Auditory processing disorder
Auditory processing disorder is a neurodevelopmental disorder affecting the way the brain processes sounds. Individuals with APD usually have normal structure and function of the ear, but cannot process the information they hear in the same way as others do, which leads to difficulties in recognizing and interpreting sounds, especially the sounds composing speech. It is thought that these difficulties arise from dysfunction in the central nervous system.
A subtype is known as King–Kopetzky syndrome or auditory disability with normal hearing, characterised by difficulty in hearing speech in the presence of background noise. This is essentially a failure or impairment of the cocktail party effect found in most people.
The American Academy of Audiology notes that APD is diagnosed by difficulties in one or more auditory processes known to reflect the function of the central auditory nervous system. It can affect both children and adults, and may continue to affect children into adulthood. Although the actual prevalence is currently unknown, it has been estimated to impact 2–7% of children in US and UK populations. Males are twice as likely to be affected by the disorder as females.
Neurodevelopmental forms of APD are different than aphasia because aphasia is by definition caused by acquired brain injury. However, acquired epileptic aphasia has been viewed as a form of APD.
Signs and symptoms
Individuals with this disorder may experience the following signs and symptoms:- speaking louder or softer than is situationally appropriate;
- difficulty remembering lists or sequences;
- needing words or sentences to be repeated;
- impaired ability to memorize information learned by listening;
- interpreting words too literally;
- needing assistance to hear clearly in noisy environments;
- relying on accommodation and modification strategies;
- finding or requesting a quiet work space away from others;
- requesting written material when attending oral presentations; and
- asking for directions to be given one step at a time.
Relation to attention deficit hyperactivity disorder
| ADHD | APD |
| 1. Inattentive | 1. Difficult hearing in background noise |
| 2. Distracted | 2. Difficulty following oral instructions |
| 3. Hyperactive | 3. Poor listening skills |
| 4. Fidgety or restless | 4. Academic difficulties |
| 5. Hasty or impulsive | 5. Poor auditory association skills |
| 6. Interrupts or intrudes | 6. Distracted |
| 7. Inattentive |
There is a co-occurrence between ADHD and APD. A systematic review published in 2018 detailed one study that showed 10% of children with APD have confirmed or suspected ADHD. It also stated that it is sometimes difficult to distinguish the two, since characteristics and symptoms between APD and ADHD tend to overlap. The systematic review also described this overlap between APD and other behavioral disorders and whether or not it was easy to distinguish those children that solely had auditory processing disorder.
Relation to developmental language disorder and developmental dyslexia
There has been considerable debate over the relationship between APD and developmental language disorder, previously called specific language impairment.SLI is diagnosed when a child has difficulties with understanding or producing spoken language, and the cause of these difficulties is not obvious. The child is typically late in their language development and may struggle to produce clear speech sounds and produce or understand complex sentences. Some theorize that SLI is the result of auditory processing problems. However, this theory is not universally accepted; others theorize that the main difficulties associated with SLI stem from problems with the higher-level aspects of language processing. Where a child has both auditory and language problems, it can be difficult to sort out the causality at play.
Similarly with developmental dyslexia, researchers continue to explore the hypothesis that reading problems emerge as a downstream consequence of difficulties in rapid auditory processing. Again, cause and effect can be hard to unravel. This is one reason why some experts have recommended using non-verbal auditory tests to diagnose APD. Specifically regarding neurological factors, dyslexia has been linked to polymicrogyria, which causes cell migrational problems. Children that have polymicrogyri almost always present with deficits on APD testing. It has also been suggested that APD may be related to cluttering, a fluency disorder marked by word and phrase repetitions.
Some studies found that a higher than expected proportion of individuals diagnosed with SLI and dyslexia on the basis of language and reading tests also perform poorly on tests in which auditory processing skills are tested. APD can be assessed using tests that involve identifying, repeating, or discriminating speech, and a child may perform poorly because of primary language problems. In a study comparing children with a diagnosis of dyslexia and those with a diagnosis of APD, they found the two groups could not be distinguished. Analogous results were observed in studies comparing children diagnosed with SLI or APD, the two groups presenting with similar diagnostic criteria. As such, the diagnosis a child receives may depend on which specialist they consult: the same child who might be diagnosed with APD by an audiologist may instead be diagnosed with SLI by a speech-language therapist, or with dyslexia by a psychologist.
Causes
Acquired
Acquired APD can be caused by any damage to, or dysfunction of, the central auditory nervous system and can cause auditory processing problems. For an overview of neurological aspects of APD, see T. D. Griffiths's 2002 article "Central Auditory Pathologies".Genetics
Some studies have indicated an increased prevalence of a family history of hearing impairment in these patients. The pattern of results is suggestive that auditory processing disorder may be related to conditions of autosomal dominant inheritance. In other words, the ability to listen to and comprehend multiple messages at the same time is a trait that is heavily influenced by genes. These "short circuits in the wiring" sometimes run in families or result from a difficult birth, just like any learning disability. Inheritance of auditory processing disorder refers to whether an individual inherits the condition from their parents, or whether it runs in families. Central auditory processing disorder may be hereditary neurological traits from the mother or the father.Developmental
In the majority of cases of developmental APD, the cause is unknown. An exception is acquired epileptic aphasia or Landau–Kleffner syndrome, where a child's development regresses, with language comprehension severely affected. The child is often thought to be deaf, but testing reveals normal peripheral hearing. In other cases, suspected or known causes of APD in children include delay in myelin maturation, ectopic cells in the auditory cortical areas, or genetic predisposition. In one family with autosomal dominant epilepsy, seizures which affected the left temporal lobe seemed to cause problems with auditory processing. In another extended family with a high rate of APD, genetic analysis showed a haplotype in chromosome 12 that fully co-segregated with language impairment.Hearing begins in utero, but the central auditory system continues to develop for at least the first decade after birth. There is considerable interest in the idea that disruption to hearing during a sensitive period may have long-term consequences for auditory development. One study showed thalamocortical connectivity in vitro was associated with a time sensitive developmental window and required a specific cell adhesion molecule for proper brain plasticity to occur. This points to connectivity between the thalamus and cortex shortly after being able to hear as at least one critical period for auditory processing. Another study showed that rats reared in a single tone environment during critical periods of development had permanently impaired auditory processing. In rats, "bad" auditory experiences, such as temporary deafness by cochlear removal, leads to neuron shrinkage. In a study looking at attention in APD patients, children with one ear blocked developed a strong right-ear advantage but were not able to modulate that advantage during directed-attention tasks.
In the 1980s and 1990s, there was considerable interest in the role of chronic otitis media in causing APD and related language and literacy problems. Otitis media with effusion is a very common childhood disease that causes a fluctuating conductive hearing loss, and there was concern this may disrupt auditory development if it occurred during a sensitive period. Consistent with this, in a sample of young children with chronic ear infections recruited from a hospital otorhinolaryngology department, increased rates of auditory difficulties were found later in childhood. However, this kind of study will have sampling bias because children with otitis media will be more likely to be referred to hospital departments if they are experiencing developmental difficulties. Compared with hospital studies, epidemiological studies, which assesses a whole population for otitis media and then evaluate outcomes, found much weaker evidence for long-term impacts of otitis media on language outcomes.