Tips for Masking Bone Conduction ABR

This video tutorial provides some tips, tricks, suggestions and ideas for how and when to apply masking noise to the bone conduction ABR. You can read the full transcript below.

Introduction

This video is designed to provide some tips and tricks on masking the ABR, with a specific focus on masking bone conduction. It refers to the Masking Calculator, a tool developed by Dr. Guy Lightfoot in the UK.

Perceived difficulty of masking the ABR

Many clinicians often find the concept of masking the ABR difficult. This is because there are more uncertainties and less threshold information readily available in ABR testing to correctly predict the required level of masking.

When do you not have to mask?

Fortunately, there are many situations when masking is not required.

Let’s start here. In most cases, you will most likely have performed air conduction first. If a hearing loss is detected, then you have two options. One, test the bone conduction in the same ear, or two, test the air conduction in the other ear.

If you test the bone conduction in the same ear, you will not have enough information to calculate the masking level, as you haven't yet tested the air conduction in the non-test ear. However, it is important to note that if a response is recorded at a not-masked bone conduction value that is below the level of interaural attenuation, then masking is not required.

Be aware that these values change depending on stimulus and age. For instance, when testing 4 kilohertz narrowband CE-Chirp bone conduction on a 6-to-8-week-old, a response at 10 dB nHL, which converts to 15 dB eHL, will not require masking.

However, if there isn't a response until you increase the stimulus level to 15 dB nHL, which equates to 20 dB eHL, masking will be needed. This is because the sound is now loud enough to potentially be crossing over to the other ear and for a response to be generated by that cochlea.

A sensible place to start bone conduction testing is at the loudest level within the normal range, which doesn't require masking. This will give you the biggest chance of seeing a response. In the example just given, this would be 10 dB nHL. Remember, the level where masking is not required will vary according to age and stimulus.

If you establish a bone conduction response within the normal range of hearing, which does not require masking, there is no need to measure at quieter levels to establish exact thresholds. If the not masked bone conduction suggests a sensorineural loss, where it is the same as the air conduction, then masking is also not required in this situation, and you can progress with your testing.

However, if the not masked bone conduction testing reveals a mixed loss or a value greater than the level of interaural attenuation, then masking noise is required, and you will need to assess the air conduction level of the opposite ear before you can apply masking.

If you have information about the air conduction levels for both ears, you can perform masked bone conduction testing. It is still advisable to perform non-masked bone conduction first, as this may eliminate the need for masking, which can risk unsettling the baby as you introduce an additional transducer and stimulus.

When to mask an air-bone gap

If air conduction levels have been obtained for both ears and you are ready to perform masked bone conduction, now is the time to enter these values into the masking calculator. It's important to enter the basic information first. This includes selecting the ABR system, transducers, stimulus type, and the corrected age of the patient.

The stimulus level is the bone conduction value that you wish to test. Usually, you would start with the not-masked value that you have recorded to assess whether this continues to display a response when masking noise has been applied.

Now, there are three remaining boxes to complete in the calculator. The test-ear air-bone gap only applies for air conduction masking, so we can leave this as zero for this example. The non-test-ear air-bone gap and the non-test-ear BC ABR threshold are both unknown values at this moment.

You can make use of your not-masked bone conduction response to guide you at this point if you have recorded a threshold. This will be the lowest possible bone conduction value for either ear, so you can use this to estimate the value for these two boxes.

If you have only recorded a minimum response level as opposed to a threshold, then you must be aware that the true bone conduction threshold may be much quieter, leading to greater amounts of masking being required.

You can also use other information, such as tympanometry, to guide you. However, this is an inexact way of estimating these values. It is recommended that any such estimations are used to guide you as to the risk of cross-masking and the likelihood of cross-hearing, which are displayed within the calculator.

In the example we have been looking at so far, you can see that the risk of cross-hearing without masking is low, although masking is still required. However, if we take another example where the stimulus level is higher, the risk increases substantially. This also opens the range of potential options for the non-test-ear BC threshold.

You can enter some different estimated values into the calculator and then judge the effect of these. Remember, the masking calculator is designed to be used to guide you. It is the responsibility of the clinician to use their clinical judgment when deciding on test strategy.

When in doubt, it is recommended to err on the side of caution and adopt a more conservative approach. For instance, when the bone conduction value for the non-test ear is unknown or you have a minimum response level rather than a threshold, the calculator recommends you enter a value of 0. This will ensure a greater amount of masking is applied in order that you can be confident that the non-test ear is being sufficiently masked and is not therefore contributing to the response.

Once you have entered your values into the calculator, assess the risk of cross-masking and the likelihood of cross-hearing. The calculator will provide you with two results for each scenario that you explore, a result and an offset. The result is the dB dial level of masking noise that you should apply to your non-test ear, and the value that you should enter into the Eclipse software.

This is an absolute value for the conditions that you have entered into the calculator. The offset is the dB noise level relative to the stimulus level. If you increase your stimulus level, the offset remains the same, but the result value will vary.

On the Eclipse, it is possible to use either of these values by entering them in the temporary test setup protocol. It is up to you as the clinician to take the clinical responsibility for the final level of masking that you decide to apply to your ABR measurement.

Bear in mind that just as in pure tone audiometry masking, you will sometimes encounter masking dilemmas to which there is no solution. Remember, it is always best to adopt a conservative approach and apply a greater level of masking if there is an element of uncertainty.