Question
I have a few questions about ABR. I think it is unclear when to use masking when you are testing ABR. Are there any rules when you should use it, and how do you know how much masking to use? How does the masking noise affect the result?
Answer
A common assumption is that contralateral masking is unnecessary when using insert earphones because of larger interaural attenuation values compared to supra-aural headphones. For the majority of patients, this is probably true and contralateral masking will not be required. However, if there is significant asymmetry between ears, contralateral masking might be warranted. The general rule of thumb is to employ contralateral masking any time you think a signal will cross over. Clinicians may have the advantage of knowing the audiometric degree and configuration prior to the ABR test, and this is quite helpful. However, if the clinician is blind to the sensitivity of each ear, the situation becomes a bit more complex. When there is no a priori knowledge of audiometric degree and configuration, clinicians will often be compelled to determine which of the two ears is the poorer of the two and ABR is one way of making this determination. If there is profound or total unilateral hearing loss, stimulus levels as high as 95 dB nHL may effectively cross over and the resultant ABR generated by the non-test ear will show a false ABR with abnormal latencies due to the time delay of crossing over the head. This is a sure sign that that contralateral masking is needed. If, however, the stimulation of the poorer test ear shows a wave I with expected latency, then it can be assumed that crossover has not occurred. Because the interaural attenuation of insert earphones is about 65 dB and greater, presenting contralateral masking in the non-test ear 50 to 60 dB below the click intensity level in the test ear will generally be all that is needed, whether or not it is warranted. However, it is not recommended that contralateral masking levels ever exceed 50 dB nHL in order to avoid potential central masking effects on the ABR.
Samuel R. Atcherson, Ph.D. is assistant professor at the University of Arkansas at Little Rock with joint appointment at the University of Arkansas for Medical Sciences. He received his Ph.D. from the University of Memphis in 2006. Dr. Atcherson's research and clinical interests are in the areas of auditory electrophysiology and electrodiagnostics, central auditory processing, and hearing assistive technology. He is former President of the Association of Medical Professionals with Hearing Losses (AMPHL), board member of the Arkansas Hands & Voices chapter, a proud colleague of a thriving group of hard-of-hearing audiologists.
