Overview of Auditory Brainstem Response
The Auditory Brainstem Response (ABR) test, also known as the Brainstem Auditory Evoked Potentials (BAEP) test, is a special test that tracks how sound travels from the nerves in the ear to the brain. It’s a way for doctors to measure the function of the hearing pathways located in your brain. This test is particularly useful in assessing the hearing abilities of individuals who may not be able to undergo ordinary hearing tests.
The ABR test works by detecting electrical activity in your auditory (or hearing) nerve and other parts of your brain involved in hearing. To do this, doctors attach electrodes (small devices that record electrical activity) to your scalp, forehead, and near your ears. These capture the brain’s responses to sounds for about 10 milliseconds after a sound is made.
The results of the test are represented by a series of positive wave peaks (identified as I-VII) with negative dips or troughs in between. The test has been used since the 1970s, and its importance has grown as modern guidelines now recommend testing newborns for hearing loss within three months of birth.
Today, the ABR test is critical in diagnosing hearing loss, and for identifying specific types of tumors called acoustic tumors and cerebellopontine angle tumors which may be affecting a person’s hearing.
Anatomy and Physiology of Auditory Brainstem Response
The Auditory Brainstem Response, or ABR, is a type of test that measures the activity of the nerve pathways in the ear and brain that are associated with hearing. When we hear something, that sound is transformed from a physical noise into an electrical signal. This transformation happens in an organ in your ear, called the cochlea. The electrical signal then travels to your brain along the auditory nerve, which is sometimes known as the vestibulocochlear nerve–or Cranial Nerve VIII in medical terms.
As the electrical signal travels from the cochlea to the brain, it passes through a sequence of different parts of the brain, or “nuclei”, such as the cochlear nucleus, superior olive, lateral lemniscus, inferior colliculus, and medial geniculate body. The tiny “hair cells” in your cochlea are very important because they are the cells that turn the physical sounds into electrical signals. Then these signals are sent to the spiral ganglion, a part of the auditory nerve.
Interestingly, different areas inside your cochlea respond to different sound frequencies. High-frequency (high-pitch) sounds stimulate the base, or bottom part, of your cochlea. On the other hand, low-frequency (low-pitch) sounds stimulate the tip, or apex, of your cochlea. The nerves that carry signals from these different parts of your cochlea then run along either the outer or inner part of the spiral-shaped auditory nerve.
The first stop inside your brain for the electrical information from your ear is the cochlear nucleus. This part of the brain is located in the backside of the brainstem and divided into two sections based on the pitch of the sound information it has received.
After the cochlear nucleus, most of that electric information crosses to the other side of the brain and goes to a part of the brain called the superior olivary complex. The electrical signals are then sent on to a part of the brain called the inferior colliculus. From there, the signals are sent to the medial geniculate nucleus in the thalamus, another part of the brain. This part of the brain is tonotopically organized, meaning it is arranged based on the pitch of the sounds it is processing.
Why do People Need Auditory Brainstem Response
The Auditory Brainstem Response (ABR) test is a useful tool in identifying potential issues with the hearing nerves in your brain, known as the cranial nerve VIII, and the pathways that sound travels through in your ears. This test is particularly helpful for those who can’t accurately provide information about their hearing abilities through other hearing tests. ABR is often used to confirm hearing loss in babies who don’t pass their newborn hearing tests. The ABR is typically done while the baby is sleepy or under light sedation, and the results match well with traditional hearing tests.
Not just for infants, this test is also used for young children and adults who may not be able to complete traditional hearing tests. For instance, kids who passed their newborn hearing tests but have been exposed to certain treatments or medications that could damage their hearing. These may include chemotherapy and some heavy duty antibiotics. The ABR test helps to monitor them for potential hearing loss.
In addition to detecting hearing loss, the ABR test can be used to diagnose other conditions affecting the ears or brain. These include vestibular schwannomas (tumors that occur on the nerve responsible for hearing and balance), damage to the brainstem (the part of the brain that connects to the spinal cord), de-myelination diseases (conditions that affect the protective coating of nerve cells), and auditory neuropathy (a hearing disorder where the sound signals from the ear to the brain are disrupted).
ABR testing is highly efficient in detecting larger acoustic tumors, those bigger than 1.5 cm, with a 92-98% accuracy rate. However, it might miss smaller tumors, less than 1.5 cm, in about 30% cases. This discrepancy is because smaller tumors might not disrupt high-frequency sound signals enough to show abnormal results in the ABR test.
ABR is also useful in identifying nerve problems in non-tumor-related diseases, like stroke, multiple sclerosis, syphilis, Wilson’s disease, and viral infections. An Auditory Neuropathy Spectrum Disorder is another type of hearing disorder where the signals from the ear to the brain are disrupted, despite the hearing nerve appearing normal on a brain MRI. These individuals might experience difficulty understanding speech and could show varying degrees of impaired speech detection on hearing tests.
When a Person Should Avoid Auditory Brainstem Response
The Auditory Brainstem Response test (ABR) is not needed if a patient can have a normal hearing test. This test might not be safe for people with certain health conditions including blockage in the throat or nose, slowing of the breath due to brain issues, epilepsy, lung infection, heart failure, a heart condition called prolonged QT syndrome which can cause a fast, chaotic heartbeat, kidney failure, and a rare genetic disorder that affects the nervous system and the skin called porphyria. Furthermore, if a patient needs to be put to sleep for the ABR test, certain medications like phenobarbital or hydroxyzine dihydrochloride may not be safe to use.
Equipment used for Auditory Brainstem Response
When conducting certain medical procedures, particularly those involving the ears, doctors need to use a specific set of tools and devices. Here is a brief rundown:
– Surface electrodes: These are small devices that stick onto your skin to record electrical activity, such as the signals from your brain.
– Earphones: They are used to deliver specific sounds during the test.
– Electrode gel: This is a special type of gel that helps the surface electrodes pick up electrical signals more effectively.
– Laptop with ABR analyzing software: This laptop is equipped with special software used to analyze the electrical signals from your brain (also known as an Auditory Brainstem Response test or ABR).
– Otoscopic microscope: This is a particular type of microscope designed to look directly into your ear.
– Myringotomy surgical tray: This tray contains all the tools a doctor needs to perform a myringotomy, a surgical procedure to remove fluid from the middle ear. The tools include an ear speculum (an instrument to widen your ear canal for examination), curettes (small scooping tools), and alligator forceps (a type of tweezer with a long handle and short, sharp jaws).
Who is needed to perform Auditory Brainstem Response?
The right team is important for a successful surgery. This team involves:
An audiologist, who is a professional that specializes in diagnosing and treating hearing and balance problems.
An otolaryngologist, often also known as an ear, nose and throat doctor (ENT), who focuses on problems related to structures of the head and neck.
An anesthesiologist, who is a medical specialist in charge of giving you medication so you won’t feel pain during the surgery.
A surgical technician, that is a healthcare professional who assists in the operation. They are responsible for preparing the surgery room and other related tasks.
And a circulating nurse, who is also a crucial part of the team. This person doesn’t directly assist in the surgery but they make sure everything else in the surgery room runs smoothly.
Preparing for Auditory Brainstem Response
ABR testing, or hearing tests, for infants are usually done while the infant is asleep and the test lasts for the duration of their nap. The babies are kept awake longer before the test so they would sleep longer during the procedure. For babies who find it difficult to sleep long enough for the test, mild sedation or even general anesthesia might be needed. However, when adults take the test, they don’t need to be asleep. They just have to be quiet and relaxed to avoid any unnecessary movement or muscle activity.
Before the test, the doctor would need to know the patient’s complete health history. This includes prior medical conditions, current medications, any allergies to medicines, and any previous negative reactions to drugs. The patient should also abstain from eating or drinking 8 hours before the procedure if sedation is a possibility.
Before the test begins, the doctor would carefully check the ears using a special microscope. The doctor then removes any ear wax or debris carefully to avoid injury that could affect the hearing test. If there’s fluid in the middle ear, the doctor may have to do a minor procedure called ‘myringotomy’ to remove the fluid. This is because the presence of fluid in the middle ear could lead to inaccurate test results.
How is Auditory Brainstem Response performed
Doctors use a method called the International 10-20 system to place electrodes on your scalp for exams, such as electroencephalography and polysomnography. These are tests that help evaluate your brain activity. The electrodes are placed on different parts of your head like the forehead, top, back or sides. Whether the electrode is placed on your right or left side is determined by whether the number labeling it is even or odd.
During an auditory brainstem response (ABR) test, which tests the hearing nerve and pathways to the brain, these electrodes play a crucial role. A main electrode is attached at the top of your head or high on your forehead, and two more are placed on your ear lobes or near your ear, on the bone behind your ear, known as mastoids. A last one, known as the ground electrode, is placed at the back of your head or on your lower forehead.
For a hearing assessment, headphones are used to deliver sound to your ears at a loud but safe level. The sound level can be adjusted to a slightly quieter level if the person does not have normal hearing. Hearing Level (HL) refers to the hearing ability of a normal person, while Sensation Level (SL) refers to the hearing ability of the person who is being tested.
In order to ensure that sounds are only heard by the ear being tested, a gentle white noise is played in the non-tested ear to prevent any sound confusion. The responses of your brain to these sounds are recorded for a very short period of time after the sound is played, and sometimes this time is extended if there is a delay in the brain’s response.
The ABR test gives information on the timings, size and intervals between the brain’s responses to these sounds. The timing of these responses can depend upon the person’s age and the loudness of the sound. A larger response is typically seen when more areas of the brain are activated at once.
ABR can help to estimate levels of hearing and reveal any hearing loss if it exists. The faintest intensity level at which your brain shows a response is typically a good indicator of your hearing ability.
Usually, a sound known as a ‘broadband click’ is used for most ABR tests to estimate someone’s ability to hear between certain frequencies. But certain other frequencies can be used to test hearing ability beyond this range. By testing these specific frequencies, a more accurate picture of someone’s hearing ability can be established.
It is believed that each ‘wave’ or ‘peak’ in the brain’s response comes from various areas of the brain. However, it is generally agreed upon that wave I comes from the part of your hearing nerve closest to the ear, wave II comes from the part of your hearing nerve closest to your brain and an area called the cochlear nucleus, wave III comes from the cochlear nucleus and another spot called the superior olivary complex, and so on.
Possible Complications of Auditory Brainstem Response
If your child needs to go through an ABR (auditory brainstem response) test and requires general anesthesia or sedation, it’s crucial to be aware of the potential risks. Research has shown that even a few hours of exposure to anesthesia can cause damage to the developing brain in young animals, effects that often last into their adult life. This may also apply to young children.
Further studies have linked prolonged exposure to anesthetics in children to learning disabilities later in life. Because of these potential effects, the US Food and Drug Administration (FDA) has advised that the use of general anesthesia and sedation in children under 3 years of age should be reduced as much as possible. If there’s a need for these drugs, the exposure should not exceed a total of 3 hours.
What Else Should I Know About Auditory Brainstem Response?
The Auditory Brainstem Response (ABR) test is a safe and painless procedure specifically designed to identify any issues in hearing. One of the key uses of the ABR is to discover any hearing loss in newborns and young children, who are usually unable to undergo regular hearing tests due to their age. Identifying hearing loss as soon as possible is incredibly important because early detection provides the opportunity for immediate treatment options. This could be through hearing aids, devices that transmit sounds directly to the inner ear, or therapy programs that target speech and language development.
On the other hand, the ABR test is also useful in diagnosing a specific type of benign brain tumor known as a vestibular schwannoma. This tumor occurs when cells that are responsible for insulating and supporting nerve fibers, grow on the nerve that connects the inner ear to the brain. This can cause delayed responses in the ABR waveforms and disrupt normal activity along the auditory pathway, leading to hearing loss.
To identify a vestibular schwannoma, doctors look at two measurements from the ABR test. If the difference in response time between the two ears is longer than 0.3 milliseconds, and the time it takes for a specific response (known as wave V) is longer than 4.4 milliseconds, it might indicate a vestibular schwannoma. The ABR test can also be used during surgery to operate on tumors in the brain, as it can help doctors prevent damage to the auditory nerve.
In addition, the ABR test can help diagnose lesions on the brainstem, which often result from conditions like multiple sclerosis, a disease where the immune system accidentally attacks the protective sheath that covers nerve fibers. Multiple sclerosis can cause non-synchronous neural activity and an absent wave V on the ABR test.
