Brainstem auditory-evoked responses to different rates of clicks in small-for-gestational age preterm infants at term.

AIM: To clarify further the influence of intrauterine growth retardation (IUGR) on early neural development. METHODS: In 30 small-for-gestational age (SGA) preterm infants at term, brainstem auditory-evoked responses (BAERs) were recorded with clicks of different repetition rates. All infants had a birthweight < 3rd centile, without any other major perinatal complications. RESULTS: Compared with the BAER in 36 appropriate-for-gestational age (AGA) term infants, the preterm SGA infants did not show any abnormalities at 21 s(-1) clicks, except for a slight increase in wave III amplitude. At 51 s(-1) clicks, there was an increase in III-V/I-III interval ratio (ANOVA p < 0.05). At 91 s(-1), the I-III interval shortened, whereas the III-V interval and III-V/I-III interval ratio increased (all p < 0.05). Wave V amplitude tended to increase slightly at all repetition rates of clicks used, although this increase did not reach statistical significance. The III-V interval and III-V/I-III interval ratio in the preterm SGA infants at different click rates correlated inversely with occipitofrontal head circumference at the time of testing, i.e. the smaller the head the longer the III-V interval (all p < 0.01). Wave III amplitude at 21 s(-1) also correlated inversely with head circumference (p < 0.01), suggesting that the slight increase in this amplitude in the preterm SGA infants is related to their relatively small head size. CONCLUSION: There were no major abnormalities in the BAER up to 91 s(-1) clicks at term in preterm SGA infants. The slight increase in III-V interval at high-rate stimulation suggests a subtle degree of central neural dysfunction, which is associated with small head size following IUGR.

One-third of term babies after perinatal hypoxia-ischaemia have transient hearing impairment: dynamic change in hearing threshold during the neonatal period.

AIM: To examine the process of change in hearing threshold during the neonatal period after perinatal hypoxia-ischaemia. METHODS: The threshold of brainstem auditory evoked responses (BAER) was measured serially during the first month after birth in 92 term babies who suffered hypoxia-ischaemia. RESULTS: The mean BAER threshold in these babies was significantly increased on day 1 (ANOVA p < 0.001). The elevated threshold decreased progressively on days 3 and 5, but was still significantly higher than that in normal controls (p < 0.01). The elevation continuously decreased more slowly on days 10 and 15, and to a near normal level on day 30. Threshold elevation was seen in 31.7% of the babies on day 1, and 34.5% during the first three days. The rate of elevation decreased progressively thereafter. On day 30, 10.6% of the subjects still had increased thresholds. Moderate to severe elevation occurred mainly during the first week and severe elevation occurred predominately on day 1. Threshold elevation starting after days 3-5 is likely to be due to middle-ear disorders. As a whole, during the first month, 44.6% (41/92) of the babies studied had threshold elevation. BAER threshold was correlated weakly with the stage of hypoxic-ischaemic encephalopathy on days 1 and 3. The threshold was significantly higher in babies with severe encephalopathy than in those with mild or moderate encephalopathy during the first 3 d of life. CONCLUSION: Hearing threshold is elevated in about one-third of term babies after hypoxia-ischaemia. The elevated threshold decreases progressively after birth, and returns to normal by one month in most babies. The threshold correlates weakly with the severity of encephalopathy.

Hearing impairment in preterm very low birthweight babies detected at term by brainstem auditory evoked responses.

UNLABELLED: Seventy preterm babies who were born with a birthweight <1500 g were studied with brainstem auditory evoked responses (BAER) at 37-42 wk of postconceptional age. The data were compared with those of normal term neonates to determine the prevalence of hearing impairment in preterm very low birthweight (VLBW) babies when they reached term. The BAER was recorded with click stimuli at 21 s(-1). Wave I and V latencies increased significantly (ANOVA p < 0.01 and 0.001). I-V and III-V intervals also increased significantly (p < 0.05 and 0.001). Wave V amplitude and V/I amplitude ratio did not differ significantly from those in the normal term controls. Ten of the 70 VLBW babies had a significant elevation in BAER threshold (>30 dB normal hearing level). Eleven had an increase in I-V interval (>2.5 SD above the mean in the normal controls) and one had a decrease in V/I amplitude ratio (<0.45). These results suggest that 14% (10/70) of the VLBW babies had a peripheral hearing impairment and 17% (12/70) a central impairment. Three babies had both an increase in I-V interval and an elevation in BAER threshold, suggesting that 4% (3/70) had both peripheral and central impairments. Thus, the total prevalence of hearing impairment was 27% (19/70). CONCLUSION: About one in four preterm VLBW babies has peripheral and/or central hearing impairment at term. VLBW and its associated unfavourable perinatal factors predispose the babies to hearing impairment.

Comparison of brainstem auditory evoked responses recorded at different presentation rates of clicks in term neonates after asphyxia.

UNLABELLED: This study examined whether high presentation rates of clicks while recording brainstem auditory evoked responses (BAER) can improve the detection of central auditory impairment in asphyxiated neonates using the BAER. The BAER was analysed at different presentation rates of clicks within the first week after birth in 38 term neonates who suffered perinatal asphyxia. At the routinely used 21 s(-1) clicks all BAER wave latencies increased significantly (ANOVA, p < 0.05-0.01). After excluding five neonates who had a significantly elevated BAER threshold, only wave V latency increased slightly (p < 0.05). The interpeak intervals of I-V and III-V also increased slightly (both p < 0.05). Similar results were found at 51 s(-1) clicks. As the clicks were increased to 91 s(-1), the III-V interval increased more significantly (p < 0.01) and the III-V/I-III interval ratio also increased significantly (p < 0.01). In particular, wave V amplitude reduced more significantly than that in normal term controls (p < 0.01). Compared with values in the controls, wave V amplitude reduced by 4.5%, 12.2% and 24.7% at 21, 51 and 91 s(-1) clicks, respectively. CONCLUSION: Although a moderate increase in the rate (e.g. 51 s(-1)) while recording the BAER did not improve the detection of hypoxic-ischaemic auditory impairment, a significant increase (e.g. 91 s(-1)) did, which mainly indicates an abnormal reduction in wave V amplitude.

Maximum length sequence brainstem auditory evoked responses in term neonates who have perinatal hypoxia-ischemia.

Maximum length sequence brainstem auditory evoked response (BAER) was studied within the first week after birth in 28 term neonates who had perinatal hypoxia-ischemia, or asphyxia. In the BAER recorded using conventional averaging techniques (click rate 21/s), the only abnormality was a slight increase in III-V interval, in addition to an increase in wave latencies when including those who had an elevated threshold (t test, all p<0.05). In the maximum length sequence BAER, however, both the III-V and I-V intervals in the asphyxiated infants were significantly increased at all the 91/s, 227/s, 455/s, and particularly 910/s click rates (p<0.05-0.001). The I-III interval was also increased significantly at 455/s and 910/s click rates (both p< 0.05). Wave V amplitude was significantly reduced at all the click rates used (ANOVA, p<0.05-0.001), particularly at 910/s, which sometimes was the only abnormality indicative of brain damage. Both the amplitude ratios V/I and V/III were significantly decreased at 455/s and 910/s click rates (p<0.01 or 0.001). A general trend was that BAER abnormalities after hypoxia-ischemia became more prominent as click rate was increased. Significant abnormalities occurred mainly at very high click rates (455/s and 910/s), which can be achieved using the maximum length sequence technique but not by using conventional averaging techniques. Thus, this technique, which can be used at the cribside, appears to be a better method for the early detection of brain damage after hypoxia-ischemia than using conventional averaging techniques, enhancing the diagnostic value of the BAER.

Brainstem auditory evoked response recorded using maximum length sequences in term neonates.

Brainstem auditory evoked response (BAER) was studied in term neonates using maximum length sequences (MLS), which allows presentation of acoustic stimuli at much higher rate than possible with conventional averaging technique. The response waveforms, especially for waves III and V, were well defined at the click rates of 91-455/s. Wave latencies and interpeak intervals at various click rates were all significantly longer and wave V amplitude was smaller in the neonates than in the adults. As click rate was increased, wave latencies increased and amplitudes progressively reduced. The slopes of wave latency-rate functions were progressively steeper for the later waves. The slopes of both III-V and I-V interval-rate functions were significantly steeper in the neonates than in the adults, indicating greater rate-dependent changes in the central components of the neonatal MLS BAER. These rate-dependent changes in the neonatal MLS BAER are generally similar to those in neonatal conventional BAER, although there are some differences. The results indicate that the neonatal brainstem can process rapid acoustic stimulation up to 455/s, although this ability remains immature.

Immaturity of electrophysiological response of the neonatal auditory brainstem to high repetition rates of click stimulation.

Changes in brainstem auditory evoked response (BAER) with varying stimulus repetition primarily represent neural processes concerning the efficacy of synaptic transmission in the brainstem auditory pathway. In this study the BAER to different rates of clicks was recorded from 16 term neonates. The results were compared with those from 16 adults to examine whether the degree of maturation of synaptic transmission of the neonatal brainstem auditory pathway parallels that of general function of the pathway. All BAER wave latencies and interpeak intervals increased linearly and wave amplitudes reduced with increasing click rate. The absolute rate-dependent changes in BAER measures were much greater in the neonates than in the adults, reflecting a significant immaturity in the efficacy of synaptic transmission in the neonatal auditory brainstem and in the ability of the neonatal brainstem to process rapid acoustic stimulation. When the data obtained at higher click rates at various age groups were analyzed as percentages, using the BAER measurements at conventionally used slow rate (21/s) of clicks as the denominators, the changing rates (%), or relative changes, of most BAER measures at higher rates in the neonates were still greater than those in the adults. Therefore, the rate-dependent BAER changes in the neonates are relatively less mature than general aspects of the BAER, reflected by the BAER elicited with conventionally used slow rates of clicks. These findings suggest that synaptic efficacy in the neonatal brainstem auditory pathway is relatively less mature than general function of the pathway and thus may be more susceptible to unfavourable perinatal conditions.