[Otoacoustic distortion products in infants and adults: a comparative study]

Distortion Products (DP) are otoacoustic emissions evoked by emitting two pure tones. The principle advantage of this method vs. transient evoked otoacoustic emissions is the specificness of frequency. The purpose of the present work was to determine whether this technique could be applied in objective study of cochlear function in infants. This was done by setting standard data for infants and comparing them with the data obtained in a control population of adults. The input/output functions of the DP and the DP audiograms for 8 f2 frequencies ranging from 696 to 6006 Hz were studied in a group of 15 healthy, full-term infants. The data obtained were statistically compared to the data taken from a control group of 8 normal hearing adults. The infants showed significantly more intense DPs in the intermediate frequencies, with a greater detection threshold and dynamic range. On the other hand, at the high frequencies the amplitude of infant DPs was the same or lower than (f2 = 5005) those found in the adults. These differences can partially be attributed to the fact that the outer auditory channel is smaller in infants thus the probe and the middle ear are better matched. There is significantly greater background noise in infant DPs, particularly at low frequencies. This may be due to the fact that the skin of the outer auditory duct--which acts as a low-pass filter allowing suction and movement of the cervical muscles to contaminate the tracing--is thinner in infants. In conclusion, the study of DPs has shown its potential in the study of cochlear function in infants because it is non invasive and objective, it can be performed quickly and shows frequency specificness. Unfortunately, it cannot be considered a valid method for determining cochlear function at the low frequencies.