Re: tympanometry

["Shahnaz, Navid" <nshahnaz@xxxxxxxxxxxxxxxxxx>]

Hi
Peak pressure refers to the position of the tympanometric peak (maximum admittance peak-Ya) on the pressure axis. Since the probe tip of the admittance measurement system is remote from the surface of the tympanic membrane, admittance  measured at the probe tip jointly reflects the admittance of the external auditory canal and the admittance of the middle ear.  The dimensions of the external auditory canal vary depending on the depth of insertion of the probe tip as well as individual differences in ear canal size.  This produces substantial variation in the admittance due to the external ear and thus to the overall measurement of admittance at the plane of the probe tip.  Therefore, to derive a measure of middle ear admittance it is necessary to subtract the admittance due to the external ear canal from the overall admittance measure. Therefore, the purpose of tympanometry is to accurately estimate the middle-ear admittance; however, the accuracy of the middle-ear admittance estimate relies on obtaining an accurate estimate of the effects of the ear canal. A principle determinant of these effects is the volume of the air between the probe tip and the tympanic membrane. This volume (commonly referred to as Vea) is affected by numerous factors such as the depth of insertion of the probe tip, the dimensions of the ear canal, and the amount of volume occupied by cerumen.  The most commonly used method for estimating the volume was proposed by Terkildsen and Thomsen (1959). They suggested the use of a high positive pressure (200 daPa, 1 daPa=10 Pa) during the measurement, which could drive the admittance of the middle ear toward zero (in theory-in reality you could still hear the probe tone meaning that the middle ear has not been knocked out). As a result, the admittance measured at the probe tip could be attributed, assuming that the canal walls are rigid, to the air trapped in the ear canal itself. Shanks and Lilly (1981) reported that in adults the volume of the trapped air is more accurately estimated from the susceptance (B) tail than from the admittance (Y) tail. Although the Vea estimated from either the positive or the negative pressure is always greater than the actual ear-canal volume (Rabinowitz, 1981; Shanks & Lilly, 1981), Shanks and Lilly (1981) suggested that Vea is more accurately estimated from the negative tail at ?400 daPa than from the positive tail at +400 daPa. The compensated static admittance is typically higher when extreme negative (rather than extreme positive) pressure is used to estimate ear canal volume (Shanks & Lilly, 1981).  This variation is due to an inherent asymmetry in the tympanogram such that the volume estimate at extreme negative pressure is typically lower compared to the volume estimate at extreme positive pressure (Margolis & Shanks, 1985).  This asymmetry is caused by the reduced contribution of conductance, i.e., increased resistance at extreme negative pressures.  It should be noted that a range of ear canal pressures may be used to estimate ear canal volume and that somewhat lower canal volume estimates (and hence higher compensated static admittance) may be observed as the ear canal pressure used to correct the volume is increased. Despite the known errors, however, in clinical measurements the most common estimation of Vea is still taken from the admittance positive tail due to better test-retest reliability (Margolis & Goycoolea, 1993) and a more consistent measure of tympanometric width (Margolis & Shanks, 1991). It should be noted that subtracting the admittance tails (either positive or negative) from the peak admittance, often called the baseline method, is typically performed by tympanometers to derive the middle-ear admittance. However, this subtraction is not physically meaningful (e.g. Margolis & Hunter, 2000), as the admittance is a complex number which includes an imaginary part, the susceptance, and a real part, the conductance. Tympanometric width (also referred to as tympanometric gradient), refers to the width of tympanogram (in daPa) measured at one half the compensated static admittance (DeJonge, 1986; Koebsell & Margolis, 1986).  This measure provides an index of the shape of the tympanogram in the vicinity of the peak; it quantifies the relative sharpness (steepness) or roundness of the peak.
For more detailed information refer to:
Van Camp,K., Margolis, R., Wilson, R., Creten, W., & Shanks, J. (1986).  Principles of tympanometry.  ASHA Monographs, 24.

Margolis, R., & Shanks, J. E. (1985).  Tympanometry.  In J. Katz (Ed.),         Hanbook of clinical audiology  (pp.438-475).  Baltimore:  Williams & Wilkins.
Cheers
Navid

---------------------------------------
Navid Shahnaz, Ph.D., Aud. (C)
Assistant Professor
School of Audiology & Speech Sciences
Faculty of Medicine
University of British Columbia
2177 Wesbrook Mall, Friedman Building
Vancouver, BC Canada V6T 1Z3

Tel. 604- 822-5953
Fax.604-822-6569
E-mail: nshahnaz@xxxxxxxxxxxxxxxxxx
Website: http://www.audiospeech.ubc.ca/navid/

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http://www.usnews.com/articles/business/best-careers/2008/12/11/best-careers-2009-audiologist.html



-----Original Message-----
From: AUDITORY - Research in Auditory Perception on behalf of Mark Shaver
Sent: Sun 4/12/2009 12:42 PM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: tympanometry
 
Does anyone know if the relationship between typanometric peak pressure
and other immittance values (i.e., tympanometric width, tympanometric
gradient, acoustic admittance, ear canal volume) has been investigated.
A classmate of mine has been instructed by his advisor to pursue this
project but he was not able to find any references on Medline. Can
anyone point me in the right direction so I could pass on some info to
him?


Thank you in advance,



Mark Shaver