Loudness decay

Eric Smith erc at cinenet.net
Mon Dec 4 07:37:57 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.

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

>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.

>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?

>>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?

>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.

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

>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.

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