Feasibility of acoustic distortion product testing in newborns

Acoustic distortion products (ADP) have been used as a neonatal screen for newborn, full-term infants resident on maternity wards. ADPs were measured across frequency with f2 (the higher frequency tone) between 1 and 8 kHz and the f1 (the low frequency tone) determined by f2/1.225. Testing was attempted on 77 infants. Of these, 13% were untestable, 69% were tested in both ears and 18% had one ear tested; 48% gave data from both ears and 25% gave data from one ear. If results from both ears were required to pass the test, 52% would fail. Of the 67 babies that could be tested, 27 out of 120 ears gave no result: a failure rate of 22.5%. It was often not possible to record ADP across all the frequencies tested, but only four babies out of the 31 who were tested at each frequency bilaterally showed no ADP at any frequency. Older babies (4-6 days old) gave distortion at more frequencies than younger babies (0-3 days old). ADP were most easily recorded with f2 at 3 and 4 kHz. Lower incidence of ADP at frequencies below 3 kHz could be explained by greater levels of background noise. Left or right ears and method of delivery did not affect the ability to record ADP, but, unexpectedly, boys showed a higher incidence of ADP (though not higher level) than girls. Behavioural testing of the infants was carried out at 7 months of age without knowledge of the previous ADP test results. It yielded three referrals (out of 52 who could be followed up), one of whom was re-tested and passed. The two remaining infants were among the four who gave no distortion bilaterally, indicating accurate prediction by the ADP measure. It was concluded that ADP measurement alone at birth would lead to an unnecessarily large number of referrals for further investigation. This high failure rate may be reduced by measurement methods designed to avoid or cancel infant-generated noise, and by sound-treated facilities, but the lack of co-operation by the infant coupled with limited test time poses a more serious problem.