Many peripheral and central mechanisms likely contribute to the origin of tinnitus, including discordant hair cell damage, dorsal cochlear nucleus hyperactivity, tonotopic map reorganization, cochlear synaptopathy, increased spontaneous neuronal firing, increased neuronal synchrony, and failure of inhibitory pathways.1-4 Nevertheless, the trigger for tinnitus is usually some type of damage to the cochlea.5 However, “cochlear pathology is not always expressed in the audiogram.”2 A normal audiogram can be obtained even when there is up to 30 percent diffuse loss of outer hair cells.6,7 In such cases, an individual may be able to detect faint tones and repeat monosyllabic words in a quiet sound booth. Understanding complex real-world speech, however, requires greater cochlear specificity than that required by a simple hearing test; as a result, communication often fails.
The prevalence of tinnitus among veterans is over twice as high as that among non-veterans (11.7% vs. 5.4%).8 In a review of over 2,500 consecutive audiology referrals at the Atlanta VA Medical Center, 26 percent were for the primary complaint of tinnitus, and nearly half of these (45%) had normal or near-normal hearing through 8,000 Hz.9 Learning theory concepts of working memory and cognitive load may help explain the hearing complaints of tinnitus patients with normal or near-normal hearing.
PROGRESSIVE APPROACH TO TINNITUS MANAGEMENT
The Department of Veterans Affairs and the Department of Defense have adopted the Progressive Tinnitus Management (PTM) program,10,11 a five-level hierarchical process for the identification and provision of the least intensive tinnitus management services that sufficiently provide adequate relief. Patients proceed from one PTM level to the next more intensive level only if their tinnitus management needs have not been met by actions in their current PTM level.
In PTM Level 1 (Triage), the audiologist determines which patients may need specific tinnitus management services. In PTM Level 2 (Audiological Assessment), the audiologist identifies patients with aidable or potentially medically treatable hearing problems. Referrals are made as necessary and hearing aids and assistive devices (such as bedside sound generators) are issued when appropriate. A hearing aid's streaming capabilities are taken into consideration to facilitate sound enrichment if indicated. Veterans with normal or near-normal hearing are offered PTM Level 3 (Tinnitus Skills Education), as are veterans who report insufficient tinnitus relief from hearing aid use on outcome measures. This level provides strategies and skills for managing the negative reactions to the tinnitus signal. PTM Level 4 (Multidisciplinary Evaluation) involves mental health services since the number of mental health diagnoses increases as patients proceed through the PTM levels and require more intensive interventions.12 Because of the significant relationships between various mental health diagnoses and perceived tinnitus severity,12,13 receipt of ongoing mental health services is verified when a patient is diagnosed with a mental health condition. Finally, PTM Level 5 (Individualized Support) includes directive counseling, as well as the use of specific management protocols. Patients who proceed to PTM Level 5 are those who demonstrate the most severe tinnitus-related distress.
IMPACT ON WORKING MEMORY & COGNITIVE LOAD
At the Atlanta VA, baseline measures of tinnitus-related distress are completed prior to the start of PTM Level 3. In response to frequent reports of subjective hearing complaints from normal-or near-normal-hearing tinnitus patients, we began to include the Hearing Handicap Inventory for the Elderly-Screening Version, or HHIES,14 as one of our baseline measurements.
Among 63 consecutive tinnitus patients with normal or near-normal hearing thresholds who attended Tinnitus Skills Education (PTM Level 3) during a specified period, 90 percent reported significant hearing handicap, with half of those providing responses indicating severe subjective hearing handicap.15 Furthermore, the median HHIES score of the 63 subjects was not significantly different from that of the 127 hearing-impaired patients (responding to how they heard without their hearing aids) who attended Tinnitus Skills Education during the same period (p >.05).
Learning theory, including the concepts of working memory and cognitive load, may help explain the subjective hearing handicap reported by normal-or near-normal-hearing tinnitus patients. Working memory is a brain function responsible for attention, problem-solving, and memory.16 It encodes, stores, and retrieves information chunks required to carry out complex cognitive tasks such as learning, reasoning, and comprehension.17 Working memory capacity is limited: It can encode, store, and retrieve only five to nine chunks of information at any one time.18
Complex tasks require mental effort, and the amount of mental effort required at any one time is referred to as cognitive load.18 The more complex the task, the greater the number of information chunks that must be processed and the greater the cognitive load. As the number of information chunks that must be processed by working memory increases, the mental effort required, or cognitive load, also increases. Increased cognitive load decreases working memory capacity and the number of information chunks that it can process.
When those information chunks interact and must be processed simultaneously, working memory capacity is reduced even further. For example, driving a car involves processing the simultaneous interaction of information chunks such as the exact pressure required to brake, the exact amount to turn the steering wheel, staying alert to changes in traffic conditions, adjusting for weather conditions, and so on.
People with impaired working memory can only process a small number of information chunks at a time. As a result, they have difficulty with multitasking, require more time to recall information, and are slower at taking in and making sense of information. Such individuals also have trouble suppressing irrelevant information, such as background noise.19
The ability to retain, process, and respond to speech is directly related to working memory capacity.20 The rapid nature of speech requires rapid auditory processing, which increases cognitive load and reduces the capacity of working memory. Auditory processing speed is inversely related to working memory capacity.21 Reduced working memory capacity reduces the speed at which rapid speech can be processed. For those with impaired working memory, attending to, encoding, retaining, and processing this rapid input essentially fills up their impaired working memory such that all the necessary information chunks required cannot be processed, and communication fails.
Environments where speech is degraded, such as noisy places, put an even greater burden on working memory,22 resulting in a further increase in cognitive load, which can “interfere with operations such as language processing and memory for what is being heard.”23
Tinnitus has been shown to reduce working memory capacity.24 Sleep deprivation—one of the most common complaints of tinnitus patients—has been shown to reduce working memory capacity.25 Tension, stress, and anxiety, which are common reactions to tinnitus, also reduce working memory capacity.26,27
Mental health status impacts working memory capacity, and mental health disorders are more common among tinnitus patients than non-tinnitus patients.28 Individuals with mental health disorders report greater perceived tinnitus severity than those without.12 In one study, 85 percent of U.S. veterans with tinnitus who proceeded to PTM Level 5 had at least one mental health diagnosis, the most common of which were PTSD, depression, and anxiety.12 Depression and PTSD have been shown to reduce working memory capacity,29,30 and the relationship between coexisting PTSD and tinnitus is bi-directional13: Increased severity of one can increase the severity of the other.
For these reasons, it seems reasonable to conclude that patients with tinnitus who proceed to PTM Level 5—typically those with severe perceived tinnitus severity—may indeed have impaired working memory capacity.
Problems caused by reduced working memory capacity and those caused by tinnitus overlap. Reduced working memory capacity and increased cognitive load related both to tinnitus and its associated problems (e.g., sleep deprivation, tension, stress, anxiety), and, in many cases, to mental health disorders, may contribute to the subjective hearing problems reported by normal- or near-normal hearing tinnitus patients.
The vast majority of individuals with normal or near-normal hearing who experience significant tinnitus-related distress also experience significant subjective hearing handicap. This subjective hearing handicap is not due to auditory processing deficits, but may be explained by reduced working memory and increased cognitive load. These tinnitus-related cognitive changes also contribute to mental health problems (e.g., sleep deprivation, tension, stress, anxiety). Training strategies used by experts in learning theory improve the working memory among people with hearing impairment,31 attention deficit hyperactivity disorder (ADHD),32 and learning disabilitites.33 Future research may employ such training on normal-or near-normal-hearing tinnitus patients with subjective hearing problems.