Acoustic Reflex Thresholds
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Purpose
The acoustic reflex threshold measurement is a means of assessing the integrity of the acoustic reflex pathway. Dysfunction in this pathway can point to specific areas of pathology, especially when combined with tympanometry, pure-tone sensitivity and speech audiometry data.
Terminology
In the earliest days of acoustic reflex threshold testing, technology had not yet been developed to record the reflex measurement in the same ear that the stimulus was presented (the ipsilateral reflex). Instead, the stimulus tone was presented to the test ear, while the reflex was recorded in the opposite ear. The primary use of this measure was a s an assessment of retrocochlear function. The reflex was referred to as a left reflex when the stimulus was presented to the left ear and a right reflex when the stimulus was presented to the right ear. Later, the ability to elicit and measure the reflex in the same ear allowed for interference of middle ear function. to differentiate the pathways, the reflex that was elicited in the same ear in which the stimulus was presented became known as the ipsilateral reflex. The reflex that was elicited in the ear opposite that in which the reflex was elicited became known as the contralateral reflex. Because the stimulus elicits a reflex in both ears simultaneously , both the ipsilateral and contralateral reflexes occur due to stimulation of the test ear. When the right ear is stimulated and the reflex is recorded is recorded in the left ear, this is called a right contralateral reflex. When the right ear is stimulated and the reflex is recorded in the right ear, this is called a right ipsilateral reflex. When the left ear is stimulated and the reflex is recorded in the left ear, this is called a left ipsilateral reflex.
Materials
Tympanometer with Acoustic Reflex Measurements
Performing Acoustic Reflex Threshold Testing
A loudspeaker in the form of a probe or insert earphone is also placed into the contralateral ear for presentation of the contralateral stimulus. The same probe that is used to measure a typanogram is used to determine the acoustic reflex threshold. In addition to having the components described earlier (i.e., air pump/manometer, microphone, and probe-tone loudspeaker), the probe unit contains another calibrated loudspeaker for delivery of the stimulus used to activate the ipsilateral acoustic reflex.The measurement probe is placed into the ear in the same manner as for tympanometry, with an airtight seal being necessary. The patient should be instructed to remain as still and quiet as possible. The patient should be informed that a series of loud sounds will be heard. The pressure utilized in acoustic reflex testing should be that of the tympanometric peak pressure, allowing for optimal transfer of sound energy into the middle ear space. The probe-tone is used to elicit a baseline admittance level. When a stimulus of sufficient intensity is presented, the acoustic reflex response occurs. The contraction of the stapedius muscle causes a stiffening of the ossicular chain and a resulting change in the admittance of sound energy into the middle ear space. The change in the level of admittance over the time course of the occurrence of the stimulus presentation is graphically represented. The stimulus is presented at several different intensity levels for each stimulus frequency. The lowest intensity level at which a demonstrable change in admittance occurs is the acoustic reflex threshold. Average threshold level for the acoustic reflex is 85 dB SPL, with a normal range from 70 to 100 dB SPL. Reflexes are considered to be elevated when they exceed 100 dB SPL. Reflexes are considered to be absent when a reflex threshold cannot be elicited at the highest intensity level that can be generated by the equipment for the activator signal. This is generally around the level of 110 dB SPL.
Admittance of the probe-tone sound energy is plotted as a function of time. As the stimulus signal causes a contraction of the stapedius muscle, the tympanic membrane stiffens, resulting in a decrease i the admittance of sound energy into the middle ear space. This reduction is observed for the duration of the stimulus signal. Each stimulus signal plotted has a different intensity, which creates a different degree of contraction of the stapedius muscle. The lowest intensity, 75 dB SPL, results in no apparent contraction of the muscle. At 80 dB SPL, a demonstrable change in admittance is observed. The acoustic reflex threshold in this case is 80 dB SPL.
The Acoustic Reflex
The stapedius tendon is connected to the head of the stapes in the middle ear. The stapedius muscle, from which the tendon emanates, is innervated by the stapedial branch of the facial nerve (cranial nerve VII). When the stapedius muscle contracts, the stapes footplate is displaced slightly in the oval window. This displacement causes a stiffening of the ossicular chain, which results in a decrease in admittance of sound energy from the ear canal to the middle ear space. The acoustic reflex is a contraction of the stapedius muscle in response to loud sound. The contraction of the stapedius muscle occurs bilaterally. So, even when the stimulus is presented only to one ear, both stapedius muscles will contract. The reflex that occurs in the ear where the stimulus is presented is called the ipsilateral reflex. The reflex that occurs in the ear opposite to where the stimulus is presented is called the contralateral reflex.
The Acoustic Reflex Pathway
Different neural pathways are involved in eliciting the ipsilateral and contralateral acoustic reflexes.
Ipsilateral Acoustic Reflex Pathway
The ipsilateral acoustic reflex pathway includes the following components: Ipsilateral middle ear
ipsilateral cochlea
ipsilateral VIIIth cranial nerve (vestibulocochlear) to the level of the ipsilateral ventral cochlear nucleus (VCN)
ipsilateral VCN to the level of the superior olivary complex (SOC)
ipsilateral SOC to the ipsilateral facial motor nucleus (MN VII)
Ipsilateral VIIth cranial nerve to the stapedius muscle in the ipsilateral middle ear
Contralateral Reflex Pathway
The contralateral acoustic reflex pathway includes the following components: ipsilateral (to the stimulus) middle ear
ipsliateral cochlea
ipsilateral VIIIth (vestibulocochlear) cranial nerve to the level of the ipsilateral ventral cochlear nucleus (VCN)
crossover from the ipsilateral VCN to the contralateral superior olivary complex (SOC)
contralateral SOC to the contralateral facial motor nucleus (MN VII)
contralateral VIIth cranial nerve to the stapedius muscle in the contralateral ear
Acoustic Reflex Threshold
The acoustic reflex threshold is the lowest intensity at which the acoustic reflex is elicited with a particular stimulus. The stimulus levels are typically presented in 5 dB increments, just as they are for audiometric evaluation. Clinically, the acoustic reflex threshold typically is elicited both ipsilaterally and contralaterally to evaluate both acoustic reflex pathways. Frequencies typically tested include 1000 and 2000 Hz for ipsilateral reflexes and 500, 1000, and 2000 Hz for contralateral reflexes. Other stimuli, such as broadband noise, may be used as well.
Interpretation/Site of Lesion
Normal Acoustic Reflex Patterns
An acoustic reflex pattern is considered normal when all four reflexes, right ipsilateral, right contralateral, left ipsilateral and left contralateral are present at normal threshold levels. This demonstrates the functional integrity of all components of both the ipsilateral and contralateral reflex pathways bilaterally.
Abnormal Acoustic Reflex Patterns
Unilateral Middle Ear Disorder
In the case of unilateral middle ear disorder, an acoustic reflex will not be recorded from the probe in the disordered ear, due to abnormal function of the middle ear mechanism, a "probe effect." If there is a problem of admittance of sound energy into the middle ear space prior to the initiation of the stimulus tone, even if the stimulus could be made loud enough to cause a contraction of the stapedius muscle, no change reflective of the acoustic reflex can be recorded. Consider the following example: Imagine that the right ear has fluid in the middle ear space.
- The responses to right ipsilateral stimulation will be absent, because the response cannot be recorded due to middle ear dysfunction.
- The responses to right contralateral stimulation are elevated because there is a conductive hearing loss in the right ear, which attenuates (reduces the force or effect) the intensity of the stimulus.
- The left ipsilateral responses will be normal.
- The responses to left contralateral stimulation also will be absent because the response cannot be recorded due to middle ear dysfunction.
Bilateral Middle Ear Disorder
In the case of bilateral middle ear disorder, there is a "probe effect" bilaterally so that no responses are present in any configuration. Consider the following example. Imagine that both the right and left ears have fluid in the middle ear space.
- The stiffening effect of the fluid does not allow a change in stiffness to be recorded when, or if, the acoustic reflex is activated by the stimulus.
Sensorineural Hearing Loss
In the case of sensorineural hearing loss, the resulting acoustic responses will be dependent on the degree of hearing loss. For hearing losses less than about 50 dB HL, there generally will be no change in the acoustic reflex response. For thresholds between about 50 and 80 dB HL, acoustic reflex responses may be elevated. For thresholds greater than this, acoustic reflex responses will be absent. Consider the following example. Imagine that the right ear has a moderate sensorineural hearing loss and the left ear has a severe hearing loss,.
- The effect of reduced sensation level affects the presence and level.
- The effect of reduced sensation level affects the presence and level of the acoustic reflex response.
Retrocochlear Function
Dysfunction of the VIIIth cranial nerve (vestibulocochlear) such as a vestibular schwannoma, can result in an absence of acoustic reflex responses when the stimulus is presented to the affected ear. Consider the following example. Imagine that the right ear has retrocochlear dysfunction due to a lesion of the VIIIth cranial nerve.
- The responses to right ipsilateral stimulation will be absent because the response cannot be recorded due to lesion of R VIIIth nerve.
- The responses to rightcontralateral stimulation will be absent because the response cannot be recorded due to lesion of R VIIIth nerve.
- The responses to left ipsilateral stimulation will be present.
- The responses to left contralateral stimulation will be present.
Facial Nerve Dysfunction
Dysfunction of the VIIth cranial nerve (facial) results in the absence of an acoustic reflex response in the ear with the facial nerve damage, regardless of which ear is stimulated - a classic probe effect. Consider the following example: Imagine that there is facial nerve dysfunction on the right side, with the left side being normal.
- There are no responses to right ipsilateral stimulation because the stapedius muscle on the right side does not contract.
- The responses with stimulation to the right side, recorded on the left side (right contralateral), are normal because this pathway is intact.
- There is an absence of response to stimulation of the left ear with contralateral recording (left contralateral) because the stapedius muscle on the right does not contract, even with stimulation to the left ear.
- The left ipsilateral thresholds are normal.
Third Window Effect
A reflex pattern may occur that demonstrates normal middle ear function in the presence of an apparent difference between air and bone conduction thresholds, known as an air-bone gap. The normal reflexes occur because there is normal function in reflex pathway. However, the air-bone gap may occur because of a phenomenon known as a third window effect where there is a pathologic area of mobility in the cochlear/semicircular canal structures. The result of a third window effect is that bone-conduction thresholds are artificially better when measured than they actually are (i.e., they are lower than they should be). This artificial lowering of the bone-conduction threshold creates an air-bone gap by decreasing the level of the measured bone-conduction threshold relative to the air-conduction threshold. In the case of a pure third window effect, there is an artificial air-bone gap, but no conductive hearing loss and no middle ear dysfunction. This results in a pattern of an air-bone gap on audiometric testing but a reflex pattern consistent with normal middle ear function.
Management
Resources
Gelfand, Stanley A.Essentials of Audiology. Thieme, 2016.
DeRuiter, Mark, and Virginia Ramachandran.Basic Audiometry Learning Manual. Plural Publishing Inc., 2017
FAQs
What is the threshold for acoustic reflex? ›
An Acoustic Reflex Threshold test lets the audiologist know whether your child's acoustic reflex is working correctly. In mammals, the acoustic reflex is triggered by loud noises. In humans, the range is usually between 65 dB and 95 dB. Muscles in the inner ear contract to help protect the eardrum from damage.
What does it mean to have absent acoustic reflexes? ›Definition. Absence of the acoustic reflex, an involuntary contraction of the stapedius muscle that occurs in response to high-intensity sound stimuli. [ from HPO]
What is low acoustic reflex thresholds? ›Abstract. The acoustic reflex threshold can be detected at lower than normal sound pressure levels by means of facilitation. This procedure entails simultaneous presentation of a high-frequency facilitating tone at a level just below reflex threshold and a second reflex-eliciting tone.
What are the results of acoustic reflex testing? ›Acoustic reflex threshold (ART) helps audiologists test for proper middle ear functioning by testing the ear's natural reflex to lower the volume of very loud sounds. A normal result means your child falls within the usual range. An abnormal ART may show some kind of a neurological disorder or nerve damage.
What are normal hearing thresholds? ›Normal hearing is a range of decibel levels starting from 0 dBHL to 20 dBHL. If you can hear sounds in this range at every pitch, then you have normal hearing. Some people can even hear sounds below 0 dBHL! Since the normal hearing range is average, some people can hear sounds that are actually below 0 decibels.
What is threshold limit of hearing? ›Humans have a hearing threshold of around 0 decibels. Above this threshold, sounds with higher sound pressure levels are heard as louder noises. Sounds above 90 dB can lead to chronic hearing damage if people are exposed to them every day or all the time.
Why is acoustic reflex testing important? ›The auditory activation of the tensor tympani muscle in humans occurs only as part of a startle response to extremely intense sounds. Acoustic reflex measurements can assist in the diagnosis of: Conductive hearing loss. Retrochochlear lesion.
Are acoustic reflexes affected by hearing loss? ›Patients with mild to moderate cochlear sensorineural hearing loss have reflexes bilaterally at about the same intensity level as those with normal hearing, but patients with severe or profound hearing loss have absent reflexes when the affected ear is stimulated.
Why is acoustic reflex important? ›In general, the acoustic reflex is important for separating the auditory signal from other internal or environmental noises and for controlling the attenuation of low-frequency speech sounds, thus favoring the perception of high-frequency sounds, the attenuation of voiced sounds, and the recognition of strong-intensity ...
What acoustic reflex thresholds are ototoxicity? ›Audiometric thresholds from 3 to 8 kHz, and acoustic reflex threshold changes were found to be the next most sensitive to ototoxicity at cumulative dosages of 251 to 400 mg/m2. No significant threshold changes were noted for frequencies from 250 Hz to 2 kHz.
What is the threshold of acoustic sensation pain threshold and boundary frequencies? ›
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Acoustics/Threshold of Hearing/Pain.
| Threshold of pain | |
|---|---|
| SPL | sound pressure |
| 140 dBSPL | 200 Pa |