Cochlear implants (CIs) have become a widely successful option for many individuals with severe to profound sensorineural hearing loss, providing them with better access to their surrounding acoustic environment. Improvements in technology have allowed for advancements in CI processing, coding strategies, and configurations. These advancements provide CI audiologists with greater flexibility in programming each patient's CI for better performance outcomes. While some features may be unique to a specific CI manufacturer, the audiologist can manipulate fundamental programming parameters across a manufacturer's software, including speech-processing strategy, electrical stimulation levels, stimulation rate, and stimulation mode, during programming appointments.
Using a combination of objective programming techniques and psychophysical input from the patient (e.g., loudness judgment of threshold [T] and upper levels of comfort [C/M levels]), a program can be developed to provide a CI user with optimal benefits. CI programming should provide users with a comfortable sound as well as access to the entire spectrum of speech sounds, which is critical for speech perception (Topics in Lang Disord. 2003;23(1):46). Despite the availability of non-behavioral programming measures such as electrically-evoked compound action potential (ECAP) and electrically-evoked stapedial reflex threshold (ESRT) testing, a survey of current practice patterns among CI audiologists revealed dependence on psychophysical loudness judgments made by the patient when programming the device and setting appropriate stimulation levels through the implant (ScientificWorldJournal. 2014 Feb 4;2014:501738; J Am Acad Audiol. 2017; https://doi.org/10.3766/jaaa.17011).
PSYCHOPHYSICAL CHALLENGES OF CI USERS WITH NEUROLOGIC ISSUES
Psychophysical measurements of loudness growth are difficult even for people with normal hearing sensitivity (Ear Hear. 1997;18(5):388). People with sensorineural hearing loss experience even greater difficulty in making loudness judgements, given the damage to their sensory structures responsible for transduction of the auditory signal through the system (Ear Hear. 2010;4(4):567; J Acoust Soc Am. 2015;137(4)1899). Noting the difficulty in categorizing loudness even among individuals with no known significant medical history, one might assume that the reliability may be even more variable in individuals with a history of neurologic impairment.
Neurologic impairment is a broad term that refers to a variety of disorders affecting the peripheral and central nervous systems (World Health Organization, 2016). These disorders can lead to functional limitations and changes in patient perception. Many types of disorders that fall under this category, such as traumatic brain injury (TBI) and stroke, have been shown to have an impact on the auditory system (Bamiou. In Celesia & Hickok, eds. Elseiver, 2015; J Neurotrama 2004;31:251; SIG 6 2012;16:18). For example, studies have shown that many individuals with TBI have greater sensitivity to more intense sounds, resulting in lower loudness discomfort levels in the presence of normal peripheral hearing sensitivity compared with individuals without TBI (J Neurotrauma, 2004; SIG 6, 2012).
Although no information regarding the reliability of psychophysical loudness judgment in patients with neurologic impairment is available to date, it may be reasonable to assume that these patients would also struggle with providing reliable psychophysical loudness judgments. The purpose of this study was to examine the relationship between psychophysical and physiological measures of loudness perception among CI users with a history of neurologic impairment compared with CI users without neurologic impairment. Eight CI users—four with a history of neurologic impairment, specifically a history of traumatic brain injury (TBI) or stroke (mean age = 76.33 years old, range = 66.08-84.16 years old), and four age range-matched (mean age = 75.92 years old, range = 67.08-80.33 years old) controls—completed electrically-evoked stapedial reflex threshold (ESRT) measurements and two runs of categorical loudness scaling at 500 Hz, 1 kHz, 2 kHz, and 4 kHz, as well as speech perception testing using AzBio sentences in quiet with their CI prosthesis only.
Figure 1 shows categorical loudness scaling data for the measured electrode corresponding to 1 kHz for participants who demonstrated traditional loudness growth, as well as individual data showing outliers. Participants shaded in orange were part of the control group, while participants shaded in purple were part of the NI group. Individual data for those who did not demonstrate traditional loudness growth were obtained from two participants from the NI group. These two participants not only demonstrated abnormally non-monotonic loudness growth, but also exhibited variability in their loudness growth between runs. Figure 2 shows the relationship between ESRT and behaviorally measured C/M level for all participants. Data points in orange represent information from the control group, while those in purple represent data from the NI group. Participants in the control group demonstrated a consistent relationship between ESRT and Behavioral M level that has been previously established in the literature, except for one participant who demonstrated a higher behaviorally measured M level compared with the stimulation level where an ESRT response was first recorded. Overall, participants in the NI group demonstrated greater variability between ESRT and behaviorally measured C/M level; however, no clear trend emerged from this study.
Finally, Figure 3 shows the performance on AzBio sentences in quiet and CI-only condition of all participants. Again, the bars shaded in orange represent individuals in the control group, while bars shaded in purple represent scores of individuals in the NI group. Study participants with a history of neurologic impairment demonstrated significantly poorer performance (p > 0.002) on AzBio sentences in quiet compared with age-matched controls, demonstrating that performance may not be optimal for individuals with neurologic impairment.
Results from this study showed that CI users with a history of neurologic impairment may demonstrate greater variability when making loudness judgements through their CI compared with CI users without any neurologic impairment. No demographic variables were determined to contribute to the variability in loudness judgements observed in participants who took part in this study. Although based on a small sample size, the results suggested a need to consider appropriate CI programming procedures for CI patients with a history of neurologic impairment.