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October 2003 In some pediatric teaching programs, most residents graduate without a practical knowledge of objective tests for middle ear function. The 1997 “Guidelines for the Diagnosis and Management of Acute Otitis Media (AOM) and Otitis Media with Effusion (OME)” stress the importance of diagnostic accuracy for middle ear diseases. The guidelines recommend using tympanometry and acoustic reflectometry to verify the presence of middle ear effusion, a sine qua non of AOM or OME. Pediatricians can accurately interpret tympanogram patterns and diagnose children’s middle ear infections during daily practice. This article will review basic tympanometry, an objective technique to verify middle ear conditions, measure the mobility of the tympanic membrane and improve diagnostic accuracy. Acoustic reflex thresholds will not be described.
Tympanometry, the older and more widely used technology, is available in one or more models from at least 10 U.S. manufacturers. A list can be found on the Internet at http://miscpaper.com/tympanometry.htm. During the performance of the test, the air pressure in the sealed ear canal is varied by means of an automatic pressure pump. This examines the range of mobility of the tympanic membrane under varying pressures, similar to the performance of pneumatic otoscopy. At the same time, the sound processor (transducer) continuously emits a pure tone of 226 Hz with an intensity of 85 dB. The pressure pump first introduces positive pressure to +200 dePa (deca-pascals) that is quickly rarefied down to a pressure of –400 dePa over a few seconds. One dePa is about equal to 1 mm water pressure. At a pressure of +200 dePa, the distance from the x-axis of the graph to the beginning of the tympanogram pattern is called the canal admittance, important because it approximates the volume of air (mmohs) of the ear canal. Normally, canal admittance (volume), should range between 0.2 to 1.5 mmohs. If the middle ear contains air only, the tympanogram pattern displays air pressure in the abscissa (x-axis) and admittance (previously called compliance) on the ordinate (y-axis). Static admittance is measured in mmohs (the reciprocal of milliohms). The highest point in the tympanometric pattern on the y-axis is labeled as the tympanometric peak, the point of maximum admittance. This is the point where the pressure in the middle ear and the pressure in the ear canal are equal. When there is a definable tympanometric peak, ie, no middle ear effusion, the location of the peak along the abscissa (x-axis) provides a measure of the pressure in the middle ear. Normal middle ear pressure should be somewhere between +50 to –150 dePa (mm water). The probe tip tone is directed to the tympanic membrane during the two seconds of the pressure change described above. Sound energy is maximally absorbed into the middle ear and minimally reflected, when the air pressure on both surfaces of the tympanic membrane is equalized. The proportion of sound reflected backward toward the instrument will increase as the pressure in the middle ear falls away from the peak compliance of the TM and the TM has reduced mobility. If there is no impedance to the transmission of sound energy such as middle ear effusion, a compliance peak pressure (peak amplitude in dePa) is shown on the graph paper. Thus, tympanometry measures middle ear pressure by locating on the ordinate, the peak amplitude of the pressure tracing. It also measures any impedance to passage of sound energy, namely, middle ear fluid. It does not test hearing acuity. The compact acoustic impedance instrument consists of a pressure pump, a tone-producing sound probe unit, a microphone and usually a strip printer for a hard copy of the tympanogram tracing. A cord with two flexible plastic tubes exits from the instrument and ends in a metal probe tip with two holes in the end. One tube is for outgoing probe tone and the other tube is for the pressure pump. Several sizes of cover seals fit over the sound-emitting metal probe. However, infants younger than 5 or 6 months are difficult to test because of small ear canals and highly compliant canal walls. Young infants may have a normal type “A” tympanogram even with confirmed middle ear effusion. Almost all infants with Down syndrome require experience and patience and specialized equipment to obtain a valid tracing. The auditory canal and tympanic membrane should be examined before tympanometry. Large clumps of earwax and squamous debris should be removed before testing, although sound is able to bypass smaller cerumen accumulations. There are several sizes of soft probe tips. The proper size of the probe tip is required to obtain a valid tracing. To insert the soft probe tip, the child’s aural helix should be grasped and pulled up and backward to straighten out the ear canal and allow insertion of the soft probe tip. When the probe tip is inserted properly and there is a hermetic seal, the automatic recording device will be triggered. The probe tip must be left in position unti the test is concluded. The procedure should be repeated if results are unsatisfactory or do not match findings on pneumatic otoscopy. After a clear tracing is obtained, the procedure is repeated in the contralateral ear. The soft probe tips are inserted into a child’s external auditory meatus and form a seal. The wave-like pressure variation can be a bit uncomfortable for babies who move or cry, negating the test results. During the test, the patient cannot cry, move, open or close the jaw, swallow, or startle. Realistically, a valid test can be quickly obtained for about 85% of infants in a pediatric office. In the normal state of middle ear function, the strip recorder tracing shows a pressure peak at some point between –150 mm water and +50 mm water. Tympanometry requires a hermetic pressure seal by the replaceable external ear tip and about 30 seconds of relative cooperation from the patient. A leak of air pressure anywhere in the system, particularly due to a poor seal between the soft probe tip and the external auditory meatus, will abort the test and create a flat line tracing. Thus, children with elliptical shaped auditory meatal openings or stenotic ear canals may not demonstrate reliable tympanogram tracings. When there is a relative vacuum in the middle ear there will be a peak pressure from –200 to –400 dePa pressure, depending on the degree of negative pressure in the middle ear space. When there is such negative pressure, the TM is pulled medially toward the promontory of the temporal bone. The lower the middle ear pressure, the greater the degree of TM retraction. There are three major patterns of tympanograms, which indirectly reflect the three degrees of TM mobility. Type “A” pattern is a sharp peaked pattern with the peak somewhere between +50 to – 150 dePa. “AS” ( S=shallow) tympanogram pattern has a low peak amplitude of compliance suggesting a stiffened middle ear system. AD (D=deep) tympanogram pattern occurs when the TM is very flaccid or when there is disarticulation of the ossicular chain. Type “B” tympanogram reflects impaired compliance or increased impedance of the motion of the TM over the continuum of pressure gradient. Type “B” tympanograms are sometimes subdivided into two subtypes: shallow or blunted peak and no peak at all, which shows on the graph as a flat line. The blunted type “B” tympanogram can be seen when there is a small middle ear effusion. A middle ear cavity that contains a large effusion instead of air will reflect most of the sound energy backward and have a flat line type “B” tympanogram. The strip recorder prints a flat line instead of a sharp or rounded peak. Type “C” tympanogram is similar to type “A” but the peak is to the left of normal pressure or in other words more negative than –150 dePa. Type “C” tympanograms usually have no clinical importance unless the pressure peak is less than –300 dePa when it indicates extreme eustachian tube dysfunction. If the middle ear pressure is more negative than –400 dePa, the tympanogram tracing may appear flat, even without a middle ear effusion. Type “B” tympanogram pattern is not diagnostic of middle ear effusion. The same pattern can also be caused when the probe tip hole is occluded by cerumen or by contact with the canal wall. A type “B” pattern will also occur when there is a perforation in the TM, including a tympanostomy tube. Any middle ear mass, including a cholesteatoma will impede transmission of sound energy and may also be associated with a flat tympanogram. There is a simple method of differentiating an occluded probe tip from a TM perforation from middle ear effusion but one should refer to the source material. Repeat any abnormal tracing, particularly during the operator’s learning curve.
Some tympanometer instruments provide a measurement of the ear canal volumes. The average ear canal volume for a child is 0.3 to 1 ml. Adolescent ear canal volumes can exceed 1.5 ml. This measurement has value when there is a type “B” tympanogram pattern that can occur when the probe tip is occluded, when there is middle ear effusion and when there is a perforation of the TM or a functioning tympanostomy tube. When the ear canal volume is much smaller than 0.3 ml, the type “B” pattern represents occlusion of the probe tip; when the volume is 0.3 to 1/5 ml, the “B” pattern represents middle ear effusion or a middle ear mass. When the ear canal volume exceeds 2.0 ml, there is a TM perforation. Expect to pay about $3,000 for new tympanometry equipment. Reimbursement by third party payers is reasonable, even when the procedure is used routinely to verify the diagnosis. Most tympanometer handheld units are attached to a recording device that plugs into the wall. MicroTymp II is the exception. This unit is small, cordless and easy to use. The instrument prints a hard copy for attachment to the patient’s chart. Because the Welch Allyn instrument is user friendly and small, it is a reasonable choice for a private practice. Tympanometry provides verification to pneumatic otoscopy. The procedure does not substitute for visualization of the position, contour, mobility and color of the TM. It is a valuable adjunctive test that improves diagnostic otoscopic skills and decreases unnecessary antibiotic treatment and follow-up examinations. I recommend that primary care pediatricians purchase a reliable tympanometer. The pediatrician must use it frequently enough to feel comfortable with the interpretations of common tympanometric patterns. Before referral to an otolaryngologist, all children should have a current tympanogram to verify the presence of middle ear effusion. For more information: |
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