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Background
Auditory processing disorder (APD), also known as central auditory processing disorder, supposedly interferes with both the input and integration of verbal information, and results in a potentially permanent cognitive dysfunction during the developmental period of acquisition of language. The prevalence of APD in the general population has not been firmly established. Chermak and Musiek (1997) estimated that APD occurs in 2 to 3 % of children, with a 2-to-1 ratio between boys and girls, while Cooper and Gates (1991) estimated the prevalence of adult APD to be 10 to 20 %. Neurological disorders, diseases, and insults, including neurodegenerative diseases, probably account for most acquired APD in adults; however, such disorders probably account for 5 % or fewer of diagnosed cases of APD in children, especially as APD relates to learning disability (Musiek et al, 1985, 1992).
Although the exact cause(s) of APD remains unclear, it does not appear to be caused by peripheral hearing impairment. The diagnosis of APD remains controversial, largely because of the purported co-morbidity with associated conditions such as attention-deficit/hyperactivity disorder, learning disabilities, and speech-language impairment, as well as the diversity of signs and symptoms associated with this disorder. Some of the more common diagnostic tests for APD include Staggered Spondaic Word (SSW) Test, the SCAN Screening Test for auditory processing disorders, and the Multiple Auditory Processing Assessment (MAPA). Moreover, there is no clear acceptance of a "gold standard" test battery for evaluating this disorder.
According to a review (Cacace and McFarland, 1998), the rationale to evaluate for APD in school-aged children is based on the premise that an impairment in auditory perception can be the underlying cause of many learning problems, including specific reading and language disabilities. However, there is insufficient scientific evidence to validate this proposition. There is no ICD-9 code for APD (ICD-9-CM, 6th Edition, 2003). Furthermore, this disorder is not recognized as a unique entity affecting school-aged children (DSM-IV-TR, American Psychiatric Association, 2000).
In a review on APD, Bamiou and associates (2001) concluded that APD may be a feature of both neurological and developmental disorders. However, whereas APD appears to be well documented in relation to certain syndromes, in other disorders the quality of existing evidence is inconclusive and the relation of APD to the coexisting disorder is poorly understood. Further research into the interface between APD and neurological and developmental disorders is needed. Clear insight into the nature of the auditory processing deficit may have implications for appropriate management, in agreement with the trend to provide multimodal intervention for these disorders. Moreover, a detailed understanding of the structural and functional substrate of auditory processing disorders will enable phenotypic evaluation specifically for the purposes of genetic research.
A review on auditory processing and the development of language and literacy (Bailey and Snowling, 2002) found that evidence for basic auditory processing impairments associated with dyslexia and specific language impairment is inconsistent. It appears that not all children with language difficulties have non-verbal auditory processing impairments, and for those who do, the impact on language development is poorly understood. The authors stated that advances in the understanding of the role of auditory processing in the genesis of language difficulties have been hampered theoretically by a lack of agreement regarding the relationship between basic auditory skills, speech perception and phonological processing abilities, and also methodologically by frequent uncontrolled group differences in experimental studies. Well-designed studies are needed to ascertain the extent to which there are auditory-specific learning disabilities.
There are no established therapies for the treatment of patients with APD. Current approaches include signal enhancement, linguistic and cognitive strategies, auditory training (including auditory integration therapy), as well as medication. Signal enhancement strategies aim to improve the signal to noise ratio. This can be achieved by minimizing background noise or by using frequency-modulated systems in the classroom. Linguistic and cognitive strategies aim to increase use of compensatory strategies.
Auditory training utilizes the brain's plasticity and can be formal or informal. Formal auditory training may include computerized commercial programs such as Fast ForWord (Scientific Learning, Oakland, CA) and Earobics (Cognitive Concepts, Inc., Evanston, IL), which alter speech acoustics and adaptively speed up neural processing; or training in the audiology clinic with modified central auditory tasks. Informal strategies can be applied at home or at school and include tasks such as vowel/consonant training, simple games such as "Simon", etc. However, there is little scientific evidence on the effectiveness of various formal and informal auditory training programs. More research is needed to ascertain whether these interventions actually enhance temporal processing abilities and the magnitude of language gains.
Methylphenidate (Ritalin), a drug traditionally prescribed for the management of patients with attention deficit/hyperactivity disorder (ADHD), has been used to treat children with APD. However, it is unclear whether methylphenidate can improve auditory processing, thus, methylphenidate and related stimulant medications should not be prescribed routinely for treatment of APD in the absence of ADHD.
Given the problems associated with diagnosing APD, any therapies should be viewed cautiously. The National Institute on Deafness and Other Communication Disorders (2001) stated that it is important to know that much research is still needed to understand auditory processing problems, related disorders, and the best interventions for each child or adult. This is in accordance with the review on this subject by Chermak (2002) who concluded that "additional controlled case studies and single-subject and group research designs are needed to ascertain systematically the relative efficacy of various treatment and management approaches".
The National Institute on Deafness and Other Communication Disorders (2004) also stated that auditory integration training may be promoted by practitioners as a way to retrain the auditory system and decrease hearing distortion. However, current research has not proven the benefits of this treatment.
In a review of the etiology of speech and language disorders in children, Carter and Musher (2006) state that "[e]valuation for a central auditory processing disorder (CAPD) in school-aged children is based upon the assumption that an auditory-specific perceptual deficit is the foundation of learning problems such as reading and language disabilities. However, the diagnosis, management, and even the existence of a modality-specific dysfunction are controversial."
Aetna’s policy on APD is based upon the limited evidence for APD as a distinct pathophysiologic entity, upon a lack of evidence of established criteria and well validated instruments to diagnose APD and reliably distinguish it from other conditions affecting listening and/or spoken language comprehension, and upon the lack of evidence from well designed clinical studies proving the effectiveness of interventions for treating APD. The reported frequent co-occurrence of APD with other disorders affecting listening and/or spoken language comprehension suggests that APD is not, in fact, a distinct clinical entity.
von Suchodoletz (2009) noted that the clinical relevance of CAPD is highly controversial. The author stated that available literature reveals that numerous authors have described subnormal auditory abilities in groups of children with developmental language disorders, dyslexia or ADHD. However, little or no relationship between the severity of clinical impairment and auditory deficits has been found. Thus, auditory deficits do not appear to be causally related to learning disorders or conduct disorders. With respect to the diagnostic process, this review made clear that the validity of the diagnosis CAPD is low. There is no agreement about diagnostic criteria, and the reliability of most auditory tests is insufficient. Moreover, while an auditory training can only improve the directly trained auditory functions, there is no transfer effect to learning ability of behavior. Altogether, there is little evidence for a significant relevance of CAPD in child and adolescent psychiatry.
In a review on APD in relation to developmental disorders of language, communication and attention, Dawes and Bishop (2009) stated that "APD, as currently diagnosed, is not a coherent category, but that rather than abandoning the construct, we need to develop improved methods for assessment and diagnosis, with a focus on interdisciplinary evaluation".
Lemos and colleagues (2009) verified the existence of scientific evidence confirming the effectiveness of personal frequency modulation (FM) systems in the treatment of APD. For this purpose, a systematic review of the literature was made, using data found in electronic databases (Medline, Lilacs and Cochrane library) as well as on the internet. The articles retrieved were analyzed according to the CONSORT statement and then classified by their evidence level. The search resulted in 1,589 references out of which only 19 met the inclusion criteria. All of the analyzed articles were classified as having low level of evidence (expert opinion or case studies). Strong scientific evidence supporting the use of personal FM systems for APD intervention was not found. Since such device is frequently recommended for the treatment of APD, it becomes essential to carry out studies with high scientific evidence that could safely guide clinical decision making on this subject.
Rosen et al (2010) evaluated auditory and cognitive abilities in a group of children referred for an auditory evaluation on the grounds of a suspected APD (susAPD), and in age-matched children who were typically developing, in order to determine the extent to which any deficits in cognitive abilities could be related to auditory deficits. A battery of auditory and cognitive tests was applied to 20 susAPD school-age children, all reported as having listening/hearing problems but performing within normal limits for standard audiometric assessments. Also tested was a group of 28 age-matched controls. The auditory tasks consisted of 2 simple same/different discrimination tasks, one using speech, and one non-speech. The cognitive evaluation comprised a vocabulary test, a test of grammar and 4 non-verbal IQ measures. Symptoms of ADHD were assessed in the susAPD group through a standardized questionnaire. A significant proportion of susAPD children appeared to display genuine auditory deficits evidenced by poor performance on at least 1 of the auditory tasks, although about 1/3 had no detectable deficit. Children in the susAPD group scored consistently lower than the controls on cognitive measures that were both verbal (vocabulary and grammar) and non-verbal. Strikingly, susAPD children with relatively good auditory performance did not differ in cognitive ability from susAPD children with poor auditory performance. Similarly, within-group correlations between auditory and cognitive measures were weak or non-existent. Measures of ADHD did not correlate with any aspect of auditory or cognitive performance. The authors concluded that although children suspected of having APD do show, on average, poorer performance on a number of auditory tasks, the presence or absence of an auditory deficit appears to have little impact on the development of the verbal and non-verbal skills tested here. Furthermore, Lagace and colleagues (2010) stated that APD is a complex and heterogeneous disorder for which the underlying deficit is currently unclear.
Moore and associates (2010) tested the specific hypothesis that the presentation of APD is related to a sensory processing deficit. Randomly chosen, 6- to 11-year-old children with normal hearing (n = 1,469) were tested in schools in 4 regional centers across the United Kingdom. Caregivers completed questionnaires regarding their participating children's listening and communication skills. Children completed a battery of audiometric, auditory processing (AP), speech-in-noise, cognitive (IQ, memory, language, and literacy), and attention (auditory and visual) tests. AP measures separated the sensory and non-sensory contributions to spectral and temporal perception. AP improved with age. Poor-for-age AP was significantly related to poor cognitive, communication, and speech-in-noise performance (p < 0.001). However, sensory elements of perception were only weakly related to those performance measures (r < 0.1), and correlations between auditory perception and cognitive scores were generally low (r = 0.1 to 0.3). Multi-variate regression analysis showed that response variability in the AP tests, reflecting attention, and cognitive scores were the best predictors of listening, communication, and speech-in-noise skills. The authors concluded that presenting symptoms of APD were largely unrelated to auditory sensory processing. Response variability and cognitive performance were the best predictors of poor communication and listening. These investigators suggested that APD is primarily an attention problem and that clinical diagnosis and management, as well as further research, should be based on that premise.
Fey and colleagues (2011) evaluated the peer-reviewed literature on the efficacy of interventions for school-age children with APD. Searches of 28 electronic databases yielded 25 studies for analysis. These studies were categorized by research phase (e.g., exploratory, efficacy) and ranked on a standard set of quality features related to methodology and reporting. Some support exists for the claim that auditory and language interventions can improve auditory functioning in children with APD and those with primary spoken language disorder. There is little indication, however, that observed improvements are due to the auditory features of these programs. Similarly, evidence supporting the effects of these programs on spoken and written language functioning is limited. The authors concluded that the evidence base is too small and weak to provide clear guidance to speech-language pathologists faced with treating children with diagnosed APD, but some cautious skepticism is warranted until the record of evidence is more complete. Clinicians who decide to use auditory interventions should be aware of the limitations in the evidence and take special care to monitor the spoken and written language status of their young clients.
Miller (2011) provided information that may aid in understanding and interpreting research literature on the role of auditory processing in communication disorders. A narrative review was used to summarize and synthesize the literature on auditory processing deficits in children with APD, specific language impairment (SLI), and dyslexia. The history of auditory processing theories of these 3 disorders was described, points of convergence and controversy within and among the different branches of research literature were considered, and the influence of research on practice was discussed. The theoretical and clinical contributions of neurophysiological methods were also reviewed, and suggested approaches for critical reading of the research literature were provided. The author concluded that research on the role of auditory processing in communication disorders springs from a variety of theoretical perspectives and assumptions, and this variety, combined with controversies over the interpretation of research results, makes it difficult to draw clinical implications from the literature. Neurophysiological research methods are a promising route to better understanding of auditory processing. Progress in theory development and its clinical application is most likely to be made when researchers from different disciplines and theoretical perspectives communicate clearly and combine the strengths of their approaches.
In a review of the evidence for auditory processing disorder, Kamhi (2011) stated that there are compelling theoretical and clinical reasons to question whether APD is in fact a distinct clinical entity. The author noted that, not only is there little evidence that auditory perceptual impairments are a significant risk factor for language and academic performance, there is also no evidence that auditory interventions provide any unique benefit to auditory, language, or academic outcomes. The author concluded that, because there is no evidence that auditory interventions provide any unique therapeutic benefit, clinicians should treat children who have been diagnosed with APD the same way they treat children who have been diagnosed with language and learning disabilities. The author stated that the theoretical and clinical problems associated with APD should encourage clinicians to consider viewing auditory deficits as a processing deficit that may occur with common developmental language and reading disabilities rather than as a distinct clinical entity.
Ahmmed and colleagues (2014) identified the factors that may underlie the deficits in children with listening difficulties, despite normal pure-tone audiograms. These children may have APD, but there is no universally agreed consensus as to what constitutes APD. These investigators therefore referred to these children as children with suspected APD (susAPD) and aimed to clarify the role of attention, cognition, memory, sensorimotor processing speed, speech, and non-speech auditory processing in susAPD. It was expected that a factor analysis would show how non-auditory and supra-modal factors relate to auditory behavioral measures in such children with susAPD. This would facilitate greater understanding of the nature of listening difficulties, thus further helping with characterizing APD and designing multi-modal test batteries to diagnose APD. They performed a factor analysis of outcomes from 110 children (68 males, 42 females; aged 6 to 11 years) with susAPD on a widely used clinical test battery (SCAN-C) and a research test battery (MRC Institute of Hearing Research Multi-center Auditory Processing "IMAP"), that have age-based normative data. The IMAP included backward masking, simultaneous masking, frequency discrimination, non-verbal intelligence, working memory, reading, alerting attention and motor reaction times to auditory and visual stimuli. SCAN-C included monaural low-redundancy speech (auditory closure and speech in noise) and dichotic listening tests (competing words and competing sentences) that assess divided auditory attention and hence executive attention. Three factors were extracted: (i) "general auditory processing", (ii) "working memory and executive attention", and (iii) "processing speed and alerting attention". Frequency discrimination, backward masking, simultaneous masking, and monaural low-redundancy speech tests represented the "general auditory processing" factor. Dichotic listening and the IMAP cognitive tests (apart from non-verbal intelligence) were represented in the "working memory and executive attention" factor. Motor response times to cued and non-cued auditory and visual stimuli were grouped in the "processing speed and alerting attention" factor. Individuals varied in their outcomes in different tests. Poor performance was noted in different combinations of tests from the 3 factors. Impairments solely related to the "general auditory processing" factor were not common. The authors concluded that the findings of this study identified a general auditory processing factor in addition to 2 other cognitive factors, "working memory and executive attention" and "processing speed and alerting attention", to underlie the deficits in children with susAPD. Impaired attention, memory, and processing speed are known to be associated with poor literacy and numeracy skills as well as a number of neurodevelopmental disorders. Individuals with impairments in the "general auditory processing" tests along with tests from the other 2 cognitive factors may explain the co-occurrence of APD and other disorders. The variation in performance by individuals in the different tests noted was probably due to a number of reasons including heterogeneity in susAPD and less-than ideal test-retest reliabilities of the tests used to assess APD. They stated that further research is needed to explore additional factors, and consensus is needed to improve the reliability of tests or find alternative approaches to diagnose APD, based on the underlying factors.
Furthermore, an UpToDate review on “Etiology of speech and language disorders in children” (Carter and Musher, 2014) states that “Evaluation for a central auditory processing disorder (CAPD) in school-age children is based upon the assumption that an auditory-specific perceptual deficit is the foundation of learning problems such as reading and language disabilities. However, the diagnosis, management, and even the existence of a modality-specific dysfunction are controversial”.
Methods used to diagnose auditory processing disorder lack a clear evidence base, and the diagnosis of any particular child with APD is determined more by the referral route than by the symptoms (Moore, et al., 2013). A study identified nine different sets of diagnostic criteria for auditory processing disorder and the resulting rates of diagnosis of auditory processing disorder ranged from 7.3% to 96% (Wilson, et al., 2012).
Substantial evidence regarding test performance (e.g. reliability, validity, sensitivity, and specificity) is lacking for most commonly used behavioral tests of auditory processing (Keith, 2009).
There are no widely accepted criteria as to when electrophysiologic tests should be included in the clinical evaluation of auditory processing disorder (Schochat, et al., 2010). There are currently no available electrophysiological measures of sufficient utility and reliability to be useful in the clinical assessment of auditory processing disorder (McFarland & Cacace, 2012; Hornickel et al, 2012). A plethora of measures and stimuli is used inconsistently from study to study, with no clear evidence of replicability across studies (Moore, et al., 2013).
Without established diagnostic criteria, the best methods for identifying and managing auditory processing disorder remain elusive. Auditory processing disorder will often co-exist with attention, language and learning impairments as well as autism spectrum disorder (Bellis, 2008; Dawes & Bishop, 2010; Witton, 2010). The status of auditory processing disorder as a distinct disorder has been questioned due to the overlap between clinical profiles of children diagnosed with auditory processing and those with other forms of learning disability (Jerger, et al., 2009). Studies comparing children with a diagnosis of dyslexia and those with a diagnosis of auditory processing disorder found the two groups could not be distinguished (Ferguson, et al., 2011; Dawes & Bishop, 2010; Miller & Wagstaff, 2011) and obtained similar findings in studies comparing children diagnosed with specific language impairment or auditory processing disorder (Corriveau, et al., 2007; Dlouha, et al., 2007).
Data specifically addressing the efficacy of interventions for auditory processing disorder are lacking and many of the recommendations commonly made are based on theory or inferred from approaches validated in other populations, e.g. specific language impairment and dyslexia. The scientific evidence for the use of personal FM systems in auditory processing disorder is of low quality (Lemos, et al., 2009). Computer-based auditory training programs that originally developed and marketed for children with language, learning and reading difficulties have also recently been recommended for children with a specific diagnosis of auditory processing disorder, despite limited research evidence to support this (Loo, et al., 2010; Thibodeau, 2007). Phonological awareness training is widely used as an intervention for children with reading disability and there are programs that have been developed and evaluated using randomized controlled studies (Otaiba, et al., 2009). However, auditory processing disorder status has not been considered in these studies. There is a lack of studies evaluating formal auditory training programs for persons with the diagnosis of auditory processing disorder. Other programs and methods have no or a very low level of evidence or conflicting expert opinion. In a systematic evidence review, Fey, et al. (2011) found only weak evidence on the efficacy of auditory processing disorder training and auditory/language interventions for children.
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