Key points
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Lack of binaural input results in difficulties with sound localization and understanding speech in noisy backgrounds.
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Children with unilateral hearing loss (UHL) are at risk for speech-language delay, poor academic performance, and decreased quality of life compared with children with normal hearing.
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As yet, no study has shown any specific intervention that can definitely mitigate the negative effects of UHL.
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Because evidence-based guidelines are lacking, individual patient and family needs and preferences must be considered to recommend interventions for children with UHL.
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Interventions may include preferential seating in class, individualized education program (IEP) or 504(c) plan, frequency-modulated (FM) systems, amplification devices (hearing aids or contralateral routing of signal [CROS] devices), osseointegrated bone conduction hearing devices, or cochlear implantation, tailored to fit the needs of the child and to set the child up for success in academic and social settings.
| BHL | Bilateral hearing loss |
| CI | Cochlear implantation |
| CT | Computed tomography |
| FM | Frequency modulated |
| IEP | Individualized educational program |
| QOL | Quality of life |
| UHL | Unilateral hearing loss |
Introduction and background
Epidemiology
The prevalence of UHL is estimated at 1 per 1000 children at birth, increasing with age due to delayed-onset congenital hearing loss and acquired hearing loss. Because the prevalence of UHL can vary significantly according to the definition applied, it is useful to consider that at least one-third of all children born with hearing loss have UHL; epidemiologically, an estimated 3% to 6% of school-aged children in the United States have UHL. By adolescence, the prevalence of UHL is as high as 14.0% if thresholds greater than 15 dB are considered and 2.7% if only thresholds 25 dB or higher are considered. Furthermore, approximately 10% of children born with UHL eventually develop bilateral hearing loss (BHL).
Etiology and Evaluation
The incidence of temporal bone anomalies in congenital UHL is high compared with congenital BHL. Enlarged vestibular aqueduct (EVA) and cochlear nerve aplasia or hypoplasia are increasingly identified using high-resolution computed tomography (CT) and MRI. Among children with severe to profound UHL, the prevalence of cochlear nerve aplasia or hypoplasia approaches 50%. Other temporal bone abnormalities commonly reported include enlarged vestibular aqueduct, Mondini deformity, cochlear and vestibular malformations, and common cavity malformation, although the likelihood of identifying an abnormality may depend on the severity of UHL, with more severe losses associated with a greater percentage of anatomic abnormalities. Although genetic causes are predominant for BHL, this is not true for UHL. Investigators have identified variants of genes associated with BHL, including Pendred syndrome (SLC26A4) associated with EVA, but have not found them to be major determinants of UHL. Although families with sensorineural UHL have been reported, the genetic mutations associated with them have not been identified. Other syndromic causes of childhood hearing loss may initially present, or simply be associated with, a unilateral loss, for example, branchio-otorenal syndrome and Waardenburg syndrome.
Other important causes of sensorineural UHL include congenital cytomegalovirus (CMV) infections, meningitis, and trauma. Although children with symptomatic congenital CMV infection are more likely to have BHL, children with asymptomatic CMV infection are more likely to have UHL. Congenital mumps and measles are infrequent in the developed world because of childhood immunization schedules but should be considered for families who choose not to immunize their children or for children adopted without clear immunization history. Temporal bone trauma, as a result of motor vehicle accidents, falls, or other head trauma, is a common cause of acquired postlingual UHL.
Important causes of conductive UHL include unilateral aural atresia, cholesteatoma, chronic otitis media, otosclerosis, ossicular discontinuity, and congenital ossicular malformations. Otitis media, labyrinthitis, and cholesteatoma are possible causes for UHL, usually mild to moderate in severity, diagnosed on physical examination. Hearing loss associated with these entities must be evaluated by audiogram. Congenital ossicular malformations can be identified on high-resolution temporal bone CT scans, and otosclerosis or ossicular discontinuity can be verified at the time of a middle ear exploration or planned stapedectomy or ossicular reconstruction.
As noted in the diagnostic algorithm suggested by Preciado and colleagues, temporal bone imaging is the most likely test that reveals a cause in children with UHL. Whether CT or MRI scan is the best modality is still under debate. Pragmatically, deciding which scan to order may depend on the age of the child, family history, type and severity of hearing loss, and other presenting symptoms. A child who could stay still in the scanner for 15 minutes but not 45 minutes might be better having a CT scan and forgo the sedation needed for a longer MRI sequence, but an infant/toddler may have an MRI with sedation out of concern for radiation exposure to the head. If another member of the family is known to have enlarged vestibular aqueduct, the sibling with hearing loss should also undergo an imaging study (CT or MRI, although CT seems to show this anomaly a bit more readily). A CT scan may also be more helpful in a child with conductive or mixed UHL, but an MRI would rule out a brainstem lesion in a child with UHL and ataxia.
Advantages of Binaural Hearing
The audiologic disability resulting from lack of binaural hearing can be summarized as difficulty picking out a desired signal in the midst of background noise and trouble with identifying the source of a signal in 2- or 3-dimensional space. The combination of binaural summation, binaural release of masking, and the head shadow effect phenomena contribute to the ease of listening and comprehension enjoyed by those with bilateral normal hearing. These concepts, along with sound localization, are described in more detail in the following section.
Bilateral summation refers to a listener’s greater sensitivity to sound when 2 ears hear simultaneously rather than just 1 ear, stemming from central auditory processing. Experimental data show that this summation is 2 to 3 dB at threshold, when sounds are just detectable and increases up to 10 dB at 90 dB sensation level. Because speech discrimination scores improve at a rate of 6% per dB, 2 to 3 dB can result in improved speech discrimination score of 12% to 18%. However, binaural summation may not occur when the hearing levels of the 2 ears differ by as little as 6 dB. But it is clear that many derive benefit from bilateral hearing aids or bilateral cochlear implants, and binaural summation may be part of this added improvement in hearing.
Binaural release of masking or squelch allows for understanding spoken language in the midst of background noise or conversation. For a pure-tone signal at 500 Hz, the advantage of 2 ears over 1 ear is 12 to 15 dB; for speech signals, the advantage is typically 3 to 8 dB. Improvement in signal to noise ratios (SNRs) of 3 dB improves word recognition scores about 18%. Empiric studies have documented that binaural hearing benefits speech intelligibility particularly when there are multiple spoken interferences at different locations from the listener.
The head shadow effect results from sound being attenuated by the head sitting between the 2 ears. Empirically, there is a 6.4-dB reduction of speech intensity from one side of the head to the other. For pure tones, the head shadow effect is greatest in high frequencies, 20 dB for 5000 to 6000 Hz. For those with UHL, the head shadow effect results in a bad ear side and good ear side depending on whether the sound signal originates from the side with the UHL (bad ear side) or normal hearing (good ear side). Children with UHL often describe awkward social moments when they are completely unaware of others speaking to them from the bad ear side; unless they develop compensatory behaviors to cope, they may be accused of not paying attention or ignoring others.
Sound localization in the horizontal plane depends on interaural time and level (or intensity) differences and is much more easily accomplished by 2 ears than 1 ear. Low-frequency sounds are localized with interaural time differences, whereas high-frequency sounds are localized with interaural level differences. In general, children with UHL have difficulty identifying a sound source and make more errors on tests of sound localization, but there is a great deal of variability. Some are able to learn how to use vertical interaural level differences using the spectral cues that result from the shape of the pinna or residual high-frequency hearing to retain the ability to localize sound.
Children with hearing loss in general are known to experience increased fatigue from the extra cognitive effort expended to detect, decode, process, and comprehend speech. They also experience more difficulty with learning new words and multitasking, which can result in possible negative results in school settings. In addition, infants and young children require a greater signal to noise ratio than adults to comprehend speech sounds in the presence of masking noise. Thus, young children with UHL may experience more difficulty with speech in noise than adults with UHL and certainly more difficulty than their normal-hearing peers.
Introduction and background
Epidemiology
The prevalence of UHL is estimated at 1 per 1000 children at birth, increasing with age due to delayed-onset congenital hearing loss and acquired hearing loss. Because the prevalence of UHL can vary significantly according to the definition applied, it is useful to consider that at least one-third of all children born with hearing loss have UHL; epidemiologically, an estimated 3% to 6% of school-aged children in the United States have UHL. By adolescence, the prevalence of UHL is as high as 14.0% if thresholds greater than 15 dB are considered and 2.7% if only thresholds 25 dB or higher are considered. Furthermore, approximately 10% of children born with UHL eventually develop bilateral hearing loss (BHL).
Etiology and Evaluation
The incidence of temporal bone anomalies in congenital UHL is high compared with congenital BHL. Enlarged vestibular aqueduct (EVA) and cochlear nerve aplasia or hypoplasia are increasingly identified using high-resolution computed tomography (CT) and MRI. Among children with severe to profound UHL, the prevalence of cochlear nerve aplasia or hypoplasia approaches 50%. Other temporal bone abnormalities commonly reported include enlarged vestibular aqueduct, Mondini deformity, cochlear and vestibular malformations, and common cavity malformation, although the likelihood of identifying an abnormality may depend on the severity of UHL, with more severe losses associated with a greater percentage of anatomic abnormalities. Although genetic causes are predominant for BHL, this is not true for UHL. Investigators have identified variants of genes associated with BHL, including Pendred syndrome (SLC26A4) associated with EVA, but have not found them to be major determinants of UHL. Although families with sensorineural UHL have been reported, the genetic mutations associated with them have not been identified. Other syndromic causes of childhood hearing loss may initially present, or simply be associated with, a unilateral loss, for example, branchio-otorenal syndrome and Waardenburg syndrome.
Other important causes of sensorineural UHL include congenital cytomegalovirus (CMV) infections, meningitis, and trauma. Although children with symptomatic congenital CMV infection are more likely to have BHL, children with asymptomatic CMV infection are more likely to have UHL. Congenital mumps and measles are infrequent in the developed world because of childhood immunization schedules but should be considered for families who choose not to immunize their children or for children adopted without clear immunization history. Temporal bone trauma, as a result of motor vehicle accidents, falls, or other head trauma, is a common cause of acquired postlingual UHL.
Important causes of conductive UHL include unilateral aural atresia, cholesteatoma, chronic otitis media, otosclerosis, ossicular discontinuity, and congenital ossicular malformations. Otitis media, labyrinthitis, and cholesteatoma are possible causes for UHL, usually mild to moderate in severity, diagnosed on physical examination. Hearing loss associated with these entities must be evaluated by audiogram. Congenital ossicular malformations can be identified on high-resolution temporal bone CT scans, and otosclerosis or ossicular discontinuity can be verified at the time of a middle ear exploration or planned stapedectomy or ossicular reconstruction.
As noted in the diagnostic algorithm suggested by Preciado and colleagues, temporal bone imaging is the most likely test that reveals a cause in children with UHL. Whether CT or MRI scan is the best modality is still under debate. Pragmatically, deciding which scan to order may depend on the age of the child, family history, type and severity of hearing loss, and other presenting symptoms. A child who could stay still in the scanner for 15 minutes but not 45 minutes might be better having a CT scan and forgo the sedation needed for a longer MRI sequence, but an infant/toddler may have an MRI with sedation out of concern for radiation exposure to the head. If another member of the family is known to have enlarged vestibular aqueduct, the sibling with hearing loss should also undergo an imaging study (CT or MRI, although CT seems to show this anomaly a bit more readily). A CT scan may also be more helpful in a child with conductive or mixed UHL, but an MRI would rule out a brainstem lesion in a child with UHL and ataxia.
Advantages of Binaural Hearing
The audiologic disability resulting from lack of binaural hearing can be summarized as difficulty picking out a desired signal in the midst of background noise and trouble with identifying the source of a signal in 2- or 3-dimensional space. The combination of binaural summation, binaural release of masking, and the head shadow effect phenomena contribute to the ease of listening and comprehension enjoyed by those with bilateral normal hearing. These concepts, along with sound localization, are described in more detail in the following section.
Bilateral summation refers to a listener’s greater sensitivity to sound when 2 ears hear simultaneously rather than just 1 ear, stemming from central auditory processing. Experimental data show that this summation is 2 to 3 dB at threshold, when sounds are just detectable and increases up to 10 dB at 90 dB sensation level. Because speech discrimination scores improve at a rate of 6% per dB, 2 to 3 dB can result in improved speech discrimination score of 12% to 18%. However, binaural summation may not occur when the hearing levels of the 2 ears differ by as little as 6 dB. But it is clear that many derive benefit from bilateral hearing aids or bilateral cochlear implants, and binaural summation may be part of this added improvement in hearing.
Binaural release of masking or squelch allows for understanding spoken language in the midst of background noise or conversation. For a pure-tone signal at 500 Hz, the advantage of 2 ears over 1 ear is 12 to 15 dB; for speech signals, the advantage is typically 3 to 8 dB. Improvement in signal to noise ratios (SNRs) of 3 dB improves word recognition scores about 18%. Empiric studies have documented that binaural hearing benefits speech intelligibility particularly when there are multiple spoken interferences at different locations from the listener.
The head shadow effect results from sound being attenuated by the head sitting between the 2 ears. Empirically, there is a 6.4-dB reduction of speech intensity from one side of the head to the other. For pure tones, the head shadow effect is greatest in high frequencies, 20 dB for 5000 to 6000 Hz. For those with UHL, the head shadow effect results in a bad ear side and good ear side depending on whether the sound signal originates from the side with the UHL (bad ear side) or normal hearing (good ear side). Children with UHL often describe awkward social moments when they are completely unaware of others speaking to them from the bad ear side; unless they develop compensatory behaviors to cope, they may be accused of not paying attention or ignoring others.
Sound localization in the horizontal plane depends on interaural time and level (or intensity) differences and is much more easily accomplished by 2 ears than 1 ear. Low-frequency sounds are localized with interaural time differences, whereas high-frequency sounds are localized with interaural level differences. In general, children with UHL have difficulty identifying a sound source and make more errors on tests of sound localization, but there is a great deal of variability. Some are able to learn how to use vertical interaural level differences using the spectral cues that result from the shape of the pinna or residual high-frequency hearing to retain the ability to localize sound.
Children with hearing loss in general are known to experience increased fatigue from the extra cognitive effort expended to detect, decode, process, and comprehend speech. They also experience more difficulty with learning new words and multitasking, which can result in possible negative results in school settings. In addition, infants and young children require a greater signal to noise ratio than adults to comprehend speech sounds in the presence of masking noise. Thus, young children with UHL may experience more difficulty with speech in noise than adults with UHL and certainly more difficulty than their normal-hearing peers.
Consequences of unilateral hearing loss
Educational Impact
In their landmark study, Bess and Tharpe reported that children with UHL had a 10-fold rate of repeating grades (35%) compared with a 3% rate for typical school children, usually in kindergarten and first grade. Their findings were corroborated by other investigators who found 22% to 24% rates of failing at least 1 grade compared with district-wide averages of 2% to 3%. In addition, 12% to 41% of children with UHL were noted to receive additional educational assistance, and a high rate had educational or behavioral problems (20%–59%). Using the Screening Instrument for Targeting Educational Risks, a teacher-based questionnaire to screen for educational difficulties, investigators documented that children with UHL had more academic problems than the normal-hearing controls and even children with moderate to severe BHL. One possible explanation for these findings is that because the children did not receive any acknowledgment that UHL might handicap their learning, they did not receive extra services that helped the children with moderate to severe BHL. Subsequently, children with aural atresia and conductive hearing loss unilaterally have also been found to require more speech therapy and educational resources in school. These findings are alarms challenging the status quo assumptions that hearing with 1 ear is sufficient for normal development of speech and language and thus educational attainment.
Speech and Language Consequences
Additional studies have documented delays in speech and language in children with UHL since the reports from the 1980s and 1990s. There are limited data regarding the effects of UHL on speech-language development in preschool children but more in elementary school children. Kiese-Himmel reported that among the 31 children with UHL in her study, the average age of first word spoken was not delayed (mean 12.7 months, range 10–33 months), but the average of first 2-word phrase spoken was delayed (mean 23.5 months, range 18–48 months). Compared with children with normal hearing, fifty-eight 4- to 6-year-old children with UHL had significantly delayed language in Sweden. Another study comparing preschool children with UHL or mild BHL (n = 10) with children with moderate to profound BHL (n = 19) and normal-hearing controls (n = 74) similarly found that all children with hearing loss had lower comprehension and expressive language scores. Furthermore, there were no statistically significant differences between the 2 groups of children with hearing loss.
Among school-aged children, a series of controlled studies have documented a robust, negative effect of UHL on speech-language scores. Compared with 74 normal-hearing siblings, 74 children with UHL were found to have significantly poorer scores on the Oral and Written Language Scales, a 2.6 greater odds of having received speech therapy, and a 4.4 greater odds of receiving extra help at school via an IEP. Verbal intelligence quotient (IQ) scores were also lower among the children with UHL. A subsequent study with a larger group confirmed these findings ( Fig. 1 ). On following up a subgroup of these children with UHL, their language and verbal IQ scores improved significantly with time but their school performance and need for IEPs or extra help did not diminish. In a small study of adolescents that included some of the earlier studied children with UHL, Fischer and Lieu reported that the gap in language scores between those with UHL and normal-hearing siblings did not diminish with time but perhaps even widened. Thus, speech-language development seems to be delayed or diminished in children with UHL on average.
Cognition and Executive Functions
Concerns about the potential effect of UHL on cognition began to appear as children with UHL were reported to have lower IQ scores, usually among children with profound or right-sided UHL. Several studies have found lower scores on standardized cognitive tests in children with UHL compared with children with normal hearing (see Fig. 1 ). A pilot study of working memory and phonological processing suggested deficits in executive function in children with UHL compared with their normal-hearing siblings. These studies suggest that unilateral auditory stimulation of central pathways may result in altered central cortical processing.
MRI studies of children with UHL, including functional MRI (fMRI), resting state functional connectivity MRI (rs-fcMRI), and diffusion tension imaging, have been used to show differences in the gray and white matter of brains of children with UHL compared with children with normal hearing. Task-based fMRI studies have demonstrated that in response to noise chirps at different frequencies delivered to the poorer-hearing ear, children with UHL have decreased activation of auditory regions and no activation of auditory association areas and attention networks. In addition, speech-in-noise tasks activated only the secondary auditory processing areas in the left hemisphere, whereas in controls, these areas were activated bilaterally. A subsequent fMRI study that evaluated cross-modal processing with the visual system noted that children with UHL displayed decreased activation in visual processing areas and reduced deactivation of the default mode network compared with normal-hearing peers. They also increased activation of the left posterior superior temporal gyrus, which is involved in secondary auditory processing. A study of passive rs-fcMRI showed that resting-state brain connections were altered in the presence of UHL, particularly in areas that are thought to be involved with task-based executive functions. Finally, even white-matter connections within the brain were found to be significantly altered in the setting of UHL, differences that were associated with some educational outcome variables.
Impact on Quality of Life
For many years, it has been acknowledged that adults with acquired UHL experience significant social and emotional decrements in quality of life (QOL) because UHL disrupts their ability to interact with others. Qualitative reports of how UHL affects QOL include social interactions (ie, one-on-one preferred to group interactions) and difficulty with conversations (eg, pretending to hear what was said, concentrating really hard to understand, or misunderstanding words). Studies have shown that children and adolescents with UHL report their own hearing-related QOL to be significantly poorer than children and adolescents with normal hearing ( Figs. 2 and 3 ). Moreover, their level of QOL was similar to that reported by children and adolescents with BHL. The only domain where adolescents with UHL had statistically better QOL than BHL was in the social interactions domain.
