Does general anaesthesia affect the child's auditory middle latency response (MLR)?

Low-frequency component of the gerbil brainstem response: response characteristics and anesthesia effects

The auditory brainstem response (ABR) was recorded with epidural electrodes in awake and anesthetized gerbils. Low- and high-frequency components of the ABR were separated by analog filters and compared as functions of stimulus intensity, frequency, repetition rate, and effects of anesthesia. In response to 0.5 kHz tone bursts, thresholds of the low-frequency component (LF-ABR) were significantly lower than that of the most prominent peak of the high-frequency components (wave P4). At both 2 and 4 kHz, thresholds of the LF-ABR and wave P4 were not significantly different. Changes in stimulus intensity over a 70 dB range produced similar changes in peak amplitudes and latencies for the LF-ABR and P4. However, while the amplitude of the LF-ABR was inversely related to stimulus frequency, the amplitude of P4 was reduced at 0.5 kHz, as compared to 2 and 4 kHz. Increases in stimulus rate from 7 to 100 bursts/s produced little change in the amplitude of the LF-ABR. At rates of 80 and 100 bursts/s, the LF-ABR was sinusoidal in appearance due to the proximity of successively generated potentials. In contrast, the amplitude of P4 varied inversely with stimulus rate between 20 and 100 bursts/s. Administration of ketamine and xylazine produced minor changes in the amplitudes and latencies of both the LF-ABR and wave P4. The response characteristics of the gerbil LF-ABR are similar to those of the low-frequency component of the ABR in humans and cats. The LF-ABR provides an estimate of hearing threshold at low frequencies (0.5 kHz) as well as higher frequencies (2-4 kHz). A major advantage to the LF-ABR is that it can be recorded at high stimulation rates in awake and anesthetized animals, thus providing an efficient measure of auditory function.

Low frequency hearing threshold determination in newborns, infants and mentally retarded children by middle latency responses

Middle latency (10-50 ms) responses (MLR) evoked by tone-pips (1,000 Hz 500 Hz) and early (0-10 ms) auditory evoked potentials (EAEP) evoked by chicks were recorded on 68 newborn babies (premature or at term), infants and children, some with central nervous system or psychiatric disorders, who had normal or impaired hearing. MLR were obtained either during sleep, chloral-hydrate sedation or ketamine anesthesia. Thresholds estimated from MLR and EAEP were compared to those from subsequent psychoacoustic pure tone testing. We confirm that MLR provide good threshold estimates for hearing in the low frequency range, whereas click evoked EAEP are good threshold indicators only for high frequencies.

The effects of anterior temporal lobectomy (ATL) on the middle-latency auditory evoked potential (MLAEP)

The anatomical and physiological origins of the middle-latency auditory evoked potential (MLAEP) are not well understood. The present investigation was conducted to determine whether the MLAEP derives its origins in part from the anterior temporal lobe. Twelve subjects with intractable seizures were evaluated with the MLAEP pre and post excision of the anterior-mesial temporal lobe (ATL) unilaterally. In our study, component Pa latency was unaffected by the ATL. The Na latency and the Na/Pa amplitude showed significant increases after ATL. The results we interpreted as being consistent with currently held beliefs regarding the origins of Pa. The changes in Na latency and Na/Pa amplitude are hypothesized to reflect a loss of the modulating influence of the cortex on the subcortical generators of Na.

Brain stem (ABR) and middle latency (MLR) auditory evoked responses in infants

We examined ABRs and MLRs for click and/or 500-Hz tone pip stimuli in infants (36-44 weeks conceptional age) admitted to the NICU and measured the threshold and detectability of those responses. We took the threshold indicators of those responses as Jewett V for ABR measurement and wave PO Nal Na2 for MLR measurement. Thresholds of MLRs for click stimuli were almost equal to those of ABRs for click stimuli. On the other hand, thresholds of MLRs for 500-Hz tone pip stimuli were slightly worse than ABRs for click stimuli, the average differences being less than 10 dB. Wave Po Nal Na2 are the best threshold indicators of MLRs in infants, but the diagnostic significance of wave Pa was questionable in our measurements.

Frequency selectivity of the middle latency response

A forward masking paradigm was used to assess the frequency selectivity of the middle latency response (MLR). Tuning curves of the MLR were obtained in unanesthetized gerbils. Changes in the amplitudes of MLR waves A, B, and C with latency values of 10 to 13 ms, 14 to 17 ms, and 20 to 25 ms, respectively, were analyzed as a function of masker frequency and intensity. Tuning curves of the MLR were also compared to tuning curves of the auditory brainstem response (ABR), which was recorded simultaneously with the MLR. The MLR and ABR differed in their response to forward masking. The MLR was reduced in amplitude or eliminated by masker stimuli that had minimal or no effect on the ABR. Forward masking often caused variable and non-monotonic changes in the amplitude of the MLR. Tuning curves of the MLR indicate that the MLR is less frequency selective than the ABR. The MLR is an electrophysiological measure of auditory function central to the auditory brainstem. Therefore, it may provide information concerning central components of normal and pathological auditory function. However, because of the variability of MLR amplitudes with forward masking, tuning curves of the MLR are difficult to obtain and are not efficient for routine measurements of frequency selectivity.

Auditory steady-state responses in the rabbit

The authors have studied auditory brainstem (ABRs), middle latency (MLRs) and steady-state potentials (SSRs) in 15 adult male rabbits weighing between 2.5 and 3 kg in order to verify if SSRs are due to a mere superimposition of ABRs and MLRs or to a resonance phenomenon. Ten of them were awake while 5 were studied under urethane anesthesia. Acoustic stimuli consisted in 0.1-ms square-wave pulses delivered at presentation rates ranging between 1 and 80/s at a stimulus intensity of 80 dB p.e. SPL. Our data show that reliable auditory SSRs can be obtained in the rabbit at a presentation rate of 30 stimuli/s, probably due to the superimposition of ABRs and MLR Pb waves which show an interwave interval of about 35 ms. The nonlinear aspects which can be detected are probably due to the effect of decreasing interstimulus intervals on the duration and amplitude of the Pb wave. It can then be concluded that SSRs in the rabbit are due more to a superimposition of ABR and MLR waves than to a resonance phenomenon.

Middle latency response: frequency and intensity effects

Auditory middle latency responses (MLR) and auditory brainstem responses (ABR) were measured with epidural electrodes in unanesthetized gerbils. Response thresholds of simultaneously recorded MLRs and ABRs, and latencies and amplitudes of MLR peaks were analyzed with respect to stimulus intensity (10-80 dB SPL) and frequency (0.5, 1, 2, 4, 8 and 16 kHz). Only minor changes in the latencies of the MLR were associated with increases in stimulus intensity. Changes in latencies were more apparent for waves A and B as compared to wave C, and were significant only at low intensities. Latencies did not change significantly as a function of stimulus frequency. Amplitudes of the MLR were highly variable between animals, particularly waves B and C, and showed complex changes with intensity. In general, wave amplitudes were inversely related to stimulus frequency. The gerbil MLR resembles MLRs recorded under similar conditions in guinea pig, cat, and rat. Some qualitative similarities between gerbil and human MLRs are apparent. Results indicate that the MLR is a less sensitive measure of hearing threshold relative to the fast waves of the ABR at frequencies above 1 kHz. However, clearly defined MLRs are elicited with a wide range of stimulus frequencies. Because the surface recorded MLR reflects activation of central auditory pathways, including the cortex, it may provide an electrophysiological measure which can be utilized to study central components of normal and pathological auditory function.

Low-frequency auditory brainstem response threshold

Auditory brainstem thresholds have been determined in 35 non-cooperative, anaesthetized children using a 'two-point audiogram' paradigm. The high-frequency point was found with a 2 kHz tone-burst without masking, and the low-frequency with a 0.5 kHz tone-burst together with 1 kHz high-pass noise masking. Great variability was found in the low-frequency thresholds, and only 3 of 18 ears with normal high-frequency thresholds had low-frequency thresholds below 70 dB nHL. It is concluded that the 0.5 kHz tone-burst with 1 kHz high-pass noise masking is not a reliable method for routine assessment of low-frequency auditory threshold at the brainstem level.

High-pass filter settings affect the detectability of MLRs in humans

Auditory middle latency responses (MLRs) have been recorded in 217 patients ranging in age from 6 days to 20 years. The probability of obtaining MLR components Na and Pa was higher with a high-pass filter setting of 15 Hz, 12 dB/octave as compared to 3 Hz, 6 dB/octave. This effect was found at all ages tested. Age-related latency effects were apparent with 3 Hz but not 15 Hz filtering.

Age-related morphological changes in auditory middle-latency response

Age-related changes in the waveforms of the middle latency response (MLR) were investigated in 9 adults and 28 children aged between 4 and 14 years. The children were classified into three groups according to their age. For obtaining characteristic configurations in the responses for each group, composite group averaging was performed by summating the individual recordings in each group. With high-pass digital filtering at 20 Hz, composite MLR for adults showed a well-defined Na-Pa-Nb-Pb complex with peak latencies at about 17, 30, 45 and 63 ms, respectively. The composite response for children aged 4-7 years was characterized by a broad positive deflection (Pa) followed by a negative peak (Nb) at about 40 and 60 ms after stimulus onset, respectively. The peak latency of Pa was close to the adult value in the composite MLR for subjects aged 8-11 years, while the complete adult pattern in the later part of the response was not reached even in the composite response for subjects aged 12-14 years.