Case History

A male veteran (MV) in his early 50s recently presented to a Veterans Affairs (VA) audiology clinic stating that he had noticed a substantial decrease in his hearing ability following his military service. The MV served in the Navy and in the Army National Guard for a total of 32 years, which included many domestic and international service missions. While deployed to Iraq, he was exposed to a total of three bomb blasts, the most severe of which occurred approximately six years prior to presenting in the VA audiology clinic. 

During this incident, the MV was in a military convoy that struck an improvised explosive device concealed in the roadway. The blast exposure left him with a traumatic brain injury (TBI), as well as ruptured discs and vertebrae throughout his spine, a broken nose, permanent damage to his right arm and knee, and loss of multiple teeth. For this encounter, he was awarded a medal for exemplary service in combat. At the time, he was seen in the VA audiology clinic, he had previously undergone cognitive rehabilitation treatment through polytrauma and speech language pathology services for concerns related to cognitive difficulties. 

His additional medical diagnoses included post-traumatic stress disorder (PTSD), anxiety, obstructive sleep apnea, chronic headaches, type 2 diabetes mellitus, sensitivity to light, colitis, chronic knee and back pain, hyperlipidemia, weakness and numbness of the right arm, and coronary heart disease. An intake interview revealed that his primary auditory complaints included difficulty hearing in noise and in the presence of multiple talkers, difficulty understanding on the telephone, problems paying attention to people speaking, and confusing similar-sounding words. 

Audiometric Findings

  • Otoscopy: clear ear canals with intact ear drums
  • Tympanometry: normal ear canal volume, middle-ear pressure, and admittance in both ears (Type A)
  • Acoustic Reflexes: contralateral and ipsilateral reflexes were present and within normal limits in both ears 
  • DPOAEs: present from 750 to 8000 Hz in both ears
  • Audiogram: see FIGURE 1; SRT was consistent with pure-tone thresholds
  • WRS: 100 percent correct in both ears
Figure 1. Audiometric findings completed at initial evaluation.
FIGURE 1. Audiometric findings completed at initial evaluation.

What Would You Do?

At first, the complaints reported by the MV were consistent with cochlear hearing loss likely due to the onset of presbycusis, noise exposure due to military service, or a combination of the two. However, the findings of normal audiometric thresholds along with present DPOAEs and excellent word recognition in quiet would appear to argue against cochlear dysfunction. Could the patient’s complaints be due to lingering cognitive dysfunction, or perhaps from chronic emotional disturbances? Had his previous blast exposures and TBI caused damage to his central auditory system?

To evaluate these options, the MV was subsequently seen for additional testing for central auditory processing disorder (CAPD). Results of the SCAN-A (Keith, 1995), the Words-in-Noise Test (Wilson et al, 2007), and the QuickSIN (Killion et al, 2004) all revealed performance within the normal range, as did the Staggered Spondaic Words Test (Katz and Smith, 1991) and the Dichotic Digits Test (Musiek, 1983). However, tests of temporal processing, including the Gaps-in-Noise Test (Musiek et al, 2005) and the Pitch Pattern Test (Musiek, 1994), revealed abnormally poor performance in both the left and right ears. 

Figure 2. Electrophysiological responses to a gaps-in-noise stimulus containing a 20 msec silent gap embedded within a broadband noise
FIGURE 2. Electrophysiological responses to a gaps-in-noise stimulus containing a 20 msec silent gap embedded within a broadband noise. The yellow area highlights the response of the auditory cortex (N1 and P2) to the presentation of the silent gap. Notice that while the age- and hearing-matched patient with no history of head injury (solid red line) shows a robust cortical response to the 20 msec gap, patient MV (broken blue line) shows a markedly reduced response. 

These tests were followed up with an additional measure evaluating the MV’s ability to benefit from having a spatial separation between a target talker and two distracting talkers. When the distracting talkers are located at 45-degree angles to the left and right of the listener and the target talker is directly in front, the average normally hearing listener can understand the target talker at a level approximately 10 dB below the level needed to achieve the same performance when the target and distracting talkers are co-located directly in front of the listener (Gallun et al, 2013). However, the MV received only a 3dB benefit from having the 45-degree spatial separation.  

Behavioral CAPD test measures can be confounded by non-auditory variables such as distraction, poor concentration, or lack of effort on the part of the patient. Thus, behavioral testing was complimented with objective electrophysiological test measures. One of these measures included obtaining passive auditory cortical responses to a gaps-in-noise paradigm similar to the behavioral test paradigm. The stimuli consisted of a broadband noise with embedded silent gaps varying in duration from two to 20 ms. During this test, the patient was seated in a recliner inside a sound attenuating chamber and instructed to watch a closed-captioned movie and ignore the auditory stimuli that were presented over insert earphones. The results of this measure, shown in FIGURE 2, confirm that the MV’s auditory cortex is considerably less sensitive to even large gap durations when compared to a normally hearing listener of the same age but with no history of blast exposure or head injury. 

Figure 3. Electrophysiological responses to the rare stimulus of a P300 oddball paradigm
FIGURE 3. Electrophysiological responses to the rare stimulus of a P300 oddball paradigm. While the age- and hearing-matched control patient with no history of TBI (solid red line) demonstrates a robust P300 response to the rare 1000 Hz tone, notice that the P300 response obtained in patient MV (broken blue line) is absent. 

Lastly, an auditory oddball P300 test was administered. For this test, the stimuli consisted of a 500 Hz “standard” tone presented during 80 percent of trials and a 1000 Hz “rare” tone presented randomly during 20 percent of trials. The MV was asked to silently count the number of deviant tones presented. Electrophysiological responses from the MV, as well as an age-matched individual with no history of blast exposure or head injury, are shown in FIGURE 3. Although the MV could achieve the same level of accuracy at detecting the deviant tone as the non-injured patient, the P300 response clearly demonstrates that his brain is processing changes in sound over time in a vastly different way. 

Diagnosis: Putting It All Together

MV was subsequently identified as having difficulties with temporal processing. This conclusion was based upon his poor performance on the behavioral Gaps-in-Noise Test and the Pitch Pattern Test, and was augmented by electrophysiological findings indicating poor sensitivity to changes in sounds over time. The addition of electrophysiological test measures also helped to rule out the possibility that the MV’s poor performance was due to cognitive deficits or to reduced effort. His reduced temporal acuity likely accounted for poor recognition of auditory temporal patterns, as well as lack of benefit that most listeners receive from spatial separation between talkers of interest and competing background sounds. 

The temporal smearing of sounds resulting from poor temporal acuity was likely responsible for his reported difficulties hearing in complex listening environments, on the telephone, as well as his confusion of various word sounds. Notice that these deficits would have been missed if the clinician had used only standard tests of speech-in-noise understanding that do not include natural features such as spatial separations between sound sources. 

Course of Care

Overall, the MV’s temporal processing impairment indicates that he requires a higher signal-to-noise ratio to understand speech in difficult listening environments compared to what a non-injured patient with normal hearing sensitivity would likely need to achieve the same performance. To improve his functioning, his audiologist prescribed a two-pronged approach including the use of low amplification hearing aids with a Bluetooth streaming system, as well as counseling on environmental modifications and communication strategies to improve signal-to-noise ratios during listening. The low-amplification hearing aids, including directional microphones and the Bluetooth assistive listening accessories would, in practice, increase the intensity of speech without commensurate increases in the intensity of competing sounds. This effect would putatively assist in difficult listening conditions. Communication strategies discussed included counseling on the types of environments that are most conducive to listening, making his communication partners aware of his hearing issues and asking them to get his attention before speaking to him, conversing in well-lit areas where his communication partner’s face could be easily seen, and recommendations to schedule important meetings earlier in the day when he is well rested and less likely to be fatigued. 

TABLE 1. International Outcome Inventory for hearing Aids (abbreviated questions).

QUESTION

RESPONSE

Hours of daily HA use?

four to eight hours

Perceived benefit of HA in difficult listening situations? 

helped very much

Remaining problems in difficult situations even with HA?

slight difficulty

HA worth the trouble?

very much worth it

With HA use, does hearing loss still affect things you can do?

affected slightly

With HA use, were others bothered by your hearing loss?

bothered slightly

HA effects on enjoyment of life?

very much better

 

Eight weeks later, the MV was seen for a follow-up visit. Datalogging indicated that he was consistently wearing his hearing aids an average of eight hours per day and he described them to be “perfect.” Although it may be assumed that patients with normal hearing thresholds and PTSD would be poor candidates for low-amplification hearing aids due to their increased startle response, the MV stated that he felt his hearing aids provided him a better sense of his surroundings which reduced his overall level of anxiety and propensity to be startled. His responses on International Outcome Inventory for Hearing Aids (IOI-HA) (see TABLE 1) and the Client Oriented Scale of Improvement revealed significant improvements in his function in background noise and while on the telephone, and he reported less fatigue at the end of the day because it was “easier to hear.” His family had also noted positive improvements, not only in his communication abilities but also regarding reduced frustration. Two years later, the MV is still wearing his hearing aids regularly. 

Discussion

MV’s perceived benefit from hearing aids likely stems from multiple factors. First, the noise reduction algorithms and directional microphones employed by the hearing aids, as well as use of the Bluetooth assistive listening accessories probably resulted in a more favorable signal-to-noise ratio, thus reducing the MV’s listening effort. Second, the nonlinear fast-acting compression characteristics of modern hearing aids, which favor amplification of low-level signals compared to higher level signals, have been shown to facilitate discrimination of speech signals from noise background and to improve listeners’ ability to “listen in the dips” of fluctuating background noise (Gatehouse et al, 2003). Lastly, multiple lines of evidence suggest that higher signal levels and lower levels of background noise are associated with more robust and synchronous neural firing in response to auditory stimuli (Dallos and Cheatham, 1976; Billings et al, 2009). Thus, it is conceivable that the MV’s temporal processing issues were somewhat ameliorated by the slight increase in signal levels provided by the mild amplification of the hearing aid as well as the improved signal-to-noise ratio.


References

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