[Morphological study of brainstem auditory evoked potentials. The effect of the position of the reference electrode]

In view of the discrepancy between the siting of electrodes and the effects on BAEP, we made a morphological study of 30 healthy volunteers, 15 men and 15 women.

In each person we obtained the BAEP with the active electrode on the ear lobe ipsilateral to the stimulated ear and the reference ear successively at four different sites: vertex (Cz), high frontal (Fz), earlobe contralateral to the stimulated ear and the spinous process of the seventh cervical vertebra (CV7). The I, III and V waves were present in all cases and tests, followed in frequency by II, IV and VII. In Cz and Fz the first five deflections were almost always seen, and these are the most useful for neurodiagnosis. The waves of greatest amplitude were 1 in both sexes in the ear lobe and CV7 for Fz and Cz in men and V for women. Regarding the complex morphological variants, the greatest incidence in the III IV V complex was observed in Fz in both sexes. In CV7 there was a greater range of variants, as found by other workers, but with greater morphological separation of waves IV and V. They were infrequent in waves I II and VI VII and least common in Fz and Cz.

We consider that the BAEP obtained with reference to Cz and Fz are ideal for interpretation, since there is greater constancy, global amplitude and morphological clarity of the first five deflections and less morphological complexity of the other waves.