Amblyaudia

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Amblyaudia (amblyos- blunt; audia-hearing) is a brain based hearing disorder that results from auditory deprivation during critical periods of brain development.[1] Individuals with amblyaudia have normal hearing sensitivity (in other words they hear soft sounds) but have difficulty hearing in noisy environments like restaurants or classrooms. Amblyaudia can be conceptualized as the auditory analog of the better known central visual disorder amblyopia. The term “lazy ear” has been used to describe amblyaudia although it is currently not known whether it stems from deficits in the auditory periphery (middle ear or cochlea) or from other parts of the auditory system in the brain, or both. A characteristic of amblyaudia is suppression of activity in the non-dominant auditory pathway by activity in the dominant pathway which may be genetically determined[2] and which could also be exacerbated by conditions throughout early development.

Physiology[edit]

Amblyaudia is a developmental disorder related to brain organization and function rather than what is typically considered a “hearing loss” (damage to the cochlea). When animals are temporarily deprived of hearing from an early age, profound changes occur in the brain. Specifically, cell sizes in brainstem nuclei are reduced,[3][4][5][6] the configuration of brainstem dendrites are altered[7][8][9] and neurons respond in different ways to sounds presented to both the deprived and non-deprived ears[10][11][12][13] (in cases of asymmetric deprivation). This last point is particularly important for listening tasks that require inputs from two ears to perform well. There are multiple auditory functions that rely on the computation of well calibrated inputs from the two ears. Chief among these is the ability to localize sound sources and separate what we want to hear from a background of noise. In the brainstem, the auditory system compares the timing and levels of sounds between the two ears to encode the location of sound sources (sounds that originate from our right as opposed to left side are louder and arrive earlier in our right ear). This ability to separate sound sources not only helps us locate the trajectories of moving objects, but also to separate different sound sources in noisy environments.

Symptoms[edit]

Children with amblyaudia experience difficulties in speech perception,[14] particularly in noisy environments, sound localization,[15] and binaural unmasking[16][17][18][19][20] (using interaural cues to hear better in noise) despite having normal hearing sensitivity (as indexed through pure tone audiometry). These symptoms may lead to difficulty attending to auditory information causing many to speculate that language acquisition and academic achievement may be deleteriously affected in children with amblyaudia. A significant deficit in a child's ability to use and comprehend expressive language may be seen in children who lacked auditory stimulation throughout the critical periods of auditory system development. A child suffering from Amblyaudia may have trouble in appropriate vocabulary comprehension and production and the use of past, present and future tenses.

Clinical Diagnosis[edit]

A clinical diagnosis of amblyaudia is made following dichotic listening testing as part of an auditory processing evaluation. If performance across two or more dichotic listening tests is normal in the dominant ear and significantly below normal in the non-dominant ear, a diagnosis of amblyaudia can be made.[21] The diagnosis can also be made if performance in both ears is below normal but performance in the non-dominant ear is significantly poorer, thereby resulting in an abnormally large asymmetry between the two ears. Amblyaudia is emerging as a distinct subtype of auditory processing disorder (APD).

Risk Factors[edit]

Abnormal auditory input during the first two years of life increases a child’s risk for amblyaudia, although the precise relationship between deprivation timing and development of amblyaudia is still unclear. Recurrent ear infections (otitis media) are the leading cause of temporary auditory deprivation in young children.[22][23][24] During ear infection bouts, the quality of the signal that reaches the auditory regions of the brains of a subset of children with OM is degraded in both timing and magnitude.[25][26] When this degradation is asymmetric (worse in one ear than the other) the binaural cues associated with sound localization can also be degraded. Aural atresia (a closed external auditory canal) also causes temporary auditory deprivation in young children. Hearing can be restored to children with ear infections and aural atresia through surgical intervention (although ear infections will also resolve spontaneously). Nevertheless, children with histories of auditory deprivation secondary to these diseases can experience amblyaudia for years after their hearing has been restored.[16][27]

Treatments[edit]

A number of computer-based auditory training programs exist for children with generalized Auditory Processing Disorders (APD). In the visual system, it has been proven that adults with amblyopia can improve their visual acuity with targeted brain training programs (perceptual learning).[28] A focused perceptual training protocol for children with amblyaudia called Auditory Rehabilitation for Interaural Asymmetry (ARIA) was developed in 2001[29] which has been found to improve dichotic listening performance in the non-dominant ear and enhance general listening skills. ARIA is now available in a number of clinical sites in the U.S., Canada, Australia and New Zealand. It is also undergoing clinical research trials involving electrophysiologic measures and activation patterns acquired through functional magnetic resonance imaging (fMRI) techniques to further establish its efficacy to remediate amblyaudia.

See also[edit]

References[edit]

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  2. ^ Morell RJ, Brewer CC, Ge D, et al. (August 2007). "A twin study of auditory processing indicates that dichotic listening ability is a strongly heritable trait". Hum. Genet. 122 (1): 103–11. doi:10.1007/s00439-007-0384-5. PMID 17533509. 
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  8. ^ Gray L, Smith Z, Rubel EW (July 1982). "Developmental and experimental changes in dendritic symmetry in n. laminaris of the chick". Brain Res. 244 (2): 360–4. doi:10.1016/0006-8993(82)90098-1. PMID 7116181. 
  9. ^ Smith ZD, Gray L, Rubel EW (October 1983). "Afferent influences on brainstem auditory nuclei of the chicken: n. laminaris dendritic length following monaural conductive hearing loss". J. Comp. Neurol. 220 (2): 199–205. doi:10.1002/cne.902200207. PMID 6315783. 
  10. ^ Silverman MS, Clopton BM (November 1977). "Plasticity of binaural interaction. I. Effect of early auditory deprivation". J. Neurophysiol. 40 (6): 1266–74. PMID 925728. 
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  15. ^ Besing JM, Koehnke J (April 1995). "A test of virtual auditory localization". Ear Hear 16 (2): 220–9. doi:10.1097/00003446-199504000-00009. PMID 7789673. 
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  17. ^ Moore DR, Hutchings ME, Meyer SE (1991). "Binaural masking level differences in children with a history of otitis media". Audiology 30 (2): 91–101. doi:10.3109/00206099109072874. PMID 1877902. 
  18. ^ Gravel JS, Wallace IF, Ruben RJ (March 1996). "Auditory consequences of early mild hearing loss associated with otitis media". Acta Otolaryngol. 116 (2): 219–21. doi:10.3109/00016489609137827. PMID 8725518. 
  19. ^ Pillsbury HC, Grose JH, Hall JW (July 1991). "Otitis media with effusion in children. Binaural hearing before and after corrective surgery". Arch. Otolaryngol. Head Neck Surg. 117 (7): 718–23. doi:10.1001/archotol.1991.01870190030008. PMID 1863436. 
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  28. ^ Levi DM, Li RW (October 2009). "Perceptual learning as a potential treatment for amblyopia: a mini-review". Vision Res. 49 (21): 2535–49. doi:10.1016/j.visres.2009.02.010. PMC 2764839. PMID 19250947. 
  29. ^ Moncrieff DW, Wertz D (February 2008). "Auditory rehabilitation for interaural asymmetry: preliminary evidence of improved dichotic listening performance following intensive training". Int J Audiol 47 (2): 84–97. doi:10.1080/14992020701770835. PMID 18236240. 

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