Otoacoustic Emissions
Otoacoustic emissions (OAE's) are vibrations generated as by-products of the mechanical action of the outer hair cells of the cochlea. These pressure waves travel "backward" through the structures of the ear, vibrate the tympanic membrane and generate very low-intensity sounds that can be measured in the ear canal. Measurement of OAEs provides an objective means of determining the functionality of outer hair cells.
The outer hair cells are responsible for hearing the softest sounds. Their presence or absence can be valuable clinically for a number of reasons. These include, but are not limited to, understanding function of hearing in children and difficult to test populations, identifying functional or exaggerated hearing loss, monitoring hearing of those taking ototoxic medications, and in some cases, distinguishing between cochlear and retrocochlear pathology.
The outer hair cells are responsible for hearing the softest sounds. Their presence or absence can be valuable clinically for a number of reasons. These include, but are not limited to, understanding function of hearing in children and difficult to test populations, identifying functional or exaggerated hearing loss, monitoring hearing of those taking ototoxic medications, and in some cases, distinguishing between cochlear and retrocochlear pathology.
What are Otoacoustic Emissions?
Otoacoustic Emissions are vibrations generated by the mechanical processes of the outer hair cells of the cochlea. These pressure waves are transmitted through the structures of the inner, middle, and outer ear in the opposite direction to the forward transmission of sound vibrations that generated them.
During forward transmission of sound vibrations, the basilar membrane of the cochlea is displaced by the forward "traveling wave." In a healthy ear, at the area of maximal displacement of the basilar membrane, the outer hair cells contract to bring the tectorial membrane closer to the inner hair cells. This changes the movement of fluid in the cochlea, increasing the likelihood of inner hair cell stimulation to generate an action potential. The effect of this outer hair cell activity is to elicit stimulation at a lower intensity level and at a more finely tuned frequency than would otherwise occur.
The contraction of the outer hair cells also creates movement of the cochlear fluids, and this motion is propagated toward the round and oval windows. When these pressure waves reach the oval window, the ossicles are set into motion, which in turn creates motion of the tympanic membrane. The tympanic membrane movement acts as a loudspeaker, creating pressure waves in the ear canal, which result in sound. This sound is called an otoacoustic emission.
During forward transmission of sound vibrations, the basilar membrane of the cochlea is displaced by the forward "traveling wave." In a healthy ear, at the area of maximal displacement of the basilar membrane, the outer hair cells contract to bring the tectorial membrane closer to the inner hair cells. This changes the movement of fluid in the cochlea, increasing the likelihood of inner hair cell stimulation to generate an action potential. The effect of this outer hair cell activity is to elicit stimulation at a lower intensity level and at a more finely tuned frequency than would otherwise occur.
The contraction of the outer hair cells also creates movement of the cochlear fluids, and this motion is propagated toward the round and oval windows. When these pressure waves reach the oval window, the ossicles are set into motion, which in turn creates motion of the tympanic membrane. The tympanic membrane movement acts as a loudspeaker, creating pressure waves in the ear canal, which result in sound. This sound is called an otoacoustic emission.
How are Otoacoustic Emissions related to Auditory Disorder
Otoacoustic emissions reflect the integrity of outer hair cell function. Ears that are functioning normally will produce measureable OAEs.
Most sensorineural hearing loss begins with the loss of outer hair cell function. These cells seem particularly susceptible to noise, toxins and the aging process. so patients with sensorineural hearing loss will most often have a loss of outer hair cells, resulting in a loss of OAEs. As a general rule, when a sensorineural hearing loss reaches 20 dB the probability of measuring OAEs becomes quite low and falls off precipitously as hearing loss increases.
Less commonly, sensorineural hearing loss is caused by problems beyond the outer hair cells. In such cases, the patient will present with a sensorineural hearing loss but with preserved OAEs. In these rare cases the loss is caused by inner hair cell loss, a disorder of the VIIIth nerve, or a disorder of the communication between inner hair cells and VIIIth nerve fibers.
Conductive hearing loss, caused by outer and middle ear disorder has a variable effect on OAEs. For example, in more involved cases of middle ear disorder, the OAE will occur in the cochlea but will not be transmitted effectively back through the middle ear, and the OAEs will not be measurable. In milder cases, the OAEs may be of sufficient intensity to be recorded in the ear canal.
Most sensorineural hearing loss begins with the loss of outer hair cell function. These cells seem particularly susceptible to noise, toxins and the aging process. so patients with sensorineural hearing loss will most often have a loss of outer hair cells, resulting in a loss of OAEs. As a general rule, when a sensorineural hearing loss reaches 20 dB the probability of measuring OAEs becomes quite low and falls off precipitously as hearing loss increases.
Less commonly, sensorineural hearing loss is caused by problems beyond the outer hair cells. In such cases, the patient will present with a sensorineural hearing loss but with preserved OAEs. In these rare cases the loss is caused by inner hair cell loss, a disorder of the VIIIth nerve, or a disorder of the communication between inner hair cells and VIIIth nerve fibers.
Conductive hearing loss, caused by outer and middle ear disorder has a variable effect on OAEs. For example, in more involved cases of middle ear disorder, the OAE will occur in the cochlea but will not be transmitted effectively back through the middle ear, and the OAEs will not be measurable. In milder cases, the OAEs may be of sufficient intensity to be recorded in the ear canal.
Types of OAEs
OAEs can occur spontaneously. In a healthy ear they are also generated in response to the occurrence of sounds that are heard by the patient. In order to make use of the OAEs that occur in the ear canal to understand the health of outer hair cell function in the cochlea, it is helpful to evoke their presence so that they can be systematically measured. OAEs can be evoked by presentation of calibrated sound stimuli into the ear canal. When OAEs occur as a function of stimulation, they are known as evoked otoacoustic emissions (EOAEs).
There are several ways in which OAEs can be evoked and measured. The EOAEs that are in widespread clinical use are transient evoked otoacoustic emissions (TEOAEs) and distortion product evoked otoacoustic emissions (DPOAEs).
TEOAEs are evoked with a transient stimulus (click). The click stimulus is broad spectrum in nature, allowing for the majority of the basilar membrane to stimulated nearly simultaneously. Thus, information about the outer hair cell function along much of the basilar membrane is elicited with each stimulus.
DPOAEs are evoked using two tones of fixed frequency and intensity. When tones have a particular frequency ratio to one another, distortions are created by the active mechanism of the cochlea. The mechanics of the basilar membrane movement cause the stimulation of outer hair cells at frequencies other than the stimulating tones that are mathematically related to the frequencies of the stimulating tones. Vibrations that occur due to outer hair cell function at these additional frequencies are known as distortion product otoacoustic emissions. When DPOAEs are measured in the ear canal, they provide information about the health of the outer hair cells at the frequencies of the stimulation tones. There are numerous DPOAEs that are generated in response to stimulation by two tones of a particular frequency ratio.
For general clinical use, two tones are presented known as F1 and F2, with F1 being the lower frequency tone and F2 being the higher frequency of the two tones. It has been empirically determined that the optimal frequency ratio of these stimulus tones also have empirically determined intensity levels, known as L1 and L2 (corresponding to the respective frequencies of F1 and F2). The most commonly used intensities of these two tones for general clinical use are 65 dB SPL for L1 (F1) and 55 dB SPL for L2 (F2). The distortion product that is most commonly used is known as the cubic difference distortion product. The formula for its frequency is 2F1-F2.
There are several ways in which OAEs can be evoked and measured. The EOAEs that are in widespread clinical use are transient evoked otoacoustic emissions (TEOAEs) and distortion product evoked otoacoustic emissions (DPOAEs).
TEOAEs are evoked with a transient stimulus (click). The click stimulus is broad spectrum in nature, allowing for the majority of the basilar membrane to stimulated nearly simultaneously. Thus, information about the outer hair cell function along much of the basilar membrane is elicited with each stimulus.
DPOAEs are evoked using two tones of fixed frequency and intensity. When tones have a particular frequency ratio to one another, distortions are created by the active mechanism of the cochlea. The mechanics of the basilar membrane movement cause the stimulation of outer hair cells at frequencies other than the stimulating tones that are mathematically related to the frequencies of the stimulating tones. Vibrations that occur due to outer hair cell function at these additional frequencies are known as distortion product otoacoustic emissions. When DPOAEs are measured in the ear canal, they provide information about the health of the outer hair cells at the frequencies of the stimulation tones. There are numerous DPOAEs that are generated in response to stimulation by two tones of a particular frequency ratio.
For general clinical use, two tones are presented known as F1 and F2, with F1 being the lower frequency tone and F2 being the higher frequency of the two tones. It has been empirically determined that the optimal frequency ratio of these stimulus tones also have empirically determined intensity levels, known as L1 and L2 (corresponding to the respective frequencies of F1 and F2). The most commonly used intensities of these two tones for general clinical use are 65 dB SPL for L1 (F1) and 55 dB SPL for L2 (F2). The distortion product that is most commonly used is known as the cubic difference distortion product. The formula for its frequency is 2F1-F2.
Equipment
Equipment used for OAE testing generally comes in two forms: screening and diagnostic. Screening equipment generally does not allow for manipulation of testing paramters and in some cases may not provide a result beyond "presence" or "absence" of emissions. Diagnostic equipment generally allows for multiple types of emissions to be measured, allows for manipulation of testing parameters and provided numeric results for interpretation by the clinician.
OAE testing equipment consists of a computer that is used to generate a calibrated acoustic signal. The digital signal is transmitted via a cord that connects to a probe where the acoustic signal is generated. The probe has a loudspeaker for delivery of the test signal and a highly sensitive microphone to measure the sounds occurring in the ear canal.
The signals that occur in the ear canal include the test stimuli, ambient and physiologic noise, and all of the evoked OAEs that occur in response to the test stimuli. Measurement of the test stimuli is helpful to ensure that the signal presented into the ear canal is of the desired frequency and intensity. These signals are measured, and on diagnostic equipment they can be inspected to ensure that the measurement was made using appropriate stimuli.
There is an additional large amount of extraneous sound in the ear canal from environmental and physiologic sources that is random relative to the time-locked evoked otoacoustic emission. In order to reduce the amount of unwanted noise relative to the OAE of interest, a signal averaging system is employed. The test stimuli are presented numerous times and the responses recorded each time. The number of times that test stimuli are presented depends on the parameters set by the clinician and the "stopping rules" that are criteria that include intensity level of the noise floor, intensity level of OAEs, differences between these values and duration of the test time. These responses are averaged, thereby reducing sounds emanating from random extraneous sources. In contrast, the signal of interest
OAE testing equipment consists of a computer that is used to generate a calibrated acoustic signal. The digital signal is transmitted via a cord that connects to a probe where the acoustic signal is generated. The probe has a loudspeaker for delivery of the test signal and a highly sensitive microphone to measure the sounds occurring in the ear canal.
The signals that occur in the ear canal include the test stimuli, ambient and physiologic noise, and all of the evoked OAEs that occur in response to the test stimuli. Measurement of the test stimuli is helpful to ensure that the signal presented into the ear canal is of the desired frequency and intensity. These signals are measured, and on diagnostic equipment they can be inspected to ensure that the measurement was made using appropriate stimuli.
There is an additional large amount of extraneous sound in the ear canal from environmental and physiologic sources that is random relative to the time-locked evoked otoacoustic emission. In order to reduce the amount of unwanted noise relative to the OAE of interest, a signal averaging system is employed. The test stimuli are presented numerous times and the responses recorded each time. The number of times that test stimuli are presented depends on the parameters set by the clinician and the "stopping rules" that are criteria that include intensity level of the noise floor, intensity level of OAEs, differences between these values and duration of the test time. These responses are averaged, thereby reducing sounds emanating from random extraneous sources. In contrast, the signal of interest
Environmental and Patient Conditions
The transmission of vibrations "backward" through the structures of the cochlea and middle ear is fare less efficient than "forward" transmission through these structures. Because of this, the OAEs recorded in the ear canal are very low in intensity. The measurement of such low-intensity sounds requires that the environment in which OAEs are measured to be very quiet. A sound booth is the ideal location for such measurements to occur, but OAEs can be measured elsewhere, as long as the environment is made as quiet as possible.
In addition to a quiet environment, a quiet patient is essential to the process of effectively recording OAEs. this can typically be achieved by instructing a compliant adult or older child that they will be hearing sounds and that their job is to sit as still and quiet as possible throughout the testing. In infants, the testing is best carried out in natural sleep but can be accomplished as long as the infant is awake but quiet. An infant who has been fed and is comfortable is typically necessary for this. A pacifier or soother is often helpful for calming awake infants. For young children the clinician must ensure that the child does not purposefully or inadvertently pull the earphone out of the ear. The child must also be kept as still and calm as possible. The parent or caregivers's assistance is typically necessary and most young children do well sitting on the lap of this person. Any number of techniques may be employed to distract the child and maintain the child's quiet attention, including showing the child a picture book, allowing the child to watch an animation with no sound, having the child hold quiet toys, and so on.
In addition to a quiet environment, a quiet patient is essential to the process of effectively recording OAEs. this can typically be achieved by instructing a compliant adult or older child that they will be hearing sounds and that their job is to sit as still and quiet as possible throughout the testing. In infants, the testing is best carried out in natural sleep but can be accomplished as long as the infant is awake but quiet. An infant who has been fed and is comfortable is typically necessary for this. A pacifier or soother is often helpful for calming awake infants. For young children the clinician must ensure that the child does not purposefully or inadvertently pull the earphone out of the ear. The child must also be kept as still and calm as possible. The parent or caregivers's assistance is typically necessary and most young children do well sitting on the lap of this person. Any number of techniques may be employed to distract the child and maintain the child's quiet attention, including showing the child a picture book, allowing the child to watch an animation with no sound, having the child hold quiet toys, and so on.