By Sherman Lord Au.D.
Tympanometry has been available clinically since the 1960’s after Terkildsen and colleagues developed the procedure in 1959. For many decades it has been a dependable, routine part of the audiological test battery.
There have been numerous peer-reviewed articles on the limitations of single frequency probe tone tympanograms in differential diagnosis of middle ear disorders. In light of these limitations, plus what is known about how the middle ear functions with respect to the mechanical transfer of acoustic energy to the inner ear, and despite recent advances in technology, audiologists and otolaryngologists continue to rely solely on single frequency tympanograms for middle ear assessment.
The purpose of this article is to explain the diagnostic value of performing tympanometry using a broad-band signal, in essence the acoustic click, in lieu of either 226Hz or 1KHz probe tones.
Audiological testing using pure tones, acoustic reflex testing, and otoacoustic emissions assess hearing and parts of the auditory system across a broad range of frequencies. Most frequently between 125Hz-8KHz. In some cases, also extending into the frequency range beyond 8KHz. Yet only one frequency is used for tympanometry. Why is that?
When the first “tympanometers” (by the way, the term “tympanometry” was coined in 1964) were developed there were several technological limitations affecting the linearity of the microphones and receivers available at the time. There was also concern about the impact the probe tone frequency might have on the unwanted elicitation of the acoustic reflex affecting the ability to measure valid acoustic reflex thresholds. Therefore, for these reasons and for convenience purposes, 220Hz was selected as the probe tone frequency. You might be interested in learning that the selection of 220Hz as the probe tone frequency had nothing to do with its potential diagnostic value.
In 1970, Grason-Stadler introduced the GSI1720 which had two probe tone frequencies, 220Hz and 660Hz and displayed the two admittance components, B (susceptance) and G (conductance). Since then, most manufacturers of both diagnostic and screening tympanometers offer additional probe tone frequencies as an option, including 678Hz, 800Hz, and/or 1KHz. Over the decades, many peer-reviewed articles have shown the added diagnostic value of using high frequency probe tones to more effectively assess the complex nature of the middle ear system. Despite all this evidence, good ‘ole 226Hz still is the probe tone frequency of choice.
Oh, did I say 226Hz? Where did that additional 6Hz come from? The probe tone frequency was switched to 226Hz in the ANSI 1987 standard for calibrating aural acoustic immittance instruments. Again, not for diagnostic value but for ease of equipment calibration. Nothing about the standard has changed since that time.
But things are starting to change!
As already mentioned, the limitations of single frequency tympanometry are well-known. Also well-known is how the middle ear system functions differently depending upon the frequency make-up of the stimulus. We all know that it isn’t feasible to perform tympanometry at multiple single-frequency probe tones. But what if there was a method to test a broad range of frequencies with one pressure sweep? Well, there is, and it is known as Wide-Band Tympanometry (WBT).
WBT is a member of a family of measurements known as Wide Band Acoustic Immittance. It not only includes WBT but also Wide Band Reflectance and its alter ego, Wide Band Absorbance. In fact, to better understand WBT, it’s better to start thinking in terms of acoustic energy absorbance. In other words, if a stimulus of some sort, e.g., a 226Hz probe tone or an acoustic click (wide band signal) is delivered into an ear canal, what percentage of that signal is actually “absorbed” by the middle ear? Since we have no way of placing a microphone at the oval window to measure the absorbance, it is calculated based upon how much acoustic energy is reflected. That can be measured in the ear canal similar to measuring an otoacoustic emission.
Performing a WBT test is no different from performing a single frequency tympanogram. It takes the same amount of time, but the amount of data is significantly greater and is more diagnostically sensitive.
The good news is a WBT which looks very cool can be sliced and diced to provide all the information we are accustomed to seeing (226Hz or 1KHz tymp) on the standard tympanogram plus a new tympanogram referred to as the “average tympanogram”. The average tympanogram encompasses a wide frequency range with normative data for adults and children. It is most helpful when a younger child is fussy during the test. By using a wide band stimulus and reviewing the average tympanogram, you can overcome artifact associated with a less than cooperative patient.
Another calculation made during the 2-3 second WBT test, is an estimate of the patient’s middle ear resonance frequency. Why is this important? In cases when the patient shows significant air/bone gaps but a tympanogram that is essentially normal, estimating the resonant frequency of the middle ear can indicate whether the cause of the ABG is related to an ossicular chain fixation or disarticulation.
The final piece of information the WBT provides is the “absorbance curve”. This may turn out to be the result that has the most diagnostic value (personal opinion). Recall absorbance is a calculation of the percentage of the stimulus that is being “absorbed” by the middle ear system and transferred into the inner ear. Absorbance can be measured at both ambient and tympanometric peak pressure. This is particularly helpful when a patient exhibits negative middle ear pressure. The absorbance measurement at ambient pressure will should show less sound absorbance.
Another benefit of the absorbance measurement when conventional audiometry is not possible is the ability to perform pre and post absorbance measurements results at ambient pressure to document improvement. For example, after successful insertion of a PE tube, a conventional tympanogram will be flat in the presence of a larger than normal ear canal volume. But the tympanogram provides no useful information to document improvement. The WBT test will document ear canal volume and a patient PE tube and reveal improvement in the absorbance test confirming a successful procedure.
Another example is an otosclerosis patient who undergoes a stapedectomy. Most physicians are reluctant to perform pressurized testing on a post otosclerosis ear. But the absorbance performed separate from a WBT is safe to use and reveal what should be a normal absorbance curve.
Finally, different middle ear disorders yield different absorbance curves. An absorbance curve for middle ear effusion will look much different from one for a discontinuity of the ossicular chain. To end with a teaser, there is evidence that absorbance test results may, someday, be able to predict the amount of CHL that is present.
There are FDA approved commercial devices available today that include the Interacoustics Titan and GSI Tympstar Pro V2. Both perform WBT and absorbance. Please contact your e3 representative to schedule a demonstration or to request a quotation and additional literature on these devices.
I also encourage you to visit the Interacoustics Academy website interacousticsacademy.com) where you will find several videos and presentations on WBT and WB Absorbance. Some have CEUs available.
Before I go a word about insurance billing. Since the CPT code uses the word tympanometry in its physician fee schedule, in order to get reimbursed for the WBT, the absorbance would have to be administered as part of a WBT procedure. As long as a tymp is performed, then you are able to bill using the tympanometer codes.
With all that’s known about the limitation of single probe tone tympanometry it is a wonder why more clinicians are not investing in WBT equipment. It is just as fast as single frequency probe tone tympanometry and much more diagnostically sensitive. And it is not cost prohibitive.