Longitudinal changes in real-ear to coupler difference measurements in infants

The real-ear-to-coupler difference (RECD) measurement is a commonly used clinical procedure that quantifies the difference in sound pressure level between a 2 cc coupler and an individual's ear canal. The SPL levels in infant ears are highly variable and significantly higher than the SPL levels present in average adult ears, making the quantification of SPL levels in infant ears extremely important for threshold determination and fitting of amplification. It is unknown how much the RECD changes in an individual infant over time, whether that change is within test-retest reliability of the RECD measure, and whether RECD values are predictable from other outer and middle ear measures.

The purposes of this study were to examine longitudinal changes in RECD values in newborn infants to determine whether a significant change in RECD values takes place over a one-month period, how the change in RECD relates to test-retest variability of the measure, and whether RECD values are predictable from the infant's corrected age, or measures of static admittance and equivalent ear canal volume (EECV).

Fourteen infants (seven females, seven males) aged 7 to 25 days were recruited through community prenatal classes, physicians, hospital nurseries, and word of mouth. All infants had normal middle ear status.

Infants were tested on two separate visits, first when the infant was approximately two to three weeks old and then approximately one month later. Each visit lasted one to two hours, during which time otoacoustic emissions, 226 Hz and 1000 Hz probe-tone tympanometry, and two RECD measures were made for each ear.

A multivariate analysis of variance revealed a significant change in RECD values over a one-month interval. Regression analyses revealed that final RECD values were partially predictable from age, initial RECD value, static admittance; and EECV. RECD test-retest variability was not large but larger than longitudinal RECD changes over a one-month period.

It may be unnecessary to remeasure an RECD to account for changes in ear canal acoustics for repeated assessments, no more than one month apart, when the same test transducer is used. The RECD should, however, be measured at one of these assessments because RECD values are not predictable from an infant's age or measures of static admittance and ear canal volume.