenburg syndrome, adds upper extremity muscle dysplasia and contracture, in addition to features in type I. Type IV is also called Shah-Waardenburg syndrome and adds congenital megacolon to type II (Nayak and Isaacson, 2003). Types I and II are the most commonly seen both in China and abroad. The pathogenic gene is PAX3 (a transcription factor) in the former and MITF in the latter, which encodes a tyrosinase catalyst that is critical in melanin production and interacts with the PAX3 gene (Watanabe et al., 1998). The pathogenesis in WS may be related to lack of melanocytes in the stria vascularis. Mutations of relevant genes cause disruption in melanin production and can affect the hair, skin and eye, as well as deposit of melanin in neural crest cells which is the foundation of formation of stria vascularis (seen as atrophy of the Organ of Corti and stria vascularis on temporal bone histology) (Watanabe et al., 1998; Nakashima et al., 1992). Cochlear implant has been used as an intervention in WS with profound sensorineural hearing loss. Bony labyrinth anomaly is not common in WS, but can present as abnormal or missing semicircular canals, which calls for caution during implant procedures but is not a contraindication (Zhang et al., 2009). WA patients have shown above average results in postimplant closed and open set word tests. Reports from China and overseas have stated that WS patients enjoy better cochlear implant outcomes than those with inner ear malformation or non-syndromic deafness with “normal” cochlea (Zhang et al., 2009; Cullen et al., 2006). In a report from University of North Carolina at Chapel Hill, five of the seven patients who had received cochlear implant showed 100% closed set word recognition score and greater than 50% open set word recognition scores soon after implantation (Cullen et al., 2006). A follow up report on WS1 patients from Iran showed similar results, with all children being able to attend regular education. However, Pau et al. (2006) noticed that in 20% of 20 WS cases implanted between 1985 and 2001, ABR results were abnormal. They concluded that the significantly worse speech recognition results were related to “co-existing” auditory neuropathy that adversely impaired cochlear implant outcomes.
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6. JervelleLangeeNielse syndrome (JLNS) JLNS is featured by emotional syncope, sudden death, arrhythmia and congenital sensorineural hearing loss, with seizure from severe bradycardia or ventricular tachycardia being frequently seen (Siem et al., 2008). On ECG, QT interval is often >0.44 s in male patients and >0.46 s in female patients, which can lead to ventricular tachycardia/fibrillation or torsive ventricular tachycardia or even sudden death, often triggered by excitement, strenuous exercise or noise that leads to high levels of catecholamine release (as seen when swimming in about 16% of such cases) (Chorbachi et al., 2002; Daneshi et al., 2008). b blockers are often used to control arrhythmia, while pace-maker and defribrillator implants are used to reduce sudden death (Chorbachi et al., 2002; Yanmei et al., 2008). JLNS-related genes are KCNQ1 and KCNE1 (LQT1), both encode the slow components in the delayed rectifier Kþ channel compound (90% and 10% respectively). Delayed rectifier Kþ channels pump Kþ out of stia vascularis cells to maintain Kþ stability in the endolymph (Neyroud et al., 1997). Temporal bone histology studies have shown collapse of Reissner's membrane, saccule, utricle and membranes near the ampulla which can lead to blockage of the endolymphatic system in the scalae tympani and vestibuli (Friedmann et al., 1968). CI is considered an effective treatment for deafness from JLNS. As mentioned above, cochlear implant bypasses the dysfunctional Organ of Corti and stimulates the spiral ganglion directly. Chorbachi et al. (2002) presented a case of JLNS in three brothers born to parents in a consanguineous marriage, all with congenital profound deafness and prolonged QT intervals. The two younger brothers enjoyed good hearing results from cochlear implant. Yanmei et al. (2008) reported a 3 years old child who achieved a CAP score of 7 and a SIR score of 5 at 36 months after cochlear implant. In a follow up study by Daneshi et al. (2008) with three children younger than 3 years, the CAP and SIR scores were 6 and 4, respectively, at 48 months after cochlear implant, and all three children were able to attend regular education. Some literatures have reported symptoms of vestibular involvement in JLNS. Green et al. (2000) reported a child whose open set word test scores and articulation improved following cochlear implant but demonstrated a delay in speech development compared to normal children by about 1.5 years. Siem et al. (2008) reported a series of 8 children aged from 17 months to 7.5 years who had undergone cochlear implantation. Two of these children died of heart conditions and the rest children showed good hearing test results, although with delayed gross motor function development that might indicate involvement of the vestibular system. In the 1960s, Friedmann et al. (1968) observed fibrosis and dysplasia of vestibular epithelium on temporal bone histology. However, our understanding of vestibular involvement in JLNS remains limited even today. For patients with JLNS, QT interval evaluation before cochlear implant is important and preoperative ECG test is
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considered necessary in patients with congenital hearing loss to prevent potential cardiac complications. Anesthetics with anti-arrhythmia properties should be carefully selected and b blockers are usually needed. For those whose torsive tachycardia can be triggered by noise, induction should be conducted in a quiet environment. With careful preparation for possible adverse incidents, safe implant procedures for improved hearing in these patients are feasible. 7. Auditory neuropathy (AN) Auditory neuropathy (AN) is a hearing disorder caused by dysfunction of the inner hair cell/auditory nerve synapse and/or the auditory nerve itself, with essentially normal functioning from the external ear canal to outer hair cells. It has unique clinical presentations and audiometric features, and is also called auditory nerve spectrum disorder (ANSD) for its rather complex etiologies (Manchaiah et al., 2011). ANSD can be classified as non-syndromic and syndromic, with the non-syndromic type being more common and presenting with simple hearing disorders without other neurological abnormalities in contrast to the syndromic type which presents with other neurological disorders in the form of a syndrome. Etiologies for AN are complex and hereditary factors play an important role. Non-syndromic ANSD is often associated with OTOF, DIAPH3 and PJVK genes, and GJB2 and mitochondrial DNA 12rRNA genes have also been reported in AN, although rare (Santaretlli et al., 2008; Wang et al., 2005). The OTOF gene encodes the protein otoferlin which plays an important role in promoting exocytosis of auditory ribbon synapse vesicles. Its mutation can result in DFNB9. The DIAPH3 gene encodes a hyaline protein compound that is important to cellular morphology polarity and adhesion, and its mutations can lead to over-expression of the protein and AN. The PJVK gene encodes Pejvakin which is expressed in the afferent neurons in the cochlea and auditory pathway, and its mutations affect activities of hair cells and auditory neurons, leading to DFNB59 (Bae et al., 2013). AN is characterized by absent or abnormal ABRs with preserved cochlear responses (OAEs and CMs). Hearing loss on pure tone audiometry in patients with AN is usually bilateral and may vary from mild to profound in severity, often with flat or ascending curves on audiogram, although other patterns are also possible. Speech recognition in these patients is usually poor, even with only mild or moderate loss on pure tone audiometry. Cochlear implant for ANSD has been controversial, although good results have been reported in some patients. Recent years have seen increasing reports of satisfactory outcomes with cochlear implant in ANSD patients. Pelosi et al. (2013) compared InfanteToddler Meaningful Auditory Integration Scale (IT-MAIS) and open set word test results among patients diagnosed with ANSD at Vanderbilt University between 2009 and 2011who either were fitted with hearing
aids with satisfaction or received cochlear implant after failing hearing aids, and found no significant difference between the two groups. They therefore concluded that cochlear implant may help improve speech recognition in patients who cannot benefit from hearing aids. Budenz et al. (2013) studied cochlear implant outcomes in children with AN only and in those with AN complicated with cognition development disorders, and found outcomes in the former are superior to those in the latter and similar to those in children who have received cochlear implant for cochlear disorders. Breneman et al. (2012) also concluded that cochlear implant outcomes in children with ANSD but preserved auditory nerve were not different from those in non-ANSD children, although some studies show that vowel recognition is slightly worse with significantly worse consonant recognition in the former than in the latter following cochlear implant (Miyamoto et al., 1999). Currently, it is believed that outcomes in patients with ANSD are related to the location of the ANSD lesion and the number of spiral ganglion cells involved. Poorly developed, absent or atrophied cochlear nerve can adversely affect cochlear implant outcomes. Preoperative imaging for auditory nerve development evaluation and EABR assessment of spiral ganglion cells may be necessary before cochlear implant in ANSD patients (Wang and Yin, 2006). Hereditary factors are an important etiology for deafness. Many genetic etiologies for deafness have been determined and characterized, including Cx26 mutation related deafness, PDS-related deafness, Usher syndrome, mitochondrial DNA-related deafness, Waardenburg syndrome, JervelleLangeeNielsen syndrome, etc. Etiologies for AN are complex, and only a few non-syndromic etiologies have been determined. From reviewing the literature, contraindications to cochlear implant are rare and may include advanced age and inability to pronounce. Cochlear implant intervention, especially early intervention, in individuals with hereditary deafness, can result in obvious hearing and speech rehabilitation as the spiral ganglion and central pathways remain relatively intact. Further studies involving large hereditary deafness populations are needed to confirm the efficacy of cochlear implant in these patients. Even more important is appropriate genetic screening and consultation in individual deafness patients. In summary, genetic studies remain a forefront in medical science research and expanding exploration into genetic etiologies for human diseases is extremely valuable while certainly facing numerous unknown challenges.
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Pare, H.J., Lee, S.J., Jin, H.S., et al., 2005. Genetic basis of hearing loss associated with enlarged vestibular aqueducts in Koreans. Clin. Genet. 67, 160e165. Pau, H., Gibson, W.P., Gardner-Berry, K., et al., 2006. Cochlear implantations in children with Waardenburg syndrome: an electrophysiological and psychophysical review. Cochlear Implants Int. 7 (4), 202e206. Pelosi, S., Wanna, G., Hayes, C., et al., 2013. Cochlear implantation versus hearing amplification in patients with auditory neuropathy spectrum disorder. Otolaryngol. Head Neck Surg. 148 (5), 816e821. Pennings, R.J., Damen, G.W., Snik, A.F., et al., 2006. Audiologic performance and benefit of cochlear implantation in Usher syndrome type I. Laryngoscope 116 (5), 717e722. Prezant, T.R., Agapian, J.V., Bohlman, M.C., et al., 1993. Mitochondrial ribosomal RNA mutation associated with both antibiotic-induced and nonsyndromic deafness. Nat. Genet. 4 (3), 289e294. Propst, E.J., Papsin, B.C., Stockley, T.L., et al., 2006. Auditory responses in cochlear implant users with and without GJB2 deafness. Laryngoscope 116 (2), 317e327. Raut, V., Sinnathuray, A.R., Toner, J.G., 2002. Cochlear implantation in maternal inherited diabetes and deafness syndrome. J. Laryngol. Otol. 116 (5), 373e375. Rosenberg, T., Haim, M., Hauch, A.M., 1997. The prevalence of Usher syndrome and other retinal dystrophyehearing impairment associations. Clin. Genet. 51 (5), 314e321. Rosenthal, E.L., Kileny, P.R., Boerst, A., 1999. Successful cochlear implantation in a patient with MELAS syndrome. Am. J. Otol. 20 (2), 187e190 (Discussion 190e191). Santaretlli, R., Cama, E., Scimemi, P., et al., 2008. Audiological and electrocochleography findings in hearing impaired children with connexin 26 mutations and otoacoustic emissions. Eur. Arch. Otorhinoaryngol. 265, 43e51. Siem, G., Fruh, A., Leren, T.P., et al., 2008. Jervell and LangeeNielsen syndrome in Norwegian children: aspects around cochlear implantation, hearing, and balance. Ear Hear. 29 (2), 261e269. Sinnathuray, A., Raut, V., Awa, A., et al., 2003. A review of cochlear implantation in mitochondrial sensorineural hearing loss. Otol. Neurotol. 24 (3), 418e426. Tono, T., Ushisako, Y., Kiyomizu, K., et al., 1998. Cochlear implantation in a patient with profound hearing loss with the A1555G mitochondrial mutation. Am. J. Otol. 19 (6), 754e757. Ulubil, S.A., Furze, A.D., Angeli, S.I., 2006. Cochlear implantation in a patient with profound hearing loss with the A1555G mitochondrial DNA mutation and no history of aminoglycoside exposure. J. Laryngol. Otol. 120 (3), 230e232. Usami, S., Abe, S., Weston, M., et al., 1999. Non-syndromic hearing loss associated with enlarge vestibular aqueduct is caused by PDS mutations. Hum. Genet. 104, 188e192. Wagenaar, M., Schuknecht, H., Nadol Jr., J., et al., 2000. Histopathologic features of the temporal bone in Usher syndrome type I. Arch. Otolaryngol. Head Neck Surg. 126 (8), 1018e1023. Wang, L., Yin, S., 2006. Cochlear implant and auditory neuropathy. Chin. Sci. J. Hear. Speech Rehabil. 05, 28e30 (Published in Chinese). Wang, Q., Li, R., Zhao, H., et al., 2005. Clinical and molecular characterization of a Chinese patients with auditory neuropathy associated with mitochondrial 12S rRNA T1095C mutation. Am. J. Med. Genet. 133A (1), 27e30. Watanabe, A., Takeda, K., Ploplis, B., et al., 1998. Epistatic relationship between Waardenburg syndrome genes MITF and PAX3. Nat. Genet. 18 (3), 283e286. Xu, X., Chi, F., 2003. Advances in research on large vestibular aqueduct syndrome (LVAS) Chines. J. Otol. 1 (2), 61e63 (Published in Chinese). Yamasoba, T., Tsukuda, K., Oka, Y., et al., 1999. Cochlear histopathology associated with mitochondrial transfer RNA(leu(UUR)) gene mutation. Neurology 52 (8), 1705e1707. Yan, Y., Li, Y., Yang, T., et al., 2011. Cochlear implant outcomes in children with GJB2 or SLC26A4 gene mutations. Chin. J. Otol. 9 (2), 163e167 (Published in Chinese).
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Young, N.M., Johnson, J.C., Mets, M.B., et al., 1995. Cochlear implants in young children with Usher's syndrome. Ann. Otol. Rhinol. Laryngol. Suppl. 166, 342e345. Zhang, Z., Cao, K., Wei, C., et al., 2009. Outcomes of cochlear implant in patients with Waardenburg syndrome. J. Audiol. Speech Pathol. 17 (4), 372e375 (Published in Chinese).
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ScienceDirect Journal of Otology 9 (2014) 156e162 www.journals.elsevier.com/journal-of-otology/
Cochlear implants in genetic deafness Xuezhong Liu Department of Otolaryngology, University of Miami Miller School of Medicine, 1120 NW 14th Ave., Miami, FL 33136, USA Received 12 August 2014; revised 17 August 2014; accepted 1 September 2014
Abstract Genetic defects are one of the most important etiologies of severe to profound sensorineural hearing loss and play an important role in determining cochlear implantation outcomes. While the pathogenic mutation types of a number of deafness genes have been cloned, the pathogenesis mechanisms and their relationship to the outcomes of cochlear implantation remain a hot research area. The auditory performance is considered to be affected by the etiology of hearing loss and the number of surviving spiral ganglion cells, as well as others. Current research advances in cochlear implantation for hereditary deafness, especially the relationship among clinic-types, genotypes and outcomes of cochlear implantation, will be discussed in this review. Copyright © 2015, PLA General Hospital Department of Otolaryngology Head and Neck Surgery. Production and hosting by Elsevier (Singapore) Pte Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Connexin Cx26/Cx30; Usher syndrome; Mitochondria genes; Waardenburg syndrome; Auditory neuropathy
Deafness is the most common birth defects in humans, with a rate of 3/1000 live births. About 1/1000 children exhibit hearing loss before school age. Over 60% of pre-lingual deafness is related to hereditary factors, while the rest is attributed to environmental or iatrogenic factors. Nowadays, newborn deafness gene screening for potential early intervention is a common practice all over the world. Conventional amplification devices may benefit some patients with hereditary deafness, but not for all types of hearing loss. Some patients with hereditary hearing loss need cochlear implant to acquire useful hearing. To examine outcomes of cochlear implant in various types of hereditary deafness, literatures on hereditary deafness are reviewed extensively, including syndromic and non-syndromic deafness. Most reports found in the relevant literatures are case reports or small cohort cochlear implant studies, with less than conclusive evaluation on the overall outcomes for hereditary deafness. The current reports
E-mail address: [email protected]. Peer review under responsibility of PLA General Hospital Department of Otolaryngology Head and Neck Surgery.
attempt to assess indications and treatment outcomes of cochlear implant in various types of hereditary deafness in relations to their clinical manifestation, genetic etiologies and temporal bone pathology. 1. Deafness related to the Cx26 and Cx30 genes Connexin is an important component of the connecting channels in cellular gap junction, facilitating intercellular electrolytes exchange. Of the known 21 types of connexins, five are expressed in the mammalian cochlea. Regulation of electrolytes across cellular membranes and balance of potassium gradient inside and outside cells in the cochlear spiral ganglion and stria vascularis cells are crucial for the generation of hair cell action potentials. Connexin dysfunction leads to potassium retention and microcirculation disorders, subsequently resulting in hair cell malfunction and degeneration (Propst et al., 2006). In addition to regulating intra- and extracellular potassium concentrations, in vitro studies have shown that channels involving Cx26 may participate in transporting secondary messenger IP3 which regulates Caþþ
http://dx.doi.org/10.1016/j.joto.2015.01.001 1672-2930/Copyright © 2015, PLA General Hospital Department of Otolaryngology Head and Neck Surgery. Production and hosting by Elsevier (Singapore) Pte Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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distribution. In cells with channels containing Cx26 and Cx30, Caþþ move faster than in cells with other types of channels. Mutation of Cx26 genes can result in disordered Caþþ movement and distribution in tissues (Beltramello et al., 2005). Cx26 gene mutations, however, do not affect vestibular functions. At this time, more than 100 types of mutation have been reported regarding Cx26, with certain racial features. For example, c.235delC (or c.233delC) is mostly seen among Asians, c.35delG (or c.30delG) among Caucasians, and c.167delG among Jews. These are all frame shift mutations, leading to premature ending of protein synthesis and hence dysfunction of connexins. Studies in China that compare cochlear implant treatment outcomes in patients with and without Cx26 gene mutations show that results in those with Cx26 are superior to those without regarding meaningful auditory integration scale, auditory behavioral grade, speech intelligibility and standardized speech learning period assessment (Yan et al., 2011). Matsushiro et al. (2002) evaluated cochlear implant outcomes in four Japanese children with homogenous Cx26 233delC mutation and profound hearing loss, and reported that speech understanding in these children were better than in deafness patients without Cx26 mutation. In addition to c.235delC mutation, multiple studies on non-syndromic deafness have also demonstrated better results in deafness patients with 35delG mutation than in those without, showing faster improvement in speech and speech understanding with better consistency. Reading capability studies have shown that patients with Cx26 mutations do not carry other complicating etiologies, such as disorders of the VIII cranial nerve, central auditory structures or cognitive centers at higher levels (Connell et al., 2007; Bauer et al., 2003). Propst et al. (2006) studied electrically evoked auditory nerve compound action potentials in patients with cochlear implant and found that intact spiral ganglion cells were present inside the cochlea in patients with Cx26 mutations. However, Cx26 mutations are not the sole accurate indicator of cochlear implant outcomes (Connell et al., 2007; Bauer et al., 2003). Compared to children with Cx26 mutations as the clear etiology for deafness, structural and molecular etiologies for deafness in nonDFNB1 children are more complex. Statistical analysis suggests that the length of implanted electrodes may be a better indicator of cochlear implant outcomes than hereditary factors (Connell et al., 2007).
2. PDS (SLC26A4) gene-related deafness The earliest records of Pendred syndrome (PDS) were two Irish sisters presenting with thyroid goiters and deafness (hence also known as goiter-deafness syndrome). The responsible gene is PDS (SLC26A4) which encodes an iodide/chloride transporter protein called pendrin in the solution carrier family. Pendrin is found in both thyroid and cochlea. Usami et al. (1999) showed that the PDS gene was also responsible for large vestibular aqueduct syndrome (LVAS). There are about 140
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types of PDS mutations discovered among patients with PDS or LAVS, with IVS7-2A > G and p.H723R being the most common among East Asia populations (Dai et al., 2008; Pare et al., 2005). Non-syndromic deafness patients carrying PDS double mutant alleles often manifest severe fluctuating hearing loss, but it is unclear if PDS mutations increase hearing loss severity or how enlarged vestibular aqueduct is related to hearing loss. Research indicates that PDS mutations lead to changes in pendrin protein structure and function, resulting in disorders in chloride transportation and inner ear fluid circulation, and subsequently enlarged vestibular aqueduct and increased endolymphatic pressure, which can cause hair cells damage and hearing loss (Kraiem et al., 1999). There are several speculations on the pathogenesis of hearing damage in LVAS. Some suggest that enlarged vestibular aqueduct disturbs endolymphatic circulation, resulting in high osmotic pressure in the endolymphatic sac and retrograde flow into the cochlea which causes damage to cochlear neuroepidermal cells and subsequently neural hearing loss. Others believe that there are weak fragile areas in the vestibule in patients with LVAS which may break when sustaining trauma or sudden CSF pressure change, leading to mix of endo- and perilymphatic fluids or perilymph fistula, and subsequently neural hearing loss (Xu and Chi, 2003). For patients with PDS mutations and severe or profound hearing loss, cochlear implant is an effective treatment to improve hearing. Cochlear implant started in 1992 and has seen significant improvement in implant devices and surgical techniques since first two successful cases in 1995, with overall improved outcomes. Since children with enlarge vestibular aqueduct tend to have gradually increasing hearing loss, they show a much broader range of age at first implant compared to those with congenital severe or profound hearing loss. Research, however, shows that postoperative hearing thresholds in these children are not significantly different from those in patients with normal inner ear anatomy (El-Amraoui and Petit, 2005).
3. Usher syndrome Usher syndrome is a recessive autosomal hereditary disease with dysfunction of both the ear and eye, manifested as congenital sensorineural hearing loss and progressive pigmentary retinitis and later retinal degeneration. At an age of 2e4 years, retinal electrophysiology test can show abnormalities with the photoreceptors, night blindness shows up at 10 years of age with loss of vision field, and finally the patient becomes blind during teen years (Rosenberg et al., 1997). Usher syndrome is the most common deafnesseblindness syndrome in human. Based on its clinical presentation, Usher syndrome can be classified in three types. Type I (UshI) is the most severe type and accounts for 30e40% of Usher syndrome cases. It is characterized by congenital severe to profound hearing loss at birth, vestibular dysfunction and impaired activity development at very young age, and progressive retinal pathological
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changes with vision deterioration e from loss of peripheral vision and central visual field sensitivity at a young age to complete blindness in early adulthood. Residual low frequency hearing in this type of Usher syndrome is usually in the range of 90e100 dB HL, unable to benefit from hearing aids and requiring cochlear implant. UshI patients demo nstrate abnormal vestibular functions and electroretinograms at an early stage (Loundon et al., 2003). However, electroretinography is a sophisticated test requiring general sedation in children and therefore not routinely used in the clinic (Liu et al., 2008). Pennings et al. (2006) conducted gene tests in 14 CI patients and found mutations in half of these patients. They concluded that gene testing could be used as one of the early diagnosis options in these patients. Molecular studies on USHI mutations show that all defective proteins appear during the time of formation of auditory hair bundles (kinocillia and sterocillia). UshI defective proteins (myosin VIIa, PDZ domain containing protein Harmonin, cadherin 23, pro-cadherin 15 and scaffolding protein Sans) are believed to be involved in the formation of hair bundle connection and adhesion. Disturbed hair bundle structures have been reported in rats lacking UshI homologous gene proteins (El-Amraoui and Petit, 2005). Wagenaar et al. (2000) examined two UshI patients and found severe degeneration in the cochlea with atrophy of the basal turn and stria vascularis and decreased number of spiral ganglion cells. In contrast to their normal counterparts, the rate of their cochlear neurons decay is as high as 68% on average, and severe degeneration of the macula sacculi can be seen in patients with genetically Ush1D or Ush1F diagnosis. Age is probably the most critical prognostic indicator in cochlear implant for Usher syndrome. Usher syndrome can cause dysfunction of both auditory and visual organs, and while sign language communication is possible at early ages, it eventually becomes impossible after loss of vision and the patient has to rely on verbal communication. Cochlear implant is therefore especially important as a way to acquire hearing and speech functions in these patients. Liu et al. (2008) compared speech rehabilitation results following cochlear implant among a group of patients with USHI syndrome aging from 2 to 15 years. Both open and closed sets word test scores showed the greatest increase in children <3 years of age when receiving cochlear implant than in those receiving implant at an age older than 6 years, indicating that early intervention before loss of vision is crucial to development of effective hearing and speech capabilities. Blanchet et al. (2007) showed that children who received cochlear implant before 3 years of age were able to benefit from regular education. Loundon et al. (2003) concluded that cochlear implant before 9 years of age yielded significant benefits, with all patients (100%) showing statistically significant improvement in open set word test scores, although only 25e75% in closed set scores. Furthermore, Young et al. (1995) concluded through their research that cochlear implant outcomes were also related to the degree of hearing loss as well as the type and intensity of rehabilitation. Clinical phenotype does not appear to be closely linked
to types of genetic mutation in UshI syndrome (Blanchet et al., 2007).
4. Mitochondria-related deafness Mitochondria DNA encodes numerous messages as well as tRNA and is at the core in coding production of ATP, which provides energy in almost all organs but especially those at high rate of metabolism. Cochlear outer hair cells and the stria vascularis are high metabolism structures. Motion of hair bundles of hair cells comes from intracochlear action potentials that are maintained by the Naþ/ Kþ-ATP pump in the stria vascularis. The basal turn of the cochlea, which responds to high frequency sounds, exhibits even higher metabolic needs than other parts of the cochlea. Mitochondrial dysfunction affects high frequency hearing first, followed by spreading to other frequencies and eventually hearing loss at all frequencies (Sinnathuray et al., 2003). Studies with temporal bone histology show that outer hair cells suffer more rapid and severe damages than inner hair cells when the number of spiral ganglion cells declines and that cochlear dysfunction progresses from the basal to apical turns (Yamasoba et al., 1999). Mitochondrial deafness can be syndromic or nonsyndromic. Those relevant to cochlear implant include “mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome”, maternally inherited diabetes and deafness (MIDD) syndrome, KearnseSayre syndrome (KSS), chronic progressive external ophthalmoplegia (CPEO) syndrome, etc. Non-syndromic mitochondrial deafness is more common than syndromic and often related to aminoglycoside ototoxicity, especially in the A1555G mutation (Sinnathuray et al., 2003; Prezant et al., 1993). Cochlear implant outcomes are generally good in cases of non-syndromic mitochondrial deafness. Tono et al. (1998) reported a case of A1555G mutation. The patient acquired sensorineural hearing loss 28 years ago following using aminoglycoside drugs and achieved 78% (54% when blinded) monosyllable test scores and 84% (76% when blinded) word test scores 10 months after cochlear implant. Ulubil et al. (2006) presented a 35 years old patient with hearing loss and A1555G mutation but no history of aminoglycoside exposure. At 1 month following cochlear implant, his average open set CUNY score was 60% (65% in HINT). Results in syndromic deafness cases are also encouraging. In a case of MELAS reported by Rosenthal et al. (1999), the 20 years old male patient achieved a 60% CID sentence test score at 6 months after cochlear implant. In a report by Yasumura et al., the 29 years old female patient demonstrated 72% and 94% scores in closed set word and sentence tests respectively, and 44% and 34% scores in open set testing, at 10 months postoperatively (Karkos et al., 2005). Cochlear implant is also effective in MIDD syndrome. A 42 years old woman in a report by Raut et al. (2002) produced a 90% BKB sentence test score. Similarly, Counter et al. (2001) reported a patient with MIDD syndrome, whose CUNY score
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increased from 5% to 84% at 3 months following cochlear implant. 5. Waardenbury syndrome (WS) WS is a relatively common autosomal hereditary syndrome, also known as auditoryepigmentary syndrome. Its clinical features include inner canthus shift, thick eye brow, heterochromia iridium, premature canities and sensorineural hearing loss of various degrees. The rate of WS among newborns is about 1/40,000 and accounts for about 2e5% of congenital deafness (Cullen et al., 2006; Nayak and Isaacson, 2003) (but only 0.44e0.46% in China (Gao et al., 2000; Yang et al., 2010)). Clinically, there are 4 subtypes within WS. Type I carries inner canthus shift, sensorineural hearing loss, heterochromia iridium, premature canities, hypopigmentation, synophridia, etc. There is no inner canthus shift in type II. Type III, also known as Klein-Waardenburg syndrome, adds upper extremity muscle dysplasia and contracture, in addition to features in type I. Type IV is also called Shah-Waardenburg syndrome and adds congenital megacolon to type II (Nayak and Isaacson, 2003). Types I and II are the most commonly seen both in China and abroad. The pathogenic gene is PAX3 (a transcription factor) in the former and MITF in the latter, which encodes a tyrosinase catalyst that is critical in melanin production and interacts with the PAX3 gene (Watanabe et al., 1998). The pathogenesis in WS may be related to lack of melanocytes in the stria vascularis. Mutations of relevant genes cause disruption in melanin production and can affect the hair, skin and eye, as well as deposit of melanin in neural crest cells which is the foundation of formation of stria vascularis (seen as atrophy of the Organ of Corti and stria vascularis on temporal bone histology) (Watanabe et al., 1998; Nakashima et al., 1992). Cochlear implant has been used as an intervention in WS with profound sensorineural hearing loss. Bony labyrinth anomaly is not common in WS, but can present as abnormal or missing semicircular canals, which calls for caution during implant procedures but is not a contraindication (Zhang et al., 2009). WA patients have shown above average results in postimplant closed and open set word tests. Reports from China and overseas have stated that WS patients enjoy better cochlear implant outcomes than those with inner ear malformation or non-syndromic deafness with “normal” cochlea (Zhang et al., 2009; Cullen et al., 2006). In a report from University of North Carolina at Chapel Hill, five of the seven patients who had received cochlear implant showed 100% closed set word recognition score and greater than 50% open set word recognition scores soon after implantation (Cullen et al., 2006). A follow up report on WS1 patients from Iran showed similar results, with all children being able to attend regular education. However, Pau et al. (2006) noticed that in 20% of 20 WS cases implanted between 1985 and 2001, ABR results were abnormal. They concluded that the significantly worse speech recognition results were related to “co-existing” auditory neuropathy that adversely impaired cochlear implant outcomes.
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6. JervelleLangeeNielse syndrome (JLNS) JLNS is featured by emotional syncope, sudden death, arrhythmia and congenital sensorineural hearing loss, with seizure from severe bradycardia or ventricular tachycardia being frequently seen (Siem et al., 2008). On ECG, QT interval is often >0.44 s in male patients and >0.46 s in female patients, which can lead to ventricular tachycardia/fibrillation or torsive ventricular tachycardia or even sudden death, often triggered by excitement, strenuous exercise or noise that leads to high levels of catecholamine release (as seen when swimming in about 16% of such cases) (Chorbachi et al., 2002; Daneshi et al., 2008). b blockers are often used to control arrhythmia, while pace-maker and defribrillator implants are used to reduce sudden death (Chorbachi et al., 2002; Yanmei et al., 2008). JLNS-related genes are KCNQ1 and KCNE1 (LQT1), both encode the slow components in the delayed rectifier Kþ channel compound (90% and 10% respectively). Delayed rectifier Kþ channels pump Kþ out of stia vascularis cells to maintain Kþ stability in the endolymph (Neyroud et al., 1997). Temporal bone histology studies have shown collapse of Reissner's membrane, saccule, utricle and membranes near the ampulla which can lead to blockage of the endolymphatic system in the scalae tympani and vestibuli (Friedmann et al., 1968). CI is considered an effective treatment for deafness from JLNS. As mentioned above, cochlear implant bypasses the dysfunctional Organ of Corti and stimulates the spiral ganglion directly. Chorbachi et al. (2002) presented a case of JLNS in three brothers born to parents in a consanguineous marriage, all with congenital profound deafness and prolonged QT intervals. The two younger brothers enjoyed good hearing results from cochlear implant. Yanmei et al. (2008) reported a 3 years old child who achieved a CAP score of 7 and a SIR score of 5 at 36 months after cochlear implant. In a follow up study by Daneshi et al. (2008) with three children younger than 3 years, the CAP and SIR scores were 6 and 4, respectively, at 48 months after cochlear implant, and all three children were able to attend regular education. Some literatures have reported symptoms of vestibular involvement in JLNS. Green et al. (2000) reported a child whose open set word test scores and articulation improved following cochlear implant but demonstrated a delay in speech development compared to normal children by about 1.5 years. Siem et al. (2008) reported a series of 8 children aged from 17 months to 7.5 years who had undergone cochlear implantation. Two of these children died of heart conditions and the rest children showed good hearing test results, although with delayed gross motor function development that might indicate involvement of the vestibular system. In the 1960s, Friedmann et al. (1968) observed fibrosis and dysplasia of vestibular epithelium on temporal bone histology. However, our understanding of vestibular involvement in JLNS remains limited even today. For patients with JLNS, QT interval evaluation before cochlear implant is important and preoperative ECG test is
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considered necessary in patients with congenital hearing loss to prevent potential cardiac complications. Anesthetics with anti-arrhythmia properties should be carefully selected and b blockers are usually needed. For those whose torsive tachycardia can be triggered by noise, induction should be conducted in a quiet environment. With careful preparation for possible adverse incidents, safe implant procedures for improved hearing in these patients are feasible. 7. Auditory neuropathy (AN) Auditory neuropathy (AN) is a hearing disorder caused by dysfunction of the inner hair cell/auditory nerve synapse and/or the auditory nerve itself, with essentially normal functioning from the external ear canal to outer hair cells. It has unique clinical presentations and audiometric features, and is also called auditory nerve spectrum disorder (ANSD) for its rather complex etiologies (Manchaiah et al., 2011). ANSD can be classified as non-syndromic and syndromic, with the non-syndromic type being more common and presenting with simple hearing disorders without other neurological abnormalities in contrast to the syndromic type which presents with other neurological disorders in the form of a syndrome. Etiologies for AN are complex and hereditary factors play an important role. Non-syndromic ANSD is often associated with OTOF, DIAPH3 and PJVK genes, and GJB2 and mitochondrial DNA 12rRNA genes have also been reported in AN, although rare (Santaretlli et al., 2008; Wang et al., 2005). The OTOF gene encodes the protein otoferlin which plays an important role in promoting exocytosis of auditory ribbon synapse vesicles. Its mutation can result in DFNB9. The DIAPH3 gene encodes a hyaline protein compound that is important to cellular morphology polarity and adhesion, and its mutations can lead to over-expression of the protein and AN. The PJVK gene encodes Pejvakin which is expressed in the afferent neurons in the cochlea and auditory pathway, and its mutations affect activities of hair cells and auditory neurons, leading to DFNB59 (Bae et al., 2013). AN is characterized by absent or abnormal ABRs with preserved cochlear responses (OAEs and CMs). Hearing loss on pure tone audiometry in patients with AN is usually bilateral and may vary from mild to profound in severity, often with flat or ascending curves on audiogram, although other patterns are also possible. Speech recognition in these patients is usually poor, even with only mild or moderate loss on pure tone audiometry. Cochlear implant for ANSD has been controversial, although good results have been reported in some patients. Recent years have seen increasing reports of satisfactory outcomes with cochlear implant in ANSD patients. Pelosi et al. (2013) compared InfanteToddler Meaningful Auditory Integration Scale (IT-MAIS) and open set word test results among patients diagnosed with ANSD at Vanderbilt University between 2009 and 2011who either were fitted with hearing
aids with satisfaction or received cochlear implant after failing hearing aids, and found no significant difference between the two groups. They therefore concluded that cochlear implant may help improve speech recognition in patients who cannot benefit from hearing aids. Budenz et al. (2013) studied cochlear implant outcomes in children with AN only and in those with AN complicated with cognition development disorders, and found outcomes in the former are superior to those in the latter and similar to those in children who have received cochlear implant for cochlear disorders. Breneman et al. (2012) also concluded that cochlear implant outcomes in children with ANSD but preserved auditory nerve were not different from those in non-ANSD children, although some studies show that vowel recognition is slightly worse with significantly worse consonant recognition in the former than in the latter following cochlear implant (Miyamoto et al., 1999). Currently, it is believed that outcomes in patients with ANSD are related to the location of the ANSD lesion and the number of spiral ganglion cells involved. Poorly developed, absent or atrophied cochlear nerve can adversely affect cochlear implant outcomes. Preoperative imaging for auditory nerve development evaluation and EABR assessment of spiral ganglion cells may be necessary before cochlear implant in ANSD patients (Wang and Yin, 2006). Hereditary factors are an important etiology for deafness. Many genetic etiologies for deafness have been determined and characterized, including Cx26 mutation related deafness, PDS-related deafness, Usher syndrome, mitochondrial DNA-related deafness, Waardenburg syndrome, JervelleLangeeNielsen syndrome, etc. Etiologies for AN are complex, and only a few non-syndromic etiologies have been determined. From reviewing the literature, contraindications to cochlear implant are rare and may include advanced age and inability to pronounce. Cochlear implant intervention, especially early intervention, in individuals with hereditary deafness, can result in obvious hearing and speech rehabilitation as the spiral ganglion and central pathways remain relatively intact. Further studies involving large hereditary deafness populations are needed to confirm the efficacy of cochlear implant in these patients. Even more important is appropriate genetic screening and consultation in individual deafness patients. In summary, genetic studies remain a forefront in medical science research and expanding exploration into genetic etiologies for human diseases is extremely valuable while certainly facing numerous unknown challenges.
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