Loudness decay

Jeffrey G. Sirianni sirianni at UTS.CC.UTEXAS.EDU
Sat Dec 2 12:17:48 EST 1995


>In article <49isa9$m1k at usenetw1.news.prodigy.com>,

>To repeat the symptoms, steady tones seem to fade away
>gradually till they are perceived as being completely gone, but when
>the tone is then interrupted for a fraction of a second, it is
>perceived as coming back at full loudness.

This is a classical example of tone decay.  But since you know this already,
there are some other questions that need answering.

1. Can the patient hear the tone for a full 60 seconds before it fades away
and before you reach the limit of the audiometer?  If yes, then it may
indicate normative tone decay associated with SNHL, if no then it indicates
a retrocochlear lesion.

2. Is there less decay in the low versus high frequencies?  Same indications
as above.

3. If this a bilateral verus a unilateral symptom?  An assumption that
acoustic neuromas usually occur bilaterally.  Is there any asymmetry in the
audiometric thresholds?  You could be looking at a lesion of the brainstem
where bilateral input is recieved...SOC(?)...

>Also, before it has faded away completely, it seems to change from a >tone
to a remote din.  In other words, it doesn't fade to a faint tone >before
fading away, but rather loses its tonal quality first.

>Another symptom, which I may not have mentioned before, is that while
>there is severe recruitment at low frequencies, it's just the opposite
>at high frequencies.  In other words, when 30 dB is added to a high
>frequency tone that sounded very faint at threshold, it still sounds
>faint, as if only 5 or 6 dB were added.

Classic example of decruitment, another symptom of a retrocochlear lesion.

>When it's louder, it starts to sound very slightly painful, but still
>doesn't have the loudness quality of a loud and clear tone.  In other
>words, it seems faint but still causes a small amount of the kind of >pain
that comes from hearing a too-loud sound.

>The fact that this deafness is bilateral and long-progressive indicate
>that it's probably not caused by an acoustic neuroma, although that is
>worth asking about.  But the real question right now is what else could
>it be?  Only by knowing what it could be would I know what possibilities
>to ask about.

What are this patient's word recognition scores?  Did you perform roll-over
testing? a PIPB function?

>Another thing I may have forgot to mention is that this deafness has
>progressed to the point where all frequencies above 1500 have
>thresholds at or above 100 dB.  So, for example, in the above mentioned
>test, where 30 dB was added to threshold, I'm talking about adding 30
>dB to 100 dB, with the resulting 130 dB tone still sounding faint, even
>though it was 30 dB above threshold.

My gut feeling here is that at 1500 Hz and above, you may be testing in a
region with no functioning inner and outer hair cells (no possiblility of
tonal perception).  At 130 dB HL, you may be stimulating a distant portion
of the cochlea, or even getting sensation from the vestibular mechanism.

>This test used some special test equipment not normally found in an
>audiologist's office, so you might suspect that the equipment itself is >at
fault.  However, that's not the case, and similar results were >obtained
with hearing aids.  For example, with hearing aids that output >138 dB at
high frequencies, loud high frequency tones still sound faint >even though
they are 30+ dB above threshold.  If the loudness is >increased gradually
from threshold to 30+ dB above threshold, the >perception is of a much
slower increase in loudness, resulting in a >total increase of maybe 5 or 6 dB.

Are there any other neurological signs and symptoms? Headaches, vertigo,
gait problems?  What is the age of this patient?  Why not refer for an MRI?
Your patient will thank you for setting his/her mind at ease.

****************************************
* Jeff Sirianni                        *
* University of Texas at Austin        *
* Communication Sciences and Disorders *
*                                      *
* sirianni at uts.cc.utexas.edu           *
* jgsaudio at aol.com                     *
****************************************




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