Auditory Processing Disorder (APD) is a hearing disorder in which the ears process sound normally but the hearing centres and circuits of the brain don’t correctly process incoming information. This can affect understanding, especially in challenging listening situations such as in the presence of other distracting sound, or when listening to complex information or instructions. APD is thus often referred to as a hearing problem in which “the brain can’t hear”.

The formal definition of APD adopted in the New Zealand Guidelines on Auditory Processing Disorder (2019) is as follows.

Auditory processing disorder is a generic term for hearing disorders that result from atypical processing of auditory information in the brain. Auditory processing disorder is characterised by persistent limitations in the performance of auditory activities and has significant consequences for participation.

The NZ definition is adapted from the definition in the Canadian Guidelines on APD (Canadian guidelines on auditory processing disorder in children and adults: Assessment and intervention, 2012). The Canadian definition reflects the World Health Organisation approach which considers activity and participation limitations that result from disabilities.

The American Speech Language Hearing Association (2005) and the American Academy of Audiology (2010) define APD as follows:

APD refers to difficulties in the perceptual processing of auditory information in the central nervous system and the neurobiologic activity that underlies that processing and gives rise to the electrophysiologic auditory potentials.

The definition of the Committee of UK Medical Professionals Steering the UK Auditory Processing Disorder Research Program is as follows:

"APD results from impaired neural function and is characterised by poor recognition, discrimination, separation, grouping, localisation, or ordering of speech sounds. It does not solely result from a deficit in general attention, language or other cognitive processes."

The terms Auditory Processing Disorder (APD), Central Auditory Processing Disorder (CAPD) and (Central) Auditory Processing Disorder ((C)APD) are interchangeable. An earlier name for APD was “central deafness”. The term “Central” emphasises that the hearing deficits are in the central auditory nervous system (i.e., the brain), and not in the outer, middle or inner ear. The terms “CAPD” and “(C)APD” are commonly used in the USA, despite the Consensus Conference on the Diagnosis of Auditory Processing Disorders in Dallas, 2000, adopting the term APD. The term “APD” is commonly used in the UK, Canada, Australia and New Zealand. In 2016 the term “CAPD” was recognised with a US International Classification of Diseases (ICD) code, ICD-10 H93.25. Current terminology discussion focuses on developing a range of terms for central hearing and listening difficulties, with less use of a “disorder” label.

APD is a form of hearing impairment, but standard hearing tests aren’t sensitive to APD. Thus many people with APD have normal hearing thresholds for pure tones, that is, a normal pure tone audiogram. This unfortunately leads to people with APD sometimes being incorrectly described as having “normal hearing” and APD. People with APD may have a normal pure tone audiogram, but they should not be described as having normal hearing.

To hear when there is more than one sound present we need to be able to disregard some sounds and concentrate on others. Both ears need to contribute in challenging listening conditions but our hearing system can’t work optimally if one ear dominates.

Amblyaudia is a hearing weakness on one side when the cause is not in the ear itself but in the brain. Amblyaudia is the hearing equivalent of amblyopia or ‘lazy eye’. Up to half of children with APD show weaker performance in one ear on certain tests. The two ears don’t work together for optimal hearing.

SoundSkills audiologists carry out specialised “dichotic” testing to detect amblyaudia. In dichotic tests, different information is played to the two ears simultaneously. Many children with APD score lower in one ear on dichotic tests (amblyaudia). Usually the left ear is the weak one. The hearing centres and pathways associated with the dominant ear may actually be suppressing auditory input from the weak ear.

Fortunately amblyaudia can be improved or fully corrected with sometimes as little as one month of specialised auditory training. The training involves attending to speech signals in the weak ear while competing speech is slowly increased in volume in the dominant ear. SoundSkills carries out research on Amblyaudia and is the leading APD clinic in New Zealand for Amblyaudia assessment and treatment.

If one or more of the following hearing or listening problems are present, and standard hearing test results are normal, APD may be the cause and an auditory processing evaluation may be indicated.

  • difficulty following spoken directions unless they are brief and simple
  • difficulty attending to and remembering spoken information
  • slowness in processing spoken information
  • difficulty understanding in the presence of other sounds
  • being overwhelmed by complex or “busy” auditory environments e.g. classrooms, shopping malls
  • undue sensitivity to loud sounds or noise
  • poor listening skills
  • preference for loud television volume
  • insensitivity to tone of voice or other nuances of speech

An auditory processing evaluation should also be considered if any of the following commonly related conditions are present.

  • brain injury
  • neurological disorders affecting the brain
  • history of frequent or persistent middle ear disease (otitis media, ‘glue ear’)
  • difficulty with reading or spelling
  • suspicion or diagnosis of dyslexia
  • suspicion or diagnosis of language disorder or delay

Some causes of APD are listed below:

  • hereditary developmental abnormalities
  • maturational delay
  • antenatal, perinatal and postnatal factors including prematurity and low birth weight, prenatal anoxia, prenatal exposure to cigarette smoke or alcohol, hyperbilirubinemia
  • diseases, toxins and neurological conditions affecting the brain including space-occupying lesions; Moyamoya disease and other cerebrovascular disorders; multiple sclerosis and other neurodegenerative diseases; bacterial meningitis; herpes simplex; encephalitis; Landau Kleffner Syndrome and other seizure disorders; Lyme disease; metabolic disease; heavy metal exposure; solvent exposure
  • traumatic brain injury
  • blast injury
  • auditory deprivation
  • aging

There is growing evidence, some from animal research, indicating that auditory deprivation secondary to otitis media during critical early developmental periods can result in central auditory deficits including amblyaudia.

(NZ Guidelines on Auditory Processing Disorder, 2019)

APD frequently occurs in conjunction with other conditions or disorders including in particular dyslexia, autism spectrum disorder (ASD), and developmental language disorder (DLD). In a University of Auckland study 94% of children with APD also had Language Impairment and/or Reading Disorder. APD sometimes co-occurs with attention disorders (ADD and ADHD). Because APD can affect the correct and rapid recognition of phonemes, the sounds of speech, it can be an underlying cause of language, phonological awareness, spelling, and reading problems including dyslexia.

Conditions commonly associated with APD include:

  • brain injury
  • neurological disorders affecting the brain
  • history of frequent or persistent middle ear disease (otitis media, ‘glue ear’)
  • difficulty with reading or spelling
  • dyslexia
  • autism spectrum disorder
  • language disorder or delay

People with APD miss parts of speech if it is too fast or too complex or if there is other competing sound present. They may completely miss, or misunderstand, spoken information. We need to be able to distinguish sounds of importance from all the sound around us. We need to be able to rapidly, correctly and automatically identify all the different phonemes that make up our language without even thinking about it, irrespective of the voice, tone, accent, or softness or loudness of the speaker. People with APD have difficulty processing the sounds of speech sufficiently quickly. Apart from not always hearing correctly, children growing up with APD have difficulty correctly learning the phonemes which make up our language. Poor phoneme recognition and phonological awareness in turn contribute to learning difficulties, especially with spelling and reading.

People with APD may also have poor skills at detecting nuances of language such as changes in the meaning of statements denoted by a change in pitch or emphasis, for example to change a statement into a question, a demand or a joke.

Because APD can be caused by dysfunction in any of a number of hearing pathways, circuits and centres in the brain, there are many possible effects. Each case of APD will have its own areas of weakness. Here are some effects of APD that can be seen on auditory processing tests.

  • unilateral weakness or inhibition of one ear (amblyaudia)
  • impaired pattern recognition (being able to recall and repeat simple patterns of high and low pitch notes)
  • poor directional (spatial) hearing and the related ability to hear against background noise
  • incorrect hearing of very fast changes at the start of speech sounds, which can affect correctly identifying the sound
  • inability to detect rapid changes in speech sounds (less than 20 milliseconds/thousandths of a second) as measured by “gap detection” tests
  • poor memory of what has just been heard and the order of the sounds (auditory memory)
  • reduced ability to maintain attention to auditory information (auditory attention)
  • difficulties in identifying and discriminating between speech sounds (phoneme discrimination, phonological awareness)

Differences in brain function in people with APD can be seen on physiological tests, for example:

  • poor consistency of auditory brainstem electrical responses to speech sounds
  • delayed electrical responses to certain speech sounds at the cortex of the brain
  • reduced brain activity to certain sounds on Functional Magnetic Resonance Imaging (FMRI)

Adults with APD, particularly if it arose in adulthood so they have prior experience of good hearing, can provide insight into the experience of hearing with APD.

Dr Louise Carroll QSO, JP, GDPPA , MPM, previously Chief Executive Officer of the National Foundation for Deaf and Hard of Hearing, has Auditory Processing Disorder and uses hearing aids and a remote microphone (RM) system. She describes her hearing experience as follows.

“Without my hearing aids or RM system, speech seems fast, fragmented and confusing. Voices lack tonality. My directional hearing is poor and voices from behind are particularly difficult to hear. It’s very difficult to distinguish a voice from any other sound that is present. For example, if the refrigerator switches on (a sound barely noticeable to most people) it seems to me to swamp anyone speaking. With my hearing aids I hear much better, losing only perhaps 25% of speech. With both my hearing aids and RM system I can usually hear 100%. But I am still exhausted from listening at the end of the work day and want to take my hearing aids off as soon as I get home.”

Click here to listen to an Audio Simulation of APD

A number of factors affect the age at which APD can be diagnosed with certainty. The central auditory system continues to mature into adolescence. In children under seven years there is considerable variance in the maturation of the auditory system. Also some tests have been evaluated and proven on younger children than others (“test norms”).

With current tests Auditory Processing Disorder can be readily diagnosed from about six years of age and considerable information about auditory processing skills can be measured at age five. Some information can be determined at age four. Below age seven however a confirmed diagnosis of APD should be made with caution. Sometimes where there are strong indicators of APD at a young age the audiologist may make a “provisional” diagnosis of APD or state that the child is “at risk” of APD. What is important is that as much information as possible is collected and treatment is commenced as early as possible if the child is clearly experiencing auditory impairment. The earlier treatment begins the better.

The results of auditory processing tests cannot be properly evaluated in isolation. In order to diagnose Auditory Processing Disorder in a child the audiologist must take into account any factors that could affect the APD test results including:

  • “peripheral” hearing loss (hearing loss in the outer, middle or inner ear)
  • cognitive ability
  • language ability

Cognitive and language ability must be sufficient to complete the tests. Further, if cognitive or language skills are below (or above) the typical range, possible influences on the APD test results must be considered in reaching a diagnosis.

If basic audiological assessment and cognitive and language evaluations by specialists such as audiologists, educational psychologists and speech-language therapists respectively are not provided, evaluation should also include:

  • standard hearing tests
  • cognitive screening
  • language screening

Questionnaires about auditory skills and listening behaviour are also usually requested from the teacher and parents to provide additional information about hearing ability in school and other environments.

In the minority of cases where the Auditory Processing Disorder is due only to maturational delay the child may grow out of it. But there is no sure way of knowing if maturational delay is the cause. Meanwhile the child is losing ground at school. So treatment to help the child catch up in auditory skills and language development is still usually advisable.

Yes. Because of neuroplasticity APD is very treatable. Some treatments address the hearing disorder itself, and some treat the language and learning consequences that result from not having heard correctly in the past.

Treatments for APD can be grouped into three types:

  • Auditory training
  • Amplification
  • Language therapy

Auditory training encompasses a variety of in-clinic and home-based hearing exercises, activities, games and software programmes to take advantage of neuroplasticity to improve hearing skills. Audiobook listening and learning a musical instrument (including voice) also fall into this category. Treatments need to be tailored to each individual’s needs. Unfortunately not all auditory training programmes and therapies on offer are evidence-based.

Amplification encompasses treatments using hearing technologies to mildly amplify spoken language and reduce the effect of background noise. Remote microphone (RM) hearing aid systems are commonly used with children. With older children and adults conventional hearing aids with an accessory remote microphone are often more suitable. Remote microphones markedly improve ability to hear against background noise. Due to neuroplasticity, use of amplification over time can lead to permanent improvement in hearing skills.

The term “language therapy” is used in a broad sense in the context of APD treatment to encompass phonemic and phonological perception training, phonological awareness, and any aspects of language therapy a particular child may require. As well as language therapy, phonics and reading tuition may be needed by children with APD.

Auditory training and amplification directly treat the hearing disorder. Language therapy addresses consequences of the hearing disorder.

Remote microphone hearing aid (RMHA) systems consist of small hearing aids worn by the child and a small transmitter microphone worn by the speaker (for example; parent, teacher, coach, friend). The remote microphone transmits the speaker’s voice direct into the hearing aids. RMHA systems (previously referred to as FM systems) have a double effect. RMHA systems assist children to hear while at the same time stimulating neuroplastic changes in the brain leading to long term improvement in hearing ability.

RMHA systems greatly improve the ability of children with APD to hear, learn and participate in multiple life situations. Benefits that have been verified in research studies include improved classroom attention and participation, academic achievement, phonological awareness, reading and social development. Improvements in confidence and self-esteem are seen, and children are less fatigued after school.

RMHA systems assist children to hear while at the same time stimulating neuroplastic changes in the brain leading to long term improvement in hearing ability. This therapeutic effect of using RMHA systems means that most children may not need hearing assistance long term. In a study carried out by SoundSkills and the University of Auckland the average duration of use was two years and nine months but individual cases varied markedly. In a small proportion of cases use of hearing technology may need to continue through tertiary education and even into employment.

Auditory training (such as amblyaudia correction or training of hearing in noise) directly addresses auditory disorders, and fitting of remote microphone hearing aid systems both assists hearing in challenging conditions and has been shown to also improve auditory skills.

Alongside these measures to address hearing difficulties, language therapy is extremely important in treating the effects of the hearing impairment such as deficient phonological awareness and its consequences on comprehension, spelling, vocabulary, language and literacy. Language therapists can also treat abnormal appreciation of pitch and intonation (prosody), another common consequence of APD. Poor prosody perception can result in misunderstanding whether spoken language is intended to convey subtleties such as a demand, a question or humour. Literacy (reading) therapy may also be needed for some children with APD.