The Effect Of Bronchopulmonary Dysplasia And Chronic Sublethal Hypoxia On Very Immature Brainstem

Bronchopulmonary Dysplasia(BPD) is a major lung disease in infants born very preterm.Increasing evidence suggests that the survivors of BPD have high incidence rates of neurologic impairment and developmental deficits,such that BPD has become one of the greatest risk factors of neurodevelopmental problems in infants.Early detection of neurologic impairment,can provide important information for clinical management of infants with BPD.The pathophysiological processes underlying neurologic impairment and developmental deficits after BPD remain poorly understood.Animal experiments with BPD revealed that prolonged or chronic sublethal hypoxia,which occurs during the course of BPD,may result in severe impairments in corticogenesis in the developing brain and a significant decrease in subcortical white matter.Hypoxemia affects the functional integrity and development of the immature brain,and brainstem auditory neurons are particularly sensitive to severe hypoxemia.Infants who suffer BPD often experience frequent episodes of hypoxemia or prolonged hypoxemia.It is possible that frequent episodes of hypoxemia or prolonged hypoxemia impair the functional integrity of the neonatal auditory brainstem.The neonatal brainstem auditory evoked response(BAER) reflects the functional integrity and development of the auditory brainstem.This response is very sensitive to arterial blood oxygen levels and hypoxia.Maximal length sequence(MLS) BAER is a relatively new technique,which can increase acoustic stimuli at much higher repetition rates(up to 1000 clicks per second or even higher),providing a much stronger physiological challenge to brainstem.To investigate the effect of BPD and chronic sublethal hypoxia on neonatal immature brainstem,we firstly studied risk factors for functional impairment of auditory brainstem in very preterm babies at corrected term age to find whether BPD is one of the greatest risk factor of neurologic impairment;meanwhile,for peripheral auditory threshold may be associated with brainstem auditory function,we also determined the correlation between them beforehand.Then we further explored the effect of BPD on auditory braistem function in very preterm babies at term age and also in a two year follow up study;Finally,to further elucidate some adaptive changes during postnatal chronic sublethal hypoxia,we employed an animal model of chronic sublethal hypoxia to test whether chronic sublethal hypoxia results in brainstem myelination arrest in the developing rat model.Part 1 Correlation between brainstem auditory function and peripheral auditory threshold in preterm infants at term ageObjective:To determine whether central auditory function in preterm infants correlates with peripheral auditory threshold and whether threshold elevation affects central auditory function.Methods:Brainstem auditory evoked response(BAER) was recorded at term age using maximum length sequence(MLS) with 91-910/s clicks in 133 preterm infants (gestation 28-36 weeks).Results:The latencies and amplitudes of all MLS BAER waves correlated significantly with BAER threshold.However,no correlation was found between MLS BAER interpeak intervals and BAER threshold at any rates.In preterm infants with a threshold＞20 dB nHL(n=30),MLS BAER wave latencies were all significantly longer than in those with a threshold≤20 dB nHL(n=103)(P＜0.01-0.001).MLS BAER wave amplitudes were significantly smaller than in those≤20 dB nHL(P＜0.05-0.001).However,no interpeak intervals differed significantly between the two groups of infants.Ⅴ/Ⅰamplitude ratio was similar in the two groups.These findings were true of all click rates.Click rate-dependent changes in MLS BAER of the preterm infants with an elevated BAER threshold are generally similar to those with a normal threshold.Conclusions:Brainstem auditory function does not closely correlate with peripheral auditory threshold at term in preterm infants.Part.2 Risk factors for functional impairment of auditory brainstem in very preterm babies at term ageObjectives:Very preterm infants are at a high-risk of developing neurological impairment and later disabilities.To gain deeper understanding of neurologic impairment mechanism,we employed MLS BAER to determine which risk factors associated with neurological impairment in very preterm babies at term age.Methods:Brainstem auditory evoked response(BAER) was recorded at term age using maximum length sequence(MLS) with 91-910/s clicks in very preterm infants (gestation≤30weeks).The relationship between MLS BAER variables and risk factors was analyzed with multiple regression analysis of repeat measure data.Results:There were 182 very preterm infants with gestational age＜30 weeks who were included into this study.Multiple regression analysis of repeated measure data showed that BPD was independent risk factor of waveⅤlatency,Ⅰ-Ⅴinterpeak interval,Ⅲ-Ⅴinterpeak interval andⅢ-Ⅴ/Ⅰ-Ⅲinterpeak interval ratio,which represent central function of auditory brainstem;At any acoustic rate of MLS BAER, the stardard partial regression coefficient of BPD was the largest among all risk factors.Other risk factors associated with MLS BAER variables included PDA, PROM,hypoglycemia,GA and SGA.Conclusions:The most important risk factor for functional impairment of auditory brainstem in very preterm babies at term age is PDA.Other relative risk factors are, PDA,PROM,hypoglycemia,GA and SGA.Part 3.Auditory Brainstem Maturation in Very Preterm Babies with Bronchopulmonary Dysplasia:a two-year follow up studyObjectives:Bronchopulmonary dysplasia(BPD) is proved to be independent risk factor of neurodevelopmental disorders in very preterm infants.We hypothesize that central auditory brainstem maturation and neurodevelopment are delayed in very preterm infants with BPDMethods:To assess the effect of BPD on neonatal auditory brainstem maturation,we studied MLS BAER in 32 very preterm infants who suffered BPD but no other major perinatal complications or problems at postconceptional ages of 34,36,38,40,44,52, 66,72,118,144weeks.BAER was recorded with clicks,delivered at 91,227,455 and 910/s and≥40db above BAER threshold of each subject.Results:1) At postconceptional ages of 34 and 36weeks,compared with healthy,very preterm control subjects,waveⅤlatency in very preterm infants with BPD tended to be longer,though no significant difference was detected.However,Ⅲ-Ⅴ,Ⅰ-Ⅴintervals andⅢ-Ⅴ/Ⅰ-Ⅲratio were significant longer in BPD infants than that in control group, indicating more central components of auditory brainstem were affected before term.2) At term,compared with term control subjects and also very preterm control babies, waveⅤ,Ⅲ-Ⅴintervals andⅠ-Ⅴintervals in BPD infants increased significantly at all click rates 3) In the follow-up study,the wave latency in BPD and very preterm control infants caught up with that in term babies rapidly.However,Ⅲ-ⅤandⅠ-Ⅴintervals in BPD babies recovered relatively slowly to normal value.They still tended to be longer than those variables in term control babies at postconceptional age of two years.Ⅲ-Ⅴ/Ⅰ-Ⅲintervals ratio,reflecting more central function,significantly increased at all occasions of age in BPD group in comparison of term control group.Increasing the stimulus rates improved the detection of neuropathology.4) The decaying exponential functions model fit sufficiently robust both applied to the age-group means and to all data in term control babies and very preterm babies with BPD.The time constants of waveⅤ,Ⅲ-ⅤandⅠ-Ⅴ,tended to be longer in BPD group than the other group.Conclusions:The results suggest long-term impairment of brainstem function in very preterm infants with BPD,resulting in delayed auditory brainstem maturation.MLS BAER provides a valuable tool to early detection of the impairment and prediction of later outcome.Part 4.Brainstem Myelination Maturation in the Developing Rats with Chronic Sublethal Hypoxia InjuryObjectives:To further elucidate some adaptive changes during postnatal chronic hypoxia,we employed an animal model of chronic sublethal hypoxia to test the hypothesis that chronic sublethal hypoxia results in brainstem myelination maturation arrest in the developing rat model. Methods:Litters of SD rats were put into hypoxic chamber(oxygen level maintained at 9.5%) with their dams starting on day 3 postnatal(P3).Age matched normoxic rats were used as control animals.Two groups were sacrificed at P13 and prepared for further analysis in four sections of brainstem(cochlear nucleus,superior olivary nucleus, lateral lemniscus,and inferior colliculus).We performed electron microscope and cresyl violet staining for morphometric analysis,immunohistochemical assays to visualize specific stages in the oligodendrocyte(OL) lineage.Western analyses of myelin basic protein(MBP) were also determined in three parts of brainstern,the midbrain,pons and medulla.Results:Measurement of axonal sheath size at P13 revealed that axonal sheath of brainstem was significantly smaller in hypoxia compared with control rats. Neuropathological analysis of the brainstem of the developing rats in chronic hypoxia demonstrated a marked reduction in staining for MBP with residual myelination was patchy in distribution.Staining with the O1 and O4 antibodies,markers of premyelinating and early myelinating OLs,also showed a significant reduction of axon sheaths in brainstem.To determine the effect of chronic hypoxia on the changes in brainstem expression of MBP,we also performed Western blotting in whole and three regions of brainstem:midbrain,pons and medulla.We observed that MBP expression was reduced in overall brainstem in hypoxia-reared rots.Among medulla,pons and midbrain,MBP expression was highest in medulla and lowest in midbrain,whether in hypoxia or normoxic group.Conclusions:We conclude that neonatal chronic hypoxia 1) reduces the extent of myelination in the brainstem,whose process of maturation is sequential from lower to higher brainstem centers of immature animal;2) the mechanism of myelination failure in chronic hypoxia is related to delayed preOLs maturation.