Unilateral congenital hearing loss in children: Challenges and potentials
Introduction
Humans are equipped with two ears for effective communication. Good coordination between the two ears facilitates, among other things, sound localization, speech in noise understanding, spatial awareness, ease of listening, and spoken language development (e.g. Boons et al., 2012a, Boons et al., 2012b, Van Deun et al., 2010, Jacobs et al., 2016, Litovsky and Gordon, 2016). The estimated incidence of sensorineural hearing impairment >40 dBHL at birth is 1.86 per 1000 newborns in developed countries of whom 30–40% are unilateral (Morton and Nance, 2006, Van Kerschaver and Stappaerts, 2011, Giardina et al., 2014 for SSD; Fitzpatrick et al., 2017). Lack of binaural input and diminished audibility negatively impact the abovementioned factors, thereby affecting communication and quality of life (e.g., Härkönen et al., 2015, Roland et al., 2016, Sladen et al., 2017b).
Newborn hearing screening (NHS) programs are pivotal to the diagnosis of congenital hearing impairment (HI) at birth. Following diagnosis of profound bilateral HI, treatment is offered early in life to enable exposure to sound for the development of speech and language. Although the brain is able to adapt to experiences throughout life, its plasticity is greatest during the first few years (Kral and Sharma, 2012). Most children with bilateral HI and two cochlear implants outperform peers with one cochlear implant in hearing and in spoken language, and the younger the age of implantation the better their performance (Van Deun et al., 2010, Boons et al., 2012b). Bilateral stimulation provided at a young age may, therefore, also be beneficial for children with profound unilateral hearing loss (UHL).
In Belgium, as in many other countries worldwide, there is no standard care for children with UHL, thereby assuming that the normal ear provides sufficient sensory cues for speech understanding. However, an increasing body of research shows that children with UHL not only experience difficulties localizing sounds and understanding speech in noise, but also have difficulty mastering complex skills for spoken language (Anne et al., 2017). Even mild HI places a child at risk for lasting language delays (Carew et al., 2017), although it is also true that language impairments are not an inevitable consequence of mild/moderate HI (Halliday et al., 2017).
Mild and moderate sensorineural HI in children are often managed with hearing aids, while conductive hearing loss can be treated with a bone anchored hearing aid (e.g. Snik et al., 2008, Nelissen et al., 2016). Profound sensorineural HI in one ear, also termed single sided deafness (SSD) could be treated by a cochlear implant (CI), especially if it is provided at a young age to maximize learning opportunities. Over the past decade, an increasing number of adults and children with SSD have received a CI. The reported performance is often variable, presumably due to small sample sizes, different onsets of hearing loss, and different ages at implantation. Moreover, most research focusses on spatial and binaural hearing, not on neurocognitive factors, such as spoken language and executive functioning. In order to draw evidence-based decisions on whether or not to provide a CI to children with SSD, it is important to consider the functional and neural consequences of UHL, as well as the window of opportunity for cochlear implantation to maximize performance of the child.
Section snippets
Objectives of the current review
The objectives of this paper are threefold. The first objective is to discuss the importance of two ears for speech perception and communication, and the consequences of UHL on several performance factors in the developing child. Inevitably, the presented difficulties in spatial hearing, speech understanding and spoken language will be much more subtle for children with UHL than for children with profound bilateral HI. It is expected that children with SSD, the most prominent form of UHL, will
The importance of two ears for speech perception and communication
In normal hearing, a specialized circuitry of neurons in the brainstem and midbrain processes the binaural cues presented to the two ears based on well-aligned input from the periphery (Colburn et al., 2006, Grothe et al., 2010). These interaural time and level cues are used to identify and separate sound sources. Thereby, they improve the signal-to-noise ratio (SNR) and, hence, provide cues for the development of communication skills. In the case of UHL the sensitivity to these cues is
Neurosensory restoration of the deaf ear combined with a normal hearing ear
A cochlear implant (CI) offers the potential to (partially) restore binaural hearing, whereas other interventions such as the CROS hearing aids or bone anchored hearing aids do not. While cochlear implantation (CI) has been standard of care and a life changing opportunity for bilaterally deaf adults and children for several decades (O'Donoghue, 2013), it has only gained attention for persons with SSD after treatment for incapacitating tinnitus by Van de Heyning et al. (2008). Due to stimulation
Cochlear implant in infants and toddlers with one deaf ear
The above-mentioned reviewed data indicate that several developmental factors are compromised in children with unilateral hearing and that these may, to some extent, be mitigated by a CI in the deaf ear. Given this, as well as the importance of early implantation, a multicenter project has been initiated to provide a cochlear implant in infants with one congenitally deaf ear.
The main objective of the multicenter collaboration (Leuven, Antwerp, Gent) is to fundamentally investigate the
Summary
This paper focusses on the consequences of UHL for several factors, including spatial perception, speech intelligibility (in noise), and (the development) of spoken language. The reviewed literature on (adults and) children with (profound) UHL clearly demonstrates deficiencies in several domains, which do not appear to resolve with age without hearing intervention (Lieu, 2015, Rohlfs et al., 2017).
It is very important to obtain a good understanding of the consequences of reduced auditory input
Acknowledgements
We gratefully acknowledge funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007–2013/under REA grant agreement n° FP7-607139 (iCARE) and funding from the FWO (TBM project LUISTER, T002216N). We thank Ann Dierckx and Ellen Boon, clinical audiologists at UZLeuven, for their expertise concerning the mapping of the CI.
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