[Characteristics of middle-latency auditory evoked potentials in awake and anesthetized patients]

Late auditory evoked potentials (50-500 msec latency) can be evoked by tonal stimuli but their instability prevents their use in many clinical situations. In contrast, the early responses (up to 10 msec) are stable, regardless of sleep or sedation, but they can only be evoked by transient, non-tonal stimuli. Therefore neither type may be used for an evaluation of frequency thresholds. The middle latency responses represent a compromise: first, they can be evoked by tonal stimuli of the tone-pip type and, second, they are relatively stable. For these reasons the authors have carried out a general study on the middle latency responses in subjects with normal hearing, in patients with impaired hearing and in patients under a variety of surgical anesthesia. The middle latency response evoked by clicks and tone-pips (4 sines of 500, 1000, 2000 and 4000 Hz) have a series of characteristic waves named, according to their latency, A(V), B(No), C(Po), D(Na), E(Pa). The threshold of visual detection for these potentials is situated near that of the psycho-acoustic threshold (0-20 dB). The authors point out the clinical utilization of the middle latency response especially in cases in which one desires an objective evaluation of the auditory threshold in the lower frequency range. It is important to know that the middle latency response remains present using some types of anesthesia (Ketalar, ataralgesy), disappears partially with neuroleptanesthesia and completely with inhalation anesthesia (fluothane, halothane).