Auditory Processing Disorder

Possible Causes

Factors associated with APD include a history of middle ear disease with accompanying transient conductive hearing loss, neurotoxic factors such as lead exposure, neonatal risk factors such as prematurity and anoxia, and altered myelination, synaptic connectivity, and neurotransmitter function associated with nutritional deficiencies. Neuroimaging studies show an association between auditory processing ability and cortical white matter and brain connectivity. These and other neuromorphological abnormalities affecting central auditory pathways could account for differences in auditory-evoked potentials observed in people with APD compared with age-matched controls with no hearing difficulties.

Hearing Difficulties Associated With APD

A common manifestation of APD is difficulty following spoken information when listening conditions are poor—for example, as a result of background noise or room reverberation. APD is also referred to as central auditory processing disorder, abbreviated as (C)APD or CAPD. People with APD can have difficulty picking up new spoken information and as a result may frequently ask for repetitions. In classrooms where background noise and reverberation levels are often high, it is difficult for children with APD to follow instructions presented orally, but these children have minimal difficulties when listening conditions are ideal.

Adults with APD may be aware of their auditory processing difficulties only when listening in background noise or trying to listen to multiple [Page 229]speech streams. Since these listening difficulties also occur in people with high-frequency hearing loss, it is important that pure-tone hearing sensitivity is assessed prior to APD being investigated, especially in adults who are likely to develop age- or noise-related (or both) cochlear hearing loss.

People diagnosed with APD typically have normal hearing sensitivity for pure tones, but they have difficulties when tested using more sophisticated listening tasks. These include discriminating tones of different pitch (frequency discrimination) or detecting changes in the timing of sounds on the milliseconds time scale (temporal discrimination or resolution) or recognizing speech that has been distorted or that is presented at the same time as other competing sounds.

It is possible to have APD in addition to peripheral conductive or sensorineural hearing loss, but this may be more difficult to diagnose because stimulus presentation levels need to be adjusted to ensure that reduced audibility is not affecting APD test results. Several APD tests allow valid testing of people with mild to moderate audiometric hearing loss, with appropriate adjustments of stimulus presentation levels.

Specific types of APD have been identified, including spatial processing disorder, temporal processing disorder, and amblyaudia (hearing difficulty because of asymmetric hearing loss). Amblyaudia is diagnosed when dichotic listening tests show greater than expected ear asymmetry. Dichotic tests involve simultaneous presentation of different auditory stimuli to left and right ears. Listeners report back words or sentences heard in both ears. Stimuli with minimal linguistic loading are used in tests such as the Dichotic Digits Test. In typically developing young children, there is an ear asymmetry, with the right ear showing a performance advantage due to the connection to the language-dominant left hemisphere.

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