Ear-to-ear derivation in short latency brainstem auditory evoked responses.

We utilized an ipsilateral to contralateral earlobe (Ai-Ac) derivation in addition to the scalp to ipsilateral ear (Cz-Ai) and scalp to contralateral ear (Cz-Ac) derivations, in 12 normal hearing community volunteers and 36 patients with a variety of referrals and varying degrees of hearing loss. In normal subjects, the latency of wave I in the Ai-Ac derivation was identical to that in Cz-Ai, but amplitude was consistently smaller (0.21 +/- 0.1 vs. 0.30 +/- 0.13 microV, p less than 0.005). The wave III behaved in a reverse manner. These data can be easily explained based on traditional principles of near- and far-field potentials. The amplitude differences of the wave I in Cz-Ai and Ai-Ac derivations were, however, small and the phase-reversals of that wave between the 2 derivations were striking and consistent in all subjects and patients. This observation indicates that the addition of Ai-Ac derivation to the conventional 1 or 2 channel montage may aid in the identification of wave I.