Beyond Controversies: The Science Behind Central Auditory Processing Disorder

Research | May 2017 Hearing Review

There is general scientific agreement on the definition, diagnosis, and treatment of CAPD

 By Gail D. Chermak, PhD, Frank E. Musiek, PhD, and Jeffrey Weihing, PhD

Central auditory processing disorder (CAPD) is a complex and evolving subject, but remains a solid diagnostic entity. While there is some variance in diagnostic criteria across US and European professional association guidelines, this simply reflects the need for additional research with participants with confirmed CAPD. A considerable body of research exists demonstrating the efficiency of individual central auditory tests and central auditory test batteries based on performance of individuals with confirmed CANS lesions, including such lesions in children. This article provides information that will assist clinicians in making informed, evidence-based clinical decisions about CAPD.

The recommended practices for diagnosis and intervention for central auditory processing disorder (CAPD) are dynamic, undergoing review and refinement as new research emerges.¹ These recommended practices have been developed by consensus groups within the American Academy of Audiology (AAA)² and the American Speech-Language-Hearing Association (ASHA),³ with careful consideration of the merits of various positions surrounding points of disagreement. Nonetheless, a number of controversial issues continue to appear in the literature. In this article, we present our perspective on a number of these issues, with a focus on the current state of the evidence supporting our viewpoint. Our goal is to provide information that will assist clinicians in making informed, evidence-based clinical decisions.

Research Participants: “Diagnosed with” vs “Suspected of” CAPD

In our view, a number of controversial positions are the result of poorly designed research studies. Several recently published reviews of the literature pertaining to diagnosis and intervention for CAPD in children have reached conclusions critical of the very concept of CAPD, as well as the tools used to diagnose CAPD, and the benefits of auditory training.^4-7 We would argue that many of the conclusions reached in these reviews were based on reports that included participants “suspected of CAPD” and/or relied on parent or teacher reports as substitutes for diagnosis based on evidence.^8-10 Drawing conclusions based on performance of poorly defined participants poses significant threats to the validity of the research. The results of any study that uses the metric of  “suspected of CAPD” or “listening difficulties” cannot be relied upon, because one cannot be sure whether the participants in the study presented any type of true auditory deficit. Moreover, the participants may have had a wide range of unidentified issues. Efficient (ie, sensitive and specific) clinical tests of auditory processing should be used to clearly define participants and identify and describe known comorbidities so that analyses can be conducted and results interpreted accordingly.¹¹

CAPD: An ICD-10 Diagnostic Entity  

A recent opinion article challenged the veracity of CAPD as a diagnostic entity, characterizing it as “ill-defined” and “poorly understood.”⁴ While there is some disagreement between professional associations in the United States and European associations like the British Society of Audiology (BSA),¹² the two major sets of guidelines published by AAA and ASHA present consistent positions and recommendations.^2,3 Moreover, CAPD is listed in the ICD-10 under ear diseases (code H93.25) for both acquired and congenital CAPD, which confirms the physiological nature of this disorder and supports the medical necessity for care. Given the inclusion of CAPD in the ICD-10, coupled with a recent US Ninth District Circuit Court precedent-setting ruling¹³ that children with CAPD are entitled to receive services in schools under the category of “other health impaired” (OHI), and that audiologists are the professionals qualified to diagnose CAPD, the legitimacy of CAPD diagnosis for children is extremely well -supported and established for a population who has long been underserved in our schools, as well as for adults seeking help for CAPD.

Behavioral Characteristics of CAPD

Clinicians must be critical of incorrect assertions that CAPD is characterized by inattention, daydreaming, and generalized listening difficulties, and then use those characterizations to argue that these behaviors are more characteristic of other disorders (eg, attention deficit hyperactivity disorder [ADHD]) and not CAPD. In fact, these are not the most common behavioral characteristics of CAPD. Most authors and professional association guidelines place greater emphasis on more specific auditory disorders (in contrast to cognitive issues of inattention and daydreaming) and generalized “listening difficulties.” The most common behavioral characteristics of CAPD are difficulty understanding spoken language in competing message or noise backgrounds, in reverberant acoustic environments, or when rapidly presented; difficulty with similar sounding words; and difficulty following complex auditory directions/commands, among others.^2,3,14-16

Clearly, these behaviors are not exclusive to CAPD. This is why one cannot diagnose CAPD on the basis of symptoms reported on questionnaires, and why audiologists must rely on efficient test batteries that are sensitive to the integrity of the central auditory nervous system (CANS), as delineated in the AAA and ASHA guidelines,^2,3 and in Musiek and Chermak.¹⁷ Moreover, many children referred for CAPD evaluations because of “listening difficulties” actually perform quite well on central auditory processing measures,^18,19 such that the use of reports of listening difficulties as a diagnostic marker for CAPD may lead to over-identification and overuse of the diagnostic label. Since central auditory measures show a significant degree of construct validity in neurological models of CAPD, this lack of agreement between observer-report and performance-based measures indicates that subjective report of listening difficulties is not predictive of true CAPD.

Causes of CAPD

Analogies have been made comparing CAPD to a broken arm, noting that the broken arm might have been caused by multiple types of accidents, and therefore, one cannot link a particular cause with the particular injury.⁴ However, brain injury is just one of the etiologies underlying CAPD. CAPD results from a number of issues affecting the CANS: neurological lesions or compromise of the CANS, including neoplasms, neurodegenerative processes (eg, multiple sclerosis, Alzheimer’s),^20-22 or impaired cerebral circulation (eg, strokes)^23,24; aging/central presbycusis²⁵; noise exposure²⁶; and exposure to neurotoxic chemicals or heavy metals (eg, styrene, lead, mercury).²⁷

In fact, the large majority of children diagnosed with CAPD present neurodevelopmental CAPD (the result of some underlying benign, diffuse neuroanatomic/neuromorphological abnormalities) in the absence of an identifiable underlying neuropathology.²⁸ In these cases, typically there is no identifiable lesion of the CANS and no apparent prenatal or perinatal, disease, injury, or exposure-related explanation for the CAPD.

Injury is not the underlying etiology for a second type of neurodevelopmental CAPD that results from neuromaturational delay, presumed to result from the slower course (delayed) of myelination or from auditory deprivation.²⁹ The broken arm analogy ignores the fact that CAPD arises from a number of etiologies that all impact the CANS.³⁰ Moreover, regardless of the etiology, the broken arm still moves consistent with the same underlying physical principles of movement and function. AAA guidelines² support the use of an interpretive approach (as commonly used across many disciplines concerned with brain-behavior relationships) when test patterns that have been demonstrated to have good sensitivity and specificity in adults with confirmed CANS lesions^23,31-33 are seen in children or older adults presenting that same pattern of test results. While the AAA guidelines² recognize that an absolute gold standard may never exist due to the heterogeneity of the disorders affecting the CANS, accumulating evidence—including the studies by Boscariol et al^34-38 detailed below—provide strong evidence that “structural changes in an area of the brain associated with auditory and language processing can lead to changes in auditory processing, and, therefore, in language and learning, as well.[p 16]”²

Minimizing Confounds in Diagnosing CAPD

Any behavioral test, including the pure tone audiogram, imposes some cognitive load (eg, attention, working memory). Although the processing deficits seen in CAPD bidirectionally interact with cognitive (and language) deficits, the functional deficits observed in individuals with CAPD derive from bottom-up (ie, sensory, data-driven) processing deficits.² This was underscored recently by Gyldenkaerne et al³⁹ who reported that cognitive measures explain only a minority of the variance in auditory processing measures, concluding that poor auditory processing certainly is not the mere reflection of attention deficits. Similarly, Weihing et al⁴⁰ concluded that central auditory tests are not likely governed by cognition given the results of their factor analysis in which central auditory tests loaded on separate factors, suggesting a unique variance associated with each test. Grant⁴¹ reported that the performance of veterans with blast injuries differed from controls on central auditory tests, but not on cognitive tests. To distinguish between two or more conditions presenting with similar symptoms or attributes requires multidisciplinary, comprehensive assessment.

Means to minimize linguistic and cognitive confounds in the diagnosis of CAPD have been described in the literature for almost two decades⁴² and appear in our textbooks and professional association guidelines.^2,3,17 Chermak et al⁴³ reviewed approaches to minimize confounds that include using tests employing nonverbal stimuli (eg, gap detection, frequency and duration patterns, masking level difference), simple speech stimuli (eg, digits), electrophysiological procedures, and multidisciplinary assessment through which information regarding attention, memory, language, etc, is available to help differentiate CAPD from language-based or supramodal cognitive deficits, motor control issues, or learning, and leads to a diagnosis with a high degree of reliability and validity.

Using intra-test comparisons—performance-intensity functions; interaural (laterality) effects; assessing performance in non-manipulated conditions (eg, monaural versus dichotic)—helps to ensure that performance deficits seen on central auditory tests are due to the acoustic manipulations rather than to lack of familiarity with the language and/or significantly reduced memory skills. Moreover, audiologists can minimize confounds by use of central auditory tests less affected by attention in their test batteries.

Attention has relatively little effect on gap detection, as indexed by N1 amplitude. In fact, gap detection can be observed during sleep, as indexed by a large P2 amplitude.⁴⁴ Gyldenkaerne et al³⁹ reported the absence of significant correlations between sustained attention and masking level differences or the Gaps-In-Noise (GIN) test.⁴⁵ Similarly, Campbell and Macdonald⁴⁴ reported weak correlations between sustained auditory attention, dichotic digits, and frequency patterns, accounting for only 8% of shared variance. Based on a factor analysis, Weihing et al⁴⁰ confirmed that central auditory processing tests load on separate factors when tests are scored using typical clinical conventions (eg, ear specific scoring). Their findings suggest that unique variance is associated with each test, and that central auditory processing tests are not likely governed by one construct (ie, cognition).

Indeed, as established by Brenneman et al,¹¹ the relationship between central auditory tests (or any behavioral measure) and cognitive measures can appear to be stronger than actually is the case when participants with low cognition are not excluded from the analysis. Similarly, inflated cognition effects also can be obtained when participants with unknown cognitive status are included in study samples.⁹

Moreover, it is clear that CAPD does not merely reflect attention deficits. Abnormal performance on central auditory processing tests often occurs despite sustained attention within normal limits.³⁹ Maturation rates for different auditory tasks are not correlated, as would be expected if nonsensory factors (eg, attention) placed a uniform influence on performance.⁴⁶ These findings suggest that the extent of cognitive influence on central auditory processing is limited and that cognition does not drive central auditory processing performance.

Other compelling evidence that auditory and cognitive systems are identifiable and can be uniquely assessed include classic disassociation studies showing preservation of P300 in patients with damage to areas of the brain supporting memory, but absence of P300 in patients with lesions of auditory areas of the brain. More evidence is suggested by the reduction of the Na-Pa complex over the involved hemisphere in patients with temporal lobe lesions, but not in patients with cortical lesions not involving the temporal lobes,^47-49 as well as auditory training studies that demonstrate improved central auditory processing with no change in memory following head trauma involving the superior temporal gyrus and hippocampus.⁵⁰

There are reasons, however, to include behavioral measures employing speech stimuli in the CAPD test battery. Speech signals provide access to different CANS processing mechanisms than do non-speech stimuli.⁵¹ The degree of temporal processing required for accurate perception of spoken language is significantly greater than required for perception of non-speech sounds; therefore, processing of speech signals may be more vulnerable to disruption by CANS dysfunction.^52-55 Central auditory processing deficits may only be revealed with speech tasks.^56-58 Moreover, younger subjects may attend less to non-speech tasks which are more abstract than speech tasks, such that non-speech tests might actually inject the confound of attention. The upshot is that the CANS is the channel used to process speech and other acoustic signals, which is why speech and speech-in-noise tests are used in audiological evaluations and hearing aid evaluations.

The Gold Standard and the Purpose of Central Auditory Test Batteries

Some have argued that there is no gold standard CAPD diagnostic test battery.⁴ Consistent with medical terminology, which defines gold standard as a method or procedure that is widely recognized as the best available,⁵⁹ it is our position that while there might not be a universally accepted gold standard, we have a considerable body of research demonstrating the efficiency of individual central auditory tests and central auditory test batteries based on performance of individuals with confirmed CANS lesions, including such lesions in children.^34-38,60,61

It behooves audiologists to fully assess the nature of an individual’s auditory deficits using efficient central auditory tests and test batteries to determine the primary source(s) of those difficulties, be they cognitive, language processing, peripheral hearing loss, CAPD, or other potential sources. While there is some variance in diagnostic criteria (eg, number of tests, number of standard deviations) across US professional association guidelines, this simply reflects the need for additional research with participants with confirmed CAPD, like that conducted by Musiek et al⁶⁰ and Weihing et al,⁴⁰ to determine which tests, how many tests, and use of strict or lax criteria lead to the most efficient (ie, sensitive and specific) diagnosis. It would be completely inappropriate to abandon the use of CAPD as a diagnostic entity, as Wilson and Arnott⁶² proposed, due to the expected variability in diagnosis rates using different criteria and tests, especially when those criteria and/or tests diverge from evidence-based, professional association guidelines. Similarly, it is incongruous to fault CAPD tests because they do not predict listening or reading ability.⁶³

The purpose of the CAPD diagnostic test battery is to identify and categorize auditory impairment or dysfunction, not to predict behavioral or academic deficits that may or may not be directly related to CAPD. Reading and listening are influenced by many variables, some of which are far removed from the auditory domain.⁶⁴

The Role of Patient Report

There have also been suggestions that our field might be wise to dispense with the “CAPD label,” and instead describe presenting symptoms and outline a treatment approach.⁴ While patient symptoms and complaints are important and must be considered, diagnosis must be based on patient performance, not exclusively on patient report. Terminology used by patients may be vague. Moreover, consider the false negative rates possible among patients in denial or simply reticent to complain or divulge, as well as the false positives if diagnosis were based on patients’ or parents’ reports, based, for example, on general information they gleaned from the Internet. Let us not forget that diagnosis and labeling conditions allow for clear communication among professionals, suggesting at least in general terms the most appropriate directions for intervention, and that diagnosis is essential to secure coverage by insurers for services rendered.

Comorbidity and Intervention

CAPD often presents co-morbidly with other disorders, including language impairment, dyslexia, ADHD, and autism spectrum disorder; however, comorbidity does not imply unity or that one disorder encompasses the other. Differential diagnosis by a multidisciplinary team is required to accurately identify the presence of a disorder and determine intervention. A child diagnosed with CAPD—whether the disorder exists alone or comorbid with another disorder in another domain—should receive deficit-specific auditory interventions. Interventions for general developmental or cognitive disorders typically do not include direct auditory remediation, and if they do, that remediation is offered within the context of other treatments (eg, language, memory, etc) more specific to these other disorders. Understanding and disentangling the effects of cognition and sensory processing are critical for appropriate, effective, and efficient treatment and management of individuals.⁶⁴

Efficacy of Auditory Interventions

Some have criticized auditory training as ineffective because it does not improve language or reading skills^7,63,65; however, as noted above, reading and language processing are influenced by many variables, some of which are far removed from the auditory domain.⁶⁴ The purpose of auditory training is to improve auditory processes, not improve language or reading skills. Auditory training, as a bottom-up strategy, aims to improve sensory processing of individuals with auditory skills deficits^2,3,66 with the expectation that such improvement is likely to benefit real-life listening situations. Improvement in areas not directly related to the targeted auditory deficit may occur, but is not the primary goal of auditory training.⁶⁷

Several recent studies have provided behavioral and electrophysiological evidence that auditory training can improve the auditory processing skills of children with CAPD or auditory-based learning problems.^66-73 Tests and procedures are needed to enable early diagnosis of CAPD in children 7 years and younger to maximize the efficacy of intervention. To do so, however, requires research to find measures of sufficient challenge to the CANS that can still be completed by young children without such variability that makes impossible reasonable “cut off” norms. In the meantime, we and others have recommended a number of informal interventions for children who are at risk but have not yet been diagnosed with CAPD.^64,74

Conclusions

James Jerger, PhD, asserted almost two decades ago that three lines of converging evidence support the reality “of a relatively pure auditory perceptual disorder.” Dr Jerger wrote:

First, there is the accumulation, over the past 30 years, of audiological evidence from children and adults with known lesions of the central auditory system…We can make the not unreasonable inference that when similar symptomatology is observed in children with listening problems, a problem in central auditory processing in the brain is at least suspect. Second, there has been an accumulation, especially over the past decade, of in-depth studies of the specific auditory perceptual deficits of children and adults whose only complaint is an apparent inability to hear well in difficult listening situations. Third, it is becoming increasingly clear that the unique listening problems of elderly persons may, in at least some cases, be related to age-related changes in the central rather than the peripheral auditory system. These three converging lines of evidence provide, I think, more than sufficient basis for accepting the reality of the phenomenon. They demonstrate that, across the entire age range, there are individuals who seem to display symptoms of the auditory perceptual disorder so concisely described by Myklebust.[393-394]⁷⁵

As cited throughout this commentary, the research base providing the evidence supporting the three lines of converging evidence Jerger noted has grown exponentially since 1998. In more recent years, Jerger recognized (as we also discussed above) that central auditory tests may impose some cognitive load and he encouraged efforts to document auditory processing dysfunction, as well as underlying cognitive mechanisms, that could impact performance on central auditory tests.⁷⁶

We have also recognized these same interactions,⁷⁷ but as we cited above, accumulating research substantiates that:

1. The extent of cognitive influence on central auditory processing is limited, and
2. Cognition does not drive central auditory processing performance.

As noted by co-author Frank Musiek, who introduced the term “auditory footprint,” the auditory system is extensive: there is a considerable amount of neural substrate involved in processing auditory signals.⁷⁸ While it is true that the auditory system shares neural substrate with other domains, which is the source of sensory-cognitive interactions, as well as overlapping clinical profiles and co-morbidity, we have clinically useful tools available to determine the primary source(s) of listening difficulties. In addition to efficient auditory behavioral tests, we encourage the use of auditory event-related potentials (AERPs), such as the P300, to assist in clarifying the interplay among auditory, cognitive, and linguistic processes. Certainly, the CANS of individuals who successfully perform the behavioral task and produce repeatable AERPs for a given auditory task must present high auditory integrity, reflecting both the integrity of the neural substrate underlying the behavioral task as well as the neural substrate of the pathways responsible for generating the AERP.⁷⁷ Consistent with Jerger and Martin’s recommendation,⁷⁶ we have offered in this commentary (and elsewhere as cited above) clinically useful and effective approaches to disentangle these interactions.

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CORRESPONDENCE can be addressed to HR or Dr Chermak at: [email protected]

About the Authors

Gail Chermak, PhD, is Professor of Audiology and Chair of the Department of Speech and Hearing Sciences, Elson S. Floyd College of Medicine, Washington State University Health Sciences Spokane; Frank E. Musiek, PhD, is a Professor in the Department of Speech, Language, Hearing Sciences at the University of Arizona; and Jeffrey Weihing, PhD, is an Assistant Professor in the School of Medicine at the University of Louisville.

Citation for this article. Chermak GD, Musiek FE, Weihing J. Beyond controversies: The science behind central auditory processing disorder. Hearing Review. 2017;24(5):20-24.