asymmetric hearing during development: the aural preference … · asymmetric hearing during...

Asymmetric Hearing DuringDevelopment: The Aural PreferenceSyndrome and Treatment OptionsKaren Gordon, PhDa,b, Yael Henkinc,d, Andrej Krale,f,g

abstractDeafness affects ∼2 in 1000 children and is one of the most commoncongenital impairments. Permanent hearing loss can be treated by fittinghearing aids. More severe to profound deafness is an indication for cochlearimplantation. Although newborn hearing screening programs have increasedthe identification of asymmetric hearing loss, parents and caregivers ofchildren with single-sided deafness are often hesitant to pursue therapy forthe deaf ear. Delayed intervention has consequences for recovery of hearing. Ithas long been reported that asymmetric hearing loss/single-sided deafnesscompromises speech and language development and educational outcomesin children. Recent studies in animal models of deafness and in childrenconsistently show evidence of an “aural preference syndrome” in which single-sided deafness in early childhood reorganizes the developing auditorypathways toward the hearing ear, with weaker central representation of thedeaf ear. Delayed therapy consequently compromises benefit for the deaf ear,with slow rates of improvement measured over time. Therefore, asymmetrichearing needs early identification and intervention. Providing early effectivestimulation in both ears through appropriate fitting of auditory prostheses,including hearing aids and cochlear implants, within a sensitive period indevelopment has a cardinal role for securing the function of the impairedear and for restoring binaural/spatial hearing. The impacts of asymmetrichearing loss on the developing auditory system and on spoken languagedevelopment have often been underestimated. Thus, the traditional minimalistapproach to clinical management aimed at 1 functional ear should be modifiedon the basis of current evidence.

Deafness is one of the most commoncongenital impairments.1,2 Newbornhearing screening programs,implemented in many countries, havedecreased the age at diagnosis ofhearing loss. When hearing loss occursin only 1 ear, the screening resultmay be overlooked or dismissed asunimportant, particularly whenhearing in the opposite ear is normal(unilateral or single-sided deafness).The consequence will be a failure tointervene until long after majordevelopmental effects have set in,which causes significant negative

clinical implications. Screeningprograms will also miss children whoacquire deafness in 1 ear frominfection, trauma, or worsening ofpreexisting hearing loss.3–6 Acquiredunilateral deafness can go unidentifieduntil educational, social, or otherimpairments push families andcaregivers to seek medical consult.Because the prevalence of permanentunilateral hearing loss in neonates isreported to vary from 0.45 to 2.7 in10007,8 and estimates in school-agedchildren range from 30 to 56 in1000,9,10 awareness of medical

aArchie’s Cochlear Implant Laboratory, The Hospital for SickChildren, Department of Otolaryngology–Head and NeckSurgery, bUniversity of Toronto, Toronto, Canada; cHearing,Speech, and Language Center, Sheba Medical Center, TelHashomer, dDepartment of Communication Disorders,Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv,Israel; and eCluster of Excellence Hearing4all, Institute ofAudioNeuroTechnology, Hannover, Germany; fDepartment ofExperimental Otology, ENT Clinics, School of Medicine,Hannover Medical University, Hannover, Germany; andgSchool of Behavioral and Brain Sciences, The University ofTexas at Dallas, Dallas, Texas

Drs Gordon, Henkin, and Kral co-proposed the articleto the editorial office, cowrote the manuscript, andrevised the manuscript; and all authors approved thefinal manuscript as submitted.

www.pediatrics.org/cgi/doi/10.1542/peds.2014-3520

DOI: 10.1542/peds.2014-3520

Accepted for publication Feb 9, 2015

Address correspondence to Karen Gordon,Department of Otolaryngology–Head and NeckSurgery, Archie’s Cochlear Implant Laboratory, TheHospital for Sick Children, Room 6D08, 555University Ave, Toronto, ON, Canada M5G 1X8. E-mail:[email protected]

PEDIATRICS (ISSN Numbers: Print, 0031-4005; Online,1098-4275).

Copyright © 2015 by the American Academy ofPediatrics

FINANCIAL DISCLOSURE: The authors have indicatedthey have no financial relationships relevant to thisarticle to disclose.

FUNDING: Dr Kral has been supported by theDeutsche Forschungsgemeinschaft (Cluster ofExcellence Hearing4all). Dr Gordon’s work has beensupported by the Canadian Institutes of HealthResearch and the SickKids Foundation, including theBastable-Potts Clinician-Scientist Award in HearingImpairment. Dr Henkin’s work has been supportedby the Shauder grant, Sackler Faculty of Medicine,Tel Aviv University, Israel.

POTENTIAL CONFLICT OF INTEREST: The authors haveindicated they have no potential conflicts of interestto disclose.

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professionals, and especially ofpediatricians, to this hearing disorderis of crucial importance.

Asymmetric hearing loss can alsobe established in children who haveprofound hearing loss in both earsby treating only 1 side. Since theappearance of cochlear implants (CIs)in clinical medicine, this therapy forprofound deafness has become veryeffective.11 Of the .100 000 childrenwho presently use CIs worldwide,11

the majority have bilateral deafnessbut are only implanted in 1 ear12;they are, in effect, children withasymmetric hearing loss.

In this State-of-the-Art Review, wepresent evidence from basic andapplied neuroscience, audiology, andotology that points to the existenceof an impairment of the centralrepresentation of the poorer hearingear if developmental asymmetrichearing is left untreated for years.First, we review the current state ofthe problem as viewed in the clinic.Next, we consider the backgroundfrom well-controlled animal models,in which investigations have rangedfrom defined areas of the brain toindividual neurons. A review ofevidence from human brain imagingand behavioral studies complementthe picture by highlighting effectsof single-sided hearing in children.The combined data support ourcontention that a preference for 1 earis established biologically,functionally, and subjectively fromasymmetric hearing in earlydevelopment. This “aural preferencesyndrome” requires rapid diagnosisand intervention.

CURRENT STATE OF THE PROBLEM

One ear alone carries only limitedinformation regarding locations ofsound sources. Without thisinformation, hearing is degraded inadverse listening conditions. Somerooms pose particular problemsbecause acoustic sound waves easilybounce between the walls, floor, andceiling, adding reflections that are

difficult to distinguish from theoriginal sound. Other situations arechallenging because .1 sound sourceis present. Many people could bespeaking at a time such as in a typicalcocktail party13,14. For children,common complex listeningenvironments include classrooms,playgrounds, and school hallwayswhere they spend much of their dailylife. These can be more spatiallydynamic than the “cocktail party”example because both the listenerand his/her peers tend to be on themove.

Hearing from 2 ears (binaural) allowsprecise localization of sound sources.Time and level differences betweenthe ears are initially detected andevaluated in the auditory brainstemand midbrain.13 The listener usesthese cues to separate and distinguishbetween sound sources in space,thereby improving the signal-to-noiseratio for complex sounds (binauralunmasking) and separating originalsounds from their reflections(precedence effect).15 Additionalbenefits of binaural hearing are thatthe ear closer to the sound sourcecan receive up to 20 dB louder inputthan the other ear, providing anadvantage for speech comprehension(better ear or head shadow effect)and an improvement in hearingsensitivity by ∼3 to 10 dB, whichprovides increased accessibilityto sound (binaural summation/redundancy effect or diotic benefit).Although the pinna (outer ear) canprovide some localization ability from1 ear alone by using spectralcues,16,17 it is less precise than withbinaural hearing and works best if thesound is broadband (containing manyfrequencies) and is familiar to thelistener.

Without normal binaural hearing,individuals with asymmetric hearingloss have impaired sound localizationabilities, particularly in the hemifieldof the impaired ear,18 andcompromised speech understandingin noise.19,20 Deficits in the

development of speech, language,and cognition are well recognizedin children with unilateraldeafness,21–28 with reports ofincreased effects for right-earedimpairments.19,29,30 A recent large-cohort study indicated lower meanvocabulary, verbal IQ, full-scale IQ,and oral language scores in childrenwith unilateral hearing loss comparedwith normal-hearing siblingcontrols.20 These children have highrisks of educational problems,including repeating at least 1 gradeand/or receiving individualizededucational assistance.31–36

Moreover, behavioral problems aremore prevalent.37 Individuals withasymmetric hearing loss perceivethemselves to have significanthandicaps38 and exhibit reducedquality-of-life scores compared withnormal-hearing peers.39 Importantly,these issues are not captured bya typical clinical hearing test(audiogram), which measuressensitivity to sound in a quietsituation.

AUDITORY PROSTHESES PRESENTLYUSED TO AID ASYMMETRIC HEARINGLOSS

To date, treatment approaches forunilateral hearing loss range from“watchful waiting” to hearingrehabilitation by means of a variety ofhearing devices, as shown in Fig 1,depending on the child’s age, degreeand type of hearing loss, and listeningenvironment. These devices includethe following: a CI (Fig 1A), a hearingaid (Fig 1B), a bone-anchored hearingaid (BAHA) (Fig 1C), and a personalassistive listening device (Fig 1D).

The most common auditoryprosthetic is the hearing aid, whichprimarily amplifies sound so that itis audible to the impaired ear(Fig 1B). Although fitting a hearingaid to the ear with hearing losshas the potential of providingbilateral stimulation, evidence foreffectiveness is limited to smallgroups40 and is predominantly based

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on subjective reports of school-agedchildren, parents, and teachers(reviewed in refs 40 and 41).The reality is that the initialrecommendation for hearing aidamplification is rare41–43 and thatadherence to hearing aid usage ispoor.39,41–44 Underlying thesefindings are the following:considerable uncertainty regardingclinical recommendations forintervention in such cases, parents’

reluctance to fit a hearing aid in thepresence of a normal-hearing ear, andrestricted benefit in cases of severe-to-profound hearing loss in whichhigh levels of gain may actuallystimulate the better hearing earthrough bone conduction. In the caseof maximum unilateral conductivehearing loss due to microtia/atresia,a BAHA can be fitted on the affectedside (Fig 1C). A BAHA containsa sound processor coupled to the

head so that sound can be transferredto both cochleae through vibrationof the skull rather than via theexternal and middle ear.

An option that should be consideredfor unilateral severe-to-profoundhearing loss is a CI (Fig 1A), asperformed in adults whose single-sided deafness was accompanied byintractable tinnitus.45,46 The use ofCIs in individuals with single-sideddeafness is supported by significantimprovements in speechunderstanding in noise, localizationability, and subjective hearingbenefits in adults (meta-analysis47).Preliminary data on 3 children (aged4, 10, and 11 years old) withnoncongenital unilateral hearing lossafter cochlear implantation provideevidence for binaural benefits.48

An alternate solution is to send soundfrom the ear with severe-to-profoundhearing loss to the better hearingear by contralateral routing of signal(CROS) hearing aids.49 BAHAs havealso been provided on the side of theimpaired ear as an alternative toCROS hearing aids. Sounds from theimpaired side are converted by theBAHA into skull vibrations thatstimulate the opposite, betterfunctioning cochlea. The use ofBAHAs in children with profoundunilateral hearing loss remainscontroversial, despite better speechunderstanding in background noise50

and significant improvements inquality of life.51 It is important toremember that any therapeuticapproach that bypasses the impairedear, such as the CROS aid and BAHA,will leave it untreated.

There are other assistive listeningdevices to help the child withasymmetric hearing loss. Frequencymodulated (FM) technology has longbeen effectively used for increasingthe signal-to-noise ratio in individualswith hearing impairments to hearfrom a distance, in noise, and inreverberant environments.52 FMsystems transmit the input froma microphone worn by a speaker,

FIGURE 1Auditory prostheses worn by children. A, CIs are indicated for ears with severe-to-profound deaf-ness. Candidacy for cochlear implantation in children involves a comprehensive assessment of thechild, including radiologic assessment of the cochlea and auditory nerve, medical suitability forsurgery, confirmation that hearing aids do not provide adequate benefit, realistic expectations ofthe child/family, and enrollment in an appropriate educational/therapy program.12 The externalequipment is shown. The ear-worn piece includes a microphone to pick up sound and a soundprocessor that analyzes the sound for frequency and intensity over time. This information is sent tothe internal equipment through FM signals via the round transmitting coil, which sits on the child’shead. It stays in place with a magnet that is attracted to another magnet in the internal device. Theinternal device delivers electrical pulses to stimulate the auditory nerve. B, A hearing aid is shownas worn on a child’s ear. Sounds are picked up by a microphone and amplified by gains specific to thechild’s hearing loss. The amplified sounds are sent via a tube into an earmold (here in blue) to thechild’s ear. For high-gain hearing aids, the earmold must seal sounds in the ear to avoid feedback(whistling caused by a leak of amplified sounds back into the hearing aid microphone). C, A BAHA isshown. The internal device vibrates the skull to stimulate fluid movement in the cochleae. Acousticsound is picked up by a microphone on the external equipment and sent to the internal device througha direct connection (percutaneous abutment) or magnetic connection as in the device shown. Thedevice can also be secured by a soft band of elastic placed around the head. D, Two parts of an FMsystem are shown. The equipment shown on the desk contains a microphone to be worn by speaker(ie, a teacher) attached to a transmitter. The sound is transmitted by FM signal to a receiver worn bythe child in his better hearing ear (enlarged insert). The sound is sent to the ear via a tube connectedto an earmold. Little amplification is required, and thus the earmold is not sealed and allows othersounds to enter the ear.

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typically a teacher, to a receivercoupled to the open, good ear.Advantages of FM technology inimproving word recognition havebeen reported.49 Disadvantages ofFM systems include the hardwarerequired for both speaker andlistener and the presentation ofbinaural cues to only 1 ear, precludingtheir analysis and use.

ASYMMETRIC HEARING LOSS IN EARLYDEVELOPMENT

Clinical studies show that the use ofa congenitally deaf ear may be limitedlater in life even when the other earhas early access to sound. In childrenwho are bilaterally implanted insequential procedures, outcomes ofspeech perception using the second-implant ear are significantly poorerthan the outcomes with the firstimplanted ear.53–55 The difference inperformance increases as the delaybetween implantation lengthens,56–58

and gains are particularly slow inchildren who receive the secondimplant after puberty.55 Similarly,patients with unilateral atresiaexhibited a postoperative dichotic earadvantage in the nonatretic ear thatwas adjustable before, but not after,puberty.59 If the single-sided deafnessoccurs during adulthood, asymmetricperformance after treating the deafear is not prominent.45,60,61 In a largestudy involving 2251 individuals withpostlingual deafness, the implantedear was not a predictive factor foroutcomes62: results were similarwhether the ear with longer or shorterduration of deafness was implanted.This outcome difference betweensequential bilateral implantation inadults and children provides evidencefor a developmentally sensitive periodfor reorganization promoted byasymmetric hearing.

Long periods of asymmetric hearingin development also affect measuresof binaural hearing after bilateralimplantation. Whereas adults whobecame deaf after childhood perceivechanges in both interaural level and

timing cues,63 the latter cues are notdetected either by children implantedsequentially64 or by adults whosedeafness was present at birth.63

Nonetheless, both adults and childrenbenefit from using bilateral overunilateral implants due to thereduction in the head shadow effectand binaural summation.65–67

Reducing the interimplant delay inchildren appears to improve soundlocalization68,69 and speechperception in noise,70,71 providingevidence for the importance ofbilateral input during early auditorydevelopment.

PHYSIOLOGY AND PATHOPHYSIOLOGYOF UNILATERAL HEARING

The mammalian brain is immature atbirth and can be manipulated bychanges to the input it receivesduring development. The humanauditory system continues to developafter birth, with auditory areas of thecerebral cortex requiring more thana decade of life to reach maturity.72,73

The circuitry for binaural processingis inborn and functional soon afterhearing onset,74,75 but is sensitive tomanipulation of hearing,75–77 withsometimes lifelong consequences(reviewed in refs 13 and 78).

Evidence for an Aural PreferenceSyndrome

When unilateral deafness occurs inearly development, the hearing earbecomes overrepresented in theauditory system.79–81 Novel auditoryprojections from the hearing ear areformed when experimental lesionsare induced in animals near birth butdo not occur in adult animals.81–86

Moderate unilateral hearing lossleads to similar, although lessextensive, reorganization.77,87–89 Theconsequences for the opposite deafear remained unexplored until CIsentered the scene. In congenitallydeaf white cats, CI stimulationrevealed that complete (binaural)deafness reduces what has beentermed the normal “aural preference”

in the auditory cortices for thecontralateral ear.90–92 As shown inFig 2A, electrodes placed in theprimary auditory cortex of a hearingcat reveal larger and faster responsesto contralateral than ipsilateral earstimulation.92 The wiring pattern ofthe afferent auditory system canexplain this finding, in part, becausethe majority of fibers crosscontralaterally at the brainstem.Congenital deafness changes thisnormal pattern; in bilateral deafness,the normal contralateral auralpreference was reduced.92 (Fig 2B).In unilateral deafness, on the otherhand, both cortical hemispheresshowed larger and faster responsesfrom the hearing ear (Fig 2 B andD90,91). In this sense, congenitalsingle-sided deafness promotes anabnormal aural preference in whichthe representation of the better-hearing ear is “stronger” (moreextensively represented in theauditory system) and the other ear is“weaker” (less well represented in theauditory system). The early onsetof unilateral hearing thus puts thedeaf ear into a significantdisadvantage for competition forcortical resources. These effectsdecreased with increasing ageof onset of single-sided hearing,signaling an early sensitiveperiod for unilaterally drivenreorganization.90,91 Importantly, theresponses for the deaf ear were notcompletely eliminated in the felinebrain;91 this finding significantlydiffers from effects of monoculardeprivation in which projections fromthe sighted eye extensively take overneurons originally responsive to thedeprived eye or both eyes.93 In thecase of unilateral deafness, even some(although substantially weakened)extraction of binaural cues waspreserved at the cellular level,75,94

suggesting that the reorganized auralpreference is not permanent and canpotentially be reversed. This is whythe term “aural dominance,” originallyproposed for aural corticalrepresentation in the auditory


cortex,95 was softened to “auralpreference.”91

Supporting evidence fora developmental change towardabnormal cortical aural preference isavailable in humans with asymmetrichearing who did not receivetreatment. Various imagingtechniques have been used, includingelectroencephalography andfunctional MRI. A number of studiesdocumented a stronger than normalrepresentation of the hearing ear atthe cortex ipsilateral to the hearingear in adult onset of single-sideddeafness.96–101 This change resultedin a more symmetric activation ofboth auditory cortices from thehearing ear.101–107 The differencesfrom normal were larger when theloss occurred in the right ear

compared with the left ear.103,104,108

More extensive effects, includingrecruitment of additional corticalareas/networks, were observed inchildren with unilateral hearingloss,109–113 in line with the increasingeffects found in younger cats90,91

and the language, cognitive, andeducational challenges these childrenare reported to have (discussed in“Current State of the Problem”section above).

Further support for abnormal auralpreference comes from children whoreceive bilateral CIs sequentially.Although expected corticalelectrophysiologic responses weremeasured from the first implantedear, responses from the second, later-implanted ear remainedabnormal.114–117 The developing

auditory brainstem, the first point ofbinaural integration in the ascendingpathways, is already affected.Brainstem responses rapidly changeover the first year of unilateral CI usein children with early-onsetdeafness.118,119 When the opposite(second) ear was implanted after thisperiod (.1.5 years), the brainstemresponses from this ear remainedabnormally prolonged despite up to 3years of bilateral implant use(Fig 3120,121). By contrast, bilateralimplantation with minimal or nodelay in early development promotedsymmetric maturation of theresponses for both ears.120 Corticalresponses of sequentially implantedchildren revealed a reduction innormal contralateral aural preference,consistent with the animal studies

FIGURE 2Summary of the results of cortical responsiveness from binaural hearing, binaurally congenitally deaf, and single-sided hearing cats, obtained bymicroelectrode mappings (.100 recording positions) of cortical local field potentials in the primary auditory cortex (field A1) evoked by CI stimulation.Shown are maximum positive amplitudes as a function of recording position. A, In a hearing control, similar to acoustic stimulation, electrical stimulationat the contralateral ear results in larger responses and shorter latencies (color denotes onset latency) compared with ipsilateral stimulation.Consequently, there is an aural preference for the contralateral ear. B, In a single-sided hearing cat if the hearing ear is the ipsilateral ear, the auralpreference reverses: ipsilateral responses become larger and appear earlier (color). C, Summary of the binaural cats. Normal-hearing cats showa contralateral aural preference. In binaurally deaf cats, the contralateral aural preference is weakened but not reversed. D, In unilaterally hearing cats,aural preference reverses at the ipsilateral hemisphere to the hearing ear and stronger responses are observed for the hearing ear in bothhemispheres. Rudimentary responsiveness for the deaf ear is, however, preserved. Thus, a “stronger” and a “weaker” ear effect results. Data and figureswere modified from refs 91 and 90, respectively.


(Fig 4122). Importantly, this abnormalasymmetry was not found in childrenwho received bilateral implantswith ,1.5 years of interimplant delay(short delay or simultaneous bilateralimplantation). Protection againstaural preference to 1 ear thusrequires bilateral input duringdevelopment.

The change in aural preference fromthe contralateral ear to preference forthe hearing ear in both cats andchildren91,122 may reflect differentialreorganization of inhibition andexcitation in both hemispheres,ie, hemisphere-specificreorganizations.84–86,90,123 Becauseof the earlier development ofinhibitory synapses,124,125 there isa shorter sensitive period for thechange in aural preference at thehemisphere ipsilateral to the hearingear91 and a longer sensitive periodfor the hemisphere contralateral tothe hearing ear.90 In other words,single-sided deafness leads to anasymmetric brain that shows distinctadaptations at the 2 hemispheres,90

which both result in boostingresponses from the hearing ear.Stronger representation of thehearing ear in developmental single-

sided deafness will lead to a biasedinput to higher-order cortical areasand cognition. Behaviorally, it is likelyfurther aggravated by subjectivefactors such as an attentional biastoward the better-represented ear. Inthis context, the aim of treatingasymmetric hearing loss is to preventthis reorganization.

IMPORTANT FACTORS FOR A NEWTREATMENT OF ASYMMETRIC HEARINGLOSS

Single-sided hearing, due toreorganization toward the hearingear, likely protects from hearing andlanguage deficits associated with thecompletely deaf brain (reviewed inrefs 126 and 127), includingimmature cortical circuits, cross-modal reorganization, and reducedplasticity (reviewed in refs 78 and128). Initial acquisition of speech andlanguage by children with single-sided deafness has made the hearingloss difficult to identify in the absenceof neonatal hearing screeningprograms and has also fueledarguments against treatment. Yet, bythe time challenges in spatial hearingand listening in noise have beenfound, these children often are

school-aged,21,129 long after many ofthe changes reviewed above havealready occurred. The increased age/duration of deafness will thus limitthe potential benefits of hearing aids,as already reported,130 or of CIs.Treatment is thus important andcannot be delayed.

Despite the increased representationof the hearing ear in the brain, therepresentation of the deaf ear doesnot vanish completely (Fig 2).Moreover, residual sensitivity forbinaural cues persists in cochlear-implanted humans,120 inexperimental animals withasymmetric hearing,77,87 and incongenital deafness.75,94 Thus, evenin the worst condition (early onset,long duration of single-sideddeafness), there is some hope forstimulating hearing in the deaf earand establishing binaural hearing,with demonstrable benefits alreadyrealized.64–66,68–70,131–134 On theother hand, these skills remainabnormal, reflecting persistentreorganization after single-sidedhearing. Without focused training,3 to 4 years of bilateral implant usewas not sufficient to reduce thepreference of the first-implanted ear

FIGURE 3A, EABR wave eV evoked by the CI-1 (right implant) and CI-2 (left implant) are at similar latencies to the BD in a child receiving both implantssimultaneously. B, A period of unilateral CI use before bilateral cochlear implantation reduces wave eV latency evoked by CI-1 and the response fromCI-2 remains delayed despite 2 years of bilateral CI use. The BD is delayed relative to the CI-1–evoked wave eV. C, CI-2–evoked wave eVs aresignificantly prolonged relative to CI-1 when the period of unilateral CI use exceeds 2 years. Plots were reprinted from ref 120, Fig 3. BD, binauraldifference component; EABR, electrically evoked auditory brainstem response; eII, the second wave of the EABR; eIII, the third wave of the EABR; eV, thefifth wave of the EABR.


in the auditory cortices of childrenimplanted sequentially with a longdelay.122 Although these childrenlearned to detect large changes inbinaural timing cues after longperiods of bilateral implantexperience, they continued to judgeinput as coming from the side of theirfirst implant more often than childrenreceiving bilateral implantssimultaneously.134 Furthermore,although speech perception wasgained in the weaker ear, the progresswas slow and did not match thestronger ear even after 5 to 9 years ofimplant use.58 Poor speechperception55 together with absentcortical binaural interaction135 inchildren receiving the second implant

as adolescents suggest that there arecontinued difficulties in processinginput from the second-treated ear.Overall, the data indicate that an earlyperiod of monaural hearing as brief as1.5 years has long-lastingconsequences.

RECOMMENDATIONS FORIDENTIFICATION AND TREATMENT OFASYMMETRIC HEARING LOSS

Combining the available evidence,we propose the existence of an“aural preference syndrome,”characterized by a combination offollowing factors:

1. asymmetric hearing duringdevelopment;

2. asymmetric speech understandingin each ear that is resistant totreatment (ie, persisting aftercompensation of the initial asym-metry); and

3. deficits in binaural hearing, in-cluding sound localization, re-sistant to therapy of the weakerear.

Awareness of the problem isimportant. On the basis of recentevidence, a more aggressive approachto treating asymmetric hearing loss inchildren appears to be justified, withthe following objectives:

1. early identification of hearing lossthat is more pronounced in 1 earthan the other;

FIGURE 4A–D, Schematic representations of the strength of pathways from each ear (right ear in red, left ear in blue) to the contralateral and ipsilateral auditorycortices are shown. E, Mean (SE) dipoles measured in the left (blue) and right (red) auditory cortices.122 Contralateral activity is normally stronger thanipsilateral activity. Bilateral pathways are essentially symmetric in children receiving bilateral input in early development (normal-hearing, simultaneousbilateral implants, short delay between implants). Unilateral right implant use strengthens pathways to both contralateral and ipsilateral auditorycortices, increasing dipoles in the left auditory cortex and reversing aural preference to the first implanted right ear. Data from ref 122. CI-1, first implant;CI-2, second implant.


2. a reduction in asymmetric hearingby providing appropriate auditoryprostheses* in each ear withlimited delay; and

3. provision of auditory-basedtraining to limit possible effectsof “aural preference” for thestronger hearing ear.

GAPS IN KNOWLEDGE

The treatment of asymmetrichearing loss has traditionallyoccurred late or not at all. As thisimpairment becomes betterrecognized and more aggressivelytreated, we will be better able todefine when aural preferencebecomes abnormal, to determinewhich treatment is mostappropriate, and to delineate factorsthat contribute to the best outcomesof treatment. Timing of treatmentwill be essential and must considerboth the age of the child and his/herhearing experience within thecontext of critical periods ofdevelopment. More research on themechanisms of plasticity and criticalperiods as well as the exactdelineation of their limits indifferent species are requiredto understand the full potential forreversibility. For clinical purposes,further aspects require attention:Did the asymmetry of hearing existafter a period of bilateral deafness?Was the asymmetry experienced inearly or later childhood and did theasymmetry progressively increaseover time? Furthermore, how muchasymmetry in hearing will lead toabnormal aural preference andcan this condition be reversedduring or after important stagesof development? What minimalextend of the asymmetry may leadto aural preference? Finally, to whatextent can the child’s hearing devicesprovide sufficiently symmetrichearing? For example, bimodal

hearing (CI in 1 ear and a hearing aidin the other) provides benefits overunilateral listening132,136–142 butmay not necessarily avoid orreverse asymmetric auralpreference or provide accuratebinaural cues.

The potential for reversing thepreference for the stronger earexists. After developmentally mildasymmetric hearing loss wasrestored in ferrets, localizationtraining restored their spatialhearing abilities.17,143 Althoughlong durations of bilateral implantuse do improve some of thechildren’s ability to use both earsfor listening, active training is likelynecessary to overcome thesignificant developmental effects ofprevious unilateral hearing.Paradigms for training inchildren need to be developed.The treatment of asymmetrichearing loss must keep in mind thatnormal asymmetries between theears do exist, with evidence inboth normal-hearing144 andcochlear-implanted145,146 childrenof a “right ear advantage” for speechprocessing. Finally, the goal ofestablishing normally symmetricbilateral hearing in children isto promote binaural hearing.The use of independentdevices and fitting paradigmsthat presently concentrate onthe function of each deviceseparately could be improved toprovide more accurate binauralcues (eg, ref 147).

SUMMARY: TREATMENT OFASYMMETRIC HEARING LOSS

On the basis of evidence of abnormalreorganization driven by single-sidedhearing, a binaural simultaneoustherapy should become the goldstandard for early bilateral deafness.If asymmetric hearing has beenidentified, early restoration of hearingsymmetry should be the goal with theuse of appropriate auditoryprostheses.

ABBREVIATIONS

BAHA: bone-anchored hearing aidCI: cochlear implantCROS: contralateral routing of

signalFM: frequency modulation

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