How much masking should we use in this ABR case example?

We tested a 7-month old baby with a bilateral conductive hearing loss. We wanted to check whether we need to mask or not. The not-masked hearing thresholds by BC were 15 dB nHL and the AC thresholds bilaterally were 40 dB nHL. We want to know what the BC threshold is on the other side?

Answer: This is potentially very complicated because, just like in conventional audiometry, a bilateral conductive loss raises the prospect of cross masking (and therefore masking dilemma) because of the need to increase the masking level in the non-test ear to overcome the attenuation from the conductive component.

The amount of masking noise depends on what you estimate the conductive loss to be in the non-test ear (ref: the BC threshold).  A masking calculator and guidance can be downloaded by pressing the link here.  You could input the values above to calculate the required masking. Depending on the estimated size of the air-bone gap, the maximum masking level possible (for the Eclipse) might be reached.

Although the patient is 7 months old, it could be worth checking back to any earlier records for ABR data in the neonatal period to help answer the question. At earlier ages the BC values are more ear specific as the plates of the cranium are not fused. This could mean the BC thresholds give ear specific information even without applying masking,

To learn more about ABR Masking please watch the following webinars 

Masking the ABR Part 1 

 Masking the ABR Part 2

Downloads 
ABR Masking Calculator