by Frank Musiek, PhD
In a recent discussion a question came up asking if ABR was considered a central auditory test. Of course, it is – and a very good one within its limitations. However, I wondered why the question even came up. Certainly, as we all know the ABR waves III, IV, and V are generated by neurons in the pons— a central auditory structure.
Therefore, the ABR serves as an integrity check of part of the central auditory system. As I thought about it however, I realized why there could be some confusion. Some of the misunderstanding may be related to semantics, defining populations of CAPD, and or understanding of auditory disorders. Therefore, in this brief script, some selected aspects of this issue will be addressed in this month’s Pathways article.
Some of the confusion surrounding traditional ABR’s usefulness in CAPD is related to how CAPD’s populations are delineated. If a CAPD population is only defined as children with learning related problems and auditory difficulties, then ABR as a diagnostic procedure may not be highly useful. So, let’s look into this narrow view of CAPD a bit closer. When ABR is used to evaluate children with learning problems for CAPD the findings are solidly normal in the vast majority of this population.
As reported by Hall (2007) abnormal ABR results in children evaluated for CAPD is approximately 10 percent. This, of course is far less than central auditory behavioral tests or electrophysiological tests such as cortical evoked potentials (MLR, N1, P2 or P300s).
The question then arises: Why are ABRs essentially normal in children with CAPD? The answer, based on what we know, is rather straight forward. These children do not have brainstem involvement!
It is clear from multiple sources that children with CAPD likely have involvement of the cerebral regions of the brain. This is why behavioral and higher level electrophysiologic tests yield better results than ABR in these populations. However, there are important caveats that need mentioning. Before going into the specific caveats – a general statement: When there is involvement of the auditory brainstem, specifically the pontine pathway, the ABR is probably audiology’s most powerful tool. However, in commonly referred populations of children with learning problems true brainstem involvement is relatively rare hence normal ABRs in the general and learning disability pediatric population.
One caveat is related to the kind of ABR or perhaps one should say the modification of the standard ABR that is used. The complex or cABR has been shown to be sensitive to defining children with language/learning/auditory problems (see Schochat et al. 2014). Also, as was reported in a previous monthly Pathways the binaural interaction component (BIC) has some emerging evidence of sensitivity to defining children with CAPD. However, it is important to note that these two procedures go beyond the standard ABR requiring additional processing in the system –which may make them more sensitive than a standard ABR.
It is noteworthy that the standard ABR can be of high diagnostic value in certain pediatric populations that have involvement of the brainstem auditory pathway. Children with neurological problems affecting the brainstem auditory pathway such as mass, vascular, developmental and degenerative disorders will yield abnormal ABRs. In this regard, for example, adults with documented brainstem involvement (neurological) ABR has been shown to have a sensitivity of about 80 per cent. However, much is dependent on the kind and degree of brainstem involvement (Musiek et al.,1995). Thankfully these kinds of disorders have a lower prevalence than learning related problems (see Musiek et al. 2021). By the way, often these populations and their evaluations fit into a category of CAPD termed NeuroAudiology.
Therefore, in order to untangle the confusion about ABR being a strong versus weak test for CAPD populations one must understand the etiological nature and range of various disorders that warrant the label of CAPD/NeuroAudiology.
How does an audiologist, in today’s clinical environment decide when ABR should/could be applied? Well, obviously, when brainstem involvement seems a possibility. This information may or can be available from the medical and audiological history. Sometimes this requires considerable searching and tracking down information and other professionals. In addition, however, the audiologist must be aware of the range of disorders or symptoms that may affect the auditory brainstem pathway. If indecision arises it may be worthwhile to contact a medical professional that understands the issues at hand – ideally from an audiological perspective.
If ABR is not available or an audiologist is unsure about taking that next step i.e., ABR, other tests specific to brainstem auditory pathway dysfunction could be applied. There are two that come to mind –the acoustic reflex and masking level difference (MLD). These are good tests, take little time to administer but do not have the diagnostic value of the ABR.
If the information obtained doesn’t support possible brainstem involvement present or past (which would most often be the case in most audiology environments) then ABR probably is not needed as part of the CAPD battery.
Readings…..
- Hall, J., & Johnson, K., (2007) Electroacoustic and Electrophysiologic auditory measures in the assessment of APD. In: Musiek & Chermak, Handbook of Central Auditory Processing Disorder, Vol. 1., 1st Edition.
- Schochat, E. et al. (2014) Electroacoustic and Electrophysiologic auditory measures in the assessment of APD. In: Musiek & Chermak, Handbook of Central Auditory Processing Disorder, Vol. 1, 2nd Edition.
- Musiek, F. & Lee, W. (1995). The Auditory Brainstem Response in Patients with Brainstem and Cochlear Pathology. Ear and Hearing, 16(6), 631-636.
- Musiek, F., Shinn, J., Baran, J. & Jones, R. (2021). 2nd Edition Disorders of the Auditory System. San Diego, CA. Plural Publishing.
