Clinical evaluation of hearing is often focused on hearing sensitivity despite evidence showing that depressed sensitivity alone does not reflect everyday listening and communication difficulties. Assessment of auditory processing should be encouraged, along with the education of audiologists, in providing a more comprehensive evaluation.
BEYOND PURE TONE AUDIOGRAM
The pure tone audiogram remains the gold standard of hearing evaluation despite its limitations. While the audiogram is the primary tool for determining the type, degree, and configuration of hearing loss, this measure does not provide information beyond hearing sensitivity to a limited range of frequencies.1 It does not reveal the physiological state of the entire (peripheral and central) auditory system (the auditory nerve included) nor the complete physiological state of the cochlea. Auditory neuropathy provides a well-known example of the disconnect between the pure tone audiogram and auditory processing.2 The audiogram explains only a small part of the variance in speech understanding performance and self-reported hearing ability of neurologically normal older adults.3 Moreover, adults with central auditory nervous system (CANS) pathology (e.g., due to stroke, traumatic brain injury, degenerative disease) and children with auditory processing disorder (APD) and learning problems present with difficulty understanding speech in noise or competing message environments despite normal pure tone audiograms. Notwithstanding these limitations, the audiogram may be the only test to conduct on children and adults with auditory complaints when their audiograms are normal. This is a matter of concern since failure to fully evaluate and identify the sources of a patient's complaints precludes appropriate, effective, and efficient treatment and management.4 It may also mislead the audiologist and patient into believing that the patient's hearing is entirely normal.
One of the primary reasons for this disconnect between research and practice seems to be the ongoing debate on the interaction between cognition and auditory processing, with proponents on both sides of the debate arguing the neuroanatomical and neurophysiological source of APD. In this brief article, we provide evidence that attention is a term widely and easily misused to explain what might in fact be an APD or even a hearing sensitivity impairment. This misuse of terminology stems from the assumption that heterogeneity in a disorder undermines its reality. This assumption, however, is incorrect in that it is incompatible with the nature of many disorders and with clinical reality. As stated in the DSM-5: “the once plausible goal of identifying homogeneous populations for treatment and research resulted in narrow diagnostic categories that did not capture clinical reality, symptom heterogeneity within disorders, and significant sharing of symptoms across multiple disorders.”5 Moreover, the DSM-5 notes that “…a too-rigid categorical system does not capture clinical experience or important scientific observations” and that “… the boundaries between many disorder “categories” are more fluid over the life course.” The DSM-5 recognizes that the fluidity of boundaries across disorders “should permit a more accurate description of patient presentations and increase the validity of a diagnosis.”5 That APD may have diverse clinical presentations, overlapping symptoms, and comorbidities is thus consistent with the definition of disorders as described in DSM-5. It should also be noted that APD is one of a few neurodevelopmental disorders with a diagnosis based on standardized tests and not merely on symptoms or questionnaires (as is the case with attention deficit hyperactivity disorder, autism, etc.). Multiple sclerosis is also a good medical example as it can affect several systems, but each system's involvement must be diagnosed and treated to best help the patient.
WHAT'S IN A NAME?
Discussing client needs in an audiology clinic, including cases of APD, involves consideration of the contributions of interdisciplinary teams. This is most appropriate given the comorbidities often present with APD and the standard practice for several neurodevelopmental disorders, to ensure that all pertinent components of an individual's deficits are addressed. This does not undermine the diagnostic entity of APD. The diagnostic label APD is dismissed by some clinicians, some of who, interestingly, may be comfortable with using the term “hidden hearing loss,” for which the evidence in humans is limited, inconsistent, and some would assert speculative at this time. If you ask why this is the case, the probable answer is that a rather precise anatomical issue is suggested as the source of hidden hearing loss or cochlear synaptopathy (i.e., loss of afferent fiber communication at the inner hair cell ribbon synapse). However, permanent behavioral thresholds shifts are not observed until 80 to 90 percent of the synapses or inner hair cells are lost.6 Cell bodies of the affected cochlear neurons can survive for decades despite their loss of functional connection to the hair cells, which means the inner hair cells can survive long periods of time as well. Moreover, as previously stated, the evidence for hidden hearing loss in humans is limited and inconsistent at this time. As argued by Musiek, et al.,7 the most likely cause of the primary symptom presumed to reflect hidden hearing loss—difficulties in speech recognition in noise despite normal pure tone thresholds—is APD.
Sensitive and specific tests to evaluate specific components of auditory processing and underlying function of the central auditory nervous system (CANS) provide the clinical tools to examine the sources of a patient's hearing complaints, especially those related to difficulties in speech recognition in noise. We caution clinicians not to be tempted to adopt a diagnostic label—hidden hearing loss—for which there is limited evidence in humans and no clinically proven means of directly assessing (only inferring) the underlying cochlear synaptopathy. We caution clinicians not to reject APD due to its heterogeneity, a feature it shares with many other disorders, and highlight the need therefore for interdisciplinary evaluation by experienced and knowledgeable clinicians. Clinical profiles may not be simple, but that must not deter us from fully examining the underlying components. Evaluation and diagnosis of APD require a battery of efficient auditory processing tests to examine the patient's complaints and symptoms, interpreted in the context of a comprehensive history (including medical issues, past and present).4
GOLD STANDARDS FOR APD AND HEARING LOSS
The gold standard for the diagnosis of a disorder is defined as the best available evidence-based methodology for diagnosis. Current clinical practice guidelines depend on the use of test batteries due to the inherent complexity of any given disorder. This enables a more accurate diagnosis. The auditory processing test battery approach8,9 is the best available gold standard approach to APD diagnosis. This is based on the best available evidence presented in numerous published research papers and clinical experience. Moreover, there are several parallels between this APD gold standard and pure tone audiogram as the gold standard for diagnosing hearing loss, including (1) the subjective nature of testing, (2) results dependent on the clinician's ability to elicit behavioral responses that reflect an individual's true auditory ability, and (3) the need for the test results to be interpreted within the context of medical history to arrive at a conclusion regarding an individual's “hearing” status. Pure tone audiometry results may not always agree with objective (i.e., electroacoustic or electrophysiological) audiological tests, yet this does not lead to questioning the pure tone audiogram as a gold standard for determining hearing sensitivity and hearing loss. The same logic and standard apply to clinically used auditory processing testing. In addition, all behavioral tests impose a degree of cognitive load, including attention, executive function, and working memory.10 However, the extent of cognitive influence on audiometric tests, including those of central auditory processing, is limited such that cognition does not solely drive central auditory processing performance. Ironically, the influence of an auditory deficit on measured cognition is often ignored.
IS IT VALID TO EVALUATE SUSPECTED APD WHEN RESEARCHING FOR APD?
A number of controversial positions regarding the relationship between auditory processing and cognition are the result of research design weaknesses, most notably poorly defined participants. Many of the conclusions reached in these studies are based on reports that included participants suspected of APD and/or defined on the basis of parent or teacher reports instead of diagnostic evidence. Conclusions drawn from the performance of poorly defined participant samples pose significant threats to the validity and reliability of the research because one cannot be sure whether the study participants presented with any true auditory deficit or if they have other unidentified issues.4 Inflated cognitive effects can be obtained when participants with unknown cognitive status are included in study samples. The relationship between central auditory tests (or any behavioral measure) and cognitive measures can seem stronger than it actually is when participants with low cognition are not excluded from the analysis.11
COGNITION AND AUDITION
It is worthwhile to compare the interaction between APD and hearing impairment and the interaction between APD and cognition. Hearing (hearing sensitivity and auditory processing) facilitates the development of cognitive abilities of children and adults. Adults with hearing impairment use a disproportionate amount of their cognitive reservoir to support listening (i.e., perceiving auditory information), as this is particularly challenging given their sensory deficit.12 This might translate into decreased capacity of higher cognitive abilities (i.e., short-term and working memory, focused and sustained attention). Perhaps as a compensatory mechanism, the role of working memory in support of listening increases as the signal is more degraded by background noise. Assessment of cognitive capacity in hard-of-hearing individuals is influenced by their sensory deficit, as the most common approach to cognitive evaluation is through the auditory channel (i.e., oral instruction, repetition, or reordering of auditory stimuli). Since audibility does not guarantee normal auditory processing, hearing sensitivity and auditory processing must be evaluated before conducting any cognitive test to ensure accurate results.13 If this is not feasible, one must insist that a thorough audiological diagnostic evaluation follow initial cognitive testing.14 Studies of children diagnosed with APD show that only a subgroup presents with attention deficits, mostly focused auditory attention deficits, not necessarily global attention deficits.15 This indicates that the interaction between cognition and hearing (auditory processing included) is complex. In a recent study, in children diagnosed with APD, a dissociation was seen between attention and rhythm perception, demonstrating that attention is not the cause of the processing difficulty in APD. (Sidiras, et al., under review). The ability to process rhythm by children with normal auditory processing ability was very much influenced by attention. This indicates that extrapolating correlations found between typically developing children and children with APD is not a valid approach. Quoting Phillips (1990)16: “Following from the modern spotlight metaphor of selective attention, it is perhaps more likely that a deficit in selective attention is secondary to the deprivation of attentional processes from their normal neural representations of the acoustic signal.”
APD is a complex disorder with a multifactorial presentation and without a single contributing factor to account for the majority of the clinical profile in patient cohorts—and certainly not in the individual patient. Some clinicians might find the one-test, gold standard approach attractive, but such an approach is not likely to maximize the accuracy of differential diagnosis nor promote effective and efficient management of patients with APD. Consideration of each patient's unique characteristics should drive the choice of tests/assessments, overall diagnostic approach, clinical decision-making, and intervention. Clinicians must be cognizant of system interactions and the limitations of a standard, peripheral hearing evaluation in revealing more central auditory involvement. These understandings will lead to more comprehensive and accurate differential diagnoses as well as more targeted and efficient interventions.
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