Auditory‐evoked responses in normal, brain‐damaged, and deaf infants

Pediatric neurologists are well aware of the difficulty in deciding whether peripheral hearing loss, mental retardation, psychosis, brain damage, or a combination of deafness and central nervous system damage is present in an infant without consistent blink, startle, or other behavioral response to sound. Investigators have measured changes in heart rate,l skin resistance,z motor activity, and width of the palpebral fissure3 in normal babies stimulated by calibrated sounds, but the validity of these tests in abnormal babies is not well established. Operant conditioning techniques4 have met with some success in assessing auditory sensitivity in young children, but poorly controllable variables such as attention and motivation confuse the results. Pure tones are widely held to be unsuitable stimuli to test infants or even young children, whatever the technique used. A more direct approach is the recording of EEG changes in response to sound, which computer averaging has made practical. Previous work5-7 has established that this method enables one to obtain a useful estimate of the sensitivity of the cochlea and auditory nerve in children with known hearing losses. This method does not indicate whether the child can “hear,” that is, whether he is consciously aware of the stimulus and can utilize it as a useful signal to modify his behavior. However, it does enable the examiner to decide with certainty that a given stimulus has reached the brain and, consequently, that the ear and peripheral auditory pathway have been stimulated effectively. The method can be used during sleep, since it tests a physiological response which is not dependent upon conscious awareness. It has been known since the early days of EEG that loud sounds often triggered a large vertex response, the K complex, in the EEG of sleeping subjects.s,s The K complex was found the most prominent part of the averaged-evoked response to clicks during sleep.1° In newborn infants, evoked responses were recorded with clicks and tones 35 to 50 decibels or more above normal adult thre~hold.~l-~3 In the newborn infant, the two most constant waves were a vertex positive wave (P2) followed by a vertex negative wave ( N2). With loud clicks mean latency of P, was 258 msec. during the quiet phase of sleep in which the EEG consisted of high-voltage slow waves and 210 rnsec. during active sleep with low-voltage EEG and rapid eye movements. Wave N, peaked at 506 msec. in quiet sleep and 412 msec. in active ~ 1 e e p . l ~ Evoked responses to clicks were found in even the smallest premature infants (600 gm.), although their latency, shape, and distribution differed from those of more mature infants.15 The present study was undertaken to assess the usefulness of auditory-evoked responses to test peripheral auditory sensitivity to pure tones and clicks in normal and in brain-damaged infants as well as in infants suspected of deafness. Visual-evoked responses were also recorded in most infants with visual handicaps, with severe brain damage, and in those with

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