By Stephanie Browning and Elizabeth Grim
Cortical auditory evoked potentials (CAEPs) evaluate access to auditory stimuli at the level of the cortex. Although CAEPs are not often used clinically to evaluate and monitor a child’s auditory function, Hearts for Hearing (HFH), a private audiology clinic, has implemented a protocol for the use of CAEP testing to assist in the intervention plan.
At HFH, CAEPs have been used to estimate auditory access primarily for children with auditory neuropathy spectrum disorder (ANSD) and children pre- and post-cochlear implantation. We have learned that CAEPs can be a useful tool for pediatric audiologists to assist in determining a plan of care and confirming auditory access for children for whom behavioral audiometry cannot yet be reliably completed.
The primary goal with CAEP testing is to assess maturation of the auditory system. The CAEP includes three landmarks: P1, N1, and P2. P1 originates from the primary auditory cortex. This response is typically robust in children, while N1 and P2 responses are often less robust until a child reaches age seven (Sharma et al, 2015). The P1 response can be elicited via click, tone-burst, or speech stimuli.
FIGURE 1 depicts a P1 response obtained during clinical assessment of an HFH cochlear implant recipient three months post-activation. The latency of the response occurs between 50–300 msec post-stimulus onset, but varies as a function of age. For newborns, the latency of P1 is expected to be around 300 msec and will decrease significantly between one to three years of age until it reaches full maturation (50–60 msec), around ages seven to 10 years (Sharma et al, 2013).
This change in latency as a function of age allows the P1 response to serve as a biomarker for cortical maturation by comparing the response obtained from a child with hearing loss to normative data.
The first three years of a child’s life are known to be the critical window for speech and language development (Ching et al, 2013). During this time, the potential for neuroplasticity is at its greatest and, therefore, consistent access to clear speech signals is critical.
Cortical plasticity is affected greatly by the amount and quality of stimulation. If one or both are compromised, the brain’s ability to complete normal synaptic connections is disrupted and the development of the auditory cortex is compromised.
Ching et al (2013) assessed and compared the language outcomes of children at three years of age whose hearing losses were both early and late identified. They determined a significant correlation between lower age of implantation and improved language outcomes. This study highlights the importance of early detection, early intervention, and appropriately fitted technology from an early age.
In pediatric audiology, the gold-standard timeline indicates diagnosis through natural sleep auditory brainstem response (ABR) and hearing-technology intervention, if appropriate, by three months of age. Meeting this timeline, particularly for children diagnosed with ANSD, is nearly impossible, due to the inability to adequately assess the degree of dyssynchrony and behavioral hearing sensitivity. Additionally, timely audiological management of children with significant hearing loss for whom cochlear implantation might be most appropriate can be difficult.
Pediatric audiologists rely on real-ear measures and prescriptive targets to ensure appropriate programming of traditional amplification; however, aided behavioral assessment may be difficult. It is often not until a child is between eight or nine months old that behavioral testing can be reliably completed.
This timeline leaves a gap where the family, pediatric audiologist, and auditory-verbal therapist may be unsure of the status of a child’s hearing and the benefit received from current hearing technology.
The following questions are often difficult to answer until additional testing can be completed; CAEP testing could be the additional tool in an audiologist’s tool belt during this time.
Would this child with ANSD benefit from amplification?
Has this child’s hearing changed since initial diagnosis and confirmation ABR?
Is this child getting good benefit from their current technology?
Is this child a candidate for a cochlear implant?
This child was implanted at six months old; does he or she have good access with the current settings?
CAEP Test Administration in the Clinic
At Hearts for Hearing, the equipment used for CAEP testing is the Frye Electronics HearLab System. This system allows for acquisition of CAEP with a clinically conducive, single-channel recording. HearLab also performs a statistical analysis of the recording and provides automatic response detection, making the detection of a response and interpretation of that response much easier for the clinician.
This system includes calibrated speech stimuli, /m/, /g/, /t/, and /s/, to allow for the assessment of cortical responses across a broad frequency range. Using speech stimuli, as opposed to tonal stimuli, is most appropriate as it is more representative of a child’s daily environment and is handled well by hearing aid and cochlear implant signal processing.
When using the HearLab system, CAEP testing is completed while a child is awake and alert, either sitting in a parent’s lap or independently in a well-supported chair in a sound booth or other sound-treated room to reduce ambient noise.
A single-channel electrode montage is used and speech stimuli are presented through a soundfield speaker at various intensities (55, 65, or 75 dB SPL).
For an infant, it is helpful to have soft toys available to keep the baby captivated and centered, but not overly engaged. CAEP testing should be initiated no earlier than four months old due limited head and trunk control prior to that age.
Older children may prefer to play a game on a tablet, with the sound muted, or quietly play with toys during the assessment. Test assistants are helpful to facilitate quiet, calm cooperation. Of note, it is important to complete immittance to ensure a healthy middle-ear space and auditory pathway to the auditory cortex.
Auditory Neuropathy Spectrum Disorder
CAEP testing can be incredibly beneficial for assessing hearing sensitivity and creating a framework for programming hearing technology for children diagnosed with ANSD. ANSD is characterized by a dyssynchronous transmission of sound across the auditory nerve and brainstem. Once a diagnosis is made, there are limited measures to determine hearing thresholds until a child can reliably participate in behavioral testing. As mentioned previously, that may be seven or more months after initial diagnosis.
Unaided, ear-specific CAEP testing can be completed to provide more specific information regarding hearing sensitivity. This provides the family and clinician with objective data to develop an intervention plan and, should that intervention plan include amplification, CAEP testing may provide a road map for conservatively programming the technology to prescriptive targets.
As previously mentioned, CAEP testing with the HearLab system is completed through speech stimuli presented at various intensities: 55, 65, and 75 dB SPL. If a response is present at 55 dB SPL, it can be inferred that the child has no greater than a mild behavioral hearing loss within that given frequency range.
If a response is present beginning at 65 dB SPL, it can be inferred that the child has between a mild to moderate behavioral hearing loss within that given frequency range. If a response is present beginning at 75 dB SPL, it can be inferred that the child has between a moderate to severe behavioral hearing loss within that given frequency range.
Finally, if a response is absent at 75 dB SPL, it can be inferred that the child likely has at least a severe behavioral hearing loss within that given frequency range (Punch et al, 2016).
By assessing CAEPs to several speech stimuli, the clinician is provided with an outline of estimated auditory access, allowing them to conservatively fit amplification sooner rather than later. However, it is imperative to note that CAEP testing can be used as a supplementary tool to assist in the plan of care development, but does not negate the importance and value of behavioral audiometry, parent questionnaires/report, and speech and language development in guiding clinical recommendations.
CAEP testing can also be successfully implemented into routine clinical practice for audiologists working with children pre- and post-cochlear implantation. For infants diagnosed with severe to profound hearing loss who might be candidates for cochlear implantation, CAEP testing can be completed in the unaided and aided conditions.
Comparing CAEPs in the two conditions provides an objective measure to confirm limited aided benefit and supports the plan to move forward with a cochlear implant. This information can be beneficial to a clinician and family, as the approved age of implantation continues to lower and relying on behavioral assessment may not be possible. Additionally, this objective measure can be a valuable counseling tool with families, as well as a vital component when working with insurance companies to determine coverage of an implant sooner rather than later.
Again, as the age of implantation continues to lower, clinicians are relying heavily on objective tools in cochlear implant programming and verification because behavioral audiometry may be inconsistent for a period.
Post-implantation, CAEP testing can be used to assess access to soft conversational speech with current settings and support a clinician in additional programming. It is recommended that CAEP testing not be completed until at least three months post-activation.
Sharma et al (2002) recorded the P1 response in 245 children with congenitally deafness and cochlear implants. Of this group, most children who received a cochlear implant prior to three and half years of age had a P1 response latency within the normal range for their age group.
Approximately half of the children who received their cochlear implant between three and a half and seven years of age had P1 responses within normal limits, while nearly all children who were implanted later than seven years of age had P1 responses that were delayed beyond the normal range. This data highlights the importance of early detection and intervention, requiring pediatric audiologists to use all tools available.
Case Study #1
DV was referred to Hearts for Hearing after referring on her newborn hearing screening bilaterally. A natural sleep ABR performed at one month of age was consistent with no response to auditory stimuli at the limits of the equipment, confirming bilateral severe to profound sensorineural hearing loss. DV was fit with bilateral hearing aids at one month and two weeks of age.
Following the hearing aid fitting, parents reported DV was very responsive to people when they were in her line of sight but did not show signs of hearing voices or environmental sounds when she could not see the sound source.
CAEP testing was completed in the unaided and aided conditions at five months of age. In the unaided condition, P1 responses were absent at 75 dB SPL for both the /g/ and /t/ stimuli. In the aided condition, P1 responses were also absent at 75 dB SPL for /g/ and /t/ stimuli, indicating poor cortical access to speech stimuli and little to no aided benefit from current hearing technology.
DV’s first cochlear implant (right ear) was activated at nine months of age. Aided behavioral audiometry, completed one-month post-activation, revealed responses to tonal stimuli in the normal to mild range and a speech-awareness threshold in the normal range with the cochlear implant alone.
DV’s parents reported an increase in her auditory awareness and an increase in vocalizations after activation. DV’s second cochlear implant (left ear) was activated at 12 months of age. Aided behavioral audiometry was attempted one-month post-activation; however, responses were limited due to disinterest in the task. Parents again reported significant improvement in responsiveness, as well as speech and language development.
CAEP testing was completed three months post-activation of the second implant. With the left CI alone, P1 responses were present for /g/, /t/, and /s/ stimuli at 55 dB SPL. With the right CI alone, P1 responses were present for /g/ and /t/ stimuli at 55 dB SPL; inconclusive for /s/ stimuli due to patient movement and noise. Overall, results suggested excellent cortical access to speech stimuli with current hearing technology.
A speech and language evaluation was completed one week later and results indicated receptive and expressive language skills within normal limits on the Preschool Language Scales (PLS-5) and Receptive–Expressive Emergent Language Test (REEL-4). The Rossetti Infant-Toddler Language Scale indicated some expressive delays, specifically in the areas on consistent verbal imitation, use of true words, and engagement in vocal turn-taking.
DV continues to receive routine audiological management and weekly auditory-verbal therapy; prognoses to develop age-appropriate listening and spoken language skills are excellent.
Case Study #2
AG was referred to Hearts for Hearing after referring on her newborn hearing screening bilaterally. A natural sleep ABR performed at one month of age was consistent with bilateral auditory neuropathy spectrum disorder.
CAEP testing was completed at four months of age. With the left ear alone (right ear masked), P1 responses were present for /g/ stimuli at 55 dB SPL and /t/ stimuli at 65 dB SPL. With the right ear alone (left ear masked), P1 responses were present for /g/ and /t/ stimuli at 55 dB SPL. Overall, results suggested good cortical access to speech stimuli and no hearing technology was recommended. At that time, AG began receiving biweekly auditory-verbal therapy.
Behavioral audiometry at eight months of age revealed possible speech-awareness responses in the moderate hearing loss range in each ear alone with fair reliability. Responses to tonal stimuli could not be reliably obtained due to disinterest in the task.
AG continued to be seen for auditory-verbal therapy through tele-intervention and, due to the COVID-19 pandemic, was not seen for behavioral audiometry until 12 months of age. At that time, possible speech-awareness responses were noted in the moderate to moderately severe hearing loss range in each ear alone with fair reliability.
Additional CAEP testing was completed one week later. With the left ear alone (right ear masked), P1 responses were absent for /g/ and /t/ stimuli at 75 dB SPL. With the right ear alone (left ear masked), P1 responses were absent for /g/ stimuli at 75 dB SPL and present for /t/ stimuli at 75 dB SPL. Overall, results confirmed a shift in auditory access relative to initial CAEP testing and corroborated elevated behavioral responses. AG will be fit with binaural hearing aids in the near future.
CAEPs can be a useful tool to assist in determining a plan of care and confirming auditory access for children for whom behavioral audiometry cannot yet be reliably completed. Although not widely used in a clinical setting to date, the audiologists at Hearts for Hearing have come to value the information obtained through CAEP testing and hope to see the clinical implementation of this assessment increase in the field of pediatric audiology.
This article is a part of the July/August 2021 Audiology Today issue.
Ching TYC, Dillon H, Marnane V, et al. (2013) Outcomes of early- and late-identified children at 3 years of age: findings from a prospective population-based study. Ear Hear 34(5): 535–552.
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