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LESSON 110

The Acoustic Reflex Decay Test

The acoustic reflex decay test measures whether the reflex is maintained during a 10 second continuous tone presented 10 dB above the reflex threshold. In this lesson we use an example to look at how the test is done, the half-life, what a positive result means and how it relates to retrocochlear pathology.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we will learn the reflex decay test, which measures how well the acoustic reflex is maintained during a sustained stimulus.
We will talk about how the test is done, how the result is read, and how it relates to retrocochlear pathology.
Acoustic reflex decay is the decrease in reflex amplitude over time while a sustained stimulus is presented.
In a normal ear, with a sustained tone a little above the reflex threshold, stapedius contraction is largely maintained for ten seconds.
If there is a problem in the auditory nerve, its fibers cannot keep responding to the sustained stimulus, and the reflex fades quickly.
The test measures this difference; in other words, it tests how well the auditory nerve withstands sustained stimulation.
Together with reflex thresholds and patterns, it is a test in the immittance battery aimed at retrocochlear pathology.
The test became widely used in the clinic through the work of the Swedish researchers Anderson, Barr and Wedenberg.
First, the acoustic reflex threshold is found at the test frequency.
The test is usually done at five hundred and one thousand hertz.
The stimulus is a continuous pure tone presented ten decibels above the reflex threshold.
The tone lasts ten seconds without interruption, and the change in admittance is recorded during this time.
The largest initial reflex amplitude is taken as the reference, and its change over the ten seconds is followed.
The stimulus is often presented contralaterally, that is, to the ear opposite the probe.
The result is recorded as the percentage drop in amplitude and, if needed, the time taken to fall to half.
At low frequencies, the reflex in a normal ear is clearly maintained for ten seconds.
At two thousand and four thousand hertz, however, the reflex can fade quickly even in people with normal hearing.
This is why decay at high frequencies is not considered meaningful, and the test is limited to low frequencies.
Because the test needs a loud tone lasting ten seconds, it can be uncomfortable for the patient.
If the reflex threshold is already high, threshold plus ten decibels may exceed the safe limit; in that case, the test may not be done.
In patients with tinnitus or sound sensitivity, the need for the test is considered beforehand.
On the screen, we see reflex amplitude over time; the largest initial amplitude is taken as one hundred percent.
The blue curve shows a normal ear; the amplitude stays high for ten seconds.
By the common criterion, the test is positive if the amplitude falls below half of its initial value within ten seconds.
On the red curve, however, the amplitude drops quickly within a few seconds.
The time it takes for the amplitude to fall to half is called the half-life; in this example, it is about four seconds.
Some sources also treat a half-life shorter than five seconds as a more marked finding.
Because criteria differ slightly between sources, state your clinic's criterion in the report.
Positive reflex decay suggests retrocochlear pathology, that is, a problem of the auditory nerve or the brainstem.
The classic example is a vestibular schwannoma pressing on the auditory nerve in the internal auditory canal.
In cochlear hearing losses, clear decay is usually not seen at five hundred and one thousand hertz.
This is why a decay finding helps to tell cochlear and retrocochlear problems apart.
Later studies, however, showed that in ears with tumors the reflex is often completely absent, rather than showing decay.
So the decay test can only be done if the reflex can be measured; without a reflex, decay cannot be assessed.
A positive result calls for further assessment, while a negative result does not rule out a retrocochlear problem.
In the decay test, the auditory nerve being assessed is the nerve of the ear that receives the stimulus.
Even though the contralateral reflex is measured in the opposite ear, the decay finding belongs to the stimulus ear.
The report should clearly state which ear received the stimulus and which ear held the probe.
The middle ear of the probe ear must allow the measurement; if there is a conductive problem there, the reflex is absent anyway.
Ear canal pressure is set to the tympanometric peak pressure of the probe ear.
A probe leak or patient movement can cause an amplitude drop that is not real.
Patient movement, swallowing or talking create irregular deflections in the recording and can change the amplitude.
If the probe leaks slowly, the baseline drifts and the reflex seems to be fading.
If the tone stops before the full ten seconds, the result cannot be interpreted.
If the stimulus is too close to the reflex threshold, amplitude can fluctuate even in a normal ear; so threshold plus ten decibels is used.
With a doubtful result, the probe is reinserted and the test is repeated once more.
True decay is seen in a similar way on the repeat measurement.
Let us look at an example: the left ear has a mild to moderate sensorineural hearing loss, more marked at high frequencies.
Both tympanograms are type A, and reflex thresholds can be measured.
A continuous tone is presented to the left ear at one thousand hertz, ten decibels above the reflex threshold.
The reflex amplitude falls below half within four seconds; the decay test is positive.
With the stimulus in the right ear, however, the amplitude is maintained for ten seconds.
Together with the asymmetric loss, this finding calls for referral for further assessment; the example is for teaching.
The results of the sample case can be shown in the report with a table like this.
With the stimulus in the right ear, no decay was seen at either frequency.
In the left ear, there is no decay at five hundred hertz either.
At one thousand hertz, however, the amplitude fell to half in four seconds, so the half-life was also written down.
The stimulus level, the probe ear and the criterion used are also noted below the table.
A clinical tip: interpret decay only at five hundred and one thousand hertz.
At two thousand and four thousand hertz, the reflex can fade in normal ears too; such a finding would be a false alarm.
Keep the stimulus level within the safe limit, and tell the patient beforehand that they will hear a ten second sound.
A prepared patient moves less, which improves the quality of the recording.
Reflex decay is not a diagnostic test on its own; it is part of the battery.
It is read together with findings such as asymmetric hearing loss, a disproportionately low word score and unilateral tinnitus.
Rollover on the performance intensity function can also support a similar retrocochlear suspicion.
When a retrocochlear problem is suspected, the main investigation today is magnetic resonance imaging.
The auditory brainstem response also gives added information about neural conduction; we will see this in later lessons.
The task of audiological tests is to help select correctly the patients who need further investigation.
The test uses a ten second continuous tone presented ten decibels above the reflex threshold.
Only five hundred and one thousand hertz are interpreted; at high frequencies, decay can occur in normal ears too.
A drop in amplitude below half of its initial value within ten seconds is a positive result.
A positive result suggests retrocochlear pathology; a negative result does not rule it out.
Test yourself: At one thousand hertz, with a tone ten decibels above the reflex threshold, amplitude halved in three seconds. The result?
The correct answer is B: positive reflex decay.
The amplitude fell below half of its initial value within ten seconds, so the test is positive and calls for retrocochlear attention.
In the next lesson, we will look at multifrequency tympanometry and the resonant frequency of the middle ear.
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Prerequisites

LESSON 109 Acoustic Reflex Patterns and Site of Lesion

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