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Assessment of Auditory Processing Disorders

Robert W. Keith

Many practicing otolaryngologists, even Fellowship-trained otologists, are unfamiliar with central auditory processing and dysfunction. This chapter provides information on central auditory testing including the rationale for such tests. The chapter includes suggestions for ordering central auditory testing and a test battery approach is proposed. Finally, case histories showing results of central auditory testing are given.

History of Central Auditory Disorders in Otoneurologic Diagnosis

Although many physicians are not familiar with the subject of central auditory disorders, otolaryngologists conducted some of the earliest systematic clinical research on this subject. Specifically, the classic works of Bocca, Calearo, and Antonelli published in the mid-1950s provided a basis for subsequent development of central auditory testing. Their early research, summarized by Calearo and Antonelli in the first edition of Otolaryngology¹ is informative reading even today.

Calearo and Antonelli noted that sensorineural hearing loss can be classified as cochlear and retrocochlear. Because retrocochlear disorders may include eighth cranial nerve and central pathways lesions, they defined central auditory lesions as involving the pathway between the cochlear nucleus and the temporal lobe cortex. This classification remains today, with the added notion of interhemispheric transmission of auditory information through the corpus callosum.

The single difference between testing conducted earlier and now is the focus or purpose of central auditory testing. In the 1950s and 1960s, the emphasis was on the identification of auditory pathway lesions. With the development of sophisticated diagnostic imaging techniques (e.g., magnetic resonance imaging), there is presently little need for auditory diagnosis of central lesions. Therefore, the current primary focus of central auditory testing is to describe functional disorders of communication.^2,3 Terms used to describe these disorders of listening and understanding include auditory perceptual disorders, central auditory processing disorders, and more recently auditory processing disorders (APDs).⁴

A secondary purpose, also discussed by Calearo and Antonelli,¹ continues to be experimental research into communication disorders related to various language and learning problems and central auditory pathway lesions.

The last few years have seen a remarkably increase of interest in APDs by professionals and the lay public. This interest is a result of the large body of research on APD that became available during previous decades in conferences, through increased communication through the Internet, and through commercialization of remediation programs. One negative factor is the lack of consensus on a precise definition of the term APD and how this disorder is unique. Two conferences held in 1993 and 2000 addressed issues of definitions of APD.

Definition of Auditory Processing Disorder

APDs can be defined as dysfunctions of one or more of the basic processes involved in understanding spoken language, which may manifest themselves in an imperfect ability to listen.⁵ An American Speech-Language Hearing Association (ASHA) ad hoc committee on central auditory processing⁶ defined auditory processing disorders as “deficits in information processing of audible signals not attributed to impaired hearing sensitivity or intellectual impairment.” Specifically, the committee stated that “APD” refers to limitations in the ongoing transmission, analysis, organization, transformation, elaboration, storage, retrieval, and use of information contained in audible signals. More simply stated, the auditory system is responsible for the following:

• Sound localization and lateralization

• Auditory discrimination

• Auditory pattern recognition

• Temporal aspects of audition including resolution, masking, and ordering

• Performance with competing signals

• Performance with degraded signals

Deficits in processing of those signals can be considered an APD.

More recently the Consensus Conference on the diagnosis of APD in school-aged children suggested, “An APD may be broadly defined as a deficit in the processing of information that is specific to the auditory modality.”4 The panel concluded that the problem may be exacerbated in unfavorable acoustic environments. It may be associated with difficulties in listening, speech understanding, language development, and learning. “In its pure form, however, it is conceptualized as a deficit in the processing of auditory input.” These discussions about terminology suggest that APD contributes to developmental language disorders and interferes with academic achievement in the classroom.

The prevalence of APD is unknown as there is no gold standard for the disorder. Learning disabilities in children have a prevalence of 4 to 5%.^7,8 Many of these children have APD also, but the occurrence is unknown and estimates reported in the literature are speculative.

Principles of Central Auditory Dysfunction

Calearo and Antonelli¹ summarized several principles attributed to Carhart that explain how the normal central nervous system (CNS) handles auditory messages:

1. Channel separation: A signal delivered to one ear is kept distinct from a different signal in the other ear.

2. Binaural fusion: If a single auditory message is divided into two bands by filtering or by switching, and these bands are delivered binaurally and simultaneously, fusion will take place (at the brainstem level) and the subject will experience one message only.

3. Contralateral pathways: Auditory messages from one ear cross at the brainstem level, and reach the temporal lobe of the opposite side.

4. Hemispheric dominance for language: Although one cerebral hemisphere (usually the left) is verbally dominant, the other hemisphere appears to possess limited verbal abilities. Adding to Bocca and Calearo, we now understand that linguistic information reaching the nondominant hemisphere (the right hemisphere from information presented to the left ear) crosses to the dominant language hemisphere through the densely myelinated fibers in the splenium of the corpus callosum.

Related to the above principles, the following additional principles apply to central auditory assessment:

1. Most diseases affecting central hearing pathways produce no loss in threshold sensitivity. Therefore, pure-tone tests do not generally identify APD.

2. Undistorted speech audiometry is not sufficiently challenging to the central auditory nervous system to identify the presence of a central auditory lesion/disorder.

3. Only tests of reduced acoustic redundancy (distorted speech materials called sensitized speech tests by Teatini⁹) are sufficiently challenging to the auditory nervous system to identify a central auditory lesion/disorder.

The rationale for use of sensitized speech testing was further described by Jerger¹⁰ as the subtlety principle, in which the subtlety of the auditory manifestation increases as the site of lesion progresses from peripheral to central. More recently, Phillips¹¹ described these processes in a somewhat different way as “patterns of convergence and divergence in the ascending auditory pathway.”

Neurophysiologic Basis of Central Auditory Disorders

The auditory nervous system presents a complex interaction of neural signals that integrates acoustic information from both ears at nearly all levels of the central auditory nervous system from the cochlear nucleus to the auditory cortex. These neural pathways are not simply passive conductors of an electrical signal; auditory analysis takes place at all levels of the auditory system from cochlea to cortex.

The final common pathway is the auditory area of the cerebral cortex that lies in Heschl’s gyrus. Anatomically the brain contains regions that are typically different in size on the two sides. The best-defined asymmetry is in the primary auditory reception area in centers known to be associated with language comprehension. This asymmetry is present in newborns.

What we know about auditory processing and the brain is as follows:

1. Language centers are usually situated in the left hemisphere. Ninety-five percent of persons are left-hemisphere dominant for language. Although production of speech is controlled from one hemisphere, there is a continuum of lateralization for speech perception. The right hemisphere is probably capable of processing paralinguistic aspects of language such as emotional tone, context, inference, and connotation. Therefore, motor speech production is a single hemisphere function, whereas speech perception is a dual hemisphere function that has implication for interpretation of auditory perceptual test results.

2. Maturation of neural structures, especially association fibers, takes place over many years. Sensory deprivation results in failure of certain neural centers to develop at least in the brainstem and probably cortically.¹¹

3. Dominance for language is present in the left cerebral hemisphere at early ages, but the brain is sufficiently plastic to allow language to be controlled by other areas if early damage occurs.¹² Linguistic processing is more completely centered in dominant hemispheres in older persons who are less able to recover from damage to language centers.

4. Failure to establish cerebral dominance is associated with problems associated with language development, learning to read, and integration of information from the several senses. Causes of brain dysfunction include:

a. Congenital abnormalities

b. Anoxia

c. Maternal virus

d. Other birth injury or illness

e. Head trauma

f. Seizures

g. Genetic factors

h. Unknown factors

According to Ferry,¹³ delay or deviation in language development is due to disordered brain functioning. Normal speech and language development is a reflection of an intact functioning brain. A speech or language delay, or auditory processing disorder, may be the only symptom or sign of neurologic impairment.

Different categories of diagnostic central auditory tests have been proposed. For example, the ASHA Committee on Disorders of Central Auditory Processing6 used the following taxonomy:

1. Monotic (signals presented to one ear)

a. Filtered speech (e.g., low-pass-filtered)

b. Time-altered speech (e.g., time-compressed speech)

c. Pattern recognition (e.g., frequency and duration patterns)

d. Ipsilateral competing signals (e.g., auditory figure ground)

2. Dichotic (signals presented simultaneously to two ears called binaural separation tasks)

a. Digits

b. Syllables

c. Words

d. Sentences

3. Binaural (signals presented simultaneously to two ears called binaural integration tasks)

a. Binaural fusion (low- and high-filtered speech bands presented simultaneously to opposite ears)

b. Rapidly alternating speech

c. Masking-level differences

Pattern recognition tests are used to assess nondominant hemisphere function and interhemispheric transfer of information. Two examples are the frequency-patterns and duration-patterns tests.^14–16

Auditory Processing Disorders in Clinical Populations

Adults

Research has been conducted in various adult patient populations including persons who are aging^17,18 and those with Parkinson’s disease,¹⁹ chronic alcoholism,²⁰ Alzheimer’s disease,^21–23 multiple sclerosis, head trauma,^24–26 stroke,^{2,27 learning disabilities and reading disorders,28–30} and AIDS.³¹ In all of these patient groups, results of central auditory tests were poorer than predicted on the basis of peripheral hearing levels. There are several purposes for administration of a central auditory test battery to adults³²:

1. In chronic CNS disease, to assess progression and to describe functional impairment

2. In head injury and stroke, to monitor recovery and provide a framework for counseling families

3. In pre- and postoperative brain surgery patients, to determine functional disorders of communication

4. In learning-disabled adults, to describe auditory processing abilities

5. In persons with neurologic disease, to monitor degenerative cognitive function, and to assess the effectiveness of medical treatment

6. In patients with normal hearing who have histories of decreased ability to understand speech, to identify the presence of an APD

7. In aging patients, to study and describe auditory processing abilities related to changes that occur among healthy elderly persons and those with chronic disease.

Identification of central auditory disorders among these patient populations will assist when counseling affected individuals and their families about their communication abilities, identifying necessary changes in the patient’s listening environment (both physical and psychological changes), and determining specific recommendations for rehabilitation.

Children

The literature is full of examples of APDs in children that are related to language, learning, reading, and other developmental disorders. Auditory processing test results provide the following information about children:

1. The test results describe the maturation level of the central auditory pathways, and through longitudinal studies on the same child demonstrate the development of auditory processing abilities.

2. The test results provide data to document the neurologic origin presumed to exist in children with specific learning disabilities.

3. The test results can aid in ruling out abnormalities of the central auditory pathways as contributing to a language-learning problem.

4. The test results describe whether language is appropriately located in the left hemisphere, or whether there is mixed or right hemisphere cerebral dominance for language.

Many variables can affect central auditory test results. Speech intelligibility of distorted speech is reduced by both peripheral and central factors, so that hearing thresholds must always be known before interpreting auditory processing test results. Peripheral hearing loss, both conductive and sensorineural, can result in hearing asymmetry and cochlear distortion. Patients with peripheral hearing loss cannot be tested with most central auditory measures. Even a mild conductive hearing loss results in diminished performance on auditory processing tests. Subjects with low intellectual abilities perform at a level that is commensurate with their mental age, and allowance for that factor must be made when interpreting results of central auditory tests. Normal children and adults who are non-native speakers of English have difficulty performing on sensitized speech tasks presented in English, even years after being immersed in the language.33,34 Marriage et al³⁵ found that even children who speak British-accented English have difficulty understanding American-accented speech on an auditory processing test battery. Examiners should be cautious when administering any test of auditory processing that uses American-accented speech to children who are not native speakers of English.

Regarding conductive hearing loss, in the past there was little available information on the long-term effects of early and prolonged otitis media with static or fluctuating hearing loss on auditory processing abilities. There is some evidence that otitis media can cause auditory learning problems.^36–44 The residual effects can be central auditory processing problems that may cause language and learning delays long after the middle ear disease has been resolved. The relationship between histories of otitis media and auditory processing disorders does not have a perfect correlation and controversy on that topic exists. Nevertheless, it is important to watch children with histories of frequent colds or chronic middle ear disease for signs of auditory language-learning problems.

As with most learning disabilities, APD can exist in combination with or because of other disorders. For example, auditory perceptual problems may stem from neurologic problems or brain injury resulting from head trauma, meningitis or other viral infections, seizure disorders, congenital anomalies such as agenesis of the corpus callosum, low birth weight, or other factors. Maternal drug and alcohol abuse may also result in APD in offspring. Additionally, there are undoubtedly genetic factors involved because many family members have similar educational histories of auditory learning problems. Some children with attention deficit hyperactive disorder (ADHD) have comorbidity with APD, and a diagnostic assessment is sometimes necessary to identify the primary problem. In fact, recent research has focused on attempts to differentiate between APD and ADHD, which have similar behavioral patterns. In general, the literature indicates that the presence of ADHD results in a general reduction of performance on tests of auditory processing. When questioned carefully, there are behavioral differences between the two entities and it is possible to separate them.^45,46 When possible, it is important to differentiate between ADHD and APD because they receive different treatments. Specifically, one does not treat children who have APD with medication.

Behaviors of Individuals with Central Auditory Processing Disorders

The following observations are characteristic of children with auditory processing problems:

1. Most are male.

2. They have normal pure-tone hearing thresholds.

3. They generally respond inconsistently to auditory stimuli. They often respond appropriately, but at other times they seem unable to follow auditory instructions.

4. They have short attention spans and fatigue easily when confronted with long or complex activities.

5. They are distracted by both auditory and visual stimulation. These children are described as being at the mercy of their environment. Unable to block out irrelevant stimuli, they must respond immediately and totally to everything they see, feel, or hear, no matter how trivial.

6. They may have difficulty with auditory localization skills. This may include an inability to discern the source and distance of sound and an inability to differentiate soft and loud sounds.

7. They may listen attentively, but have difficulty following long or complicated verbal commands or instructions.

8. They frequently request that information be repeated.

9. They are often unable to remember information presented verbally for both short-term and long-term memory.

10. They sometimes have a significant history of chronic otitis media.

Other behavioral characteristics of children with central auditory processing disorder (CAPD) include having poor listening skills, taking a substantial amount of time to answer questions, having difficulty relating what is heard to the words seen on paper, and being unable to appreciate jokes, puns, or other humorous twists of language.

In addition to specific auditory behaviors, many of these children have significant reading problems, are poor spellers, and have poor handwriting. They may have articulation or language disorders. In the classroom they may act out frustrations that result from their perceptual deficits, or they may be shy and withdrawn because of the poor self-concept that results from multiple failures. Therefore, appropriate referrals for central auditory testing should be made for patients of any age who exhibit the symptoms listed above.

Test Battery Approach

Examiners can arrive at a diagnosis of APD using a variety of indices. The ASHA Consensus Statement on Central Auditory Processing47 recommends the following approach to assessment:

1. History of the patient’s social, birth, health, developmental status, and auditory behavior

2.

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