Audiologists are regularly faced with the patient who presents to their office with a variety of hearing complaints. They often report significant difficulty hearing, particularly in background noise. These hearing difficulties negatively affect them both occupationally and socially. However, upon assessment, the pure-tone audiogram results demonstrate “normal” peripheral hearing sensitivity. These are perhaps not the anticipated results, given the patient’s reported difficulties. In this situation, the question we should ask ourselves is: Does normal hearing on the audiogram in fact mean normal hearing? The answer is no.
Hearing is a complicated process. As audiologists, we are well versed in anatomy and physiology of the peripheral auditory system and the evaluation of hearing sensitivity. Unfortunately, many individuals lack training in central auditory function and its assessment. As a result, we all too often forget about the role that the brain plays in our ability to hear.
As a profession, admittedly few audiologists evaluate central auditory function (Chermak et al, 2007) and those who do often focus on the pediatric population. While most individuals relate auditory processing disorders to children, this should not be the case. Many adults present with auditory processing deficits that are often overlooked because of their age and normal hearing sensitivity.
The central auditory nervous system is an intricate and complicated neural network that plays a critical role in hearing. Auditory information must be preserved and transferred through an array of cells, nuclei, and pathways. If there is an interruption in transmission of the signal, the auditory system may not be able to efficiently or effectively process meaningful input, resulting in measurable perceptual deficits. This often manifests from impairment in one’s ability to code intensity, frequency, and/or temporal elements of auditory input.
Deficits in this neural pathway can result in a myriad of complaints. While the most prevalent is difficulty in hearing in complicated or challenging listening environments, other perceptual deficits reported may include difficulty localizing sound sources, changes in sound quality (i.e., distortion), and poor auditory discrimination.
Limitations of the Audiogram
As a profession, we have relied heavily on the pure-tone audiogram for the evaluation of our patients. This is our hallmark diagnostic test and has been for 90 years. While there have been significant advances in technology, the fundamental principles behind basic audiological assessment have seen little advancement.
The audiogram does in fact provide clinicians with important information regarding type, degree, and configuration of hearing loss. However, in cases where it is found to be normal, yet the patient presents with obvious auditory complaints and difficulties, it provides little value other than eliminating peripheral involvement as a contributing factor.
Musiek and colleagues (2017) provide a comprehensive perspective of the pure-tone audiogram through which they demonstrate the significant limitations of the audiogram. As outlined in their paper, the audiogram can only provide information regarding hearing sensitivity. It fails to provide meaningful functional information.
There is a growing body of literature that supports auditory difficulties among individuals with normal peripheral hearing sensitivity. Shinn et al (2016) retrospectively reviewed the audiological findings of many patients in a busy otology practice. Their analysis found that 61 percent reported hearing difficulties yet had normal audiograms. In addition, the Beaver Dam Offspring Study (Tremblay et al, 2015) found that 12 percent of individuals with normal audiograms self-reported hearing difficulties.
As audiologists, we must be keenly aware of the role that the brain plays in auditory function and dysfunction. Some of the most fascinating support for limitations of traditional audiological assessment comes from patients with central deafness (Musiek and Lee, 1998; Musiek et al, 2007; Musiek et al, 2019).
Musiek and colleagues (2007) present a patient with bilateral cerebrovascular insult. The patient’s audiological evaluation demonstrated a severe to profound sensorineural hearing loss bilaterally. Upon further examination, distortion product otoacoustic emissions, acoustic reflexes, and the auditory brainstem response (ABR) evaluation were all normal, whereas the middle (MLR) and late auditory-evoked potentials were compromised. This supports the notion that the audiogram cannot always be taken at face value and a full audiological evaluation of the entire auditory system is warranted, particularly in complicated cases. While, in this case example, the patient had residual hearing, it is not unusual for some patients to present with complete central deafness.
Abnormal Auditory Function in Normal Peripheral Hearing
There are several papers that highlight significant auditory deficits in spite of normal peripheral hearing sensitivity in individuals with auditory nervous system disorders. Karlin (1942) was perhaps one of the first to recognize that the audiogram is a poor predictor of hearing, particularly in complex listening environments. Bocca and colleagues (1954) also observed the shortcoming of the pure-tone audiogram. They demonstrated that, even with significant temporal lobe involvement, patients with normal audiograms had significant difficulty understanding degraded speech.
More recently, the Veterans Administration (VA) extensively examined auditory difficulties among individuals with blast-related injuries. Specifically, these Veterans reported hearing difficulties, particularly in noisy environments, yet demonstrated normal audiograms.
Saunders and Abrams (2009) reported that more than half of the audiologists working in the VA system reported encountering 1–3 patients monthly with auditory complaints who presented with normal audiograms.
Gallun et al (2012) identified deficits in central auditory function for both behavioral and electrophysiological measures in normal-hearing Veterans. More recently, they found that 60 percent of blast-exposed individuals demonstrated a moderate-to-severe hearing handicap, despite having normal pure-tone audiograms (Gallun et al, 2016).
Head injuries are not limited to the Veteran population. As audiologists, we often encounter patients who report hearing difficulties following such damage. Motor vehicle accidents, sports-related injuries, and blunt-force trauma to the head are among just a few of the culprits responsible for head injuries.
The challenge associated with head injuries is that damage is often not observable or measurable upon imaging. There is a growing body of literature that supports auditory-processing deficits in this population across the lifespan.
Thompson and colleagues (2018) found that children who had sustained concussions exhibited difficulty understanding speech-in-noise and decreased performance on sustained auditory tasks, as compared to their non-concussed peer group.
Central auditory dysfunction has many etiologies including, but not limited to, stroke, epilepsy, degenerative diseases, neuromaturational lags, etc. There is growing evidence that hormonal changes also play a role in auditory dysfunction.
Recently, Trott and colleagues (2019) demonstrated the effect of hormonal changes on central auditory function. It is well established that estrogen has been identified as playing an integral role in the auditory system. The authors found statistically significant differences between pre- versus peri-/postmenopausal women, particularly on listening-in-noise and electrophysiological measures. While it may be the case that peri-/postmenopausal women are not at risk for peripheral hearing loss of sensitivity, results from this study indicate auditory-processing disorders may occur secondary to hormonal changes.
Evaluation and Treatment
The purpose of an auditory-processing evaluation is to determine if there are any abnormalities present, and if so, to describe the nature and extent of the disorder for the purposes of appropriate intervention (American Academy of Audiology, 2010).
The evaluation of auditory processing should be completed after careful history, which should include obtaining information regarding audiological, otological, and neurological involvement. A careful history can help guide the clinician in test selection.
Both behavioral and electrophysiological tests are recommended (when appropriate) in the evaluation of auditory-processing disorders. The battery should be comprehensive and evaluate a variety of processes. Additionally, clinicians should use tests with good sensitivity, specificity, validity, and reliability.
After an individual has been diagnosed with an auditory-processing disorder, it may be challenging to determine patient recommendations and treatments for the deficit area(s). While most experts in this sub-specialty would support recommending deficit-driven therapy in pediatric populations, this can be more challenging in adults, as it is not always feasible given busy lifestyles. However, there are formal auditory-training approaches that have been found to be efficacious in the treatment of auditory-processing deficits in both pediatric and adult populations.
For example, dichotic interaural intensity difference (DIID) training is a therapeutic approach used to remediate binaural integration deficits (Weihing and Musiek, 2014). This treatment approach is based on the principles of constraint-induced therapy used in physical therapy rehabilitation.
The goal of DIID training is to improve performance of the weaker ear, while maintaining the performance of the stronger ear through intense auditory therapy. Patients who have undergone this therapy have demonstrated significant improvement in their auditory- processing skills (Musiek et al, 2004; Schochat et al, 2010).
Adult patients with normal hearing who have been diagnosed with auditory-processing deficits have also been found to receive benefit from mild-gain amplification (Kokx et al, 2016; Roup et al, 2019). Kokx et al (2016) report that patients identified with auditory-processing deficits and normal hearing thresholds reported benefit from the use of mild-gain amplification. More recently, Roup and colleagues (2019) present a case report of improved auditory-processing skills and quality of life after using hearing aids in an individual with an auditory-processing deficit.
Although there are no known published studies with adults, remote-microphone technology may be another option for patients reporting hearing difficulties in noisy situations.
This is a case example of a high-functioning and high academically performing senior in high school. She had chronic otitis media as a child resulting in multiple sets of pressure-equalization tubes. Due to a right-side tympanic membrane perforation and hearing loss, she required a tympanoplasty in her early teens. Her surgical intervention was successful and her hearing returned to normal.
After several years, she returned to her audiologist due to concerns regarding difficulty hearing, particularly in noise. Recent history included five sports-related concussions, one of which was severe enough to result in loss of consciousness.
She was evaluated by a neurologist at that time, due to experiencing several migraines per week and chronic fatigue. Imaging was found to be normal. Although she was performing well academically, she felt that she had to work significantly harder than had been normal prior to her head injuries.
An audiological evaluation was performed. Her pure-tone audiogram showed normal hearing thresholds and excellent word recognition bilaterally. Tympanograms were also normal. Given her normal hearing sensitivity and significant auditory complaints, it was recommended that she undergo an auditory-processing evaluation.
The auditory-processing test battery included the duration patterns, filtered words, dichotic digits, competing sentences, and listening in spatialized noise sentences (Cameron and Dillon, 2007) tests. Results from the auditory-processing testing demonstrated a right-ear deficit on the filtered words, dichotic digits, and competing sentences tests.
Given the documented deficits, auditory therapy was recommended. She underwent intense aural rehabilitation using DIID training in the clinic, under the direction of the audiologist, two times per week for six weeks (30 minutes per session). In addition, a trial with mild-gain amplification was also recommended.
Following DIID training, re-evaluation demonstrated significant improvements in binaural integration and separation. She continued to exhibit some difficulty hearing in noise and filtered speech. This was not surprising, given these areas were not targeted during therapy. She elected to pursue mild-gain hearing aids following DIID training.
The patient reported significant auditory improvements and quality of life following treatment. Specifically, she no longer needed to exert the same degree of listening effort.
In addition, she perceived making fewer auditory-processing errors. She had significant reduction in fatigue and no further migraines. She reported benefit from her hearing aids, noting a significant difference in her listening abilities with their use, particularly in difficult listening environments and large classroom situations.
As audiologists, we are charged with evaluating and treating a variety of hearing and balance-related disorders. Unfortunately, we all too often work within the borders of a confined box and attempt to clearly delineate normal from abnormal based on a diagnostic tool that has significant limitation.
If we fail to appropriately evaluate, and potentially miss, a diagnosis of auditory-processing deficits, we are doing a great disservice to our patients. We clearly still have significant strides to make in this clinical area.
With continued education and awareness, audiologists will hopefully come to recognize the entire auditory system and gain further appreciation and understanding of the hearing brain.