Perhaps the handiest weapon in the Hearing Aid Professionals’ arsenal is the tympanometer, as if you draw a straight line between the audiologic information it can reveal and how that information will impact a hearing aid fitting, you’ll be rather surprised… And will have you running to your equipment dealer or eBay to buy one.
The tympanometer performs two separate functions, which each reveal significant data those of us “in the trenches” which we can use to nail down a good fit, by performing measurements in two disparate areas:
- Tympanic membrane mobility via compliance measurement, which can yield all sorts of mechanical problems from the TM itself all the way through the Eustachian tube, & all stops in between;
- Neural measurements, through stapedial reflex thresholds; also called “acoustic” or “middle ear muscle” reflexes.
The first item, relating to mechanical (conductive) problems, such as commonly found fluid in the middle ear cavity, is something that, when we detect a problem, we refer to a physician, as it is above our pay grade. Of course, busy doctors appreciate it when we give them good data in an easy-to-comprehend format, especially if there is a nice graph. For more on tympanometry, the reader is directed to this good article, which explains the various bugbears it uncovers.
However, the purpose of this article goes to the second function of the tympanometer, namely the detection and measurement of middle ear muscle reflexes to sound stimulus, as this can be highly revealing. In normal and some impaired ears, a reflex contraction of the middle-ear stapedius muscle will occur when sound is of sufficient energy, typically 90dB. The stapedius muscle is attached by a tendon from the rear wall of the middle ear to the head of the stapes; and when the stapedius contracts, the tendon produces tension on the stapes, causing the middle-ear ossicles to stiffen. This action reduces the transmission of sound energy through the middle ear, by acting as a dampener, to protect the delicate structures of the cochlea.
One way of visualizing the acoustic reflex function is to treat it as a negative feedback process with a unity gain amplifier, with the forward path signal (the so-called “afferent” signal) applied to a non-inverting Schmitt trigger and then fed back (the so-called “efferent” signal) into the negative input of the amplifier, acting as a control vector, hence reducing gain by 10dB. Note well that the stapedius muscles on both sides contract in response to sound delivered to either ear, as the signals are summed in the cochlear nuclei:
For more detailed drawings of the neural pathways, please see reference 3
First, acoustic reflex thresholds will give you an upper limit to the Maximum Power Output (MPO; previously SSPL-90) of the hearing aids, as the operating theory is that you do not want the instruments to constantly trigger the stapedial muscle reflex, as it is tiring to the patient.1 In fact, according to this paper by Ed Overstreet, this principal is extended to cochlear implants (CI’s) to determine M/C levels;2
Second, it can be used to screen for the dreaded Auditory Neuropathy Spectrum Disorder, which will tell you if a hearing aid will even work. This will be the subject for an upcoming article; but in the mean time, any stapedial reflex thresholds – Ipsilateral (“ipsi”) or contralateral (“contra”) greater than 90dB means ANSD can be in play, and must be followed up with more testing to confirm it or rule it out.
- Using Acoustic Middle Ear Muscle Reflexes and Their Utility in Fitting Hearing Instruments by Jay B. McSpaden, PhD, BC-HIS, and Dana K. McSpaden, MSEd
- Relationship between Electrical Stapedial Reflex Thresholds and HiRes Program Settings: Potential Tool for Pediatric Cochlear-Implant Fitting by Ed Overstreet PhD, Lisa Buckler MA & Kristen Dawson MA
- Acoustic Reflex Threshold (ART) Patterns: An Interpretation Guide for Students and Supervisors Diana C Emanuel PhD
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