Auditory Processing Disorders and Dyslexic Children.

More children with learning and reading disabilities are being referred to audiologists for hearing and auditory processing evaluations. In the past, children with these problems were evaluated by educational specialists, speech-language pathologists, neurologists, psychologists and psychiatrists.

These children were often found to have various difficulties, including problems with visual-spatial organization, receptive and expressive language, phonology, attention, and in some cases, auditory processing disorders. Most tests were administered across the table from the child, in regular classrooms, with the acoustic material delivered by a cassette recorder or by the clinician, at a conversational level.

While these methods indicated a number of children had auditory processing difficulties, it was apparent that more stringent, controlled procedures, such as those typically used by audiologists, might yield better results.

Today, there is an increasing demand on the audiologist to provide useful clinical batteries for diagnosing auditory processing disorders (APDs) in children using standard audiologic test conditions.

Interestingly, children are rarely referred to the audiologist based on auditory processing issues in isolation. Typically, referred children have other problems, such as learning, speech, language, attention and/or reading difficulties. It is likely that most children with APDs have comorbid conditions and therefore, the audiologist needs to ideally provide a targeted diagnostic battery that will ultimately distinguish auditory processing difficulties from other disorders.

Today, audiologists struggle to deal with these issues, while few of our diagnostic tools provide the sensitivity and specificity required to accurately diagnose a specific auditory processing deficit.

A good example is children with dyslexia. Many parents and professionals are confused about dyslexia and often express frustration because symptoms which characterize dyslexia appear to be indistinguishable from auditory processing disorders. Some try to distinguish auditory processing problems and dyslexia based on the commonly held notion that dyslexia is primarily characterized by the visual reversal of letters during reading. Despite 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 dyslexia.

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).

Both of these definitions describe children with disabilities in the processing and acquisition of language, despite normal intelligence, normal hearing, normal vision, no known neurological impairments or deficits, and appropriate educational opportunities.

Neither definition addresses the etiology of the disability. However, 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 visual or auditory domain, but rather in the processing of information through the visual or auditory system. This is consistent with the profile of the dyslexic child with normal hearing, who has limited abilities regarding processing auditory information when the nature of the acoustic stimuli is more complex than a pure tone.

While much is known about normal processing of visual and auditory information, new advances in technology have helped us understand that our knowledge is inadequate.

In the auditory domain, we have a general base of information regarding the processing of simple types of stimuli such as pure tones and clicks. This has helped us understand peripheral mechanisms and to some extent, central mechanisms involved in auditory processing, especially within the lower brainstem.

Nonetheless, information regarding how the brain processes complex acoustic stimuli and speech, is not yet sufficiently understood for the audiologist to diagnose a specific auditory perceptual deficit when auditory processing breaks down in the brainstem and other central locations.

The deficit could occur at many points along the ascending auditory system or it could be the result of failure of auditory information to integrate with information arriving through other sensory modalities.

Arousal, attention, cognition and other factors interact with auditory input and those factors must ideally be 'filtered out', to allow the auditory component of the deficit to be isolated and differentiated from other non-auditory deficits.

A number of audiologic tests demonstrated sensitivity to central auditory nervous system disorders. Most were developed in medical settings where they were used to demonstrate functional deficits in patients with known lesions within the auditory system. Conversely, the assumption was often made that persons without known lesions demonstrating the same type of functional deficit, was possibly evidence of a disorder within the central auditory system. Most tests focused on known lesions in the temporal lobes of the cortex or the lower brainstem, leaving a large part of the central auditory nervous system poorly understood.

When dyslexic children are referred to audiologists for evaluation of auditory processing disorders, the audiologist will likely use a battery of tests utilizing simple auditory stimuli such as tones, clicks, and noise bursts, and complex stimuli such as speech. However, audiologists should be aware of a number of important considerations relating to the evaluation of dyslexic children for APDs.

A complete medical history to address non-auditory factors which may contribute to APDs such as complications at birth or early development, frequent bouts of otitis media, any type of neurological disorder or head injuries, is mandatory.

Prior to the first meeting, parents and teachers should be given checklists to help describe the child's auditory abilities and deficits. Copies of all previous evaluations, especially those done by speech-language pathologists, neuropsychologists, reading specialists and other persons who have assessed the child's disorder should be made available to the audiologist. Typical complaints include poor listening skills, easy distractibility, inability to learn or sound out new words in reading, inattentiveness, and difficulty following auditory directions.

Based on the symptoms outlined and documented above, the audiologist should initiate a battery of tests designed to assess the specific auditory deficits described.

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 non-words or unfamiliar words and the diagnosis is usually based on poor performance on standardized tests of phonology and normal performance on standardized tests of reading comprehension.

The comprehension dyslexic is more likely to have problems with irregular words that don't fit customary categories and the diagnosis is usually based on normal performance on standardized tests of phonology and poor performance on standardized tests of reading comprehension.

There is considerable debate about whether the deficits observed in dyslexic individuals are primarily language-based or whether they stem from a more fundamental auditory perceptual problem. The auditory system is crucial for the development of language.

However, there is enormous evidence that hearing impaired children have significant delays and disorders of language development, secondary to peripheral hearing loss.

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 normal acquisition of language. Unlike hearing impaired children, the disruption is not occurring at the periphery, but perhaps at some point within the ascending auditory system, the cortical level, through intrahemispheric, interhemispheric or association connections, or there may be an abnormality of function that results in the child's inability to process linguistic input.

There is evidence to suggest that dyslexic children have abnormalities within some of the auditory structures necessary for language development, including symmetry differences of the planum temporale (Hynd, et al. 1990; Kushch, et al. 1993; Larsen, et al. 1990; Leonard, et al. 1993), abnormal portions of the corpus callosum (Duara, et al. 1991; Hynd, et al. 1995), and duplicated Heschl's gyrus in the right hemisphere (Leonard, et al. 1998; Musiek & Reeves 1990; Penhune, et al. 1996).

All of these (above) occur at the cortical level and can be assessed through the behavioral dichotic listening tests which were developed on patients with known lesions of the temporal lobe. There is also evidence of cellular differences in subcortical regions of the auditory system in dyslexic individuals, primarily within the medial geniculate body (Galaburda & Livingstone, 1993).

The thalamo-cortical area is far more difficult to assess through a standard APD battery. The medial geniculate is thought to process the temporal characteristics of speech in a frequency-specific manner and is essential for the transmission of speech discrimination information to the primary auditory cortex.

With behavioral tests, it is not possible to isolate the medial geniculate from the cortex by looking at functional deficits. What is needed is a better battery of electrophysiologic measures that can evaluate different portions of the ascending auditory system in response to a variety of complex stimuli. Through an analysis of evoked potential characteristics, the audiologist could potentially assess which acoustic features are processed normally and which are not, and at what anatomic location the breakdown appears to occur.

Until this idealized battery of electrophysiologic measures is developed, the audiologist should consider which currently available measures provide the most precise diagnosis of an auditory processing disorder in a dyslexic child.

Areas 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 in the majority of dyslexic children, these are the primary areas where weaknesses will likely 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. However, at this time, no such sub-typing of dyslexia and APDs 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 and other prevalent comorbid conditions.

When records from a large number of patients can be compiled and analyzed, it may be possible to note patterns of results that occur specifically in this population. The development of a battery of auditory processing tests, together with standardized administration and scoring, are essential if the audiologist is to accurately reflect children's performance across a wide variety of clinical settings.

Efforts are under way at the University of Florida to review the auditory processing measures currently available and to provide the audiologist with standards to better diagnose these deficits in children. These standards, together with the development of electrophysiologic measures to assess auditory functions not currently assessed, will significantly enhance the audiologist's role in auditory processing evaluations in dyslexic children and in all other patients with an auditory processing disorder.

REFERENCES:
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