ON THE NEUROPATHOLOGICAL CHANGES IN THE CENTRAL NERVOUS SYSTEM FOLLOWING NEONATAL ASPHYXIA, WITH SPECIAL REFERENCE TO THE AUDITORY SYSTEM IN MAN

Impact of prematurity on neurodevelopment

The consequences of prematurity on brain functional development are numerous and diverse, and impact all brain functions at different levels. Prematurity occurs between 22 and 36 weeks of gestation. This period is marked by extreme dynamics in the physiologic maturation, structural, and functional processes. These different processes appear sequentially or simultaneously. They are dependent on genetic and/or environmental factors. Disturbance of these processes or of the fine-tuning between them, when caring for premature children, is likely to induce disturbances in the structural and functional development of the immature neural networks. These will appear as impairments in learning skills progress and are likely to have a lasting impact on the development of children born prematurely. The level of severity depends on the initial alteration, whether structural or functional. In this chapter, after having briefly reviewed the neurodevelopmental, structural, and functional processes, we describe, in a nonexhaustive manner, the impact of prematurity on the different brain, motor, sensory, and cognitive functions.

Perinatal anoxia degrades auditory system function in rats

Little is known about the neural bases of the reduced auditory and cortical processing speeds that have been recorded in language-impaired, autistic, schizophrenic, and other disabled human populations. Although there is strong evidence for genetic contributions to etiologies, epigenetic factors such as perinatal anoxia (PA) have been argued to be contributors, or causal, in a significant proportion of cases. In this article, we explored the consequences of PA on this elementary aspect of auditory behavior and on auditory system function in rats that were briefly perinatally anoxic. PA rats had increased acoustic thresholds and reduced processing efficiencies recorded in an auditory behavioral task. These rats had modestly increased interpeak intervals in their auditory brainstem responses, and substantially longer latencies in poststimulus time histogram responses recorded in the primary auditory cortex. The latter were associated with degraded primary auditory cortex receptive fields and a disrupted tonotopy. These processing deficits are consistent with the parallel behavioral and physiological deficits recorded in children and adults with a history of language-learning impairment and autism.

Prenatal alcohol and cocaine exposure: influences on cognition, speech, language, and hearing

This paper reviews research on the consequences of prenatal exposure to alcohol and cocaine on children's speech, language, hearing, and cognitive development. The review shows that cognitive impairment, learning disabilities, and behavioral disorders are the central nervous system manifestations of fetal alcohol syndrome (FAS), and cranio-facial abnormalities are also present. Delays in language acquisition, as well as receptive and expressive language deficits, are commonly reported. The cranio-facial abnormalities of FAS, which sometimes include cleft palate, make the child prone to otitis media with effusion and conductive hearing loss. The family environment in which one or both parents is a heavy alcohol user presents challenges to a child with normal intelligence, but may be especially deleterious to the child with mental retardation. Prenatal exposure to cocaine results in subtle cognitive disabilities when measured at 4 years of age. The cognitive effects may be ameliorated by a stimulating and sensitive care-giving environment. A small, deleterious "cocaine-effect" is also seen in speech and language development. The child with prenatal exposure to cocaine may be considered at increased risk for language delay or disorder. There is no evidence that prenatal cocaine exposure by itself is a risk factor for sensorineural hearing impairment, although auditory evoked potentials from the brainstem and cortex suggest some abnormalities in central auditory processing, at least during the newborn period. The strong effect of the home environment for ameliorating the effects of prenatal cocaine-exposure suggests that a family-focused approach for cognitive, language, and social-emotional habilitation would be beneficial to all.

LEARNING OUTCOMES

The learner will be able to describe the major features of fetal alcohol syndrome and how they relate to speech, language, hearing, and cognitive disorders. The learner will review the literature and determine research needs with respect to language, speech, and hearing among infants and children with fetal alcohol syndrome. Similarly, the learner will distinguish the outcomes of prenatal alcohol-exposure from those of prenatal cocaine-exposure. The learner will summarize the controversy regarding the possible stigmatization of cocaine-exposed infants. The learner will summarize the speech, language, and hearing effects of prenatal cocaine-exposure.

Evoked potentials as predictors of outcome in neonatal intensive care unit survivors: review of the literature

Neonatal intensive care unit survivors are at substantial risk for a range of neurodevelopmental sequelae, and therefore a variety of clinical diagnostic techniques have been evaluated as predictors of outcome. We summarize the prognostic value of evoked potentials in newborns at risk. A review of the literature reveals that brainstem conduction abnormalities in auditory brainstem evoked potentials are associated with neuromotor impairment; however, there are many false negative studies. Visual evoked potentials are highly accurate in predicting neurologic deficits in early childhood in asphyxiated term neonates. Sensitivity and specificity are consistently high for somatosensory evoked potentials in term newborns; however, correlations with outcome in premature infants is controversial. Several studies have compared neonatal findings on neuroimaging studies and evoked potentials, and concordant results between these two tests are highly predictive. However, neurologic sequelae often can most accurately be predicted by visual or somatosensory evoked potentials. Evoked potentials may therefore be a useful adjunct to the clinical investigation and prognostication of outcome in the high risk newborn.

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

Long-term effect of perinatal and postnatal asphyxia on the developing auditory brainstem was investigated in children, particularly those who exhibited residual neurodevelopmental deficits, by analyzing the central components of brainstem auditory evoked responses (BAER). The major abnormalities in the BAER were a reduction of wave V amplitude, followed by a decrease in V/I amplitude ratio, while abnormalities in interpeak intervals were relatively rare. These findings suggest that asphyxia could result in residual neural dysfunction of the brainstem but does not appear to exert any major long-term effect on neuronal transmission. BAER abnormalities occurred more frequently in the children with residual neurodevelopmental deficits than those without these deficits after perinatal asphyxia. The occurrence of BAER abnormalities was related to the duration as well as the degree of asphyxia. No significant difference was found in the abnormalities of the central BAER components between the children after perinatal asphyxia and those after postnatal asphyxia, suggesting that perinatal and postnatal asphyxia exerts a similar long-term effect on the developing central nervous system.

Prediction of outcome at school entry in neonatal intensive care unit survivors, with use of clinical and electrophysiologic techniques

OBJECTIVE

To determine the predictive value of multimodality evoked potentials as well as the neonatal neurobehavioral assessment in neonatal intensive care unit survivors at school entry.

STUDY DESIGN

In this prospective study, healthy (n = 24) and high-risk newborn infants (n = 78) were assessed in the newborn period with auditory brain-stem responses (ABRs) and somatosensory evoked potentials (SEPs), as well as the Einstein Neonatal Neurobehavioral Assessment Scale (ENNAS). Healthy and high-risk newborn infants were assessed in a blind fashion at 1, 3, and 5 years of age by a psychologist and a pediatric neurologist. Of those healthy (all 24) and high-risk newborn infants (72/78) with a neonatal ABR, SEP, or both, 62.5% were assessed at 5 years of age with the Wechsler Preschool and Primary Scale of intelligence, the Beery-Buktenica Test of Visual-Motor Integration, the Griffiths Locomotor Subscale, and neurologic examination. Chi-square analyses were carried out on neonatal and outcome measures, and sensitivity, specificity, and predictive values for each of the neonatal tests were ascertained.

RESULTS

The ENNAS and the ABR had good negative predictive value for cognitive (85.7% to 93.3%), locomotor (83.3%), and visual-motor (91.4% to 100%) performance. The ABR had good specificity, whereas the ENNAS was more sensitive. The SEP was an excellent prognostic tool, with high sensitivity (100%) and specificity (80% to 81.3%) for motor as well as cognitive domains. All infants with normal SEPs had favorable outcomes, whereas those with absent potentials did poorly. The SEP abnormalities were associated with findings on neurologic examination in all cases.

CONCLUSIONS

Normal neonatal evoked potentials and ENNAS are associated with favorable outcomes. The SEP most accurately predicts neurodevelopmental status at school entry.

Effects of Maternal Cocaine Abuse on Neonatal Auditory Brainstem Responses

Auditory brainstem responses (ABRs) were recorded from 18 neonates born to mothers who used cocaine during pregnancy, and from 18 control infants matched to the cocaine group on the basis of birthweight and conceptional age. ABRs were elicited using click signals presented at 40, 60, and 80 dB nHL at a rate of 33 clicks/sec and also at 11 and 89 clicks/sec at 80 dB nHL. The absolute latencies for ABR components I, III, and V and wave I-V interpeak latencies (IPLs) were measured for each signal condition.ABR absolute and IPLs for the infants who had been exposed to cocaine were prolonged relative to their age- and weight-matched controls. Neurodevelopmental compromise of the auditory system appears to result from gestational exposure to cocaine.

Prognostic significance of multimodality evoked response testing in high-risk newborns

Exposure to hypoxic-ischemic events in fetal or neonatal life may lead to permanent brain damage and subsequent neurodevelopmental deficits. Clinical and diagnostic tools have been somewhat helpful in identifying an at-risk group, particularly those patients sustaining significant neurologic sequelae. In this prospective study, the prognostic significance of multimodality evoked responses in high-risk newborns was examined. A group of 44 high-risk newborns, as well as 14 healthy newborns, were tested during the newborn period with auditory brainstem responses and somatosensory evoked responses; these tests were repeated at 2 and 6 months corrected age. A neonatal neurologic examination, the Einstein Neonatal Neurobehavioral Assessment Scale, was also conducted. At 1 year corrected age, both groups were assessed in a blind fashion by a pediatric neurologist and a psychologist to determine neurodevelopmental outcome. Results indicated that somatosensory evoked response abnormalities in particular predict an abnormal neurologic status at 1 year of age. Abnormalities that persisted or worsened correlated with severe neurologic impairment, whereas an abnormal somatosensory evoked response that improved or normalized in infancy was associated with mild to moderate neurologic sequelae. Increased brainstem conduction in the auditory brainstem responses was also associated with neurologic sequelae. Normal findings from auditory brainstem responses and somatosensory evoked responses predicted normal developmental scores in all areas, as well as a normal neurologic outcome at 1 year with negative predictive powers ranging from 85-100%. Evoked response testing appears to be an important adjunct to the neurologic investigation of high-risk newborns.

Prognostic significance of the auditory brainstem evoked response in high-risk neonates

The prognostic significance of the auditory brainstem evoked response (ABR) was examined in this prospective study of neonates at risk for neurodevelopmental sequelae. ABR testing was performed in the neonatal period (37 to 45 weeks conceptional age) and at two and/or six months corrected age. 34 high-risk newborns and 14 controls were followed to one year of age, when they received neurological and developmental assessments. Increased I to III and I to V interwave latencies predicted gross motor delay at one year, with a positive predictive power of 83 per cent and a specificity of 94.7 per cent. Increased brainstem conduction, dispersal of waves III to V and V/I amplitude ratio abnormalities predicted abnormal neurological findings at one year, with positive predictive values of 100, 100 and 80 per cent, respectively. A standard clinical examination of the newborn, performed on the at-risk and control infants at 40 weeks conceptional age, was not found to be strongly predictive of neurodevelopmental deficits at one year.

The fetal auditory brain stem response: serial measurements at two stimulus intensities

Eleven fetal lambs were tested for auditory brain stem responses (ABRs) at 105 and 90 db sound pressure level from 110 to 116 days until 133 days gestation. ABRs were elicited in response to 1200 clicks, presented at 16 clicks/sec, from a hearing aid receiver secured in the fetal external ear canal and recorded from subdermal stainless steel electrodes at the vertex (active electrode) and anterior pinna (reference electrode). Six term newborn lambs were tested similarly for ABR comparison. No fetal ABRs appeared before 117 days gestation. Thereafter, the ABRs exhibited decreasing peak latencies with increasing stimulus intensity and fetal age. Newborn ABR latency measurements were compared with predicted newborn values generated from linear regression analysis of fetal data. Newborn latencies to waves I and II approximated predicted values. Newborn latencies to waves III, IV, and V were much shorter than predicted, suggesting rapid maturation of higher brain stem and midbrain neurogenerators during late fetal development.

Changes of auditory brainstem responses in electrolyte-induced myelinolysis in rats. Role of hyponatremia and hypernatremia

Electrolyte-induced myelinolysis was produced in rats to evaluate electrophysiological derangement in the brainstem. Auditory brainstem responses (ABRs) were used to assess the brainstem function. Rats treated with hypertonic saline for 2 days had normal interpeak latencies in ABRs. Rats treated with vasopressin-induced hyponatremia alone and those with hypertonic saline after 3 days of hyponatremia had significantly prolonged interpeak latencies between waves II-III and III-IV in ABRs. These prolongations indicate electrophysiological derangement of the upper pons and mesencephalon in hyponatremic rats as well as in rats with hyponatremia followed by hypernatremic myelinolysis. In view of these data, hyponatremia may be a prerequisite for electrolyte-induced myelinolysis and electrolyte derangements such as hyponatremia, and rapidly correcting hypernatremia may be the cause of electrolyte-induced myelinolysis in rats.

Quantitative and cytometric analysis of the ventral cochlear nucleus in man

A systematic quantitative and cytometric study of the ventral cochlear nucleus has been performed on 3 groups of histologically normal brains, ranging in age from 20 weeks of gestation to adult life. Serial sections, 20 micrometer in thickness, through the length of the nucleus were obtained from paraffin-embedded blocks of the ponto-medullary junction. Every 10th section was analyzed with a Cambridge Imanco Quantimet 720 Image Analyzing system. The surface area of the nucleus was determined on each slide. Cells showing neuronal characteristics were individually counted and classified into 18 pre-determined cell size classes. The raw total number of cells was then corrected for periodicity and split cell error. The total volume of the nucleus and cell packing density were estimated. The mean total number of neurons was 26,098 in premature, 31,802 in full term and 30,440 in adult cases. There was no significant difference among these values. The mean volume of the nucleus was 1.32 mm3 in premature, 3.84 in full term and 5.43 in adult cases. Positive correlation coefficients were found comparing volume versus age. A negative correlation coefficient was detected comparing cell packing density versus age from the full term period to adult life. A significant increase in the number of cells in the largest cell classes was also found with increasing age. The mean neuronal diameter did not change significantly between premature and full term cases but was significantly different between full term and adult cases. These findings are discussed and compared to already reported quantitative studies of the ventral cochlear nucleus in man, which involved manual counting techniques.