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LESSON 251

Auditory processing disorder

Year 3
Spring semester
Auditory Processing Disorders
Auditory processing
Duration: 11:44

What should we think about a child with a normal audiogram who struggles to understand speech in noise? In this lesson we cover auditory processing disorder (APD) through the ASHA (2005) and BSA definitions, its symptoms, its links with attention, language and learning difficulties, and the debated points in diagnosis.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn what auditory processing disorder is and how guidelines define it.
Then we will look at its links with attention, language and learning difficulties, and the debated points in diagnosis.
Auditory processing disorder is usually referred to as APD.
Put simply, APD is a difficulty not in hearing sound, but in how the brain processes the sound that is heard.
These people's pure tone audiogram is often normal; the problem is not the ear's threshold sensitivity.
The difficulty involves processing in the central auditory pathways, from the auditory nerve to the auditory cortex.
This is why the term central auditory processing disorder is also used in the literature.
The problem may appear not only with speech but also in processing non-speech sounds, such as tone sequences.
However, its definition, its boundaries and even whether it is a separate disorder have long been debated.
In its 2005 technical report, the American Speech-Language-Hearing Association, or ASHA, defines APD by a set of auditory processes.
The first are the skills of locating a sound source and placing a sound to one side within the head, called lateralization.
The second is the ability to tell similar sounds apart, called auditory discrimination.
Next come the skills of recognizing sound patterns and processing the temporal aspects of sound.
Another process is listening when different signals reach the two ears at once, or when a competing sound is in the background.
The last is understanding a degraded signal, for example one that is filtered or time-compressed.
According to ASHA, APD shows as poor performance in one or more of these processes, not explained by language or cognitive problems alone.
ASHA defines APD as a difficulty in the perceptual processing of auditory information in the central nervous system.
This definition is built on the measurable auditory processes we just listed.
Diagnosis rests on a battery of behavioral and, where needed, electrophysiological tests that measure these processes.
The British Society of Audiology, or BSA, defines APD as a condition arising from impaired neural function.
By the BSA definition, recognition, discrimination, separation, localization or ordering of speech and non-speech sounds is poor.
The BSA especially stresses everyday listening difficulty and how it interacts with higher processes such as attention and language.
It also divides APD into three subtypes according to origin.
Developmental APD appears in childhood without a clear cause and occurs with normal hearing.
This is the type most often seen in the clinic and the most debated.
Acquired APD develops after an event such as head trauma, stroke or a neurodegenerative disease.
Secondary APD is linked to a peripheral hearing problem; for example, it may follow long-lasting middle ear fluid in early childhood.
This distinction helps in understanding the cause and making the right referral.
Auditory processing happens not at a single point, but across a whole network from the cochlear nucleus to the cortex.
The signal from the cochlea is processed again and again, rapidly, in the brainstem.
The superior olivary complex first compares the input from the two ears; this is the basis of sound localization.
Midbrain centers such as the inferior colliculus combine timing and frequency information.
The auditory cortex decodes the pattern, order and meaning of sounds.
When different sounds reach the two ears, transfer between the two cerebral hemispheres also becomes important.
Inefficiency in any part of this network can cause listening difficulty without affecting the audiogram.
The most common complaint is a child who copes in quiet but struggles to understand speech in a busy classroom or in noise.
Following multistep verbal instructions given one after another can be difficult.
The child may often ask for repetition, saying what or pardon, or respond slowly to spoken questions.
Their attention wanders easily, and they tire quickly in activities that require long listening.
Difficulty locating where a sound comes from may also be reported.
Some children also have difficulties with reading, spelling and music.
None of these symptoms is specific to APD; the same complaints can occur in many other conditions.
This example shows the audiogram of a child reported to struggle with understanding in noise.
In both ears, all thresholds are in the normal range.
So this child hears sounds; but this audiogram tells us nothing about how sound is processed in the brain.
A normal audiogram does not make an APD diagnosis; it only shows that a peripheral loss does not explain the complaints.
The next step is to plan a more comprehensive evaluation to understand the cause of the complaints.
APD often occurs together with other developmental conditions; this is called comorbidity.
The most common co-occurrence is with attention deficit hyperactivity disorder, or ADHD.
Poor performance on auditory processing tests is also often reported in children with developmental language disorder.
Dyslexia and other learning difficulties also frequently occur together with APD.
Similar listening difficulties are also described in some children with autism spectrum disorder.
This link works both ways: APD can worsen other difficulties, and other conditions can lower auditory test performance.
So assessment is team work, with the audiologist, speech and language therapist, psychologist and teacher working together.
In peripheral hearing loss, the audiogram shows a threshold loss.
Depending on the degree of loss, the person may miss soft sounds even in quiet.
Amplification can make the missing sounds audible again.
In APD, by contrast, the audiogram is often normal.
The difficulty becomes clear especially in noise, in reverberant rooms and with fast speech.
The problem is not in sound getting through but in processing it, so simply making sound louder is often not enough.
The most basic problem in diagnosing APD is the lack of an accepted gold standard test.
Diagnosis rests on the results of a battery of several tests and how they are interpreted.
Most of these tests are behavioral, so they are affected by the child's attention, memory, language skills and motivation.
Some researchers have argued that poor test performance reflects attention and cognitive factors more than an auditory problem.
At the center of this debate is modality specificity: is the difficulty only auditory, or is it also present in vision?
Because diagnostic criteria differ between centers, prevalence estimates in children also vary over a wide range.
A frequently cited estimate is about 2 to 5 percent, but much lower values have also been reported.
So an APD diagnosis should be made carefully, within a team, and with its limits in mind.
Assessment always starts with a full peripheral hearing evaluation.
This includes pure tone and speech tests, tympanometry, acoustic reflexes and otoacoustic emissions.
This step is needed to rule out a peripheral loss, or a condition such as auditory neuropathy, that could explain the complaints.
Because results vary, many guidelines recommend diagnostic testing from about age 7.
By a commonly used criterion, performance 2 standard deviations below the age norm on at least two tests supports the diagnosis.
Test results are interpreted together with the history, teacher and parent observations, and other specialists' assessments.
For support, environmental changes stand out, such as front-row seating, noise reduction and remote microphone systems.
In addition, auditory skills training and language-based strategies can be planned by team decision.
A tip from the field: auditory processing tests are very sensitive to attention and motivation.
A child who tires at the end of a long session may score low even without an auditory problem.
So split testing into short parts, give breaks, and before each test check with practice items that the instructions are understood.
Noting the child's behavior during testing is very valuable when interpreting the results.
No diagnosis should rest on a session where attention clearly wandered; if needed, repeat the test on another day.
We saw that APD is a difficulty processing heard sound in the central auditory pathways, often with a normal audiogram.
The ASHA definition focuses on measurable auditory processes, the BSA definition on neural function and everyday listening difficulty.
APD often occurs with attention, language and learning difficulties and is hard to tell apart from them.
Because there is no gold standard test, diagnosis should be made carefully with a test battery and team assessment.
Test yourself: when auditory processing disorder is suspected, what is the first step of the assessment?
The correct answer is B: a full peripheral hearing evaluation.
Until a peripheral loss is ruled out, the results of central auditory tests cannot be interpreted correctly.
In the next lesson we will examine the APD test battery: dichotic listening, temporal pattern and low-redundancy speech tests.
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