Long-term effect of perinatal and postnatal asphyxia on developing human auditory brainstem responses: brainstem impairment

Brainstem auditory electrophysiology is supressed in term neonates with hyperbilirubinemia

BACKGROUND

Whether hyperbilirubinemia suppresses electrophysiological activity of the neonatal auditory brainstem remains to be investigated.

AIM

To determine whether hyperbilirubinemia suppresses the brainstem auditory electrophysiology in term neonates.

METHODS

Maximum length sequence brainstem auditory evoked response (MLS BAER) was recorded shortly after confirming hyperbilirubinemia in 58 term neonates. Wave amplitudes of the response were analyzed in detail.

RESULTS

Compared with age-matched term controls, the neonates with hyperbilirubinemia showed a significant reduction in the amplitudes of MLS BAER waves III and particularly V at all click rates 91-910/s. The reduction tended to be more significant at higher than lower rates. Wave I amplitude was reduced at 910/s. V/I amplitude ratio was decreased at all click rates. Therefore, the amplitudes of MLS BAER, particularly later, waves were all reduced. The amplitudes of all MLS BAER waves tended to be reduced with the increase in total serum bilirubin level. All wave amplitudes were correlated with the level of total serum bilirubin at some or most click rates.

CONCLUSIONS

Brainstem auditory electrophysiology is suppressed in neonates with hyperbilirubinemia, which related to the severity of hyperbilirubinemia. Wave amplitudes are valuable BAER variables to detect functional impairment of the brainstem and auditory pathway in neonatal hyperbilirubinemia, and are recommended to be used in assessing bilirubin neurotoxicity to the neonatal brain.

Neurodevelopmental outcomes in very low-birth-weight infants in Korea: 1998-2007 vs 1989-1997

The authors reviewed the medical records of very low-birth-weight infants admitted from 1998 to 2007 and compared neurodevelopmental outcomes with their previously reported data from 1989 to 1997. The recent group included 824 infants, and the previous group included 471 infants. Neurodevelopmental outcomes were classified into cerebral palsy and non-cerebral palsy neurodevelopmental impairment. In the recent group, the survival rate was significantly higher (79.4% vs 66.2%), the rate of cerebral palsy was lower (7.9% vs 10.5%), and the rate of non-cerebral palsy neurodevelopmental impairment was higher (6.0% vs 4.5%) but not significant. The survival rate increased significantly over time, but there was no significant change in neurodevelopmental outcomes over time. Multivariate analysis indicated that abnormal neurosonographic findings, using assisted ventilation, vaginal delivery, and abnormal brainstem auditory evoked potential, were associated with increased risk for cerebral palsy.

Relationship between brainstem auditory function during the neonatal period and depressed Apgar score

OBJECTIVE

To understand whether infants with depressed Apgar scores are at high risk of brainstem auditory impairment, we examined the relationship between brainstem auditory function during the neonatal period and depressed Apgar score.

METHODS

Brainstem auditory evoked responses (BAERs) were recorded from day 1 to day 30 in 145 term infants with Apgar scores < or = 6.

RESULTS

For 1-min Apgar score none of BAER response wave latencies and interpeak intervals during the first 15 days correlated significantly with the score. On day 30, all wave latencies but no intervals correlated negatively with the score (all p < 0.05). For 5-min Apgar score, only on day 3 wave V latency and I-V and III-V intervals correlated negatively with the score (all p < 0.05). For 10-min Apgar score, only III-V interval correlated negatively with the score (p < 0.05). No correlation was found between BAER variables on any other days and 5 and 10-min Apgar scores. Wave V latency and I-V and III-V intervals on day 3 were all significantly longer in infants with 5-min Apgar scores < or = 6 than in those with scores > 6.

CONCLUSIONS

During the neonatal period, only on day 3 after birth a depressed 5-min Apgar score is an indicator associated with central auditory impairment. A depressed 1-min score may be associated with later peripheral auditory impairment.

Impairment of perinatal hypoxia-ischemia to the preterm brainstem

Hypoxia-ischemia is a major perinatal problem that results in severe damage to the newborn brain. This study assessed functional integrity of the brainstem at term in preterm infants after perinatal hypoxia-ischemia to shed light on the influence of hypoxia-ischemia on the preterm brainstem. We recruited sixty-eight preterm infants after perinatal hypoxia-ischemia, ranging in gestation 28-35 weeks. Brainstem evoked response was studied at term age (37-42 weeks postconceptional age) with 91-910/s clicks using the maximum length sequence technique. Compared with healthy preterm infants, the preterm infants after perinatal hypoxia-ischemia showed a significant increase in I-V interval at very high rates 455 and 910/s of clicks (P<0.05, 0.05). III-V interval and III-V/I-III interval ratio also increased significantly at 455 and 910/s (P<0.05-0.01). The slope of III-V interval-rate function was significantly steeper than in the healthy preterm infants (P<0.05). Compared with normal term controls, the preterm infants after hypoxia-ischemia showed similar, but slightly more significant, abnormalities. The differences between the preterm infants after hypoxia-ischemia and the healthy preterm and term infants generally increased with increasing click rate. These results demonstrated that central components of brainstem auditory evoked response were abnormal at very high click rates in the preterm infants after perinatal hypoxia-ischemia. Click rate-dependent change in the more central part of the brainstem is also abnormal. Apparently, functional integrity of the brainstem, mainly in the more central part, is impaired. Hypoxic-ischemic damage to the preterm brainstem is unlikely to completely recover within a relatively short period after the insult, which is of clinical importance.

Sustained depression of brainstem auditory electrophysiology during the first months in term infants after perinatal asphyxia

OBJECTIVE

To gain further insights into the pathophysiological processes of neuronal impairment in neonatal brainstem after perinatal asphyxia.

METHODS

Maximum-length sequence brainstem auditory-evoked response (MLS BAER) was recorded with clicks at 91, 227, 455 and 910/s on days 1, 3, 5, 7, 10, 15 and 30 after birth in 108 term infants who suffered perinatal asphyxia. Wave amplitude variables in the MLS BAER were analysed in detail at 40 dB above BAER threshold in 86 infants who had no peripheral hearing impairment.

RESULTS

On day 1 the amplitudes of MLS BAER waves I, III and V were all reduced significantly at all click rates, especially at higher ones (91-910/s, ANOVA P<0.05-0.001). On day 3 these amplitudes were reduced further. On days 5 and 7, the amplitude reduction persisted and did not show any significant further changes. On days 10 and 15 the reduced amplitudes were increased slightly. On day 30 all amplitudes were still reduced significantly (P<0.05-0.0001). During the first month, the reduction of wave amplitudes was more significant for the later MLS BAER components than for the earlier ones, and occurred most significantly at 455 and 910/s clicks. By comparison, the amplitude reduction in conventional BAER was much less significant.

CONCLUSIONS

During the first month after perinatal asphyxia the amplitudes of MLS BAER waves were reduced significantly and persistently, which was more significant at higher rates of clicks than at lower rates. The reduction is much more persistent than the increase in wave latencies and intervals we previously reported.

SIGNIFICANCE

There is sustained depression of brainstem auditory electrophysiology, indicating neuronal damage of the auditory brainstem, in infants after perinatal asphyxia. This may have important clinical implications.

Auditory brainstem responses as a clinical evaluation tool in children after perinatal encephalopathy

Auditory brainstem responses (ABR) reveals the neurophysiological status of the neural axis. In this study we compared the ABR of healthy children, under 1-year-old, with children who suffered from perinatal encephalopathy (PE).

OBJECTIVE

The purpose of this study was to characterize the ABR differences between children with PE and healthy children in order to identify groups with specific neurophysiological profiles, associated with their neurological condition.

METHODS

Thirty-six children with perinatal encephalopathy (PE) and 36 healthy children, ages 1-12 months, were studied. The variables considered were: latencies of waves I, II, N1, III, V, and N2; interpeak latency interval (IPL) of waves I-III, III-V, and I-V; as well as amplitudes of waves I, III, and V. The results were analyzed using ANOVA, as well as Ji(2), and Ward's cluster analysis.

RESULTS

The absolute latencies of the ABR showed an inverse correlation with the children's age. Latencies of waves I, II, N1, V, and N2, IPL III-V, and amplitude of waves III and V show significant differences (p<0.05) between healthy and PE children. Children with PE showed greater absolute latencies and larger wave amplitudes than the control group. Ward's cluster analysis, used to define the groups with similar functional characteristics, revealed three groups: fast, intermediate, and slow-responders, depending on their wave latencies and IPL wave amplitudes. These groups were gender- (p<0.03), age- (p<0.0001), and neurological damage- (p<0.01) related.

CONCLUSIONS

Our data clearly show that the ABR obtained from PE children differ from ABR obtained from healthy children. PE infants showed larger wave latencies, intervals amplitudes than the control group. Three functional profiles resulted from the groups established using the Ward's method, and these indicate their neurological functional condition.

Sub-optimal function of the auditory brainstem in term infants with transient low Apgar scores

OBJECTIVES

To assess functional integrity of the auditory brainstem in neonates with transient low Apgar scores.

METHODS

Forty-two term infants were studied with brainstem auditory evoked response (BAER) using the maximum length sequence during the first month of life. All had transient low Apgar scores but no clinical signs of hypoxic-ischaemic encephalopathy (HIE).

RESULTS

The latencies of BAER waves I and III in these infants were similar to those of age-matched normal controls at all click rates (91/s, 227/s, 455/s and 910/s) during the period studied. Wave V latency was increased at 910/s on day 1 (P<0.01), but did not differ from that in the controls on any other days. I-V interval was increased significantly at 455/s and 910/s on day 1 (P<0.01 and 0.001) and day 3 (P<0.05 and 0.01). On days 5 and 7, BAER wave latencies and intervals were similar to those in the controls. On day 30, all latencies and intervals reached the values in the controls. No abnormalities were seen in BAER wave amplitude variables on any days.

CONCLUSIONS

Neonates with transient low Apgar scores but without HIE had a significant increase in I-V interval at very high click rates on the first three days of life.

SIGNIFICANCE

Brainstem auditory function is sub-optimal during the first few days in neonates with transient low Apgar scores.

Neonatal auditory function and depressed Apgar score: correlation of brainstem auditory response with Apgar score

AIM

To examine the relationship between neonatal auditory function and Apgar score in term infants with depressed Apgar scores.

METHODS

Brainstem auditory evoked response (BAER) was recorded on day 3 after birth in term infants who had Apgar scores < or = 7 at 1 and/or 5 min. Half of the infants also had Apgar scores at 10 min, with 17 < or = 7.

RESULTS

No BAER variables correlated significantly with 1-min Apgar score. However, wave III and V latencies, and I-V, I-III and III-V intervals correlated significantly and negatively with 5-min Apgar score (p < 0.05-0.01). These BAER variables were significantly longer in infants with 5-min Apgar scores < or = 7 than those > 7. Wave V latency and all intervals also correlated negatively with 10-min Apgar score (p < 0.05-0.01). Compared to normal controls, all latencies were prolonged in infants with depressed Apgar scores (all p < 0.05-0.01). All intervals were also prolonged in those with 5-min scores < or = 7 (p<0.05-0.01). Similar results were found when defining the depression of Apgar score as < or = 6.

CONCLUSION

A depressed 5- and/or 10-min Apgar score is an indicator associated with neonatal auditory, mainly central, impairment. Apgar score < or = 7 or 6 at 1 min alone is unlikely to be associated with central impairment.

Brainstem auditory evoked response at term in preterm infants after perinatal hypoxia-ischaemia

AIM

To examine brainstem auditory function in preterm infants after perinatal hypoxia-ischaemia (HI).

METHODS

Brainstem auditory evoked response (BAER) was studied at 37-42 wk postconceptional age in preterm infants who suffered perinatal HI, and were compared with those in preterm and term infants without any major perinatal problems.

RESULTS

None of the latencies of waves I, III and V in the BAER in preterm infants after perinatal HI differed significantly from those in preterm and term controls. There were also no significant differences in the I-V and I-III interpeak intervals between preterm infants after HI and the controls. However, III-V interval in infants after HI tended to increase, and was significantly longer than in term controls (p<0.01), although it did not differ significantly from that in preterm controls. Similarly, the III-V/I-III interval ratio was greater than in term controls (p<0.05), but was similar to that in preterm controls. All amplitude variables did not differ significantly from the controls.

CONCLUSION

BAER in preterm infants after perinatal HI did not differ significantly from that in preterm infants without perinatal problems, but differed slightly from that in normal term infants. The prolonged III-V interval as compared with the term controls suggests a minor impairment in central auditory function.

Changes in BAER amplitudes after perinatal asphyxia during the neonatal period in term infants

We recorded serially brainstem auditory evoked response (BAER) during the neonatal period in term infants who suffered perinatal asphyxia. The amplitudes of BAER components was analysed at 40 dB above BAER threshold of each subject who had a threshold<or=25 dB nHL. No apparent changes in the amplitudes of waves I and III during the first 5 days after birth. The two wave amplitudes were slightly reduced thereafter. On day 30, the amplitudes were slightly smaller than in normal controls. No statistical significant differences were found in the two amplitudes between the infants after asphyxia and the controls on any days studied. In contrast, wave V amplitude showed a trend of reduction during the whole neonatal period. The amplitude was reduced slightly on the first day after birth, but reduced further on day 3 (ANOVA, P<0.01). Thereafter, the reduction persisted without any significant change. On day 30, wave V amplitude remained significantly smaller than in the controls (P<0.001). Compared to the controls, V/I amplitude ratio was slightly smaller during the neonatal period, but V/III amplitude ratio was significantly smaller on most of the days studied (P<0.05-0.01). The persistent reduction of wave V amplitude suggests a sustained neuronal damage of the auditory brainstem in infants after perinatal asphyxia.

Brainstem auditory-evoked responses in full-term newborn infants with temporary low Apgar score

CONCLUSIONS

No abnormalities, with the exception of maturational changes, in BAER were found during the neonatal period. The results suggest that a temporary low Apgar score is not accompanied by any significant auditory impairment.

OBJECTIVE

To examine brainstem auditory function in newborn infants with a temporary low Apgar score but no clinical signs of hypoxic-ischaemic encephalopathy (HIE).

MATERIAL AND METHODS

The subjects were 36 full-term infants with Apgar scores of < or =7 at 1 and/or 5 min and > or =8 at 10 min but without HIE. The brainstem auditory-evoked response (BAER) was serially recorded at click rates of 21, 51 and 91/s on Days 1, 3, 5, 7 and 30 after birth.

RESULTS

On Day 1 and Days 3-5, the latencies of waves I, III and V tended to increase slightly at all click rates but did not differ significantly from normal control values. Thereafter, all latencies tended to decrease, reaching control values on Day 30. The I-V interval was similar to the control values at all click rates during the first 5 days, tended to decrease from Day 7 and did not differ from the control values on Day 30. There were no significant changes in BAER wave amplitudes at any of the click rates on any day.

Brain-stem auditory impairment during the neonatal period in term infants after asphyxia: dynamic changes in brain-stem auditory evoked response to clicks of different rates

OBJECTIVE

To explore dynamic changes in brain-stem auditory electrophysiology during the neonatal period in term infants after perinatal asphyxia.

METHODS

Sixty-eight term newborn infants who suffered asphyxia were studied on days 1, 3, 5, 7, 14 and 30 after birth. Brain-stem auditory evoked response (BAER) was recorded with clicks, delivered at 21, 51 and 91 s(-1) and > or =40 dB above BAER threshold of each subject.

RESULTS

During the neonatal period wave I latency in the infants after asphyxia increased slightly while later BAER components changed more significantly. On the first day after birth wave III and V latencies and I-V and III-V intervals increased significantly at all rates of clicks (ANOVA P<0.01-0.001). On day 3, the latencies and intervals increased further. III-V/I-III interval ratio increased at 51 and 91 s(-1), suggesting a relatively more significant increase in III-V interval than in I-III interval at higher rates. Thereafter, wave III and V latencies and all intervals decreased progressively, although these BAER variables were still significantly longer than in normal controls on days 5 and 7 (P<0.05-0.001) On day 30, all latencies and intervals approached near normal values, with a slight increase in wave V latency and I-V and III-V intervals at 51 and 91 s(-1).

CONCLUSIONS

Perinatal asphyxia has a major effect on central auditory function, resulting in acute impairment. The impairment progresses during the first 3 days and then tends towards recovery. By 1 month the impaired auditory function has largely returned to normal. Significant increase in click rates can moderately improve the detection of auditory impairment.

SIGNIFICANCE

After perinatal asphyxia early detection of hypoxic-ischaemic damage to the central auditory system and initialisation of neuroprotective and therapeutic measures during the first hours after birth are critical to prevent or reduce deterioration of central impairment.

Time course of brainstem pathophysiology during first month in term infants after perinatal asphyxia, revealed by MLS BAER latencies and intervals

Dynamic changes in electrophysiology of brainstem auditory neurons during the first month after birth were studied in 51 term infants after perinatal asphyxia using maximum length sequence brainstem auditory evoked responses. The responses were recorded on d 1, 3, 5, 7, 10, 15, and 30 after birth. On d 1, wave III and V latencies and all interpeak intervals increased significantly at all repetition rates of clicks used (91-910/s), especially the higher rates (ANOVA, p < 0.05-0.0001). On d 3, all these latencies and intervals increased further and differed more significantly from the normal control subjects. Thereafter, the latencies and intervals decreased progressively. On d 7, wave V latency and all intervals still differed significantly from the control subjects. These dynamic changes were more significant at higher rates of clicks than at lower rates. On d 10 and 15, all intervals decreased significantly. On d 30, all wave latencies decreased to the values in the normal control subjects on the same day. The intervals also approached normal values, although the III-V and I-V intervals still increased slightly. These results indicate that hypoxic-ischemic brain damage persists during the first week, with a peak on d 3, and recovers progressively thereafter. By 1 mo, the damage has largely returned to normal. Maximum length sequence brainstem auditory evoked responses results correlated well with the stage of hypoxic-ischemic encephalopathy during the first week. The present study revealed a general time course of brainstem pathophysiology after asphyxia, although there were individual variations. Our findings can be used as a reference to monitor cerebral function and help judge the value of neuroprotective or therapeutic interventions. The first week, particularly the first 3 d, is a critical period of hypoxic-ischemic brain damage, and early intervention may prevent or reduce deterioration of the damage.

Brain-stem auditory evoked potentials in children with perinatal encephalopathies

OBJECTIVE

We sought to describe if neurological damage, in terms of brain lesions, syndrome and syndrome severity led to abnormalities in the brain-stem auditory evoked potentials (BAEPs) in order to provide a profile of children that could be used as an indicator of subsequent neurological sequelae. We analyzed the BAEPs from a group of children having prior evidence of neurological damage and determined the presence of neurological sequelae when the subjects were 3 years old.

METHODS

Brain-stem auditory evoked potentials (BAEPs) were carried out in a group of 154 children with perinatal neurological damage. The children were classified with neurofunctional (clinical and EEG alterations) or organic and neurofunctional brain disease (clinical, EEG and image alteration) and were all followed from the first month of life and serially for 3 years. We used principal component analysis (PCA), clustered analysis and linear correlation to determine association between BAEPs, risk factors and future sequelae.

RESULTS

Latencies of BAEPs decreased significantly with age, and the time of conduction was modified by the presence of neurological damage. All statistical analyses suggested positive and significant associations between risk factors (trophism and condition at birth), and the latencies of waves I, III and V as well as with IPL III-V (interpeak latency) and I-V. PCA showed that IPL I-III was also positively associated with condition at birth, severity of the neurological syndrome and encephalopathy. In addition, we found that the presence and type of sequela reflected changes in the latencies of the waves, as well as IPLs, primarily those of IPL I-III.

CONCLUSIONS

Our results suggest that statistical methods are often needed to analyze neurological damage. The relation between BAEPs, risk factors and neurological sequelae allowed us to obtain a profile of children, which can be then used as an aid in the prognosis of children having a risk of developing neurological sequelae.

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.

Outcome of brain stem auditory electrophysiology in children who survive purulent meningitis

Brain stem auditory electrophysiology was assessed in children long after purulent meningitis by examining the central components of brain stem auditory evoked response (BAER) at different presentation rates of clicks. Children with peripheral hearing loss were excluded from this study to avoid any possible influence of the loss on the measurements of BAER central components. Of the 70 children who had recovered from meningitis 2 to 8 years earlier, 9 (12.9%) showed abnormalities in BAER central components--mainly a slight reduction of wave V amplitude, at the click presentation rate of 10/s. Of the remaining 61 children who did not show any obvious abnormalities in the BAER at 10/s, an abnormally prolonged I-V interval and a reduced amplitude of wave V were seen in 5 children (8.2%) at the click rate of 90/s. The total BAER abnormality rate was therefore 20.0% (14/70 cases). The results suggest that most children who survive purulent meningitis have a favorable outcome for the brain stem auditory pathway, with about 1 in 5 having mild dysfunction or a suboptimal outcome in the pathway.

A study of auditory afferent organization in children with dyslalia

The auditory afferent (AA) control is an important feedback mechanism in the speech generation. A different organization of AA pathways in children with speech alterations is suggested. In order to investigate this possibility we recorded the auditory brainstem responses (ABR) and middle latency responses (MLR) on monoaural and binaural click stimulation in a group of 17 normal children with no alteration of the speech (N) and in 16 children with dyslalia (eight with systematic (S) and eight with non-systematic errors of the speech (NS)). All of children were normal hearing, with normal ORL and neurological status, right-handed and with the age approximately 7 years old. A lateralization effect was found in the S group. Normally, it was only found for wave I. The efficiency of both AA pathways was the same in NS group, indicating a more effective right pathway in more rostral areas. A prolonged latency (X = 0.25 ms) of wave III was registered on the right side in the NS group compared to normals, as well for wave V (X = 0.175 ms) with increased sweep rate (21 vs. 51 and 71). The effect of sweep rate augmentation was also studied (21-51-71) on latency values and inter-wave differences in these groups. A successive latency prolongation (X = 2.97 ms) of MLR wave Na was registered between the N-S-NS groups. In the S group a latency binaural interaction (BI) of MLR left wave Na was prolonged for 3.52 ms and in the NS group for a further 1.32 ms compared to normals. Only in the NS group was a prolongation of the BI of the right wave Pa detected (6.76 ms) compared to normals. Results suggest a different AA organization in children with dyslalia. Possible locations of alterations in functioning could be pons, and thalamocortical projections. ABR and MLR could evaluate the auditory-speech capability of children.

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.

Binaural interaction and the effects of stimulus intensity and repetition rate in human auditory brain-stem

Binaural interaction (BI) components in brain-stem auditory evoked potential (BAEP) and their changes with stimulus intensity and repetition rate were examined in human adult. Seven BI components were identified, which occurred between the latency range of 5 and 11 ms and coincided consistently with the latency range of BAEP waves IV-VII. Waves DV and DVII, occurring at the downslopes of BAEP waves V and VII, respectively, were the two most prominent and reproducible BI components. Wave DVII existed consistently at high, moderate and, in most cases, low stimulus intensities, suggesting that this component is neurogenic although acoustic cross-talk may account for a part of its waveform at high stimulus intensities. The latencies of all BI components increased as a function of decreasing stimulus intensity, while the interpeak intervals, especially DV-DVII, were essentially constant at different intensity levels. The amplitudes of BI components decreased slightly with decreasing intensity. As click repetition rate increased, BI wave latencies and interpeak intervals increased slightly and amplitudes decreased slightly. When repetition rate increased to above 20/s, BI components became poorly differentiated. Lower repetition rates, e.g. 10/s, are therefore preferred for routine derivation of the BI. The changes in the latency and amplitude of BI components with stimulus intensity and repetition rate were associated or concomitant with those of the corresponding BAEP components in monaural and binaural potentials. In view of the concomitant relationship between BI and BAEP latency, we designate BI components in association with the corresponding BAEP components.

Binaural interaction in human neonatal auditory brainstem

Binaural interaction (BI) in brainstem auditory evoked response (BAER) were examined in normal term neonates. The BI components coincided consistently with the latency range of BAER wave IV through wave VII. Most BI components seen in the adults could be identified in the neonates, but the later components, i.e. those with longer latency, were underdeveloped in wave form. Wave DV was the most consistent and reproducible BI component. A marked difference between the neonatal and adult BI wave forms was that wave DVII was particularly small in the neonates. It appears that neuronal responses contributing to later BI components such as wave DVII are particularly immature at birth. Wave latencies and interpeak intervals were longer and amplitudes were smaller in the neonates than in the adults, which was associated with the differences between the neonates and adults in the BAER components. Changes in the BI components with stimulus intensity and rate in the neonates were fundamentally similar to but more significant compared with those in the adults. These findings suggest that neural connections in human auditory brainstem subserving the BI are established at birth but, particularly at higher levels of the brainstem, are immature.