What is the acoustic reflex?
The acoustic reflex is an involuntary muscle contraction in response to a loud sound. When a loud sound is presented to a normal ear, the stapedius muscle will contract in both ears – regardless of the side of stimulation. An acoustic reflex threshold (ART) is the minimum intensity of a sound required to elicit an acoustic reflex. Since loud stimuli will elicit a response in both ears, ARTs can be measured both ipsilaterally, meaning stimuli is presented in the ear the reflex is measured, and contralaterally, meaning the stimuli is presented in one ear and the reflex is measured in the opposite ear. While this reflex seems like a simple response, it involves a large portion of the auditory pathway and can help in the diagnosis of a variety of lesions.
In an ipsilateral acoustic reflex, sound travels through the outer, middle, and inner ear where it is sent to the auditory nerve. The auditory nerve then sends the stimuli to the brainstem, specifically the cochlear nucleus. Stimuli then travels to the superior olivary complex (SOC) and the nuclei of the facial nerve. The stimuli being sent down the facial nerve is what causes the stapedius muscle to contract. The contralateral acoustic reflex follows a near identical pathway to the ipsilateral reflex until the SOC. It is here where the stimuli crosses over to the opposite facial nerve which contracts the stapedius muscle on that side.
How is an ART measured?
Acoustic reflex testing is built into many modern immittance devices alongside tympanometry. During testing, a probe is inserted into the ear and a 226 Hz probe tone is used to detect changes in middle ear admittance. For ipsilateral acoustic reflexes, this probe will both present the loud stimuli and measure ear drum movement. When testing contralateral acoustic reflexes, the probe will measure eardrum movement and a single-sided earphone or ear insert, will present stimuli into the opposite ear. Ipsilateral and contralateral ARTs are each valuable in their own right and provide different diagnostic information. It’s important to note, however, that while different ARTs can suggest a site of lesion, they cannot diagnose a pathology on their own and should be interpreted in relation to other tests like audiometry and imaging.
In a healthy ear, the loud stimulus will cause the stapedius muscle to contract. This contraction will pull the stapes away from the oval window, stiffening the ossicular chain. This will decrease admittance in the middle ear, which can be measured in the ear canal.
The ART test is typically performed at several frequencies, ranging from 500 Hz to 4,000 Hz. Stimuli are presented anywhere from 75 to 120 dB HL. The test is repeated at increasing sound intensities until a measurable contraction of the stapedius muscle is detected. It should be noted that there is presently no official maximal level at which ARTs can be presented, but many agree that presentation should stop around 110 dB HL. Patient preference and loudness discomfort should also be considered when performing reflex testing.
Why test acoustic reflexes?
Acoustic reflex thresholds are a valuable measure of hearing function that can provide information about the function of the entire auditory pathway and facial nerve. As such, acoustic reflexes can not only aid in differential diagnosis, but confirm type or degree of hearing loss. For example:
Differentiating between cochlear and retrocochlear hearing loss
For those with retro-cochlear hearing loss, ARTs may not follow the same pattern as with cochlear hearing loss. Specifically, acoustic reflexes may be absent despite mild to moderate hearing thresholds. Click here for more ways to differentiate between cochlear and retrocochlear loss.
Confirming Auditory Neuropathy
Studies have shown that acoustic reflexes are extremely sensitive to auditory neuropathy with one study finding absent or elevated reflexes in nearly all individuals with this pathology.
Identify malingerers
In malingerers, present acoustic reflexes can provide a clue to an inaccuracy in hearing thresholds. Use the Strenger test to identify non-organic hearing loss.
Identifying Facial Nerve pathology
In an individual with a facial nerve pathology, a unique acoustic reflex pattern will present as the reflex will be absent whenever measured on the affected side.
Identifying middle ear pathology
At times, an individual may have an abnormal tympanogram, but normal hearing. ARTs can help to confirm middle ear pathology as acoustic reflexes will be absent. Similarly, an individual can have a normal tympanogram, but a mild conductive hearing loss. Absent acoustic reflexes can help confirm this conductive component.
Most important, however, is that the acoustic reflex threshold is the only measure of central auditory function present in a traditional audiometric test battery.
This easy-to-perform measure is relatively accurate in identifying retro-cochlear pathologies. Studies have shown sensitivity to be over 80% for individuals with brainstem and auditory nerve pathologies.
This is especially important for individuals with retro-cochlear pathology who do not yet present with asymmetrical thresholds or abnormal speech scores, as abnormal acoustic reflex thresholds may be the only indicator of retro-cochlear involvement
How to interpret acoustic reflex thresholds
Acoustic reflex thresholds are interpreted at each individual frequency tested and are often classified as present, absent, or elevated. Typically, different pathologies result in different patterns of acoustic reflex threshold results in relation to their presence or absence during ipsilateral and contralateral measures. It’s important to note, however, that these patterns can vary between patients and acoustic reflexes should always be interpreted among other test results like audiometry, imaging, tympanometry, and patient history. For individuals with normal hearing, reflexes will be present around 80 – 90 dB HL ipsilaterally and about 5 dB HL above ipsilateral results contralaterally. Several different ART patterns and their corresponding pathologies are outlined below. For the purposes of this article, right and left refer to the ear being stimulated by the loud sound. Meaning that in a right contralateral reflex, for example, the loud sound would be presented to the right ear and measured in the left ear.
Note the retrocochlear pattern is identical to the cochlear pathology. When differentiating between the two, it is important to take hearing thresholds into account. In a retrocochlear loss, acoustic reflexes are more likely to be absent or significantly elevated in relation to hearing thresholds.
