Interview with Steve Smith Au.D.
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Topic: Issues in ASSR
Topic: Issues in ASSR
AO/Beck: Hi Steve. Thanks for joining me today. Before we address issues related to ASSR, would you please tell me a little bit about your formal education and your professional history?
Smith: Sure, Doug. I got my B.S. and my M.S. at the University of South Alabama and my Au.D. through the Pennsylvania College of Optometry, School of Audiology. I'm a Limited Partner with Drs. Kitchens, Chapman, & Anderson, where I serve as the Director of Audiology and the Director of Physician's Hearing and Balance Center. We see patients with a variety of hearing, vestibular and balance disorders. Additionally, I perform intra-operative monitoring and we dispense and service hearing aids. I am an Adjunct Instructor with the Pennsylvania College of Optometry, School of Audiology, Arizona School of Health Sciences, School of Audiology, and Auburn University Doctoral Au.D. program. I teach a variety of courses ranging from electrophysiological testing to intraoperative monitoring. I also consult with Trilogy Audiometrics, Inc., Bio-Logic Systems Corporation and other leading manufacturers regarding electrophysiological instrumentation and education. Doug, if you don't mind, I'd like to take a moment to personally thank you for the assistance you've provided me over the years in terms of great advice regarding electrophysiological assessments and intraoperative monitoring. Furthermore, the work you do with Audiology Online is tremendous and it's a great source of information and education for all of us in the audiology community. Thank you!
AO/Beck: Woah, thanks Steve. That's very kind of you. Sometimes I get lucky. OK, back to the topic at hand --- What does ASSR stand for?
Smith: It stands for Auditory Steady State Evoked Response. Although for many years it was called Steady State Evoked Potentials (SSEP), Steady State Response (SSR) or Amplitude-Modulation Following Response (AMFR). It's a brain response to an acoustic stimulus. Many of the things we're going to discuss today are based on what we know at this moment regarding ASSR, but it's important to understand that research is still ongoing, and we don't have all the answers! As more clinical evaluations are performed and more research is completed, we'll gain more information regarding this new and exciting technology and the clinical utilization of the same.
AO/Beck: Thanks for pointing that out - I think you're exactly right. Please tell me about the stimulus used to elicit the ASSR?
Smith: We essentially use a modulated pure tone signal. The stimulus can be frequency or amplitude modulated. For example, you can think of amplitude modulation as turning the signal on and off, and frequency modulation can be thought of as warbling the tone. A combination of amplitude and frequency modulation, referred to as Mixed Modulation (MM) may be used. Typically, you'll see MM used because it seems to yield the best and largest response. Although you'll get an expanded spectrum of the presentation tone, the frequency is more specific and narrower, than with the broad band click used in traditional ABR or the tone burst ABR stimulus.
AO/Beck: Is ASSR frequency specific?
Smith: Most definitely. That's the whole emphasis -- to be able to have an electrophysiologic response that is more frequency specific than click or tone burst ABR. I think a lot of our colleagues are stuck using ABR clicks, and they just don't give you enough information. Although the broad band click stimulus provides spectral energy from low to high frequencies, we essentially only have energy in the 1000-4000 Hz range. Tone bursts are much more frequency specific than clicks, but unless you're using the correct parameters, such as the Blackmon Ramping, correct rise and fall times, and you've got expertise in tone burst testing, you may not be obtaining the best or most reliable responses. As audiologists, we like frequency specificity, we want an audiogram! ASSR gets us closer to recreating an audiogram than anything else, and it can be used with the pediatric population, malingerers, difficult-to-test individuals and other challenging populations.
AO/Beck: Are ASSR thresholds and ABR Tone Burst thresholds equivalent?
Smith: ASSR thresholds and Tone Burst ABR thresholds have been shown to be comparable, although it appears ASSR is much easier to perform and interpret.
AO/Beck: If you're going to record an ASSR, can you use traditional ABR recording sites?
Smith: You can more or less. It's going to depend on the hardware and software you're using. In general, you're still using scalp electrodes on traditional recording sites, such as the mastoid, forehead, vertex, and the nape of the neck. Each manufacturer has a specific recommendation or requirement for electrode placement, but essentially is it very similar to our traditional ABR methods.
AO/Beck: Steve, please tell me about the response. What does a response to an ASSR look like?
Smith: Well, the response is not an ABR-like response it terms of how it looks or how it is measured. The response is actually embedded within the EEG of the patient, within the brain activity. It is not a transient response as you would see with ABR, but a steady-state response. In other words, it is evoked by periodic stimulation and the constituent frequency components maintain a stable amplitude and phase through the duration of the response. The two systems currently available in the United States are the GSI-Nicolet Audera and the Bio-Logic MASTER systems. ICS will be introducing their system very soon too. The underlying principles are similar regarding ASSR, but in terms of data acquisition, statistical analysis, software and hardware they are quite different. Probably the best thing is to have the readers contact the manufacturers directly to learn about their systems.
AO/Beck: Steve, let's add in the graphic so the readers can see what the responses look like.....
Smith: OK, Doug, here are a few illustrations (see below).
Figure 1 (below).
ONGOING BRAIN EEG
Figure 2 (below).
2000 Hz Modulated Tone at a rate of 100Hz
Figure 3 (below).
The EEG response follows the modulation envelope of the stimulus, e.g. this response was evoked by 2k Hz stimulus in the previous slide:
Figure 4 (below).
ASSR results (depicted in yellow) embedded within the brain's ongoing EEG. This graph represents four frequencies (500-4000 Hz) tested simultaneously in both the right and left ear.
AO/Beck: Steve please describe what we're looking at (above).
Smith: Essentially what you have in the first slide is a graphical display of ongoing brain EEG activity. The second slide represents the stimulus, which in this case is a 2000 Hz tone, modulated at 100Hz. The third slide represents the presence of a response which follows the modulation envelope of the stimulus. The fourth slide demonstrates what the response from both ears might look like. Remember, the ASSR response is based on frequency content rather than traditional ABR peak latencies. In other words we're not looking for waveform latencies for threshold measurement. So in essence, you can actually reconstruct the patient's audiogram using ASSR, which has been a goal for a long time.
AO/Beck: Steve, are you involved with any studies at the moment to help validate ASSR techniques?
Smith: Yes, one study I'm currently working on is taking established patients, toddlers, children and adults, and evaluating how accurately the ASSR thresholds compare to the behavioral audiometric results. Additionally, I'm comparing click and tone burst ABR thresholds with ASSR thresholds obtained on the same patients.
I have previously performed mulitiple ASSR tests on the same patients to determine the reliability of the ASSR tests. In other words, I tested patients using ASSR, obtained thresholds, and then tested the patients a few weeks later under the same test conditions and with the same ASSR instrumentation. I hope to publish these studies soon, but the bottom line is the test appears to be solid and repeatable.
AO/Beck: So, based on your experience and knowledge, the bottom line appears to be that using ASSR, I can get threshold information at 250, 500, 1k, 2k, 4k, 8k?
Smith: Yes. Actually, I've only been investigating 500 to 4000 Hz. However, the answer to your question is yes, you can evaluate 250-8k. One real advantage with ASSR, is that when we reach the severe to profound range we can still get responses. With most ABR equipment, you'll probably get no measurable response in the severe to profound range, depending on the type and degree of hearing loss. With ASSR, we can go up to higher intensities, about 120 dBHL, and that helps a lot for accurate hearing aid fittings and for determination of cochlear implantation candidacy.
AO/Beck: So you can actually stimulate up to 120 dB HL?
Smith: Yes, testing one frequency at a time or a maximum of two frequencies at a time simultaneously in both ears. The highest I've actually had to go thus far is about 115 dB HL.
AO/Beck: At 115 dB, were you able to sort out 500, 1k, 2k and 4k?
Smith: Yes, you're able to be frequency specific. In fact, I had an adult patient last Friday for cochlear implant evaluation. We wanted to figure out if he had any residual hearing. We got a response at about 115 dB at 3 and 4k, and nothing after that.
AO/Beck: Are you presuming the ASSR is essentially an outer hair cell response or is it a brain stem response?
Smith: We believe the ASSR is a derivative of the brain stem response. It does not appear to be a higher cortical response like the 40 hertz response. If you look through the literature you'll see that ASSR essentially evolved from the early 40 Hz studies. The problem with 40 Hz was that if the patient was awake or asleep, it had a dramatic effect on the response and we couldn't use it reliably in infants because if the infant went to sleep the response diminished. Modulation rates greater than 60Hz are believe to be derived from the same anatomical area or generators as the waveforms III, IV, and V of the ABR. Of course we typically use modulation rates between 60-100Hz.
AO/Beck: How long does it take if you're going to do an ASSR evaluation at let's say, 500, 1, 2, and 4k in both ears?
Smith: I've been able to get good ASSR thresholds on both ears, with a total time ranging from less than 20 minutes to about 60 minutes. When you're doing this test, it's a little like a lot of things; like doing an audiogram or doing a latency-intensity function study with ABR, or tone burst threshold assessments or the behavioral audiogram. Once you get a response, the audiologist decides the next step. Of course experience, the degree and configuration of hearing loss, and whether you are testing one or more frequencies at a time all matter. Additionally, the patient's state of arousal is important. Having the patient asleep or sedated helps you obtain the lowest thresholds and reduces test time.
AO/Beck: Have you personally correlated specific type and degree of hearing loss with ASSR results?
Smith: Yes. I have over 110 patients to date. I'm using patients with known and established hearing loss and I'm matching ASSR thresholds with behavioral thresholds at individual frequencies. I'm finding a close correlation between the ASSR estimated thresholds and the actual audiometric behavioral thresholds.
AO/Beck: Very good Steve. Who's working on standards for this?
Smith: The manufacturers have extensive research studies supporting their instrumentation and the statistical algorithm they employ. Additionally, based on their testing they have recommended threshold correction factors that are applied to the raw ASSR threshold. I actually had a phone call recently from one of the manufacturers asking what I thought about the idea of setting up a committee or a forum on ASSR standards. I think we need to guide people to use this technology appropriately, properly, and efficiently. Of the 100 patients or so I've tested, 88 of them have not been sedated, but we need to run comparative studies on sedated versus natural sleep and awake patients. Further investigation into what modulation rates provides us with the best responses would be great too. Research into the effects of Auditory Neuropathy, Multiple Sclerosis, Vestibular Schwannoma's and other brainstem dysfunctions or disorders will be needed to evaluate their effects on the ASSR responses.
So there really is quite a bit left to be done!
AO/Beck: Do you use inserts only or are you using headphones?
Smith: I almost never use headphones. We use inserts for all the traditional reasons. However, you could use headphones, insert earphones, bone conduction or sound field testing with ASSR systems. I believe the soundfield test method is still undergoing clinical evaluation, so I'd probably not depend on that quite yet. Additionally, there are some concerns regarding bone conduction with the ASSR stimulus, but from conversations with colleagues who have used bone conduction and based on my experience, I feel this is a viable ASSR method. We really need to be able to use bone conduction ASSR for patients who may have a conductive loss or a mixed hearing loss.
AO/Beck: So the bottom line is ASSR offers us the ability to gather frequency specificity, on each ear, allowing us to confirm thresholds on people who are unable or unwilling to provide behavioral information?
Smith: Yes! The primary application will probably be evaluating pediatric populations or people in whom we cannot obtain behavioral results. ASSR is frequency specific, so we can better fit and assess patients hearing sensitivity. This will provide us with better information to accurately provide the correct amplification needs or recommendations regarding cochlear implant candidacy. I'm really excited about it and I believe it is a viable test modality. Of course, we will learn more as we continue to evaluate the ASSR test. I believe ASSR will add to our overall diagnostic test protocols and fit nicely into our well established Cross Check evaluation principles. With the emphasis on Newborn Infant Screening and the need to more accurately and appropriately perform diagnostic testing on infants referred for diagnostic testing, we really need the capability to obtain accurate thresholds for as many frequencies as we can. In think we must have as many pieces of the puzzle as possible to accurately and appropriately determine the status of the patient's hearing. I believe ASSR will fulfill a critical need in our test modalities and will contribute to our overall diagnostic capabilities, much as OAEs have done over the past several years.
AO/Beck: Thanks Steve, I appreciate your time and knowledge. I'll look forward to working with you again soon.
Smith: Thank you too Doug. It's been a pleasure.
AO/Beck: Hi Steve. Thanks for joining me today. Before we address issues related to ASSR, would you please tell me a little bit about your formal education and your professional history?
Smith: Sure, Doug. I got my B.S. and my M.S. at the University of South Alabama and my Au.D. through the Pennsylvania College of Optometry, School of Audiology. I'm a Limited Partner with Drs. Kitchens, Chapman, & Anderson, where I serve as the Director of Audiology and the Director of Physician's Hearing and Balance Center. We see patients with a variety of hearing, vestibular and balance disorders. Additionally, I perform intra-operative monitoring and we dispense and service hearing aids. I am an Adjunct Instructor with the Pennsylvania College of Optometry, School of Audiology, Arizona School of Health Sciences, School of Audiology, and Auburn University Doctoral Au.D. program. I teach a variety of courses ranging from electrophysiological testing to intraoperative monitoring. I also consult with Trilogy Audiometrics, Inc., Bio-Logic Systems Corporation and other leading manufacturers regarding electrophysiological instrumentation and education. Doug, if you don't mind, I'd like to take a moment to personally thank you for the assistance you've provided me over the years in terms of great advice regarding electrophysiological assessments and intraoperative monitoring. Furthermore, the work you do with Audiology Online is tremendous and it's a great source of information and education for all of us in the audiology community. Thank you!
AO/Beck: Woah, thanks Steve. That's very kind of you. Sometimes I get lucky. OK, back to the topic at hand --- What does ASSR stand for?
Smith: It stands for Auditory Steady State Evoked Response. Although for many years it was called Steady State Evoked Potentials (SSEP), Steady State Response (SSR) or Amplitude-Modulation Following Response (AMFR). It's a brain response to an acoustic stimulus. Many of the things we're going to discuss today are based on what we know at this moment regarding ASSR, but it's important to understand that research is still ongoing, and we don't have all the answers! As more clinical evaluations are performed and more research is completed, we'll gain more information regarding this new and exciting technology and the clinical utilization of the same.
AO/Beck: Thanks for pointing that out - I think you're exactly right. Please tell me about the stimulus used to elicit the ASSR?
Smith: We essentially use a modulated pure tone signal. The stimulus can be frequency or amplitude modulated. For example, you can think of amplitude modulation as turning the signal on and off, and frequency modulation can be thought of as warbling the tone. A combination of amplitude and frequency modulation, referred to as Mixed Modulation (MM) may be used. Typically, you'll see MM used because it seems to yield the best and largest response. Although you'll get an expanded spectrum of the presentation tone, the frequency is more specific and narrower, than with the broad band click used in traditional ABR or the tone burst ABR stimulus.
AO/Beck: Is ASSR frequency specific?
Smith: Most definitely. That's the whole emphasis -- to be able to have an electrophysiologic response that is more frequency specific than click or tone burst ABR. I think a lot of our colleagues are stuck using ABR clicks, and they just don't give you enough information. Although the broad band click stimulus provides spectral energy from low to high frequencies, we essentially only have energy in the 1000-4000 Hz range. Tone bursts are much more frequency specific than clicks, but unless you're using the correct parameters, such as the Blackmon Ramping, correct rise and fall times, and you've got expertise in tone burst testing, you may not be obtaining the best or most reliable responses. As audiologists, we like frequency specificity, we want an audiogram! ASSR gets us closer to recreating an audiogram than anything else, and it can be used with the pediatric population, malingerers, difficult-to-test individuals and other challenging populations.
AO/Beck: Are ASSR thresholds and ABR Tone Burst thresholds equivalent?
Smith: ASSR thresholds and Tone Burst ABR thresholds have been shown to be comparable, although it appears ASSR is much easier to perform and interpret.
AO/Beck: If you're going to record an ASSR, can you use traditional ABR recording sites?
Smith: You can more or less. It's going to depend on the hardware and software you're using. In general, you're still using scalp electrodes on traditional recording sites, such as the mastoid, forehead, vertex, and the nape of the neck. Each manufacturer has a specific recommendation or requirement for electrode placement, but essentially is it very similar to our traditional ABR methods.
AO/Beck: Steve, please tell me about the response. What does a response to an ASSR look like?
Smith: Well, the response is not an ABR-like response it terms of how it looks or how it is measured. The response is actually embedded within the EEG of the patient, within the brain activity. It is not a transient response as you would see with ABR, but a steady-state response. In other words, it is evoked by periodic stimulation and the constituent frequency components maintain a stable amplitude and phase through the duration of the response. The two systems currently available in the United States are the GSI-Nicolet Audera and the Bio-Logic MASTER systems. ICS will be introducing their system very soon too. The underlying principles are similar regarding ASSR, but in terms of data acquisition, statistical analysis, software and hardware they are quite different. Probably the best thing is to have the readers contact the manufacturers directly to learn about their systems.
AO/Beck: Steve, let's add in the graphic so the readers can see what the responses look like.....
Smith: OK, Doug, here are a few illustrations (see below).
Figure 1 (below).
ONGOING BRAIN EEG
Figure 2 (below).
2000 Hz Modulated Tone at a rate of 100Hz
Figure 3 (below).
The EEG response follows the modulation envelope of the stimulus, e.g. this response was evoked by 2k Hz stimulus in the previous slide:
Figure 4 (below).
ASSR results (depicted in yellow) embedded within the brain's ongoing EEG. This graph represents four frequencies (500-4000 Hz) tested simultaneously in both the right and left ear.
AO/Beck: Steve please describe what we're looking at (above).
Smith: Essentially what you have in the first slide is a graphical display of ongoing brain EEG activity. The second slide represents the stimulus, which in this case is a 2000 Hz tone, modulated at 100Hz. The third slide represents the presence of a response which follows the modulation envelope of the stimulus. The fourth slide demonstrates what the response from both ears might look like. Remember, the ASSR response is based on frequency content rather than traditional ABR peak latencies. In other words we're not looking for waveform latencies for threshold measurement. So in essence, you can actually reconstruct the patient's audiogram using ASSR, which has been a goal for a long time.
AO/Beck: Steve, are you involved with any studies at the moment to help validate ASSR techniques?
Smith: Yes, one study I'm currently working on is taking established patients, toddlers, children and adults, and evaluating how accurately the ASSR thresholds compare to the behavioral audiometric results. Additionally, I'm comparing click and tone burst ABR thresholds with ASSR thresholds obtained on the same patients.
I have previously performed mulitiple ASSR tests on the same patients to determine the reliability of the ASSR tests. In other words, I tested patients using ASSR, obtained thresholds, and then tested the patients a few weeks later under the same test conditions and with the same ASSR instrumentation. I hope to publish these studies soon, but the bottom line is the test appears to be solid and repeatable.
AO/Beck: So, based on your experience and knowledge, the bottom line appears to be that using ASSR, I can get threshold information at 250, 500, 1k, 2k, 4k, 8k?
Smith: Yes. Actually, I've only been investigating 500 to 4000 Hz. However, the answer to your question is yes, you can evaluate 250-8k. One real advantage with ASSR, is that when we reach the severe to profound range we can still get responses. With most ABR equipment, you'll probably get no measurable response in the severe to profound range, depending on the type and degree of hearing loss. With ASSR, we can go up to higher intensities, about 120 dBHL, and that helps a lot for accurate hearing aid fittings and for determination of cochlear implantation candidacy.
AO/Beck: So you can actually stimulate up to 120 dB HL?
Smith: Yes, testing one frequency at a time or a maximum of two frequencies at a time simultaneously in both ears. The highest I've actually had to go thus far is about 115 dB HL.
AO/Beck: At 115 dB, were you able to sort out 500, 1k, 2k and 4k?
Smith: Yes, you're able to be frequency specific. In fact, I had an adult patient last Friday for cochlear implant evaluation. We wanted to figure out if he had any residual hearing. We got a response at about 115 dB at 3 and 4k, and nothing after that.
AO/Beck: Are you presuming the ASSR is essentially an outer hair cell response or is it a brain stem response?
Smith: We believe the ASSR is a derivative of the brain stem response. It does not appear to be a higher cortical response like the 40 hertz response. If you look through the literature you'll see that ASSR essentially evolved from the early 40 Hz studies. The problem with 40 Hz was that if the patient was awake or asleep, it had a dramatic effect on the response and we couldn't use it reliably in infants because if the infant went to sleep the response diminished. Modulation rates greater than 60Hz are believe to be derived from the same anatomical area or generators as the waveforms III, IV, and V of the ABR. Of course we typically use modulation rates between 60-100Hz.
AO/Beck: How long does it take if you're going to do an ASSR evaluation at let's say, 500, 1, 2, and 4k in both ears?
Smith: I've been able to get good ASSR thresholds on both ears, with a total time ranging from less than 20 minutes to about 60 minutes. When you're doing this test, it's a little like a lot of things; like doing an audiogram or doing a latency-intensity function study with ABR, or tone burst threshold assessments or the behavioral audiogram. Once you get a response, the audiologist decides the next step. Of course experience, the degree and configuration of hearing loss, and whether you are testing one or more frequencies at a time all matter. Additionally, the patient's state of arousal is important. Having the patient asleep or sedated helps you obtain the lowest thresholds and reduces test time.
AO/Beck: Have you personally correlated specific type and degree of hearing loss with ASSR results?
Smith: Yes. I have over 110 patients to date. I'm using patients with known and established hearing loss and I'm matching ASSR thresholds with behavioral thresholds at individual frequencies. I'm finding a close correlation between the ASSR estimated thresholds and the actual audiometric behavioral thresholds.
AO/Beck: Very good Steve. Who's working on standards for this?
Smith: The manufacturers have extensive research studies supporting their instrumentation and the statistical algorithm they employ. Additionally, based on their testing they have recommended threshold correction factors that are applied to the raw ASSR threshold. I actually had a phone call recently from one of the manufacturers asking what I thought about the idea of setting up a committee or a forum on ASSR standards. I think we need to guide people to use this technology appropriately, properly, and efficiently. Of the 100 patients or so I've tested, 88 of them have not been sedated, but we need to run comparative studies on sedated versus natural sleep and awake patients. Further investigation into what modulation rates provides us with the best responses would be great too. Research into the effects of Auditory Neuropathy, Multiple Sclerosis, Vestibular Schwannoma's and other brainstem dysfunctions or disorders will be needed to evaluate their effects on the ASSR responses.
So there really is quite a bit left to be done!
AO/Beck: Do you use inserts only or are you using headphones?
Smith: I almost never use headphones. We use inserts for all the traditional reasons. However, you could use headphones, insert earphones, bone conduction or sound field testing with ASSR systems. I believe the soundfield test method is still undergoing clinical evaluation, so I'd probably not depend on that quite yet. Additionally, there are some concerns regarding bone conduction with the ASSR stimulus, but from conversations with colleagues who have used bone conduction and based on my experience, I feel this is a viable ASSR method. We really need to be able to use bone conduction ASSR for patients who may have a conductive loss or a mixed hearing loss.
AO/Beck: So the bottom line is ASSR offers us the ability to gather frequency specificity, on each ear, allowing us to confirm thresholds on people who are unable or unwilling to provide behavioral information?
Smith: Yes! The primary application will probably be evaluating pediatric populations or people in whom we cannot obtain behavioral results. ASSR is frequency specific, so we can better fit and assess patients hearing sensitivity. This will provide us with better information to accurately provide the correct amplification needs or recommendations regarding cochlear implant candidacy. I'm really excited about it and I believe it is a viable test modality. Of course, we will learn more as we continue to evaluate the ASSR test. I believe ASSR will add to our overall diagnostic test protocols and fit nicely into our well established Cross Check evaluation principles. With the emphasis on Newborn Infant Screening and the need to more accurately and appropriately perform diagnostic testing on infants referred for diagnostic testing, we really need the capability to obtain accurate thresholds for as many frequencies as we can. In think we must have as many pieces of the puzzle as possible to accurately and appropriately determine the status of the patient's hearing. I believe ASSR will fulfill a critical need in our test modalities and will contribute to our overall diagnostic capabilities, much as OAEs have done over the past several years.
AO/Beck: Thanks Steve, I appreciate your time and knowledge. I'll look forward to working with you again soon.
Smith: Thank you too Doug. It's been a pleasure.
