Introduction
(Central) auditory processing disorders [(C)APD], as defined in the 2005 Technical Report by the American Speech-Language and Hearing Association (ASHA), are "difficulties in the perceptual processing of information in the central auditory nervous system (CANS) and the neurobiological activity that underlies that processing and gives rise to the electrophysiological auditory potentials." This disorder is evident in a wide variety of patient populations including both children and adults. It may exist alone or coexist with other disorders such as Attention Deficit Hyperactivity Disorder (ADHD) and language impairments. (C)APD may be secondary to lesions of the central auditory nervous system such as vestibular schwannomas, multiple sclerosis, strokes and other neurological etiologies.
The prevalence of (C)APD has been reported to be anywhere from 2-3% in the pediatric population (Chermak & Musiek, 1997) to nearly 70% in the older adult population (Stach, Spretnjak & Jerger, 1990). As a result, it is important for clinicians to understand that (C)AP evaluations are not reserved solely for the pediatric population. The adult population is equally at risk for presenting with auditory deficits. In particular, patients with neurological involvement with lesions within the CANS should strongly be considered for a (C)AP evaluation. This is due to the fact that there is a high possibility of auditory involvement secondary to the auditory involvement of the lesion.
Over the past decade there have been numerous conferences and consensus statements from expert panels that have addressed (C)APD. The most recent clinical practice guidelines were established by the American Academy of Audiology (AAA) and address the diagnosis, treatment and management of children and adults with (C)APD (AAA, 2010). Individuals practicing in the sub-specialty field of auditory processing should look to this very comprehensive guide for current evaluation and management considerations. This article will provide a basic overview of (C)APD based on evidenced-based research. For a more in-depth discussion of this disorder, the reader is referred to the AAA guidelines and to the references provided.
Evaluation
Patient Selection
Appropriate patient selection is a critical component of a (C)AP evaluation. While it is true that clinicians should consider the central auditory function of each patient they evaluate, that does not mean that a central auditory battery is indicated for each patient. Without utilizing a strict set of criteria, clinicians may be overwhelmed with a plethora of referrals and subsequent evaluations. As a result, clinicians need to carefully select which patients are good candidates for a (C)AP evaluation. In addition, failure to set forth such guidelines for evaluation of children and adults for auditory processing evaluations will create a diagnostic dilemma in many instances. Perhaps one the most obvious example of this is the use of neuropsychological and/or neuroeducational testing. It is recommended that only children and adults with normal cognitive abilities be considered candidates for (C)AP evaluations. There are two primary reasons for this recommendation. First, tests of central auditory function are normed on children and adults with normal cognitive abilities. Second, if a patient with low cognitive functioning scores poorly on the (C)AP battery it is impossible to differentiate whether it is because of a true (C)APD, or because of low cognitive functioning. Another diagnostic dilemma presents when clinicians agree to see very young patients (below the age of 7 years) for (C)AP testing. One can administer, for example, the Gaps-In-Noise test on a 5-year-old, but it is impossible to interpret test results because there are no normative data for this age group (Shinn, Chermak & Musiek, 2009). It is extremely difficult to evaluate young children for (C)APD due the paucity of normative data, therefore it is currently recommended that children be a minimum of 7 years and older for consideration of evaluation (Jerger & Musiek, 2000).
There are several factors with respect to language that must also be taken into consideration. One of the first is adequate receptive and expressive language skills (Richard, 2007). Additionally, the patient's native language needs to be taken into account. It is always important that clinicians administer tests in a manner consistent with the original research. For tests of (C)APD, this means administering behavioral tests to individuals who native language is English. While it is not impossible to evaluate non-native English speaking individuals, it is certainly more difficult and may require the use of techniques such as electrophysiology and non-linguistically loaded stimuli (ASHA, 2005; AAA, 2010).
Peripheral Auditory System
Finally, the integrity of the peripheral auditory system should be carefully considered when determining if an individual should be evaluated for (C)APD. The strong influence of the peripheral auditory system will most likely contribute to central auditory function and/or dysfunction. Numerous studies have demonstrated that patients with peripheral hearing loss will demonstrate poor performance on a central auditory test battery (Humes, Coughlin & Talley, 1996; Musiek, Baran & Pinheiro,1990; Musiek, Gollegly, Kibbe & Verkest-Lenz,1991). Again, while it is not impossible to do so, it takes a very skilled clinician who is well versed in this area to: 1) understand of what audiometric patterns may allow for such evaluation, and 2) interpret patterns of central test results given peripheral involvement. These examples demonstrate why patient selection is the critical first step in appropriate evaluation of patients with suspected (C)APD.
Team Approach
Evaluation of auditory function examines the efficiency and effectiveness by which the CNS processes auditory information. In order to be able to thoroughly evaluate patients' function, it is important that audiologists engaging in (C)AP evaluations surround themselves with a team of professionals such as neurologists, neuropsychologists, speech language pathologists and others that can provide a collaborative approach to the evaluation and subsequent remediation of (C)AP.
Case History
The diagnosis of (C)APD should be made on the basis of a carefully selected battery of tests that measure the patient's particular area of difficulty. Every patient should not be administered the same battery of tests. Instead of a standard protocol for all patients regardless of their symptoms, the test battery should be driven by each patient's behavioral manifestations. A through and accurate case history is therefore a critical component of the (C)AP evaluation, as the results will be used in the selection of the test battery. The case history begins with a query into the presenting complaint or symptoms.
Common complaints among patients with (C)APD include:
- Difficulty hearing in the presence of background noise
- Difficulty following directions
- Often asking for repetition, or saying "huh or what"
- Poor localization
- Easily distracted/poor attention
- Academic difficulties, particularly in the areas of reading and/or spelling
- Poor musical abilities or appreciation of music
- History of Otitis Media
While the above list only suggests a few of the more commonly reported difficulties, there are many other signs and symptoms that may be reported by a patient. As there are many different patient presentations of (C)APD, it may be difficult to capture the exact profile of complaints, thus the need for a comprehensive history for each individual.
Included in the history should be the following areas (AAA, 2010):
- Auditory and/or communication difficulties
- Family history of hearing loss and/or (C)APD
- Medical history
- Speech-language history
- Educational and/or work history
- Existence of comorbid conditions
- Social development
- Linguistic background
Test Battery
Once a through case history has been obtained, the clinician must determine what battery of tests is appropriate given the history, along with factors such as age, attention, language, and others. The clinician's knowledge of several factors associated with test selection is of the utmost importance. These include an understanding of: 1) the methods in which the test was originally created, 2) proper test administration and 3) the test's sensitivity and specificity.
Behavioral test battery.
Every evaluation should begin with a thorough peripheral assessment. Once the peripheral system has been evaluated through comprehensive audiometry, as well as tympanometry and otoacoustic emissions when indicated, the clinician should carefully choose a behavioral battery that can accurately evaluate the patient's functional auditory performance. An important concept with respect to the behavioral evaluation is examination of a variety of processes. By evaluating the auditory system across behavioral sub-domains, the clinician is able to gather information not only about different auditory processes, but also about various regions within the CANS. Clinicians should consider the evaluation of the following auditory domains (AAA, 2010): Sound localization and/or lateralization; discrimination; temporal processing; pattern processing; binaural integration and/or separation; performance in competing and/or degraded acoustic signals. While these recommended areas should indeed be considered in a (C)AP evaluation, it is clear that as a field we are lacking in clinical measures in some areas. More research and the development of tests are needed. The following discussion will provide clinicians with a brief overview of the auditory processes for which clinical tests are available.
Dichotic listening.
Dichotic listening has been used to evaluate binaural integration and separation of individuals with suspected (C)AP deficits for nearly 60 years (Broadbent, 1954; Kimura, 1961). Dichotic tests involve presenting two different stimuli to each ear simultaneously. They have employed a variety of stimuli, but perhaps the two most popular and widely-studied clinical tools are the Dichotic Digits Test (DDT) developed by Musiek (1983) and the Staggered Spondaic Word Test (Katz, 1962). The use of dichotic listening in the (C)AP battery is extremely advantageous because these measures are highly sensitivity to cortical and interhemispheric dysfunction (Gootjes, Scheltens, Van Strien, & Bouma, 2007; Hurley & Musiek, 1997) in adults as well as children (Bartel-Friedrich, Broecker, Knoergen, & Koesling, 2010; Boscariol et al., 2011)
Temporal processing.
The area of temporal processing actually gives rise to four sub-components: Temporal resolution, temporal ordering/sequencing, temporal integration and temporal masking (Shinn, 2006). It is important for clinicians to understand that when a test of "temporal processing" is administered, it is only examining one specific area of temporal processing. For example, the highly popular frequency pattern test only evaluates temporal ordering and not other temporal processes. Emanuel and colleagues (2011) recently surveyed 195 audiologists who specialize in the area of auditory processing. Of those respondents, 64% reported often measuring temporal ordering and 35-70% indicated failure to evaluate other tests of temporal processing. There are additional measures such as the Gaps-In-Noise Test (Musiek et al., 2005) that evaluates patients' temporal resolution abilities with good sensitivity and specificity. The evaluation of temporal processing is important because it evaluates the timing abilities of the CANS. These tests are useful because they provide information about the integrity of the CANS, in particular at the level of the auditory cortex, and are typically not highly linguistically loaded.
Monaural low-redundancy speech tests.
Tests that fall within the category of monaural low-redundancy speech tests are those that degrade the acoustic signal in some manner. The degradation may be achieved through filtering, compressing the signal or introducing background noise or acoustic competition. While these are highly popular measures because of their "ecological validity", they lack high sensitivity and specificity (Musiek & Baran, 2002).
Localization, lateralization and binaural interaction.
As a field we are severely lacking tests or clinical measures of localization and lateralization. While the Masking Level Difference (MLD) test is an interaction measure that does provide information regarding low-brainstem function, it is only used by an estimated 11-18% of clinicians (Emanuel, 2011). Additionally, it is not a direct measure of localization and lateralization. Another test which taps into similar sub-processes is the LISN (Cameron and Dillon, 2007).
Electrophysiological Testing
Perhaps one of the most underutilized tools in the (C)AP battery is measures of electrophysiological function. This is evidenced in the survey data presented by Emanuel in which few respondents reported performing the electrophysiological assessment "always or often" (14.9% auditory brainstem response (ABR), 7.9% middle latency response (MLR) and 4.4% cortical evoked potentials) as part of their battery. For those specializing in (C)APD, it is strongly recommended that the use of cortical potentials be included as part of the test battery when appropriate (Jerger & Musiek, 2000). Electrophysiology is a powerful tool that provides objective information regarding the integrity of the CANS from the 8th nerve through the level of the auditory cortex.
ABR.
The ABR, while a widely used and a highly useful diagnostic tool, is actually very limited in its benefit with respect to diagnosing (C)APD as it provides information only through the level of the brainstem. Certainly patients who have abnormal ABRs should be evaluated for retrocochlear involvement, however a majority of patients who present with (C)APD will likely yield normal ABRs.
MLR and LAEP.
A more useful electrophysiologic measure in the evaluation of (C)APD is the Middle Latency Response (MLR). The late auditory evoked potentials (LAEP) may also be employed in the (C)AP battery. Of note with respect to electrophysiological potentials, in particular the MLR and LAEPs, is that measurements are dependent upon intra-subject comparisons. In other words, the patient is essentially used as his or her own control. Additionally, one must consider the impact of other factors such as attention on the MLR and LAEP's. Unlike the ABR, these potentials are highly influenced by subject state and attention. Specifically these responses will be significantly reduced or eliminated if the subject is not awake.
MLR provides clinicians with information regarding the integrity of the thalomo-cortical pathway and the primary auditory cortex. In addition, with the use of the correct recording parameters, clinicians can also capture wave V of the ABR. Those that are skilled in conducting diagnostic ABRs will quickly realize that administering MLR is just as easy. The MLR has a later maturational time course and reaches adult values around 10 years of age. It is recommended if at all possible to use a minimum of two electrode sites including C3 and C4 in order to observe any laterality effects. There are several ways in which the MLR, and for that matter the late potentials, can be interpreted, including the electrode effect and/or the ear effect. If the electrode effect is present, the clinician will observe the greatest deficit in amplitude of the response at the electrode site closest to the site of lesion. In contrast, if an ear effect is observed, at times, the electrode contralateral to the lesion will demonstrate the greatest deficit.
LAEP gives rise to the N1, P2 and P300 complex. As with the other potentials, clinicians will look for patterns of abnormality with respect to latencies and amplitude measurements resulting in electrode and ear effects. Of all of the potentials, these are the most highly influenced by the effects of subject state and attention.
There have been numerous investigations which have demonstrated the utility of these potentials in both pediatric and adult populations (Jirsa & Clontz, 1990; Musiek, Baran, & Pinheiro, 1992; Schochat, Musiek, Alonso, & Ogata, 2011). For example, patients with traumatic brain injuries have been found to demonstrate prolonged latencies and decreased amplitudes on the P300 response (Lew et al., 2004). Electrophysiologic measures, while highly underutilized, are very powerful objective diagnostic tools which should be considered as part of a comprehensive (C)AP battery.
Interpretation
Interpretation of (C)AP test results can often be difficult, however, following certain recommended guidelines can make analysis of test results easier. It is recommended clinicians used norm-based interpretation with a two standard deviation cut-off value (Musiek & Chermak, 1997). This information is derived from normal listeners. Sensitivity and specificity measures, on the other hand, are based on patients with known lesions of the CANS.
It may sometimes be the case that patients will present with conflicting or grossly abnormal test results (i.e., depressed scores across all tests bilaterally). In such instances, one should consider variables such as attention, cognition or greater global involvement. In particular, differential diagnosis of patients with ADHD can be extremely difficult (Musiek, Schochat & Shinn, 2001). In such cases, it is important for the clinician to further investigate or eliminate possible confounding variables. This might include looking for patterns of abnormality that improve with reinforcement, or repeating a test on an alternative day that was perhaps the last measure administered during the test session. If such patterns are observed it is important for the clinician to determine alternative approaches to obtaining results that are valid and reliable.
(Re)Habilitation
The purpose of central auditory evaluation is to not only evaluate function, but also to provide appropriate intervention. There are several types of strategies that have been employed throughout the years. These include environmental modifications, compensatory strategies and auditory training (AT). Environmental modifications are those strategies designed to improve the patient's surroundings in order to provide a more ideal listening situation. An example of an environmental modification is the implementation of an FM system in the classroom of a child diagnosed with (C)APD. While this is an excellent environmental modification that will likely benefit the child, it is not direct deficit-driven therapy. Compensatory strategies are designed to assist individuals with techniques that they can apply to help overcome some of the daily struggles that these patients may face. For example, a child may need to "chunk" important information together to understand the message being conveyed (Bellis, 2003).
While environmental modifications and compensatory strategies may be beneficial for patients with (C)APD, they fail to provide true AT that results in physiological reorganization of the CANS. It is well established that the CANS is highly plastic, meaning that it has the ability to be altered both physiologically and anatomically based on sensory stimulation and deprivation (Tremblay & Kraus, 2002). In order for true functional, physiological and behavioral changes in central auditory system to occur, AT should be both: 1) deficit driven and 2) intense in nature (Chermak, Bellis & Musiek, 2007).
Deficit driven therapies are those therapies that provide intervention for a specific area of deficit. For example, if a patient presents with a binaural integration deficit, then it would not be appropriate to address it by administering therapies that were designed to improve temporal processing skills (i.e., FastForward® or temporal processing exercises). Rather, a more appropriate therapy option would be dichotic interaural intensity difference (DIID) training. This is a therapeutic approach which has been show to improve binaural integration in patients with dichotic listening deficits (Moncrieff & Wertz, 2008). In order to maximize plasticity, and as a result observe positive change in auditory function, therapies must be directed towards the specific deficit resulting in subsequent individualization of the treatment plans.
There are two primary modes of therapy, which can be classified as formal and informal (Musiek, Shinn & Hare, 2002). Formal auditory training generally requires special instrumentation and is oftentimes administered by an audiologist, or through commercially available programs. Informal programs are intended to be coupled with formal training programs in order to enhance their efficacy. Regardless of whether it is formal or informal, therapy should be age and language appropriate, motivating, progressive and varying in task complexity, balanced with respect to level of difficulty, and have enough dedicated time (Musiek, Chermak and Weihing, 2007). Therapy is most efficient and effective when administered with a high degree of intensity. In addition, training should be neither too hard, nor too easy. Training ones brain is no different than training the physical body: intensity, progression and variety are key (Musiek, 1999). We may explain this to patients by comparing their AT to training for the Boston Marathon, for example. If an individual wished to train, be competitive and complete such a monumental race, they would spend a significant amount of time training. You would not expect to see them running for only 30 minutes once per week, or they would not achieve their goals. It is important to mention that while there is strong evidence of auditory plasticity, there is need for further data to support the efficacy of these types of (re)habilitation programs with respect to (C)APD.
(C)APD in Everyday Practice
Consideration of central auditory function, and in turn dysfunction, is fundamental in any audiology practice. All too often, clinicians get immersed in loss of hearing sensitivity, but forget about what is occurring with respect to processing of auditory information. Due to an expertise in auditory anatomy, audiologists know that the entire auditory system from the outer ear through the auditory cortex is a major player in efficient and effective auditory processing. In other words, hearing goes well beyond the periphery. Take for example, the recommendation for the use of binaural amplification for most patients with binaural hearing loss. We tell our patients that two hearing aids are better than one, however, the question we must ask ourselves is: Are two hearing aids always better than one? Although most clinicians would say yes, it is not always the case that binaural amplification is best when one takes central auditory function into consideration.
Jerger and colleagues (1993) demonstrated that binaural interference is observed in 8-10% of the elderly population. In fact, this may not only be true for the adult population, but should also be considered in pediatric hearing aid fittings as well (Schoepflin, 2007). This phenomenon, and the contraindication of binaural amplification in cases of binaural interference, has been supported by several investigators (Carter, Noe & Wilson, 2001; Walden & Walden, 2005; Kobler, Lindblad, Olofsson, & Hagerman, 2010). What clinicians should be cognizant of in reviewing such data is that this is due not to a peripheral phenomenon, but to a central phenomenon. It is through the use of a binaural integration task, such as the dichotic digits, that these sometimes subtle deficits in auditory function may contribute to rejection of amplification. Therefore, even those clinicians who do not specialize in (C)APD may want to consider routinely screening for it using such measures in patients with symmetrical hearing.
It is critically important for clinicians to have a thorough understanding and appreciation of the auditory system and the role it plays, and incorporate this knowledge into their everyday practice. In order for this to happen, there is clearly a need for greater education and training among professionals regarding the importance of evaluating the entire auditory system, and not just the periphery. A recent survey by Chermak and colleagues revealed that while didactic courses in (C)AP have improved, there are still only a limited number of clinicians performing such evaluation (Chermak, Silva, Nye, Hasbrouck, & Musiek, 2007). Perhaps even more alarming was the report that the majority of respondents reported only spending on average one hour per week assessing central auditory function in the clinic. Training and education will be important elements in changing the course of this trend.
Summary
The intent of this article is to provide the reader with a brief overview of the current status of (C)APD. (C)APD affects many of our pediatric and adult patients, and should be considered as part of every audiologist's clinical practice. Although not all clinicians are experts in this particular area, it is important that all audiologists understand when evaluation is warranted. There is a plethora of evidence-based information available to those clinicians who currently provide services, or wish to implement (C)AP evaluation and (re)habilitation into their clinical practice. Although we have come a significant way in our ability to diagnose and treat this disorder, we continue to seek novel and innovative approaches to helping our patients overcome these deficits.
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