Hearing healthcare professionals often emphasize periphery function when prescribing amplification, paying little consideration to central auditory function and dysfunction. Immersed in treating hearing sensitivity, they may fail to consider central auditory processing even though it is well known that the auditory system, from the outer ear through the auditory cortex, is a major contributor to effective auditory processing. This focus, however, can lead to missing a phenomenon known as binaural interference, which may be why bilateral amplification is ineffective in some patients.
It has been standard practice for nearly 50 years to fit hearing-impaired patients with bilateral amplification whenever possible. (J Am Acad Audiol 2005;16:574.) Hearing healthcare providers also routinely counsel patients on the benefits of binaural amplification, emphasizing improved localization and the ability to hear with background noise. We tell patients that two hearing aids are better than one, but is this always the case? Most clinicians would say yes, and perhaps it is better peripherally, but that is not always true with central auditory function.
BILATERAL VS. UNILATERAL AMPLIFICATION
Patients experience binaural interference in their better ear when bilateral performance is poorer than unilateral performance. One study showed binaural interference in eight percent to 10 percent of the elderly population. (J Am Acad Audiol 1993;4:122.) Other investigators have also confirmed those results, and agreed that binaural amplification in cases of binaural interference is contraindicated. (J Am Acad Audiol 2001;12:261 and 2005;16:574; J Basic Clin Physiol Pharmacol 2007;18:201; Int J Audiol 2010;49:613.)
Investigators evaluating patients using the QuickSIN test, which measures the ability to hear in noise, and the dichotic digits test, which is a central auditory test that assesses binaural integration skills, found that older patients performed better when listening to speech recognition in noise with unilateral rather than bilateral amplification. (See FastLinks.) (J Am Acad Audiol 2005;16:574.) This underscores the need for using a variety of listening environments when evaluating the benefits of binaural amplification.
Another study found that binaural amplification may not always be beneficial for speech recognition in noise in elderly patients. (J Basic Clin Physiol Pharmacol 2007;18:201.) The authors systematically studied the benefits of bilateral versus unilateral amplification to compare unilateral versus bilateral speech recognition in noise and to investigate the relationship between monaural and binaural hearing aid performance and central auditory function. They concluded that bilateral amplification may not provide the best speech recognition in noise for elderly patients. This finding was made by observing behavior and through objective electrophysiological measures. (J Am Acad Audiol 1993;4:122.)
Binaural interference should also be considered in pediatric fittings. One study on a child who demonstrated asymmetric word recognition found that he began to demonstrate adverse behaviors when bilaterally amplified. (J Am Acad Audiol 2007;18:515.) The researcher documented binaural interference when patients were aided unilaterally (90% vs. 36%) and bilaterally (56%). A more recent investigation demonstrated that early intervention using amplification may improve localization in children with unilateral hearing impairment, but older children experienced interference and may not benefit. (J Am Acad Audiol 2010;21:522.)
ASSESSING AUDITORY PROCESSING
Binaural amplification may help patients in quiet situations, but a degree of binaural interference may occur with significant background noise. Clinicians should be aware that this may be attributed to a central, not peripheral, phenomenon. These sometimes subtle deficits in auditory function may cause ineffective amplification in binaural integration tasks, such as dichotic digit tests and listening in noise. Given these findings, we must go beyond the pure-tone audiogram during fittings, and look at other measures such as auditory system stress.
Clearly we need more education and training on the importance of evaluating the entire auditory system, not just the periphery. A recent survey found that while didactic courses in auditory processing have improved, only a limited number of clinicians perform these assessments. (J Am Acad Audiol 2007;18:428.) Even more alarming is that the majority of respondents reported spending only an average of one hour a week assessing central auditory function. Training and education are needed to change this trend. It is critical that clinicians thoroughly understand the auditory system and the role it plays in everyday practice. Clinicians who do not specialize in auditory processing may want to consider routinely screening for central auditory processing deficits in patients with symmetrical hearing. This will generate appropriate recommendations for the best course of treatment.
© 2012 Lippincott Williams & Wilkins, Inc.
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