Changes in the DP-gram during the preterm and early postnatal period.

OBJECTIVES: To describe and define changes in the infant DP-gram during an age continuum from the preterm period through the first 6 mo of postnatal life. This information provides normative guidelines for audiologists or hearing screeners using DPOAEs to monitor infant hearing status.
DESIGN: In this retrospective study, 2f1 - f2 DP-grams (DPOAE level x f2) were recorded with primary tones at 65/55 dB SPL, f2/f1 = 1.2, and f2 frequencies ranging from 1500 to 9000 Hz. Results from one ear of 290 healthy infants ranging in age from 31 wks postconceptional age to 6-mo-old were examined. Data were collected using both longitudinal design (repeated tests on the same infant over time) and cross-sectional methodology (a different group of subjects representing each age category). Subjects were divided into three groups according to age and experimental design. The effects of age and frequency on DPOAE level were analyzed in the three groups separately.
RESULTS: The combined results from the three databases indicate that (1) DPOAE level increased for mid-frequencies throughout the preterm period, from 31 to 33 wks until the time period associated with term birth. This change was significant for 4500 and 6000 Hz; (2) DPOAE level decreased as f2 frequency increased. In many infants, a shallow trough was observed with peak amplitude at 1500 Hz, a reduction in response amplitude through 4500 Hz, and a second peak around 6000 Hz; (3) during the postnatal period from birth through 6 mo, DPOAE level did not change significantly as a function of age and the DP-gram was relatively flat across f2 frequency; and (4) infants showed mean DPOAE levels that were 4 to 12 dB higher than adult levels.
CONCLUSIONS: The results indicate a frequency-dependent increase in DPOAE level during the preterm period in human infants. After birth, there is little change in amplitude through 6 mo. The infant DPOAE remains larger than adult amplitude at all ages tested, as shown in other reports, well into childhood, suggesting continued changes in DPOAE level during the first decade of life. Recent research suggests that immaturities of the conductive pathway may account for infant-adult differences in DPOAE level; however, it is not yet clear whether other sources contribute.