Auditory processing disorder is defined as a specific deficit in the processing of auditory information along the central auditory nervous system, including bottom-up and top-down neural connectivity (20, 23). Hearing sensitivity is in the majority of cases normal as measured by the pure tone audiometry. The deficits are thought to be infrequently associated with a macroscopic structural brain lesion identifiable by brain imaging at least in the pediatric population. However, there are pediatric cases with APD with established subtle structural abnormalities of the central auditory pathway in the presence or absence of other developmental disorders [e.g., Ref. (32–35)]. Atypical auditory processing may also be reflected in abnormal Auditory Brainstem Responses (ABR) recording in a limited number of cases, suggesting neural conduction deficits beyond the auditory nerve level (36). Auditory processing together with but beyond the early stages of cochlear amplification and auditory nerve transmission will impact on auditory perception of speech and of other complex auditory stimuli (37). Perception of such stimuli is usually not assessed by classical audiological evaluation. Thus assessing both audibility and perception of sounds with baseline audiometric tests [audiometry, Otoacoustic Emissions (OAE), ABR] in conjunction with central auditory processing tests provides a more ecological approach to auditory perception and hearing in everyday life.

The term “hidden hearing loss,” or supraliminal hearing disorders describe disorders that concern more temporal aspects of hearing impacting on the intelligibility of degraded speech by noise, reverberations, speed, limited articulation, or the localization of sounds sources rather than pure tone audiometric thresholds (38). It arises due to pathologies between the inner hair cells and auditory nerve fibers entry to the brainstem. There are also cases of progressive auditory neuropathies (e.g., in the presence of genetic or other peripheral neuropathies) that first present with auditory perceptual and processing deficits before the disease evolves and affects pure tone sensitivity (39). Conversely, there are cases diagnosed with ANSD at the time of neonatal hearing screening in whom the ABR normalize later on (40) and the audiological and clinical profile fulfils APD rather than ANSD criteria. This consensus acknowledges the overlap between APD, HHL, and ANSD, which may not always be easy to resolve with current audiological batteries; however, clinicians should attempt to localize the auditory deficit within the auditory nervous system as best as they can. It should be noted, however, that the majority of the authors of this position statement prefer the term APD; thus favoring the inclusion of individuals having peripheral mis-analysis of the acoustic cues with a secondary impact on the central auditory pathway functioning.

Defining deficits of auditory processing as “Central Auditory Processing Disorder” (CAPD) may exclude individuals whose problem starts at the level of the cochlea (HHL) but is not evident in the majority of classical audiological tests (OAE may be partially present) or cases of ANSD. In the latter deficits, usually present in speech perception and temporal resolution in association, in the majority of cases, with raised audiometric thresholds and ABR findings indicative of low-level involvement of the auditory pathway. If these clinical entities are to be included in one general disorder category then the term APD is more optimal, since processing abnormalities will be detected in all; however, the level of the pathology will be very broad and the natural course of the disease highly variable. In case we decide on a narrower disorder category that excludes inner ear and auditory nerve disorders, the term CAPD is best. There is, however, a need to consider the possibility of “mixed” presentations such as cochlear type HL and CAPD [e.g., Ref. (41)]. Defining normal hearing on the basis of a normal pure tone audiogram is at best insufficient (42). Auditory processing may start at the level of the cochlea and detailed evaluation of OAE should be incorporated with tests assessing neural function (ABR) and with results of specific diagnostic auditory processing tests (37).

Otitis media with effusion in its recurrent form (chronic) or other possible conditions of acoustic deprivation should be researched in terms of resulting transient HL and subsequent APD in longitudinal studies with specific diagnostic audiological tests.

Auditory processing evaluation in the clinical setting is largely based on psychoacoustic test batteries of verbal and non-verbal stimuli (42–50) and may be ancillary completed with electrophysiological or objective audiological measures, such as acoustic reflex thresholds, tympanometry, ABR (speech and noise ABR included), or OAEs (suppression included). APD diagnosis (Table 1) is a highly demanding time consuming procedure with multidisciplinary information on the client’s profile being essential to test selection. Test results interpretation is facilitated by carefully evaluating an individual’s behavior as measured by specific questionnaires (51) and detailed clinical interview during history taking that includes an audiological history (of auditory symptoms, early development of auditory behaviors, and APD risk factors including a family history of APD as well as of HL). When choosing the auditory processing tests, language and cognitive confounds should be minimized (52). In this respect, a detailed receptive and expressive language evaluation may in some cases be necessary to choose those language-based audiologic tests which are appropriate regarding the individual’s language development, and, second, to identify language development impairments as possibly higher level disorder which may impact audiological results. Even though it should be kept in mind that the link between language, cognition, and auditory processing is complex with one process influencing another and vice versa, it is generally advised to adopt a multidisciplinary approach (when possible) to make sure that individual audiological findings are not primarily the result of a higher level disorder. History elements concerning symptoms, specific difficulties, onset of disorder, and musical training are taken into account to conclude whether an individual has APD or a comorbid challenge.

It is essential to establish a scientifically valid evidence-based understanding concerning the direction of causality or the presence of a common causal pathology when APD is comorbid with cognitive deficits. The best way of approaching this matter at this time is to longitudinally evaluate individuals diagnosed with APD for cognitive skills by using large cohorts who are characterized in detail in terms of symptom description and other participant characteristics across different countries. These studies should also employ measures of brain activity with methods like electroencephalography, magnetoencephalography, and functional magnetic resonance imaging, which are frequently used in basic research studies. The already existing knowledge gained in such studies on the link between auditory cortex activity and performance in listening tasks in healthy subjects [e.g., Ref. (53, 54)] would then be translated to the clinical population. Comparing measures of central auditory processing in APD patients to the outcome of standard diagnostic procedures would contribute to differential diagnosis and identify predictors for long-term prognosis and management outcomes.

The benefit of such an approach with strong methodological quality will be to optimize diagnosis and arrive at a consensus of a standard diagnostic pathway that could be written down as a tree of decisions on whom to test, by which test means and when. In our joint effort to differentiate APD from other neurodevelopmental disorders or causes of listening difficulties we should not exclude the possibility of APD co-existing with other disorders or with multimodal deficits which may interact. Diagnosing APD when other disorders are present [ADHD, autism spectrum disorder, language impairment, dyslexia, learning disabilities (dementia in adults), emotional disorders] may benefit individuals by adding specific management or modifying existing instructions (i.e., FM systems, specific auditory training, individualized listening strategies/teacher based adaptations).

The interventions (Table 2) should be as individualized as possible addressing (i) environmental modifications, (ii) use of FM systems (15), and (iii) systematic auditory training (55). Management needs to be multidisciplinary, and it is important that this is implemented in the educational environment for affected individuals who are still in education, in which case teacher-based adaptations and related strategies are of the utmost importance, alongside the previously mentioned interventions. Informal auditory training and compensatory, metacognitive, and metalinguistic strategies may also be of value (56, 57). Working memory deficits may be specifically trained as they may be closely linked to auditory perception, but they are a precondition for auditory-language processing, for example examined by dichotic listening tests. The client’s needs should be kept in mind before the multidisciplinary intervention planning. The audiologic results should be reflected in a multiprofessional background to figure out the appropriate and individualized management for each child under consideration of the child’s overall picture and problems in everyday life, the child’s individual resources, and that management interventions match well with the individual main problems in everyday life. It may benefit individuals with comorbid APD to provide them with individualized management even if complete recovery to normal auditory processing may not be a realistic expectation. Data showing short-term memory and general IQ improvement following auditory training and FM use should be further investigated as they are indicative of brain plasticity (9, 58, 59)—on condition that it is not a short-term improvement and the stability of the improvements is ensured. It is, however, of clinical interest that such outcomes are being reported in the difficult to test and complex mental health patient as well as in the neurological patient and the child diagnosed with APD.

In order to implement cost-effective evidence-based APD management, there is a great need to identify specific psychoacoustic, clinical, or objective auditory markers (core measures) to guide appropriate specific management provision as opposed to using all three general approaches mentioned above. A European platform for collecting information on diagnostic approaches and APD intervention outcomes will be of great value to individuals experiencing listening, communicational, academic, and working/leisure difficulties. A European database would be used in this sense to check comparability of the prevalence of diagnosis, of symptomatology and of diagnostic test yield, usability of behavioral versus objective measures, and measure and compare management efficacy. The challenge in the European platform and database described would be to achieve verbal tests comparison across languages, instructions, and response mode uniformity with the addition of objective measures to compare across different languages and cultures.