Masking Dilemma?

Ed Luczak ed.luczak at gsfc.nasa.gov
Tue Oct 7 20:17:21 EST 1997


Can someone please explain the "Masking Dilemma", and how to best
interpret audiological test results when this is experienced?

My situation is as follows.  I have a hearing loss that has been diagnosed
as otosclerosis, and underwent a right stapedectomy in 1976.  Although the
surgery initally restored hearing, performance in the operated ear
gradually deteriorated, and I now wear hearing aids in both ears. 
Periodic audiological evaluations have shown air conduction losses in the
50-65 dB range, and masked bone conduction losses in the 10-25 dB range in
both ears.

Last week I was tested by an audiologist I had not visited before.  The
test results are summarized below (view with a monospace font!):

               250   500  1000  2000  3000  4000  6000  8000

Left
AC              60    60   65    50    55    60    60    30
BC-"BestBone"    0     0   20    20    10     5
BC-Masked       20    25   30    50    40    45

Right
AC              50    45   50    60    65    70    70    55
BC              (not recorded)
BC-Masked       30    35   40    55    35    45

The audiologist indicated that the test results exhibited the "Masking
Dilemma", and explained the testing symptoms, but I'm still confused a
bit.  She pointed out that the "BestBone" measurements indicate the
sensorineural response of at least one, and possibly both of my ears. 
However, because sound from the transducer placed on the bone behind one
ear can propagate through the skull to the other ear, masking tests are
performed to determine the sensorineural response of each ear
individually. As shown above, the BC-Masked test results are 20-40 dB
below the "BestBone", on both ears.  This seems to be a logical
inconsistency (is this the dilemma?), because the BC tests indicate at
least one ear with fairly good senso-neural performance, but the BC-Masked
tests show two ears with sensorineural losses.  

My concern in interpreting these results is two-fold:

1. Hearing aid fitting - Is there a difference in specifying the best
hearing aids for a patient having a sensorineural loss versus one with
only a conductive loss?  The audiologist recommended that I consider new
Resound hearing aids. The Resound web pages and brochures say that their
patented amplification technology "specifically addresses the need to
compenstate for the loudness sensitivity common in sensorineural hearing
impairment."  My BC-Masked measurments indicate that this might be for
me.  My BC-BestBone measurements indicate that at least one, and maybe
both of my ears don't need this rather expensive technology.  How do I
choose?

2. Surgical options - In consulting with an ENT MD after the audiological
test, he said that the BC-Masked results, especially in the 500-2000 Hz
range, indicate that little improvement would result from stapes surgery,
because much of my hearing loss is sensorineural.  But is it really?  Or
are these important measurements incorrect, due to the "Masking Dilemma"?

I'd appreciate any insight that anyone can share with me on this.  Thanks,

--Ed Luczak

-- 
NASA/Goddard Space Flight Center

The opinions expressed in the article above are not necessarily 
those of NASA or any of its contractors.  The poster is respon-
sible for the accuracy of the statements above.



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