Loudness decay (fwd)

Jeffrey G. Sirianni sirianni at UTS.CC.UTEXAS.EDU
Fri Dec 8 01:16:11 EST 1995


>In article <199512021716.LAA11393 at curly.cc.utexas.edu>,
>Jeffrey G. Sirianni <sirianni at UTS.CC.UTEXAS.EDU> wrote:
>>>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.

>Yes, more than 60 seconds.  Typically several minutes, but it depends
>on factors such as how loud the test tone is and what frequency it is.

If the patient can detect the presence for more than 60 seconds at any one
intensity, then it is not a sign of retrocochlear lesion.

>What causes the "normative" tone decay, and does that imply that all
>equivalent SNHL cases should exhibit it?

I would say that it is an adaptation effect of the inner hair cells (?) 

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

>Yes, the decay does seem to happen faster at higher frequencies, but
>that hasn't been tested carefully.

Greater decay in the high frequencies indicates a cochlear lesion and not a
retrocochlear lesion

>>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(?)...

>It's bilateral.  The audiograms are different for the two ears.  In the
>left ear, it's a ski slope.  In the right ear, it's somewhat flatter.
>At low frequencies, the left ear ski slope loss is about 10 dB less
>severe than the right.  At higher frequencies, the right ear is as much
>as 20 dB less severe than the left ear.  Thus, the left ear is better
>at low frequencies and the right ear is better at high frequencies.
>But both ears are better at low frequencies than high.
>
>What do you mean acoustic neuromas usually occur bilaterally?  Aren't
>they in the auditory nerve between a particular ear and the brain?

Ooops... Sorry, they usually occur unilaterally.... My mistake...

>>>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.

>Aha, so there is a term for it.  Is decruitment very common?  Does it
>have a lot of different possible causes, or is a lesion the usual >cause?

Looks like someone needs to review the ABLB.... :-) It's OK, people hardly
use it anymore..... Who has time ??

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

>For various reasons, the only reliable test done yet has been the
>audiogram.  Word recognition seems to be zero without speechreading,
>much higher with.

>>>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.

>The tones are perceived as faint ringing sounds up to 6000 and
>faint hissing sounds above that.

Well I guess the loss of "tonality" can occur at such a high intensity

>Could loss of inner hair cells cause decruitment?  It seems logical,
>considering that loss of outer hair cells causes recruitment.

No, I don't think so, it usually appears as recruitment, but recruitment
would imply that the inner hair cells are at least functioning.  I don't
know what to expect if they are non-functional.  Maybe decruitment...

>>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.

>There are no such symptoms at all.  The age is in the neighborhood
>of 40.  The hearing loss has been progressive since infancy.  I will
>mention an MRI to the ENT and see what he thinks.

An interesting thread for folks interested in diagnostics....

Jeff
****************************************
* 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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