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LESSON 107

High-Frequency (1000 Hz) Tympanometry in Infants

In infants younger than six months, a 226 Hz tympanogram can look normal even when there is fluid in the middle ear. In this lesson we use examples to look at the features of the neonatal ear canal, measurement with a 1000 Hz probe tone, and interpretation based on a positive peak.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we will learn why a two hundred twenty six hertz tympanogram can be misleading in infants under six months.
We will talk about measurement with a one thousand hertz probe tone, how to interpret it, and the features of the neonatal ear canal.
In earlier lessons, we saw that routine tympanometry in adults uses a two hundred twenty six hertz probe tone.
In infants under six months, however, results obtained with this probe tone are not reliable.
The main problem is that a peaked, normal looking tympanogram can be recorded even when there is fluid in the middle ear.
This lets the effusion go unnoticed, which means a false negative result.
Paradise and colleagues, in nineteen seventy six, were among the first to show this problem in young infants.
In that study, the ability of the tympanogram to detect effusion was poor, especially in infants younger than seven months.
The problem comes from the neonatal ear canal and middle ear, which are built differently from those of adults.
In the newborn, the ear canal is short and narrow, and its walls are made mostly of soft tissue.
The bony part of the ear canal develops after birth, and this development continues through the first years.
The soft walls can expand and contract with the pressure changes of tympanometry.
At two hundred twenty six hertz, this movement adds to the measured admittance and can create a peak that seems to come from the middle ear.
Also, the tympanic membrane lies more horizontally than in adults, and the middle ear cavity is smaller.
In the first days, fluid or tissue remnants from birth may also be present in the middle ear.
In the infant ear, the mechanical properties of the ear canal wall and the middle ear differ from those of adults.
As we saw in earlier lessons, at two hundred twenty six hertz the adult middle ear is stiffness dominated.
In the newborn, mass and friction play a larger role in the system.
A higher probe tone is less affected by the movement of the ear canal wall and more sensitive to changes in the middle ear.
This is why, in infants under six months, a one thousand hertz probe tone detects effusion more accurately.
The British Society of Audiology, or BSA, also recommends a one thousand hertz probe tone for this age group.
Corrected age is used for the age limit; for a preterm infant, the time born early is subtracted from the calendar age.
High-frequency tympanometry is preferred for infants whose corrected age is under six months.
After six months, two hundred twenty six hertz gradually becomes reliable and is used for routine measurement.
Some studies have found it useful to use both probe tones together in a transition period of about three to nine months.
The age limit your clinic uses should be clearly written in the protocol.
Before measurement, otoscopy is used to examine the ear canal and, if visible, the tympanic membrane.
Testing while the infant is asleep or calmly feeding is the easiest way.
Crying and strong sucking movements cause irregular deflections in the curve.
A small probe tip that fits the infant ear canal is chosen and inserted so that no air leaks.
A one thousand hertz probe tone is selected; pressure is usually swept from plus two hundred to minus four hundred decapascals.
The daily device check for one thousand hertz is done with a test cavity of a suitable size.
The report must state the probe tone, the infant's corrected age and the infant's state during testing.
A one thousand hertz tympanogram does not always look like the two hundred twenty six hertz curves, so Jerger types do not apply directly.
The key question in interpretation is simple: does the curve have a clear positive peak?
A clear peak rising above the baseline drawn between the two ends of the curve is consistent with normal middle ear function.
A flat curve, or a trough that dips below the baseline, suggests a middle ear problem such as effusion.
In Baldwin's two thousand six study, this approach detected effusion with high sensitivity and good specificity.
Complex or notched curves can also be seen; in that case, findings from other tests help the interpretation.
Numerical norms differ between studies and devices, so know the source of the norms you use.
Let us put the two probe tones side by side: at two hundred twenty six hertz, the middle ear is stiffness dominated.
This is why the Jerger classification and the norms we know are used for infants older than six months.
Only at this frequency is one milliliter of air volume equal to one millimho.
A one thousand hertz measurement, on the other hand, is less affected by the movement of the soft ear canal wall.
Interpretation rests on the presence of a positive peak rather than on a letter type.
At one thousand hertz, results are read in millimhos and are not compared with adult norms.
Let us look at an example: a three month old infant comes for diagnostic assessment after newborn screening.
In the same ear, the two hundred twenty six hertz measurement shows a peaked, normal looking curve.
The one thousand hertz measurement, however, shows a flat curve with no clear positive peak.
At this age, the one thousand hertz result is the one to rely on; the finding is consistent with possible middle ear fluid.
This finding also directly affects how the otoacoustic emission, or OAE, result is interpreted.
The example is for teaching; assessment and follow-up decisions are made with the physician and the full test battery.
One thousand hertz tympanometry is a small but important part of the infant test battery.
When OAEs are absent, it helps to tell whether the cause lies in the middle ear or in the cochlea.
In auditory brainstem response, or ABR, recordings, a conductive problem can delay the waves; the tympanogram supports this reading.
In infants, the acoustic reflex is also more often recorded with a one thousand hertz probe tone.
In this way, tympanometry, reflexes, OAEs and ABR together form a more consistent picture.
If the middle ear finding persists on repeated measurements, the infant is referred for an ear, nose and throat assessment.
One thousand hertz tympanometry is not perfect either; movement and sucking artifacts can distort the result.
If there is vernix or fluid debris in the ear canal, the probe can be blocked and a flat curve can be misinterpreted.
Instead of a single measurement, a repeat measurement after some time, when needed, gives more reliable information.
Newer methods such as wideband tympanometry add information in the infant ear; we will cover them in a later lesson.
In every case, the result is interpreted together with otoscopy, OAE and ABR findings.
A tip from practice: if the device starts at two hundred twenty six hertz by default, check the probe tone before testing an infant.
In an infant under six months, a peaked curve at two hundred twenty six hertz does not show that the middle ear is normal.
Base your decision on the one thousand hertz measurement, and write in the report which probe tone you used.
This small detail prevents an infant with effusion from being recorded as normal by mistake.
In infants under six months, a two hundred twenty six hertz tympanogram can look normal even when fluid is present.
The main reasons are the soft ear canal wall of the newborn and the different mechanics of the middle ear.
In this age group, a one thousand hertz probe tone is used and the result is read in millimhos.
Interpretation rests on a clear positive peak; a flat curve or a trough suggests a problem.
Test yourself: A three month old infant has a peaked curve at two hundred twenty six hertz and a flat one at one thousand. Which counts?
The correct answer is B: the one thousand hertz measurement.
At this age, a peak at two hundred twenty six hertz can come from the soft ear canal wall and hide fluid.
In the next lesson, we will learn about the acoustic reflex, the reflex arc and normal reflex thresholds.
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Prerequisites

LESSON 106 Tympanometric Values: Peak, Pressure, Width, Volume

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