聴性脳幹反応（ＡＢＲ）におけるマスキングの意義

Abstract

The influences of contralateral masking on the click evoked auditory brainstem response (ABR) were investigated in 16 patients with unilateral total deafness and in 44 normal adult subjects. Simultaneous recordings of auditory nerve action potentials (APs) and ABRs to clicks at intensities of 90 and 70 dBnHL were recorded by a conventional signal averaging technique with or without contralateral masking (white noise was used as a masker at intensities of 40-80 dB), and a peak amplitudes and latencies of component waves were measured. In subjects with a unilateral totally nonfunctioning ear, shadow responses (synonymous with crossover responses) were recorded in both tracings (APs & ABRs) when the click intensity exceeded 50 dBnHL. The majority of these shadow responses were eliminated by masking noise at a level equal to the click presentation level minus 30 dB. Although it was enough to eliminate these shadow responses by applying masking noise at a level equal to the click presentation level minus 20 dB, the phenomenon of overmasking was observed in the recording of auditory nerve action potentials ipsi-lateral to the stimulation. In normal subjects, the most significant influences of contralateral masking were observed with amplitude changes of wave VI. There were statistically significant differences between the responses with or without contralateral masking (p<0.05, by paired t-test), and the decrease of the amplitude was relatively propotional to the amount of contralateral masking noise. As long as the contralateral masking did not exceed the level that strongly affected the APs and wave I of ABRs, the influence of contralateral masking had little effect on both wave III and V in contrast to that of waves II, IV and VI which were strongly affected by the contralateral masking. In clinical practice, appropriate contralateral masking can eliminate shadow responses in patients with unilaterally profound deafness when tke click intensity exceeds 60 dBnHL and in patients with responses that can not show a clear distinction between wave V and the other waves, particularly wave VI.