Central Auditory Processing Disorder

Etiology

CAPD can be a difficult condition to diagnose, given that its etiology is so broad. Generally, CAPD is linked to a specific lesion or disorder, or the cause may be unknown.

There are several leading causes and risk factors for CAPD.[12] They are as listed below.

• Degeneration of the CANS
  • Most commonly due to age-related changes
• Genetics
• Neurological disease or damage
  • Cerebrovascular disorder - e.g., stroke
  • Brain injury or damage - e.g., meningitis or head trauma
  • Neurodegenerative disorders - e.g., multiple sclerosis
  • Lesions or tumors of the central nervous system (CNS)
  • Epilepsy
  • Neurotoxins - e.g., heavy metals
• Otological disease - e.g., auditory deprivation as a result of chronic otitis media
• Developmental delay
• Prenatal/neonatal risk factors
  • Anoxia
  • Hypoxia
  • Prematurity
  • Drug exposure
  • Hyperbilirubinemia
  • Cytomegalovirus

Epidemiology

Unfortunately, it is difficult to calculate the incidence and prevalence of CAPD due to the lack of standardized diagnostic criteria and guidance from different academic organizations varies. However, some studies have given estimates on prevalence.

In patients over the age of 55 years, the prevalence of CAPD has been estimated to be between 27% and 75%; however, it is as high as 95% in the population over 80.[13][14]

This extreme range of estimates in prevalence is also reflected in the pediatric population. Using different diagnostic criteria causes significant variability in the estimated prevalence of CAPD within the same population group. A 2013 study demonstrated that using nine different sets of diagnostic criteria on a population of children that completed CAPD assessment ranged from a prevalence of 7.3% with the strictest criteria to 96.0% with the most lenient.[15]

CAPD tends to affect males more than females, with a male: female ratio of 2 to 1.[16] Further studies on the epidemiology of CAPD are clearly needed, and standardized criteria that can give a more accurate representation of the actual prevalence of the condition should be developed and universally adopted.

Pathophysiology

The pathophysiology of CAPD is classified into two categories based on coexisting conditions:

1) Neurological conditions

• Tumors of the CANS
• Epilepsy
• Cerebrovascular disease
• Extrinsic damage to the brain
• Prematurity and low birth weight

Very few cases of CAPD in children will have an underlying neurological pathology; nonetheless, CAPD may be the only presenting manifestation of a space-occupying lesion. For this reason, when patients present with CAPD, there must be a high level of clinical suspicion for underlying CNS pathology, and performing a thorough neurological and developmental examination is critical.

2) Delayed central nervous system maturation or other developmental disorders

Despite the fact that the human auditory system is already fully developed at birth, myelination in the higher auditory pathways is still occurring up until the ages of 10 to 12 years.[12] Therefore any form of auditory deprivation, even in diseases such as chronic glue ear, can have disastrous effects on the organization and maturation of the central auditory pathway, a fact that may explain the correlation of CAPD with other developmental and behavioral disorders, such as ADHD.[17]

History and Physical

A thorough history must be taken to determine the nature and type of disorder, along with its impact and functional ramifications. This will also help to delineate the diagnosis and the subsequent treatment plan.

Information gathering should focus on the patient's family and genetic history; the pre-, peri-, and post-natal course; current health, including medications; developmental history, especially communication and listening behavior; psychosocial factors; education; social behavior; and any other hearing concerns.

The history can be obtained directly from the patient, although this may be difficult in a younger patient; therefore, collateral history from the parent or family members is extremely beneficial.

A thorough neurological examination should also be conducted, as other signs may be elicited that could reveal the cause of the presenting symptoms; focal neuropathy, for example, may present if there is a space-occupying lesion.

Patients presenting with CAPD will frequently present with one or several of a set of specific behavioral characteristics. They will be uncertain about what they hear or misunderstand what has been said and have difficulty understanding or listening in loud or noisy environments with frequent requests for repetition.[9] Patients may also have difficulty paying attention, are easily distracted, have difficulty localizing sounds, and have associated learning difficulties.

CAPD symptoms will become apparent in the early school years or later during the more academic stage of patients' lives as their environments change and there is increased demand for focus and concentration. In rare cases, this presentation can manifest a neurological disorder rather than isolated CAPD.[12]

Because individuals with CAPD have primary auditory difficulties, young patients are liable to develop associated language, reading, writing, and spelling disorders. Therefore, it is crucial to recognize that the behavioral characteristics associated with CAPD are not exclusive to this condition and that other diagnoses present similarly, such as ASD, ADHD, learning disorder, and language impairment; these characteristics are not pathognomonic of CAPD.[12]

Evaluation

There are two primary diagnostic modalities: behavioral and electrophysiological. To diagnose CAPD, tests have to evaluate the integrity of the CANS. Combining these modalities allows clinicians to assess the CANS via a bottom-up sensory approach and top-down cognitive evaluation.[9] A sizeable battery of tests can be utilized by an interprofessional team that includes physicians, audiologists, speech and language therapists, and psychologists. An accurate diagnosis requires careful consideration of the following parameters:[9]

• Cognitive, memory, and linguistic assessments
• Medical, education, and developmental histories
• Behavioral tests
• Electrophysiological tests
• Neuroimaging
• Speech and language assessment
• Psychological assessment

Some electrophysiological tests will require specialist audiometry equipment and must be carried out in an acoustically controlled environment under the guidance of trained audiologists. These test results will be interpreted by audiologists, who are responsible for accurately assessing hearing and balance.

Although there is no universally accepted screening program for CAPD, there is still a need to identify patients likely to benefit from further central auditory testing. Screening is performed in children who have shown delays in speech and language development, following which abbreviated test protocols can then be followed. Questionnaires and checklists are available for clinicians to determine a patient's auditory and social behaviors and then ascertain whether a more systematic approach to evaluation needs to be taken.

Initially, a peripheral cause for any hearing loss or behavioral characteristics should be sought, as peripheral auditory disorders, e.g., conductive or sensorineural hearing loss, can similarly affect an individual. However, if there is an identified cause of peripheral hearing loss, it does not preclude further assessment for CAPD. It may, nevertheless, alter the results of further testing - in patients with profound hearing loss, testing cannot be completed. Therefore, tests that utilize stimuli minimally affected by peripheral hearing loss should ideally be used.[18]

To effectively evaluate the CANS, an audiologist must select an appropriate CAPD test battery. Findings from the patient's history, language assessments, cognitive assessments, and peripheral hearing tests will inform the decision of which tests to administer to diagnose or exclude dysfunction of the CANS.[19] 

Appropriate test selection may pinpoint impairments in the neurophysiological processes that occur within the CANS, which will lead to more precisely targeted treatment plans. Tests of auditory discrimination are used to determine the ability to differentiate between similar acoustic stimuli that have different frequencies, intensities, and temporal characteristics. Tests that assess the auditory temporal processing ability monitor how the CANS analyzes acoustic events over time. Additional tests should assess the CANS' ability to separate or integrate (binaural separation or binaural integration) different sounds presented at each ear simultaneously.

The ability to recognize speech in noise, speech in competition, low pass filtered speech, and rapid speech are assessed using monaural low redundancy speech tests. Binaural interaction tests are used to evaluate the CANS' ability to combine inputs that complement each other when simultaneous and otherwise identical stimuli are distributed between the ears with different intensities, timing, or spectral differences.[20]

Essentially, behavioral tests fall into three main categories.

1) Monaural low redundancy

• This type of test presents speech stimuli to one ear, which will either be accompanied by competing signals, e.g., background noise, or degraded in terms of their frequency context. The patient is then asked to identify the correct speech stimulus.[12]

2) Dichotic or binaural interaction tests

• The stimuli are presented to one or both ears, and the patient is asked to identify the side of the stimulus.[12]

3) Temporal tasks

• The patient is asked to identify auditory stimuli in sequencing tasks.

Electrophysiological tests asses the neural processing and functioning of the auditory structures of the CANS from the vestibulocochlear nerve to the auditory cortex.[21] These tests include:

• Auditory brainstem evoked responses
• Middle latency responses
• Late cortical response
• P300
• Mismatch negativity

Electrophysiological tests tend to be preferable in the following circumstances:

• When behavioral tests are not feasible, e.g., in infants and young children or non-English speakers
• Suspicion of a neurological disorder
• To confirm behavioral test findings
• If behavioral tests are inconclusive

Treatment / Management

Each patient will have specific needs and, therefore, a unique treatment plan devised by a multidisciplinary team. The overall aim of any treatment plan is to allow the patient to communicate effectively in everyday situations, such as school and work environments.

There are currently three main treatment approaches for patients with CAPD that are recognized by the American Speech Language Hearing Association:

1. Direct skills remediation
2. Compensatory strategies
3. Environmental modifications

These three methods complement one another to provide auditory training in a bottom-up fashion along with the recruitment of higher-order brain functions, i.e., the top-down approach that helps patients compensate for the disorder. Changing the patients' environment further maximizes their opportunities to process auditory stimuli effectively.

Direct skills remediation targets the individual tasks of auditory processing. These skills may help with difficulties in the following:[22]

• Auditory discrimination
• Phoneme discrimination
• Temporal audition
• Auditory pattern recognition
• Sound localization and lateralization
• Inability to recognize auditory stimuli in the presence of background noise
• Recognition of different auditory information when it is presented into each ear simultaneously

Computer-based training methods are used for direct skills remediation and can address both auditory system and language deficits. These programs are designed to utilize the brain's plasticity within the auditory system.[23] Another category of direct skills remediation is exercises that train interhemispheric transfer, the process responsible for binaural hearing and processing.[24](B3)

Compensatory strategies are referred to as the top-down approach, and they effectively reduce the impact of CAPD on patients' language, cognition, and learning. These strategies target higher-order abilities such as memory and attention and strengthen them to develop communication, improve social and listening skills, and optimize learning outcomes. Compensatory strategies can be split into two different categories: metalinguistic and metacognitive.

Metalinguistic strategies focus on predicting elements in a message (schema induction) and using grammar to identify links and relationships between sentences. Patients can use a better understanding of context and active listening to create a broader vocabulary. Metacognitive strategies, in contrast, are more internally focused, looking at self-instruction, metamemory skills, organization skills, problem-solving, and assertiveness training.[25](B3)

Environmental modifications allow the patient to improve access to any auditory information with which they are presented. This approach can be split into two categories:

• Enhancement of the listening environment and the auditory signal (bottom-up)
• Management approaches (top-down)

These two methods aim to improve the overall clarity and the audibility of the signal that is received and to improve access to the signal's information. Enhancement of the listening environment involves making physical modifications to the patient's physical surroundings to improve the acoustics, such as covering reflective surfaces, removing any objects that generate background noise, placing absorption materials in open spaces, and using acoustic dividers.

Management approaches try to develop the compensatory ability of the higher-order neural processing centers and support the areas of the CANS that are deficient. Examples of management approaches include: advising speakers to slow their speech, emphasizing keywords, checking to understand, and using visual aids and written support.

Differential Diagnosis

Other clinically distinct disorders present similar to CAPD and can cause similar behavioral auditory function test results, producing comparable functional listening difficulties. For example, patients diagnosed with ADHD often present with deficits in tasks designed to challenge the CANS.[26] 

Patients with any learning disability, developmental delay, or psychological disorders, e.g., ADHD or ASD, should be screened for CAPD. Therefore, a multidisciplinary assessment is required to distinguish diagnoses from one another. Children, in particular, with any of these conditions not only present similarly but also tend to have similar scores on cognitive, communication, and auditory evaluations.[27]

Prognosis

The management of CAPD does have a certain degree of controversy surrounding it, as many of the proposed interventions are based on assumptions. One such assumption is that the brain's ability to process incoming auditory signals follows a bottom-up pathway, i.e., sensory-to-cognitive, and therefore deficits in the auditory process result in language impairments. It would then follow that attempting to rectify auditory deficits would lead to an improvement in language abilities.[28] 

While this may not necessarily be true, there is sufficient evidence to indicate that current management strategies for CAPD benefit patients; future research is required to determine how and why these methods work.[12]

Prognostically, children diagnosed with CAPD before their teenage years can "grow out" of the disorder because auditory information is processed in areas of the brain that can still develop until around the age of 13. Auditory processing can still improve in other patients receiving therapy for CAPD; however, in older patients, the treatment should be orientated towards developing new coping mechanisms instead of relying on neural plasticity. It is recommended that patients diagnosed with CAPD be retested every 1 to 2 years to ascertain whether or not there have been any improvements.

Complications

CAPD predominantly affects children's speech and language development, resulting in difficulty in communication. These difficulties may also affect the ability to read and write, resulting in dyslexia or other conditions associated with talking and understanding, e.g., ASD. In adults, CAPD may cause similar complications, but difficulties may manifest in a more limited fashion, such as in processing sound localization.