Overview of Auditory Steady-State Response
The Auditory Steady-State Response (ASSR) is a test used to estimate a person’s hearing abilities. Compared to other tests that measure hearing, like the Auditory Brainstem Response (ABR), ASSR is a newer method. Both of these tests give similar results regarding a person’s hearing levels, but ASSR can provide more dependable results in less time.
The researchers Galambos and his team first mentioned ASSR in 1981. They found a significant neural response to a certain kind of sound in adults with normal hearing. This response was most noticeable close to the hearing threshold, which is the softest sound that a person can hear. Galambos and his team suggested that because this response was so consistent, it could help predict whether a person has normal hearing.
Later studies showed that the dependable recording of this particular response was mainly possible in adults who were awake. This finding posed a problem because the most significant need for this type of testing is often in children, who are typically asleep or sedated during the test. Further research showed that this issue could be addressed by changing the test parameters.
Now, ASSR is an important tool in a hearing specialist’s toolkit, along with other tests like otoacoustic emissions (OAE) and ABR. The advantage of ASSR is its ability to test multiple sound frequencies at once. This feature makes it more efficient to perform when infants are asleep. Nonetheless, because ASSR is a new technology and there isn’t a widely agreed-upon standard of testing protocols and equipment, the results from this test must be interpreted with caution.
Anatomy and Physiology of Auditory Steady-State Response
The Auditory Steady-State Response (ASSR) is a measure of the brain’s activity in processing sound. These responses are believed to come from different areas of the brain, each of which responds to different sound frequencies. The frequency of a sound affects how high or low the sound seems. Here’s how it breaks down:
When sounds have a frequency of less than 20 Hz, the response comes from the primary auditory cortex, the part of the brain that controls hearing. For sounds with a frequency between 20 and 60 Hz, the response comes from several areas: the primary auditory cortex, the auditory midbrain which helps process sound, and the thalamic regions which are brain regions that help relay signals. Finally, for sounds with a frequency higher than 60 Hz, the response is mostly from the superior olivary complex, inferior colliculus, and cochlear nucleus, which are all parts of the brain and ear involved in hearing.
This variety of responses is what made the initial research into the Auditory Steady-State Response challenging for scientists like Galambos and his team. They noticed a strong response at 40 Hz in awake adults with developed neural systems (meaning their brains are fully formed and functioning). On the other hand, stimulation at 70 Hz produced responses in the brainstem (the part of the brain that controls basic functions) that were independent of the person’s condition and age, similar to the auditory brainstem response (ABR), which is another measure of auditory function.
Why do People Need Auditory Steady-State Response
The Auditory Steady-State Response (ASSR) is a technique often used to check hearing levels in people who have difficulty participating in traditional hearing tests. This difficulty might be due to an inability, or refusal, to respond in the usual hearing tests. This makes ASSR a vital tool for testing hearing levels in children, especially the very young ones. ASSR can also be used in situations where people’s collaboration is unpredictable, such as those with mental or physical disabilities or those with hearing loss due to non-medical reasons.
One of the key uses of ASSR is in setting up hearing aids, particularly for young children. The ability to hear and understand well at a young age is closely related to the development of speech skills. In the past, it often took too long to calibrate hearing aids for young children. Tests like free-field behavioural audiometry couldn’t provide reliable readings for each ear, which is crucial while setting up hearing aids. Using the Auditory Brainstem Response (ABR), another hearing test, also had its issues as it needed multiple appointments and typically lasted over an hour, which is longer than a child’s average nap time. Therefore, children often woke up before the test was done.
Recent studies have also taken the ASSR and used it to study responses in the brain for conditions like bipolar disorder, schizophrenia, autism spectrum disorder, and Alzheimer’s disease. They do this by using a 40-Hz stimulus, which primarily stimulates the hearing and prefrontal areas of the brain. These areas play a vital role in mental functions that are typically damaged in disorders related to neurodevelopment and psychiatry.
When a Person Should Avoid Auditory Steady-State Response
ASSR, also known as Auditory Steady State Response, is a test often used to understand a person’s hearing abilities. However, it’s typically not chosen for those who are prepared and able to do the conventional hearing tests. While there are no strict reasons why someone wouldn’t be able to undergo the ASSR, children with possible hearing loss might also have other health conditions making it unsafe for them to be put to sleep during the test. For instance, a study in 2022 by Urfali et al pointed out that patients suffering from mucopolysaccharidosis, which is a severe genetic disorder, can have a higher risk of breathing difficulties when they’re sedated compared to those who are healthy.
Equipment used for Auditory Steady-State Response
Understanding the results of an ASSR (Auditory Steady State Response) test, which evaluates how well your ears respond to sounds, involves getting to know some specific terms.
- Carrier Frequency (CF): This is the specific sound or tone which creates a response in the cochlea, the part of your ear involved in hearing. This concept is just like the frequencies used in pure-tone hearing tests, where you’re asked to indicate when you hear a sound. Common carrier frequencies are 0.5, 1, 2, and 4 kHz.
- Modulation Frequency (MF): This refers to how often the sound or auditory stimulus changes. For example, an MF of 100 Hz means there are response peaks every 10 milliseconds.
- Amplitude Modulation (AM): These are pure sound tones that change in volume or loudness over time.
- Frequency Modulation (FM): These are pure sound tones that change in pitch or frequency over time.
- Mixed Modulation (MM): These are pure sound tones that change both in volume (amplitude) and pitch (frequency).
The Fast Fourier Transform (FFT) is a method used to analyze the varying components of the ASSR waveform recorded during the test. It’s like breaking down a complex sound into its individual notes. The F-test is a statistical method that checks if there’s a significant response to the sounds with a certain accuracy.
ASSR test methods are continually improving. New techniques evaluate the size and timing of responses to multiple stimuli rather than just one. This has improved the rate of detecting responses and made the tests quicker. In addition, these new tests can evaluate multiple frequencies at the same time, making them more efficient.
Unlike other ear tests that use brief tones, ASSR tests use continuous or steady tones. This means they can test louder sounds up to 120 dB, allowing the test to measure even small amounts of remaining hearing in those with significant hearing loss. These tests can be used to diagnose conductive (related to the ear’s conduct of sound) or sensorineural (related to the nerves involved in hearing) hearing losses, similar to pure-tone hearing tests.
ASSR tests use varied tones, mainly amplitude modulated (AM), frequency modulated (FM), and mixed modulated (MM) tones. AM tones are sounds that change in loudness over a period, and these are frequently used in hearing tests. The amount of AM is given as a percentage, with a greater value representing more substantial changes in loudness.
FM tones only change in pitch during the test. For example, if you have a carrier frequency of 4000 Hz and a frequency modulation of 20%, the frequency values would deviate by ±20% from the carrier frequency, creating a range from 3200 to 4800 Hz.
MM tones are a combination of AM and FM. They show less frequency specificity than AM tones, but they elicit a larger response. If the AM and FM are in phase, with their peaks aligning, this results in a higher response amplitude. Thus, MM is the preferred way of delivering stimuli for ASSR tests.
Who is needed to perform Auditory Steady-State Response?
Auditory Steady-State Response (ASSR) testing is a type of hearing test. This test requires a team of trained professionals which usually includes the following people:
First, the audiologist, who is a healthcare professional trained to identify, diagnose, treat, and monitor disorders of the auditory and balance systems. They will play a big role in evaluating your hearing.
Second, an otorhinolaryngologist. This is a doctor who specializes in the ears, nose, and throat (also known as an ENT doctor). They are there to help diagnose and treat any medical problems you might have in these areas.
Third, an anesthesia provider may be present; this is a healthcare professional that gives medication to help you relax or sleep during the test, if necessary.
Finally, a nurse is often part of the team as well. They assist the doctors and provide care for you in the testing process.
All of these professionals work together to make sure your hearing test runs smoothly and that you get the care you need.
Preparing for Auditory Steady-State Response
Undergoing audiology testing can be challenging for young children because they may find it hard to stay still. Testing can sometimes be done when the child is naturally asleep, but this isn’t always reliable as they can be woken up easily. For children less than six months old, a testing method called ASSR (Automated Auditory Brainstem Response) during their regular sleep seems to work best.
For older kids, doctors can sometimes give them safe sedation medicines like phenobarbital-alimemazine or chloral hydrate to help them stay still for the test. The ASSR testing method also works well when performed under full anesthesia, which is helpful when a deeper level of sedation is needed.
To improve the chances of the child’s successful sleep or sedation during the test, parents might be asked to wake up their kids early on the day of the examination and keep them awake until the test, which helps ensure they will be tired and more likely to sleep.
How is Auditory Steady-State Response performed
The Auditory Steady-State Response (ASSR) test is a method to examine your hearing. Sounds, like tones or modulated noises, are played through headphones that you wear. Electrodes, which are devices that pick up electrical activity, are placed on your head to record how your brain responds to these sounds. This test allows experts to gather information about how well you can hear at different frequencies and if you might have any hearing related issues.
The electrodes are placed at three positions on your head: the ridge of the eyebrows (nasion), the top of your head (vertex), and either on the earlobe or just behind your ear (mastoid). Before the test begins, it’s checked that the connections between these electrodes are good. ASSR tests examine how you hear different frequencies. The most commonly tested frequencies are 0.5 kHz, 1 kHz, 2 kHz, and 4 kHz.
The loudness levels used in the ASSR test often start from what has been determined in previous tests. Then, the test is focused on finding out the quietest sound you can hear at each frequency. For people who are sleeping during the test, the changes in loudness might need to be bigger to prevent them from waking up.
An advantage of ASSR testing is that it can check how you hear multiple frequencies at the same time in both ears. This feature makes the test faster than other hearing tests, which is beneficial especially for children who might wake up easily from sleep or sedation. Some advanced systems even allow adjustments for each frequency while the testing is ongoing, making the test even faster. For example, the typical testing time for ASSR is 20 to 25 minutes for 8 frequency thresholds, which is less compared to other methods that may take up to 60 minutes.
ASSR testing is also unique because it doesn’t rely on the person conducting the test to determine if a response is present. Rather, it uses mathematical analysis for each sweep (or segment of test time) to ensure objective results. The lower the sounds, the more sweeps are generally needed.
The outcomes of ASSR testing in children depend on several factors such as the level of noise in the room, the duration of the test, and the comparison of the hearing response signals to background brain activity. Generally, infants have smaller response signals than adults, which means that tests on infants should be conducted with careful criteria for signal detection. Also, tests on healthy infants older than two weeks provide more reliable results.
Possible Complications of Auditory Steady-State Response
Your doctor might use a test called ASSR to check your hearing. However, this test could cause some side effects, especially if you need to be sedated or put to sleep for it. These side effects might include throwing up, feeling restless, staying asleep for a long time, low oxygen level, trouble breathing, slow heart rate, problems with your brain, and even stop breathing for a short time. If you are put to sleep for this test, doctors will keep a close watch on important things like your heart rate and breathing to make sure you are doing okay.
What Else Should I Know About Auditory Steady-State Response?
Auditory steady-state response (ASSR) testing is a method used to check how well our ears are working. The results of this test are shown on a chart, similar to the traditional audiogram that many people are familiar with. However, the results from the ASSR test may not always match those from the traditional hearing test called pure-tone audiometry.
A study in 1999 and another in 2016 found that ASSR often showed hearing levels that were slightly lower than those found with traditional hearing tests. This was more common in people with mild to moderate degrees of hearing loss than in those with severe loss. It’s thought that this might be caused by something called the ‘recruitment phenomenon’. This is when sounds that are louder than a certain level cause an abnormally large increase in the electrical activity picked up by the hearing test.
Researchers have suggested what normal ASSR results should look like for different sound frequencies. However, more studies are needed to better understand these results across a wider range of sound frequencies and different groups of people.
It’s important to note that different equipment can give slightly different results. Because of this, when using the ASSR test, it’s important to adjust the results based on guidelines provided by the manufacturer of the testing equipment.
Sound frequency also seems to influence the results of the ASSR test. Low-frequency sounds often give a higher hearing threshold (meaning worse hearing) compared to high-frequency sounds. This may be due to several factors, such as the influence of noise from the brain’s electrical activity, how well earphones fit, and the unique characteristics of the infant’s middle ear and hearing nerve.
It’s important to know that the ASSR test can’t be used to diagnose a specific type of hearing nerve disorder known as auditory neuropathy spectrum disorder (ANSD). This is because people with this disorder often have normal ASSR results even though their hearing is impaired. Therefore, other tests like otoacoustic emissions (OAE) and auditory brainstem response (ABR) tests are needed to correctly diagnose this condition.
Furthermore, children younger than one year usually have smaller responses on the ASSR test than adults do, which suggests that their hearing thresholds might be slightly higher. It’s recommended that the ASSR test should only be used in outpatient clinics for infants older than two weeks of age due to the continuing development of their auditory system during the early stages of life.
In summary, the ASSR test is a promising tool for measuring hearing abilities in infants. However, due to its relatively recent development, doctors should be cautious when performing the test and interpreting the results. Many experts recommend using the ASSR test alongside other tests such as the ABR test to get a more complete picture of a child’s hearing ability rather than relying solely on the results of the ASSR test. This approach aligns with the principle of using multiple tests to judge hearing ability in children.
