Auditory Processing Disorders and Dyslexia
More and more children with learning and reading disabilities are being referred to the audiologist for a hearing and an auditory processing evaluation. In the past, children with these problems were evaluated by educational specialists, speech-language pathologists, neurologists, psychologists and psychiatrists. While the methods used by these specialists did indicate that a number of children had auditory processing difficulties, it has become clear that more stringently controlled procedures typically used by audiologists might yield better results. Today, there is an increasing demand on the audiologist to provide a useful clinical battery for diagnosing auditory processing disorders in children under standard audiological testing conditions.
Because many of the children referred to the audiologist experience difficulties in addition to the listening problems characteristic of an auditory processing disorder, it is important that parents and audiologists begin to understand and separate the symptoms commonly found in different disorders. A good example of this problem is the referral of children with dyslexia. Many parents are confused about what dyslexia is and often express frustration that the symptoms appear to be indistinguishable from those that describe an auditory processing disorder. Others try to make a distinction between auditory processing problems and dyslexia on the basis of the commonly held notion that dyslexia is based primarily on the visual reversal of letters during reading. In spite of many efforts to more accurately define dyslexia, there are still a number of conflicting opinions and multiple sources of misinformation that make it difficult for parents and teachers to fully understand the nature of the reading disorder.
Dyslexia is defined by the International Dyslexia Association (2000) as a "language-based disability in which a person has trouble understanding words, sentences or paragraphs; both oral and written language are affected." An earlier definition, formulated by a dyslexia research committee with the National Institutes of Health added that the disorder was "characterized by difficulties in single word decoding, usually reflecting insufficient phonological processing abilities" that are "often unexpected in relation to age and other cognitive and academic abilities" (Shaywitz, Fletcher & Shaywitz, 1994).
What both of these definitions describe is a child with disabilities in the processing and acquisition of language in spite of normal intelligence, normal hearing, normal vision, no known neurological impairments or deficits, and appropriate educational opportunities. Neither of the definitions addresses the source of the disability, however, but a pioneer in reading disabilities (Orton, 1937) suggested that perceptual impairments either in the auditory or visual domain, or both, were at the root of developmental reading disorders. Orton recognized that the impairment was not related to absolute acuity in either the visual or auditory domain (these kids had normal hearing and vision when tested), but rather in the processing of information through the visual or auditory system.
When a dyslexic child is referred to the audiologist to be evaluated for an auditory processing disorder, the audiologist will likely use a battery of tests that utilize both simple auditory stimuli such as tones, clicks, and noise bursts and complex stimuli such as speech. Based on the symptoms presented through the information gathered from parents, teachers and other specialists, the audiologist can structure the battery of tests to assess the auditory deficits that the behavior describes. Typical complaints may include poor listening skills, easy distractibility, inability to learn new words or to sound out words in reading, inattentiveness, and difficulty with following auditory directions. If possible, it would be helpful to know how the diagnosis of dyslexia was made and whether the child is characterized as a phonologic or deep dyslexic or a comprehension or surface dyslexic. The phonologic dyslexic is more likely to have problems with nonwords or unfamiliar words and the diagnosis is usually based on poor performance on a standardized test of phonology. The comprehension dyslexic is more likely to have problems with irregular words that don't fit customary categories and the diagnosis is based on normal performance on a standardized test of phonology and poor performance on a standardized test of reading comprehension.
There is considerable debate about whether the deficits observed in dyslexic individuals are primarily language-based or whether they may stem from a more fundamental auditory perceptual problem. The auditory system is crucial for the development of language and there is an enormous amount of evidence in the population of hearing impaired children that auditory perceptual deficits at the periphery can cause significant delays and disorders of language development. It seems reasonable, therefore, to expect that for at least some of the children with phonologic dyslexia there may be a disorder within the auditory system that has disrupted the normal acquisition of language. Unlike the hearing impaired child, the disruption is not occurring at the periphery, but it is possible that at some point within the ascending auditory system or at the cortical level, through intrahemispheric, interhemispheric or association connections, there may be an abnormality of function that results in the child's inability to normally process linguistic input.
Areas that are most likely to show performance deficits include temporal sequencing of information (as assessed by pitch pattern and duration pattern tests), auditory figure ground problems (as assessed by speech in noise tests) and interaural asymmetry in competition (as assessed by dichotic listening tests). Other deficits may appear in some dyslexic children, but it is anticipated that in a majority of dyslexic children, these are the primary areas where weaknesses will be found. It is possible that results will eventually demonstrate that children with different types of dyslexia are more likely to show specific patterns on auditory processing tests, but at this time, no such sub-typing of dyslexia and auditory processing disorder has been extensively documented. For now, the audiologist can focus primarily on these three areas of auditory processing skills and develop a database of results found in children with dyslexia (as well as any other comorbid conditions that are prevalent). These same categories will be likely for children without reading disorders also. Typically, the diagnosis of dyslexia is not made until children reach the third grade in school. Children who are evaluated for auditory processing disorders at ages younger than that will also show similar types of deficits when tested. Eventually, it may be possible to test younger children and predict which ones are likely to have long-term problems as a result of an auditory perceptual deficit and target which ones will become phonologic dyslexics and which ones will become comprehension dyslexics.