Amblyaudia: Review of Pathophysiology, Clinical Presentation, and Treatment of a New Diagnosis

State of the Art Review Otolaryngology– Head and Neck Surgery 1–9 Amblyaudia: Review of Pathophysiology, Ó American Academy of Otolaryngology—Head and Neck Clinical Presentation, and Treatment of a Surgery Foundation 2015 Reprints and permission: sagepub.com/journalsPermissions.nav New Diagnosis DOI: 10.1177/0194599815615871 http://otojournal.org Alyson B. Kaplan1,2*, Elliott D. Kozin, MD1,2*, Aaron Remenschneider, MD, MPH1,2, Kian Eftekhari, MD3, David H. Jung, MD, PhD1,2, Daniel B. Polley, PhD1,2, and Daniel J. Lee, MD1,2 No sponsorships or competing interests have been disclosed for this article. Received March 18, 2015; revised September 18, 2015; accepted October 15, 2015. Abstract Objective. Similar to amblyopia in the visual system, ‘‘amblyaudia’’ earing loss is a common diagnosis in the pediatric is a term used to describe persistent hearing difficulty experi- population, estimated at 1 to 3 of every 1000 new- enced by individuals with a history of asymmetric hearing loss borns and, due to diagnostic delay, at 4 to 10 of every H 1-4 (AHL) during a critical window of brain development. Few clini- 1000 school-aged children. Despite advances in diagnosis cal reports have described this phenomenon and its consequent and treatment of hearing loss over the past several decades, effects on central auditory processing. We aim to (1) define the unilateral or asymmetric hearing loss (AHL) presents a particu- concept of amblyaudia and (2) review contemporary research lar problem, as cases are often unidentified, lost to follow-up, on its pathophysiology and emerging clinical relevance. or undertreated.5-8 Lack of treatment stems, in part, from the Data Sources. PubMed, Embase, and Cochrane databases. common belief that the contralateral ear with normal or near- normal hearing is able to compensate for a unilateral hearing Review Methods: A systematic literature search was performed loss.7 Indeed, many cases of AHL, particularly in children with combinations of search terms: ‘‘amblyaudia,’’ ‘‘conductive with acquired or transient etiologies, go unnoticed by guar- hearing loss,’’ ‘‘sensorineural hearing loss,’’ ‘‘asymmetric,’’ dians or even children themselves.8 ‘‘pediatric,’’ ‘‘auditory deprivation,’’ and ‘‘auditory develop- Several studies have shown, however, that AHL is not ment.’’ Relevant articles were considered for inclusion, includ- benign. Children with AHL have been observed to have higher ing basic and clinical studies, case series, and major reviews. rates of academic, social, and behavioral difficulties, with esti- 9-14 Conclusions. During critical periods of infant brain development, mated rates of grade repetition as high as 30% to 50%. As imbalanced auditory input associated with AHL may lead to a result, a growing number of otolaryngologists and audiolo- abnormalities in binaural processing. Patients with amblyaudia gists now advocate for correction of AHL by either amplifica- can demonstrate long-term deficits in auditory perception even tion or surgical intervention. AHL management is largely with correction or resolution of AHL. The greatest impact is in completed on an individualized basis, and definitive treatment 15 sound localization and hearing in noisy environments, both of guidelines for AHL do not currently exist. which rely on bilateral auditory cues. Diagnosis and quantifica- Recent studies have shown another alarming phenom- tion of amblyaudia remain controversial and poorly defined. enon: Long-term hearing deficits may remain after even Prevention of amblyaudia may be possible through early identifi- AHL has been treated if the correction was not completed cation and timely management of reversible causes of AHL. Implications for Practice. Otolaryngologists, audiologists, and 1Department of Otology and Laryngology, Harvard Medical School, Boston, pediatricians should be aware of emerging data supporting Massachusetts, USA 2 amblyaudia as a diagnostic entity and be cognizant of the Department of Otolaryngology–Head and Neck Surgery, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, USA potential for lasting consequences of AHL. Prevention of 3 Oculoplastic Surgery, Inc, Salt Lake City, Utah, USA long-term auditory deficits may be possible through rapid *These authors contributed equally to this article. identification and correction. Corresponding Author: Keywords Elliott D. Kozin, MD, Department of Otology and Laryngology, Harvard Medical School, Massachusetts Eye and Ear Infirmary, 243 Charles Street, amblyaudia, conductive hearing loss, amblyopia, congenital Boston, MA 02114, USA. hearing loss Email: [email protected] Downloaded from oto.sagepub.com at Harvard Libraries on January 7, 2016 2 Otolaryngology–Head and Neck Surgery within a critical time window.16,17 This concept has been interpretation of a single image—a concept known as defined in the basic science literature as ‘‘amblyaudia,’’ a ‘‘binocular vision.’’24 By utilizing the disparity between the term with a literal translation of dullness of hearing.18 The 2 images, binocular vision affords many advantages, includ- origin of the word ‘‘amblyaudia’’ stems from parallels to ing contrast differentiation, depth perception, and target the visual system and the diagnosis of amblyopia. In localization in space.25-29 amblyopia, a disparity of visual input at a young age results Any disruption of visual afferent input in a single eye in the permanent loss of visual acuity in the nondominant, during a critical period of childhood development may result amblyopic eye, if undertreated. Amblyopia, an ophthalmolo- in abnormal and persistent deficits of central visual processing gic diagnosis, is well defined, extensively studied, and often pathway, termed ‘‘amblyopia.’’24 There are 3 primary types of treatable. As with amblyopia, any etiology that causes an amblyopia: strabismic, refractive, and deprivational.30,31 Each asymmetric loss of hearing (afferent signal) during critical type of amblyopia has a slightly different pathophysiologic windows of brain development may result in lasting diffi- basis; however, all 3 result in an asymmetric mismatch in sen- culties in challenging listening environments, particularly soryinformationgatheredbyeach eye. Classic experiments by with regard to sound localization and the utilization of Hubel and Wiesel in the early 1960s first demonstrated the binaural cues to process speech signals embedded in high lasting effects of asymmetric visual input on the developing levels of background noise.19-22 visual system in a cat model: after 1 eyelid was reversibly Herein, we aim to define the clinical concept of amblyau- sutured shut from birth for a period of 2 to 3 months, the ana- dia, illustrating contemporary research on its pathophysiol- tomic and neurophysiologic organization of the primary visual ogy, prevention, and treatment. We seek to draw analogies to cortex revealed a persistent influence of monocular deprivation amblyopia, a well-studied diagnostic analogue. Finally, we that greatly exaggerated the representation of the nondeprived aim to provide a discussion of amblyaudia to assist with clin- eye at the expense of a diminished representation of the devel- ical decision making and suggest areas for future research. opmentally deprived eye.32,33 The perceptual outcome is the creation of a ‘‘dominant’’ eye with normal or near-normal Methods visual acuity and an ‘‘amblyopic’’ eye with decreased visual A review of the literature was conducted to identify articles acuity. related to the phenomenon of amblyaudia. Articles were identi- Decades of research have shown that amblyopia is a rela- 34 fied by searching the MeSH terms ‘‘hearing loss, conductive,’’ tively common disorder affecting up to 3% of the population. ‘‘hearing loss, sensorineural,’’ ‘‘hearing loss, congenital,’’ and This persistent imbalanced central representation leads to dis- ‘‘cochlear implantation’’ with combinations of ‘‘unilateral,’’ rupted binocular functioning, often resulting in inaccurate ‘‘asymmetric,’’ ‘‘pediatric,’’ ‘‘auditory deprivation,’’ and tracking, decreased contrast sensitivity, unsteady fixation, 35 ‘‘auditory development.’’ A wide array of articles was identi- prolonged reaction times, and loss of depth perception. fied, including basic and clinical studies, case series, and The impact of resulting visual difficulties on patient qual- reviews. Titles and abstracts were reviewed for potentially rel- ity of life is significant; thus, management strategies today evant articles. The reference lists of each study were also emphasize optimal visual correction with early detection 30,36-38 examined for additional relevant papers. As the etiology and and treatment. clinical standards for assessing amblyopia are already well Analogous to the visual system, the auditory system depends 39 established, a literature search was performed to identify high- on the integration of afferent signals from both ears. Sound impact articles and recent reviews. The Preferred Practice input from the periphery is transformed into neural signals in Pattern guidelines published by the American Academy the inner ear and is transmitted by the auditory nerve to the of Ophthalmology were used as a resource for additional cochlear nucleus, the first station of sound processing in the references. Finally, auditory neuroscientists and experts brain. The cochlear nucleus contains excitatory and inhibitory in hearing loss were contacted to ensure inclusion of the projections that innervate second-order auditory brainstem 39 most

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