Children with unilateral hearing loss (uHL) are at larger risk for academic, social-emotional, and speech-language difficulties, compared with children with normal hearing thresholds in both ears (e.g., McKay et al. 2008). Grade repetition has been reported to be ten times as common among children with uHL (Bess & Tharpe 1986; Bovo et al. 1988; Hartvig Jensen et al. 1989; Tharpe 2008). The speech-language difficulties include worse speech understanding and verbal expression skills (Lieu et al. 2010; Lieu et al. 2012; Kishon-Rabin et al. 2015). While it is unknown why these problems occur, it may in part be related to the degraded spatial hearing that many children with uHL experience, for example, in sound source localization (Humes et al. 1980; Newton 1983; Bess & Tharpe 1984; Bess et al. 1986) and in speech recognition in noise (Bess et al. 1986; Bovo et al. 1988; Ruscetta et al. 2005).
Perceptual consequences and intervention options differ depending on the site of lesion of uHL (Moore 1996; McKay et al. 2008; Gordon et al. 2015). Consequently, sensorineural and conductive uHL are separated by differential diagnosis in clinical practice. In Stockholm, the universal newborn hearing-screening program enables detection of hearing loss, already a few days after birth (Berninger 2007; Berninger & Westling 2011). With the high coverage rate (98%), a representative group of children with congenital unilateral sensorineural hearing loss (uSNHL) can be studied, which affects approximately 0.05 to 0.06% of the population (Mehl & Thomson 1998; Berninger & Westling 2011).
Intervention Options for uSNHL
Hearing aids (HAs) and cochlear implants are the only intervention options for uSNHL that directly stimulate the impaired ear, in order to support binaural hearing. Bone-conduction hearing devises and contralateral routing of signals HAs are other alternatives for uSNHL but were not studied here. A cochlear implant is a new intervention option for children with severe to profound congenital uSNHL, and has only been implanted to children in a few countries so far, but has already been the focus of several recent studies (Tavora-Vieira & Rajan 2015, 2016; Polonenko et al. 2017a, 2017b; Thomas et al. 2017). By contrast, even if a HA is a nonsurgical habilitation option that has been used for decades in clinical practices worldwide, HA benefit in children with uHL has been the subject of only four studies, each with small sample sizes (Updike 1994; Johnstone et al. 2010; Briggs et al. 2011; Rohlfs et al. 2017). Due to the small material to support clinical decision making, there is inconsistency in recommendations regarding HA intervention for children with uHL.
The few studies on HA outcomes for children with uHL showed neither benefit nor disbenefit in speech recognition in quiet and noise in the sound field (Updike 1994, n = 4; Briggs et al. 2011, n = 8). Questionnaires revealed HA benefit in various listening environments after short HA use (Briggs et al. 2011; 7- to 12-year-olds, n = 8). Benefit in sound localization accuracy (SLA) was dependent upon the child’s age and the age at first HA fitting (Johnstone et al. 2010). Younger children (6 to 9 years of age, n = 6) with earlier first HA fittings (at 4 to 6 years of age) received aided benefit in SLA, whereas older children (10 to 14 years of age, n = 6) with late HA fittings (at 7 to 12 years of age) demonstrated aided SLA detriment (Johnstone et al. 2010). Development largely affected the results, for example, the younger children were diagnosed earlier than the older children, and the older unaided children had significantly better SLA compared with aided younger children with uSNHL (Johnstone et al. 2010).
Central Auditory Nerve Function and Spatial Hearing
Environmental factors shape the development of the brain from infancy to adulthood, with particularly profound effects during early sensitive periods (Knudsen 2004). In hearing, a weaker auditory signal reaching the neuronal parts of the auditory system can result in abnormal neural function, as demonstrated in animals with induced conductive uHL at the brainstem level (Clopton & Silverman 1977; Silverman & Clopton 1977; Moore & Irvine 1981; Popescu & Polley 2010; Keating & King 2013), and for profound congenital uSNHL and induced conductive uHL at the cortical level (Popescu & Polley 2010; Kral et al. 2013a, 2013b; Polley et al. 2013; Tillein et al. 2016).
Stimulus-driven maturation is believed to have an important role in the development of spatial hearing in uHL. An association between atypical brain activity and SLA has been demonstrated in mammals with induced conductive uHL (Popescu & Polley 2010; Keating et al. 2013; Polley et al. 2013). Most of the existing studies induced experimental lesions to one ear and then studied how the animal used and processed spatial information during development and in adulthood (Keating & King 2013). Yet, it is unknown how children with uSNHL use and process spatial information. Normal hearing individuals primarily use interaural level difference cues and interaural time difference cues (ITDs) to localize sounds (Middlebrooks & Green 1991). ITDs are the dominant cues in horizontal sound localization (Wightman & Kistler 1992). Some adults with profound uSNHL use monaural cues for horizontal sound localization, in the absence of more effective binaural cues (Agterberg et al. 2014). However, children with mild-to-severe uSNHL have some audibility in their impaired ears and should be able to use (somewhat altered) binaural cues for horizontal sound localization, as for example, ferrets with induced mild-to-severe uHL (Keating et al. 2013, 2015).
In summary, studies on HA outcomes for uSNHL are few and provide limited information regarding why and in which situations HA benefit or disbenefit occurs. For example, to our knowledge, measurements of auditory neural function after HA intervention and HA outcomes for children with solely congenital uSNHL have not been reported before.
The aim was to study whether children with mild-to-severe congenital uSNHL benefit from HAs by measuring a wide range of variables that target everyday life listening. Specifically, aided and unaided horizontal SLA and speech recognition in competing speech was assessed in the sound field. Subjective HA benefit was measured with the abbreviated profile of hearing aid benefit (APHAB) and the parents’ evaluation of aural/oral performance of children (PEACH). The neural conduction time along the auditory nerve and brainstem was assessed by recording the auditory brainstem response (ABR) wave I to V interval. The ABR wave I to V interval, also referred to as the central conduction time (Eggermont & Don 1986), is a functional test of neural activity between the cochlea and upper brainstem (Starr & Achor 1975; Eggermont & Don 1986). Our hypothesis was that there may be an association between abnormal neural function and SLA in congenital uSNHL, as it has been demonstrated in mammals with induced conductive uHL (Popescu & Polley 2010; Keating et al. 2013; Polley et al. 2013).
MATERIALS AND METHODS
All children with nonsyndromic congenital uSNHL, aged 6 to 11 years, were identified in Karolinska University Hospital’s child and youth hearing habilitation database. Seven children fulfilled the inclusion criteria: pure-tone thresholds (PTTs) ≤20 dB HL (0.25 to 8 kHz) in the better ear, pure-tone average (PTA at 0.5, 1, 2, and 4 kHz) between 30 and 90 dB HL in the worse ear, and at least 6 months of conventional HA use in the impaired ear. One eligible subject declined the study invitation. All the remaining six subjects (9.7 to 10.8 years of age at test) participated in the study (50% males, 50% right ears). The subjects had used HAs for 1.5 to 5.8 years (further details in Table 1). All subjects were fitted with their first HA after 4.8 years of age, which we refer to as late-fitted.
Congenital origin was verified for all subjects in Stockholm City Council’s newborn hearing-screening program database, with one ear passing and one ear failing repeated transient-evoked otoacoustic emission (TEOAE) measurements (first TEOAE 2 days after birth for subjects 1 to 5, and 37 days after birth for subject 6; last TEOAE at a median of 35 days after birth [quartiles: 33 and 73 days, n = 6]). Congenital uSNHL was considered as low priority in clinical practice during the years 2005 and 2006 when the subjects were born; hence, ABR was only performed in subject 3 at birth (for program details see, Berninger & Westling 2011; Berninger 2014). According to the medical records, the hearing losses were sensorineural (diagnosis based on ear-specific audiometry with the degree of uSNHL versus frequency obtained at a median age of 57 months [quartiles: 36 and 73 months, n = 6]), which was further verified by evaluation of recent audiograms.
The subjects and one accompanying parent visited the clinic for 3 hrs.The appointment was always in the morning to reduce the variability in children’s attentiveness and fatigue. Information on the subject’s health, hearing impairment, and HA was obtained during the first 50 min. After a 10-min break, the second hour included measurements of HA amplification and speech recognition thresholds (SRTs) in competing speech, then a questionnaire session. SLA and ABRs were recorded during the last hour.
The subject’s own HA was used in the aided SRT and SLA measurements (normal hearing ear unoccluded; see Table 1 for HA details). The psychoacoustical and electrophysiological measurements were performed in two closely located audiometric test rooms (see for details, Berninger et al. 2014; Asp et al. 2016). Ambient sound levels were low, allowing PTT determinations down to −10 dB HL (ISO 8253-1 2010).
Right/left ear and unaided/aided start condition was chosen at random for all the psychoacoustic and electroacoustic measurements (based on the national ID number).
The study was approved by the regional ethics committee in Stockholm. Written informed consent was obtained for all the participants.
Otomicroscopy, Tympanometry, and Acoustic Reflex Tests
Bilateral otomicroscopic examination, tympanometry and acoustic reflex tests in the impaired ear were performed to ensure normal middle ear conditions. A Madsen Otoflex 100 tympanometer (program version 2.01.0183, GN Otometrics, Denmark) was used with a probe tone at 226 Hz (ipsilateral stimulation at 1 kHz [≤100 dB HL]).
PTTs were measured monaurally via insert ear phones (EAR Tone; Etymotic Research Inc, USA) in both ears using a computerized fixed-frequency Bekesy technique (at 0.125, 0.25, 0.5, 1, 1.5, 2, 3, 4, 6, and 8 kHz). The Bekesy technique is characterized by high reliability (Erlandsson et al. 1979; Paintaud et al. 1994; Berninger & Gustafsson 2000). Each PTT was defined as the mean of four turning-point pairs (cf. Berninger et al. 2014). The stimulus consisted of 3 dB/sec pulsating tones (see Berninger et al. 2014 for stimuli details, presentation order, apparatus and calibration protocol).
A comparison of interaural attenuation levels for insert earphones, with interaural difference for each subject’s latest PTTs, indicated that masking was not needed (≈9 dB safety margin when the interaural PTTs were compared with mean interaural attenuation levels determined by Sklare and Denenberg ).
Hearing Aid Verification, Acoustic Gain, and Speech Intelligibility Index
Before HA measurements, the HA was checked to identify defective parts (one earhook was replaced). Each subject’s real ear to coupler difference (RECD) was measured according to IEC 61669 (2015) followed by measurements of HA gain in a 2cc-coupler at input levels of 55, 65, and 75 dB SPL, according to IEC 60118-15 (2012). The input signal was the international speech test signal (ISTS). Coupler verification, using the measured RECD, was chosen over real ear verification with the HA on the ear, to minimize time spent with subject participation (see Moodie et al. 1994 for method comparison).
The RECD measurement was performed with a foam eartip in a quiet room using a personal computer connected to an Aurical Freefit, and coupler measurements were performed in an Aurical HIT test box (OTO suite, program version 4.73.01, GN Otometrics, Denmark). If the RECD was lower than −5 dB at any test frequency, it was considered unreliable and was remeasured (in agreement with Bagatto et al. ). The RECD for subject 4 could not be obtained above −5 dB at first test occasion, due to a highly sensitive ear canal. Thus, the RECD was remeasured at a scheduled medical appointment 27 days after the test session (Callisto Suite, Program version: 1.5.0, Interacoustics, Denmark).
The aided and unaided speech intelligibility index (SII) was calculated according to ANSI S3.5 (1997) to estimate speech audibility (octave band procedure), by using the unaided PTTs and the HA acoustic coupler gain for the ISTS at 65 dB SPL (similar signal and sound-pressure level as used in the SLA and SRT measurements). The SII is a value between 0 (no intelligibility) and 1 (100% intelligibility) that is highly correlated with intelligibility of speech (ANSI S3.5 1997). An estimated average (0.5, 1, 2, and 4 kHz) aided threshold was also calculated from the PTTs and the acoustic coupler gain (based on RECD) measured at the 65 dB SPL input level.
Auditory Brainstem Response
The ABRs were recorded monaurally in both ears using 100 µsec rarefaction clicks (repetition rate: 39.1 Hz; time window: 15 msec; 2000 sweeps). Clicks were presented through insert ear phones (EAR Tone; Etymotic Research Inc, USA) with Eclipse EP25 (program version 188.8.131.52, Interacoustics, Denmark). Ag/AgCl electrodes were placed on left and right mastoids (left and right), and the forehead (vertex and ground), for differential recording between ipsilateral mastoid (noninverting) and the forehead (inverting). All electrode impedances were ≤2 kΩ. The subjects were placed in a supine position and lights were turned off in the audiometric test room to minimize electric interferences.
The ABR measurements started at 70 dB nHL (calibration according to ISO 389-6 2007; IEC 60318-4 2010). If the ABR waves I and V could not be discerned at 70 dB nHL, the stimulus level was increased in 5 dB-steps up to maximally 90 dB nHL. In cases where no wave I (but a wave V) could be recorded, wave-I latency from the subjects’ normal hearing ear was used to estimate the ABR wave I to V interval in the impaired ear (to extract all data as per Eggermont and Don ).
ABRs could not be obtained for subjects 1 to 3 within the test session. Thus, ABRs were recorded shortly afterwards, with no change of hearing status since the test occasion, according to the subject and parent (30, 61, and 70 days post-test for subject 3, 2 and 1, respectively).
The third author measured all the ABR waves, blinded to which subject and ear that was evaluated. The ABR latencies were measured using narrow-band filtering (digital filter, 300 to 1500 Hz), thereby obtaining a well-defined vertex-positive peak.
Sound Localization Accuracy
Horizontal SLA was measured with an objective eye-tracking system, using a fast (≈3 min) method with high reliability (Asp et al. 2016). The subjects were seated facing 12 active loudspeaker/video display pairs placed equidistantly in the frontal horizontal plane (±55°, loudspeakers at ear level 1.2 m in front of the subject). The subjects watched a movie (auditory-visual stimulus and speech-shaped spectrum) at 63 dB SPL Aeq, as presented via one of the loudspeaker/display pairs. The sound shifted from one loudspeaker to another during the test, with a 1.6 sec sound-only period before reintroduction of the visual stimulus at the new sound location. An eye tracking system (Smart Eye Pro, Smart Eye AB, Gothenburg, Sweden) was used to record subject’s gaze at 20 Hz sampling rate in relation to the loudspeaker/display pairs. The coordinates of the video displays and loudspeakers were defined in three dimensions in the eye-tracking system (for further details of the setup and procedure, see Asp et al. 2016).
The subject’s perceived azimuth was defined as the pupil’s position relative to the active loudspeaker (median of the final 10 samples of the 1.6 sec sound-only period, i.e., 500 msec sampling period). SLA was quantified by an error index (EI) (Gardner & Gardner 1973; Asp et al. 2011) which was calculated as:
where P is the number of presentations with at least 3 recorded gaze samples in the 500 msec sampling period (P ≤ 24 in the current test paradigm), ip is the presented loudspeaker (1 to 12) and kp is the perceived azimuth (1 to 12) at the pth presentation, and n is the number of loudspeakers (12). EI = 0 corresponded to perfect match between perceived and presented azimuths. EI = 1 corresponded to pure guess (95% CI, 0.72 to 1.28 for the current test paradigm). Each subject participated in three subsequent sessions: aided, unaided (randomized order), and unaided retest.
Speech Recognition in Competing Speech
Aided and unaided SRTs were recorded in the presence of speech interferers, resembling a demanding everyday listening situation. The setup consisted of five loudspeakers. Female target speech was presented from a loudspeaker at 0° azimuth and at ear level, 1.8 m from the subject who was seated in the center of the room. The target speech was five-word sentences (Hagerman 1982). Sentences were grammatically correct with low semantic predictability in a fixed syntax (e.g., “Karin ägde fyra vackra knappar”, translated: “Karin owned four beautiful buttons”). Interfering speech was taken from a recording of a male speaker reading a novel. Four different sections of the recording were presented from each of four loudspeakers positioned at ±30° and ±150° azimuth at ear level, at a fixed overall level of 63 dB SPL Ceq (12 min recording time) as measured at the position of the subjects’ head (Asp et al. 2018, for detailed setup and procedure; Berninger & Karlsson 1999, for original setup and test–retest analysis).
Each subject participated in three subsequent sessions: aided, unaided (randomized order), and unaided retest. Each session started with a training list, followed by two randomly selected lists (9 lists available and 10 sentences each). Subjects were instructed to face the frontal loudspeaker during the test, where the target speech was presented. Target speech started at 73 dB SPL Ceq, and was then adjusted according to the number of correct repeated words with an adaptive up-down method according to Hagerman and Kinnefors (1995). For the following training sentences, the target speech level decreased in 5 dB steps up to three times, then increased in 3 dB steps up to three times, and finally in 2 dB steps until the number of correct words was ≤2. After the training list, the signal-to-noise ratio (SNR) changed with +2, +1, 0, −1, −2, −3 dB for 0, 1, 2, 3, 4, and 5 correctly repeated words, respectively. The SRT was calculated as a SNR threshold average from the last 10 sentences, corresponding to 40% speech recognition. That threshold and the adaptive scheme for level adjustment were chosen based on computer simulations and analysis of the maximum steepness of the psychometric function (Hagerman 1979, 1982; Hagerman & Kinnefors 1995).
The Abbreviated Profile of Hearing Aid Benefit
A Swedish version of APHAB was used to quantify hearing disability (Cox & Alexander 1995). APHAB was used, as we have good experience with the questionnaire for children in the same age-span as in the present study (e.g., Rance et al. 2014), and we are not aware of any pediatric version of APHAB in Swedish. The APHAB consists of 24 items divided into four subscales: ease of communication, reverberation, background noise, and aversiveness of sounds. The tester read the items and the subject responded how frequently each situation occurred: always, almost always, generally, half-the-time, occasionally, seldom, or never. The questionnaire was administered twice, with an aided and unaided condition in mind. Subjects 1 to 3 began answering the questionnaire in the unaided condition, while subjects 4 to 6 began with the aided condition.
The Parents’ Evaluation of Aural/Oral Performance of Children
A Swedish version of PEACH was used to quantify the hearing performance of the children in everyday life (Ching & Hill 2007; Brännström et al. 2014). The PEACH rating scale comprises 13 items and the result is presented in two subscales (quiet and noise) and as a total. The parent answered how frequently each situation (item) occurred on a five-point scale ranging from 0 to 4 (from never to always). The parent answered the PEACH rating scale with an aided and unaided condition in mind, starting in the aided condition.
Hearing Aid Use
The subject’s own HA in the default listening program was used in the study. All the subjects used adaptive directionality and noise reduction.
HA data logging was obtained by connecting the HA to a personal computer with a Noah system 4 (program version 4.4.0, Denmark) and the HA manufacturer’s software. The estimated HA usage time was also obtained with PEACH. The parent answered how often the subject used the HA on the PEACH scale (0 to 4, i.e., from never to always).
All the statistical analyses were performed with Statistica version 13 (Statsoft Inc., USA). Wilcoxon matched pairs test (nonparametric) for dependent samples was used to study interaural and aided-unaided differences. Regression analysis was used to study the relationship between SLA and the ABR I to V interpeak interval. The two-sided unpaired t-test was used in comparisons with previous studies.
Degree of hearing loss in the impaired ears varied from mild to moderate. The average PTT versus frequency function was quite flat, from 35 dB HL (0.125 kHz) to 44 dB HL (8 kHz) (Table 2, n = 6), though the shape of the PTT functions varied across subjects. Mean ± SD four-frequency PTA (0.5 to 4 kHz) was 45 ± 8 (range: 31 to 52) and 6 ± 4 (−1 to 10) dB HL in the impaired and normal hearing ear, respectively (n = 6).
Hearing Aid Verification, Acoustic Gain, and Speech Intelligibility Index
The HAs were verified with acoustic coupler gain measurements based on the RECD. Average RECD increased as a function of frequency (0.5 to 4 kHz), that is, 1 ± 3 dB (0.5 kHz), 6 ± 1 dB (1 kHz), 8 ± 2 dB (2 kHz), and 8 ± 3 dB (4 kHz) (n = 6). The average acoustic gain at 65 dB SPL also increased as a function of frequency (0.5 to 4 kHz), that is, 16 ± 10 dB (0.5 kHz), 21 ± 11 dB (1 kHz), 29 ± 7 dB (2 kHz), and 29 ± 16 dB (4 kHz) (n = 6).
Aided and unaided SIIs were calculated from the PTTs and the amplification at 65 dB SPL. The mean unaided SII in the impaired ear was 0.28 ± 0.14, while the mean aided SII was significantly higher at 0.82 ± 0.05 (p = 0.02, n = 6). Additionally, all aided SIIs were above 0.77, which is associated with good speech audibility for children with hearing loss in the 10 to 11 years age group (Scollie 2008). Based on PTA, the SII values should also be suitable for each fitting, as all the subjects had PTAs ≤ 52 dB HL, in contrast to severe to profound losses where there may be a difficulty in achieving appropriate speech intelligibility (Bagatto et al. 2016; Moodie et al. 2017).
Auditory Brainstem Response
The ABR waves I and V were identified in all the better ears (n = 6). The ABR wave I was discerned in four impaired ears (subjects 1 to 4, 50% males, 50% right ears), and wave V was discerned in five impaired ears (subjects 1 to 4 and 6, 40% males, 60% right ears).
The ABR interpeak I to V interval was used to determine the neural transmission time between the cochlea and upper brainstem. The mean I to V interval was 4.00 ± 0.23 (3.64 to 4.24, n = 6) and 4.24 ± 0.36 msec (3.80 to 4.56, n = 4) in the normal hearing and impaired ear, respectively. Two subjects (3 and 4) revealed I to V intervals of 4.50 and 4.56 msec in the impaired ear. On a group level, the mean interaural difference in wave I to V interval did not differ significantly from zero (p = 0.47, n = 4, 50% males; and p = 0.22, n = 5, 40% males, if the estimated wave-I latency from the normal hearing ear is used for subject 6).
The mean wave-V latency at 70 dB nHL was 5.36 ± 0.21 and 5.73 ± 0.41 msec in the normal and impaired ear, respectively (no significant difference: p = 0.14, n = 5).
Sound Localization Accuracy
The mean aided EI was significantly higher than the mean unaided EI (p < 0.05, n = 6), demonstrating worse SLA in the frontal horizontal plane when using HAs. The mean EI was 0.25 ± 0.11 and 0.36 ± 0.17, in the unaided and aided condition, respectively (n = 6; Fig. 1). All the subjects could localize sound better than chance performance (i.e., EI < 0.72), as all the unaided and aided EIs were <0.59 (n = 6).
Subjects 1, 4, and 5 with the lowest hearing thresholds at 125 to 1000 Hz (Table 2) displayed the lowest unaided EI (Fig. 1), whereas subjects 2, 3, and 6 with poor low frequency audibility (Table 2) displayed the worst unaided SLA (Fig. 1).
The subjects displayed high reliability in SLA. The unaided test–retest difference in EI averaged 0.03, and the corresponding 95% CI included zero (−0.05 to 0.12; n = 6), that is, no systematic learning effect was found.
Sound Localization Accuracy and Neural Conduction Time
Linear regression analysis revealed a significant relationship between the aided EI and the ABR interpeak I to V interval in the impaired ear (r = 0.98, p = 0.02, n = 4) (Fig. 2). A longer I to V interval was associated with poorer aided SLA. No significant relationship was found between the unaided EI and the ABR I to V interval in the impaired ear (r = 0.63, p = 0.37, n = 4). Unaided or aided EI did not depend on the ABR I to V interval in the normal hearing ear (r = −0.20, p = 0.71, n = 6; r = −0.66, p = 0.16, n = 6, respectively).
To study the largest subject group possible in further analysis of the relationship between aided SLA and the neural transmission time, post hoc analysis included subject 6 (with a discernable wave V, but no wave I). Two different approaches were used for estimation of neural function in the absence of a wave I, as suggested by Eggermont and Don (1986).
One of the methods suggests using the other ear’s wave I to estimate the ABR I to V interval in the impaired ear. This method revealed a significant relationship (r = 0.98, p = 0.004, n = 5; Fig. 2). Furthermore, the hearing loss for subject 6 should not affect the wave-I latency at the used 70 dB SPL stimulus level (Jerger & Johnson 1988), as the PTTs in the impaired ear was 36 dB HL at 4 kHz (frequency most likely to affect wave I), and the PTTs at the nearby 3 and 6 kHz were lower (12 and 29 dB HL, respectively).
The other method suggests using the impaired ear’s wave-V latencies corrected for hearing loss. Without correction the relationship between aided EI and wave-V latency was significant (r = 0.92, p = 0.03, n = 5; see Figure in Supplemental Digital Content 1, http://links.lww.com/EANDH/A535). With a wave-V correction method based on the effective click level at 4 kHz (see Fig. 2 in Jerger and Johnson ), a significant relationship was also demonstrated between aided EI and wave-V latency (r = 0.93, p = 0.02, n = 5).
Speech Recognition in Competing Speech
On average, no significant difference was found for SRTs in the unaided compared with the aided condition (p = 0.25, n = 6; Fig. 3). Mean unaided and aided SRTs were −10.4 ± 2.9 and −9.7 ± 1.5 dB, respectively.
The subjects showed high reliability in speech recognition in competing speech. No systematic learning effect was found, as the corresponding 95% CI included zero (−1.6 to 2.7 dB; n = 6) and the unaided test–retest SRT difference averaged 0.6 dB.
The Abbreviated Profile of Hearing Aid Benefit
The subjects’ perceived hearing disability in different listening situations (Fig. 4) showed that the mean aided and unaided frequency of problems with Ease of communication was 13 ± 9% and 24 ± 11%, respectively (n = 6). The difference was statistically significant (p = 0.03, n = 6), reflecting a HA benefit. No significant aided to unaided difference was found for background noise, reverberation, and aversiveness of sounds (p > 0.14, n = 6) (Fig. 4).
The Parents’ Evaluation of Aural/Oral Performance of Children
The parents rated the subjects’ overall performance significantly higher when using HAs, compared with when not using the HAs (p < 0.05, n = 6). The mean overall performance was 84 ± 7% and 73 ± 11%, in the aided and unaided condition, respectively (Fig. 5).
No significant aided to unaided difference was found for the quiet (p = 0.12, n = 6) and noise (p = 0.07, n = 6) subscales (Fig. 5).
Hearing Aid Use
The average HA usage time was 5.1 (0.7 to 12.7) hrs daily, according to the data logging (n = 6; measurement period: 0.1 to 1.7 years) (Table 1). The PEACH mean ranking for HA use was 2.0 ± 1.6, where 2 corresponds to sometimes (26 to 50% of the time).
We studied whether children with congenital mild-to-moderate uSNHL benefit from late-fitted HAs in demanding listening situations reflecting everyday life. Additionally, the neural transmission time from the cochlea to the upper brainstem was measured, to study its relationship with horizontal SLA.
The SLA results suggest that HAs introduced late (after 4.8 years of age) may disrupt the horizontal SLA for children with congenital uSNHL, as the aided SLA was significantly worse than the unaided SLA (Fig. 1). In this pilot study, the aided SLA deteriorated with increased neural transmission time (r = 0.98, p = 0.02, n = 4; Fig. 2), suggesting that the central auditory nerve function may have an important role for horizontal SLA in children with congenital uSNHL and late-fitted HAs.
Questionnaires revealed HA benefit for one-to-one listening situations in quiet (Fig. 4). HA benefit was also found for aided overall aural/oral performance (Fig. 5). However, no HA benefit was found in background noise or reverberation (Figs. 4 and 5). Similarly, no HA benefit was found for SRTs in competing speech (Fig. 3). Thus, both questionnaires and psychoacoustic tests demonstrated neither aided benefit nor disbenefit for late-fitted HAs in demanding listening situations.
Auditory Brainstem Response
No significant difference was found between the subjects’ ABR I to V interval and that of normal hearing children (Eggermont & Salamy 1988; 4.02 ± 0.16, n = 5) (normal ear: p = 0.87, n = 5, impaired ear: p = 0.26, n = 4, two-sided t-test). The comparison to 5-year-olds should be appropriate (Eggermont & Salamy 1988), as the ABR wave I to V interval matures early and is adult-like at ≈2 years of age (Ponton et al. 1996).
Interaural asymmetry in electrically evoked ABR latencies (comparable to the ABR I to V interval, as it reflects the transmission time from the cochlear implant to the upper brainstem) have been demonstrated for children with binaural hearing loss with long delay (≈3 years) between first and second cochlear implantation (Gordon et al. 2008; Sparreboom et al. 2010). In contrast, this first study of neural function at a brainstem level for congenital mild-to-moderate uSNHL did not show any significant interaural ABR I to V interval difference.
Sound Localization Accuracy
A significant difference was found between the subject’s SLA and that of normal hearing adults (Asp et al. 2016; 0.054 ± 0.021, n = 8) both in the unaided (p < 0.001, n = 6) and aided (p = < 0.001, n = 6) condition (two-sided t-test). Normal hearing 10- to 11-year-olds are predicted to have adult-like SLA, as horizontal SLA is typically mature at approximately 5 to 6 years of age (Van Deun et al. 2009). Thus, children with uSNHL demonstrated worse SLA, compared with normal hearing counterparts, which is consistent with previous research (Humes et al. 1980; Newton 1983; Bess et al. 1986; Johnstone et al. 2010).
The HA disbenefit in SLA corroborated a previous study in which the subjects were fitted with HAs somewhat later (at 7 to 12 years of age compared with 5 to 9 years of age) and the subjects were somewhat older (10 to14 years of age compared with 10 to 11 years of age) (Johnstone et al. 2010). But contrary to Johnstone et al. (2010), we included only congenital uSNHL. Additionally, we performed an analysis of speech audibility, to exclude the risk that poor-aided SLA was due to insufficient acoustic gain provided by the HAs. Compared with the PTA versus aided SII functions by Bagatto et al. (2016) and Moodie et al. (2017), the mean aided SII was also similar, or somewhat larger for our subject group (SII = 0.85 in comparison with SII ≈ 0.76 for a mean PTA of 45 dB HL). Furthermore, all subjects’ aided SIIs were >0.77, reflecting good overall audibility (Scollie 2008), and estimated aided average thresholds (0.5, 1, 2, 4 kHz) displayed a mean of 22 ± 5 dB HL (near the PTA for normal hearing subjects; estimated aided PTA in Table 2), indicating that all subjects were able to hear the SLA stimuli in both ears.
The poor-aided SLA is probably explained by distortion of the cues the subjects used during their first >5 years without a HA. Binaural localization cues are not preserved after HA amplification, which has been demonstrated to be detrimental for SLA in adults with bilateral hearing loss and bilateral HAs (Van den Bogaert et al. 2006). Unilateral fittings are expected to have even larger effects on the dominant ITD cues, as a HA delay of about 5 to 10 msec is introduced to the aided ear. However, HA benefit in sound localization has been demonstrated for children with uSNHL fitted quite early in development (at 4 to 6 years of age) (Johnstone et al. 2010), indicating that the highly plastic brains of young children with uSNHL may be able to adapt to the altered binaural cues.
The results further suggest that children with congenital uSNHL and quite low PTTs at low frequencies have better unaided SLA. The low frequency audibility allows access to low-frequency ITD cues, which have a dominant role in horizontal SLA for normal hearing subjects (Wightman & Kistler 1992). Open fittings may be beneficial in these cases, for example, subject 1 with an open vent may also have had access to unaltered ITD cues bilaterally in the aided condition. However, subjects 2 to 5 all had similar vent sizes (occluded to 1.5 mm) where the amplified sound possibly influenced the sound transmitted through the vent and the ITD cue (cf. Dillon 1991).
Sound Localization Accuracy and Neural Conduction Time
The significant effect of the impaired side’s neural transmission time between the cochlea and upper brainstem on the aided EI (r = 0.98, p = 0.02, n = 4), but not the unaided EI (r = 0.63, p = 0.37, n = 4), has not been reported previously (Fig. 2). ABR and SLA were measured with objective methods, and the ABR analysis was blinded; therefore, the influence of systematic bias from the tester is expected to be minimal. The close relationship was also demonstrated when all available data were extracted either by estimating an ABR I to V interval for subject 6 (Fig. 2), or by correction for processing time through the cochlea when using absolute wave-V latencies.
The female subjects 3, 4, and 6 displayed the longest ABR I to V intervals, minimizing the probability that the variability in the ABR I to V interval was due to typical sex differences, as female subjects normally have shorter ABR I to V intervals (Beagley & Sheldrake 1978). Furthermore, the group was homogenous on essential parameters that may affect maturation (apart from HA experience), for example, the subjects were close in age at test (9.7- to 10.8-years-old), they had nonsyndromic hearing losses, and all had congenital uSNHL.
It is likely that all subjects had access to both low- and high-frequency interaural cues in the aided condition, as the mean SII-value was 0.82 (see SII-values in Table 2 and Pavlovic ). In the unaided condition, the mean SII was 0.28 (corresponding to 28% audible sound components when taking into account the importance of various frequencies in a speech signal). Accordingly, unaided binaural processing may have occurred in some, but probably not all, frequency bands (see individual PTTs in Table 2). This between-condition difference represents one explanation for an association between central auditory nerve conduction time and aided SLA but not unaided SLA. The longer ABR I to V interval may reflect an immature central auditory nerve function, which led to ineffective integration of binaural cues in the aided condition. In cats, it has been found that inferior colliculus neurons, part of the upper brainstem where the wave V is generated in humans (Møller & Jannetta 1983), combine the information provided by different auditory spatial cues (Chase & Young 2005). Although the human auditory system differs from that of cats and other mammals, changes in binaural tuning properties of neurons in the inferior colliculus have been demonstrated in mammals with mild-to-moderate uHL (Clopton & Silverman 1977; Silverman & Clopton 1977; Moore & Irvine 1981; Popescu & Polley 2010). Keating et al. (2013) has further suggested that the inferior colliculus may be an important site for the development of spatial hearing in humans, sensitive to weaker auditory input, as it in mice (and possibly for other mammals) is a major site for convergence of many auditory pathways (Yu et al. 2007).
The APHAB subscales ease of communication, background noise and reverberation demonstrated significantly higher perceived hearing disability in both the aided and the unaided condition compared with normal hearing children (p ≤ 0.001, n = 6, two-sided t-test) (normal data from Fig. 1 in Rance et al. , n = 20). No significant difference was found for aversiveness of sounds (p > 0.49, n = 6, two-sided t-test). Thus, the subjects demonstrated more frequent problems with communication in quiet, noise, and reverberation (aided and unaided), compared with normal hearing children, and showed similar aversiveness of sounds.
The aural/oral performance with PEACH (rated by parents; Fig. 5) was significantly lower than for normal hearing children in the same age (mean = 93 ± 5, n = 9; Bagatto & Scollie 2013), in the unaided and the aided condition (aided: p = 0.01, unaided: p < 0.001, two-sided t-test). Thus, aural/oral performance, experienced by the parents, was degraded both with and without the HA.
In agreement with previous results for children with uHL (Briggs et al. 2011), a HA benefit was found for one-to-one listening situations (ease of communication subscale, Fig. 4). Briggs et al. (2011) used the Children’s Home Inventory of Listening Difficulties (CHILD), where the subjects rated their communication and listening skills, before compared to after 3 months of HA use, mostly in one-to-one situations in quiet. Different from Briggs et al. (2011), HA outcomes in the present study were measured at one occasion independent of the initial HA fitting process.
In contrast to previous research (Briggs et al. 2011), no aided benefit (nor disbenefit) was found in noisy and reverberant listening situations (APHAB background noise and reverberation subscales, Fig. 4). Briggs et al. (2011) used the Learning Inventory For Education (LIFE) to assess student classroom listening and additional situations (mostly noisy situations), and the subjects experienced significantly less listening difficulties in school situations when using the HAs (compared with before HA experience). In contrast to Briggs et al. (2011), we excluded acquired and mixed uHL. Congenital uSNHL may explain why our subjects did not experience HA benefit in noise, as the perceptual consequences for sensorineural hearing loss and conductive hearing loss differ (e.g., in time/frequency selectivity; Moore 1996).
Parents rated the subjects’ overall aided aural/oral performance higher, compared with their unaided performance (Fig. 5), which is in agreement with previous results for children with uHL (Briggs et al. 2011; measured with CHILD for parents). Generally, HA benefit by the parents was more favorable than the other HA outcomes, which may be attributed to bias by their own decision regarding HA use for their child, or by clinical recommendations. School may also be the most demanding noisy setting (aided and unaided), where the parents do not observe their child on a daily basis. On the contrary, it cannot be excluded that PEACH may target everyday HA benefit better than the other outcome measures.
Speech Recognition in Competing Speech
The mean SRTs in competing speech were significantly higher (worse) than SRTs for young normal hearing adults (−10.4 ± 2.9 and −9.7 ± 1.5 [n = 6] compared with −15.1 ± 1.6 dB SNR [n = 8]; Asp et al. 2018) (unaided: p = 0.002, aided: p < 0.001, two-sided t-test). However, the age of the subjects may influence the SRTs, as development of speech recognition in speech continue into adolescence (Wightman & Kistler 2005; Brown et al. 2010; Corbin et al. 2016; Buss et al. 2017). According to our normal material for school-aged children, SRTs improve on the average 0.6 dB/year from 6 to 15 years of age, and stabilize at 14 years of age (Berninger, unpublished results, n = 48). Aided SRTs were still significantly higher than for normal hearing young adults with the age correction, but not the unaided SRTs (unaided: p = 0.07, aided: p = 0.003, two-sided t-test).
Aided SRTs were comparable to unaided SRTs (Fig. 3), in agreement with the few previous studies of SRTs in noise for children with uHL and HAs (Updike 1994; Briggs et al. 2011).
Study Limitations and Future Research
The six subjects included in this study make up a quite small group, but similar in size to the uHL groups previously studied with HAs (Updike 1994; Johnstone et al. 2010; Briggs et al. 2011). We aimed at studying the most homogeneous group possible, by excluding children with conductive (or mixed) uHL, as they experience perceptual consequences different from those of children with sensorineural uHL (Moore 1996; McKay et al. 2008). Furthermore, we only included children with congenital uSNHL, as the effects of neural plasticity are likely to differ between subjects with acquired and congenital uHL, as observed in cats and rats (Popescu & Polley 2010; Kral et al. 2013b).
Real ear measurements with the HA on the ear might have been better than RECDs and coupler measurements at taking account of the vent in the estimation of gain and audibility. However, real ear measurements also have disadvantages, for example, being more time consuming (Moodie et al. 1994). We prioritized the time aspect, in order for the subject to participate efficiently on a wide range of measures.
A limitation with the data logging was that the measurement period varied within the group, with a risk of over or underestimation. The large variability in daily usage (0.7 to 12.7 hrs daily) was similar to that reported by Briggs et al. (2011) of 0.7 to 9.4 hrs daily (3-month trial, n = 7) for children with uHL, indicating that a large spread in daily HA use may be expected for children with uHL. However, the effect of HA use on HA benefit cannot be excluded and warrants further study.
All the subjects used behind-the-ear HAs with adaptive directionality and noise reduction (Table 1), reducing the risk that differences in HA microphone positions, microphone settings, and/or noise reduction algorithms affected our results. Yet, the children were fitted with different prescription methods for amplification (Table 1), which may have affected HA benefit. It is not yet known how these HA parameters may influence HA benefit for children with congenital uSNHL. The effect of HA parameters on HA benefit for children with uSNHL would be valuable to evaluate in future studies, along with studying potential benefits with large vents on aided SLA.
We believe that the close relationship between auditory neural function and aided SLA after HA intervention for children with uSNHL needs further attention. Longitudinal follow-up is necessary, to study the effect of maturation on the relationship between neural conduction time and spatial hearing. Furthermore, studies of very early HA intervention are needed to investigate whether early HA intervention may improve binaural cue integration and optimally provide HA benefit in demanding listening situations.
Our results suggest that children with congenital uSNHL generally do worse in SLA with than without late-fitted HAs, despite 1.5 to 5.8 years of HA use. In this pilot study, almost all variability in the aided horizontal SLA was explained by the neural transmission time between the cochlea and upper brainstem (r = 0.98, p = 0.02, n = 4), also when the fifth subject’s estimated ABR I to V interval in the impaired ear was included (r = 0.98, p = 0.004, n = 5). The close relationship indicates that the central auditory nerve function may have an important role in aided spatial hearing for children with congenital uSNHL.
Our results further suggest that 10- to 11-year-old children with congenital uSNHL who were fitted with a HA late struggle with communication in demanding listening environments, which corroborates previous research, and motivates further studies aiming at optimizing intervention for this group. Questionnaires revealed HA benefit in one-to-one communication, in agreement with previous research. However, no significant HA benefit or disbenefit was found for communication in background noise or reverberation, which was consistent with the measurements of speech recognition in competing spatially separated speech results. Accordingly, we suggest offering children with mild-to-moderate congenital uSNHL a HA trial with realistic expectations, thorough counselling, and a comprehensive test battery to evaluate individual HA benefit.
The authors express their gratitude to the test participants and their parents. They also thank Per-Olof Larsson for his excellent technical assistance.
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