Neuropathology of Certain Forms of Mental Retardation: Experiments on monkeys illustrate probable mechanisms of brain damage in human infants

Detecting brain injury in neonatal hypoxic ischemic encephalopathy: closing the gap between experimental and clinical research

Moderate to severe neonatal hypoxic ischemic encephalopathy remains an important cause of infant death and childhood disability. Early and accurate diagnosis of encephalopathy is difficult but critical for timely intervention. Thus, we have utilized a clinically relevant large animal model of asphyxia in-utero, followed by immediate lamb delivery, resuscitation and clinical care over the next 72h for assessment of potential biomarkers of brain injury. In-utero asphyxia was induced in twelve near-term lambs and outcomes compared with seven controls. Asphyxia resulted in bradycardia (97±12beats/min), hypotension (12.1±1mm Hg) and metabolic acidosis (pH6.9±0.02; base-excess -13.8±0.8mmol/l). 72h following asphyxia, cerebrospinal concentrations of malondialdehyde and S100B were elevated 2-fold and 5-fold, respectively, in asphyxic lambs compared to control lambs. Magnetic resonance spectroscopy (MRS) at 72h showed a significant decrease in n-acetyl aspartate: choline ratio in asphyxia lambs compared to that observed at 12h (0.56±0.23 vs. 0.82±0.15, respectively); lactate:choline ratio was not changed over this time. Marked neuropathology was observed in asphyxia lambs with neuronal degeneration in the hippocampus, thalamus, striatum and cortex. Astrogliosis was observed in the hippocampus and thalamus. Early blood markers of metabolic state showed limited predictive value of histological damage at 72h. MRS outcomes at 72h showed a modest but significant correlation with histological evidence of neuronal brain injury (lactate:N-acetyl aspartate ratio in the thalamus r(2)=0.2, p<0.01). MRS at 72h was best able to detect established brain injury, but a combination of biomarkers over multiple phases of injury may be able to assess the evolution of neonatal brain injury.

Management of fetal distress

Since its introduction more than 20 years ago, continuous electronic FHR monitoring has become the standard in most modern obstetric units. Practitioners well versed in FHR pattern interpretation do not question the value of fetal monitoring. Not only does this modality detect hypoxia early in its evolution, but also it allows the opportunity to understand the physiology of the hypoxia and to intervene if necessary. Although nonrandomized studies demonstrate an improvement in the perinatal death rate with continuous monitoring, most randomized studies have failed to confirm this observation. Continuous fetal monitoring has been associated in several studies with an increase in the CS rate; however, concomitant changes in obstetric practice have also raised the incidence of CS, making the interpretation of to what degree fetal monitoring is responsible for this increase difficult. Other than this association with an increased CS rate, fetal monitoring seems to present few risks. A thorough understanding of basic fetal heart abnormalities is crucial to prevent unnecessary intervention; however, although quite sensitive, FHR monitoring remains nonspecific in predicting fetal metabolic acidosis. Fetal pulse oximetry is a recent development still undergoing investigation. The ability to measure fetal oxygen saturation during labor adds critical information about fetal status and refines the interpretation of abnormal FHR patterns. If approved by the US Food and Drug Administration, it has the potential to affect dramatically the practice of obstetrics.

Brain damage after intermittent partial cord occlusion in the chronically instrumented fetal lamb

The relationship between intermittent partial occlusion of the umbilical circulation and fetal acid base status, brain function, and neuropathologic outcome was assessed in nine control and nine experimental singleton fetal lambs to determine if transient episodes of partial cord occlusion play a role in antenatal brain damage in this species. Intermittent partial occlusion of the umbilical circulation for 1 minute of every 3 minutes for 2 hours was associated with a 89% incidence of histologically confirmed damage confined to the cerebral white matter. This occurred without systemic evidence of progressive acidosis, but both fetal heart rate patterns and electrocortical activity were altered. We conclude that in the late gestation fetal lamb, umbilical cord compromise plays a causal role in a specific type of antenatal central nervous system injury.

Brain energy metabolism in two kinds of total asphyxia: an in vivo phosphorus nuclear magnetic resonance spectroscopic study

Brain energy metabolism was studied in vivo by means of 31P-NMR spectroscopy in newborn mice during and after 20-minutes exposure to either pure carbon dioxide gas or nitrogen gas. In the N2 group, the brain ATP concentration remained almost normal throughout the experiment, while it showed a 30% reduction in the CO2 group. The brain concentration of phosphocreatine dropped to about 20% of the control value during the asphyxia in both groups, and its recovery was significantly delayed in the CO2 group compared to in the N2 group. Tissue acidosis and Pi accumulation were more remarkable and prolonged in the CO2 group.

Stereotypic behavioral response of rat fetuses to acute hypoxia is altered by maternal alcohol consumption

Transient compression of the umbilical cord has been implicated as a potential cause of unexplained brain damage and/or fetal death. We induced acute fetal hypoxia by compressing the umbilical cord of the fetal rat with a microvascular clamp. Direct observation of fetal behavior immediately after cord compression identified a three-phase behavioral response to acute hypoxia, consisting of an initial suppression in motor activity, a brief period of hyperactivity, and a secondary suppression of movement. All three phases occurred in less than 2 minutes from the onset of hypoxia. The stereotypic hypoxic response of rat fetuses may be adaptive as a means of removing mechanical sources of umbilical cord compression. Among fetuses exposed to alcohol in utero, the response to hypoxia was diminished in intensity and duration. Because of their altered hypoxic response, alcohol-exposed fetuses may therefore be at greater risk from transient cord compression in utero.

Pregnancy events and brain damage

A national standard of perinatal care has been established for the early detection and appropriate management of the fetus-neonate who is at risk for distress, brain damage, or death. While perinatal and infant mortality has significantly decreased during the past 10 years, a similar reduction in conditions of severe handicap has not been observed. Undetected antenatal umbilical cord compression is suggested as a possible cause of unexplained brain damage. Basic animal experimentation and clinical experience is reviewed to support the hypothesis.

Dendritic development of cortical neurons of mice subjected to total asphyxia: a Golgi-Cox study

The effect of total asphyxia on dendritic development of the pyramidal cells in the mouse cerebrum was investigated using the Golgi-Cox method. Although neonatal asphyxia did not alter the number of basal dendrites arising from the cell bodies, the dendrites in the 20-day-old treated animals were significantly shorter than those in the controls. Even at 60 days, the number of longer dendrites in the treated mice was significantly less than that in the controls.

Persisting effects on adult brain monoamines of neonatal distress and carbon monoxide exposure

Perinatal factors causing distress or trauma to the neonate are often linked to behavioral abnormalities in development and later life; however, little is known of the long-term effects of these experiences on neurotransmitter systems. This study used carbon monoxide exposure to produce a slow onset and recovery hypoxic episode of two hours in 5-day-old rat pups. This required placement of the pups in a hypothermic environment (air exposure) isolated from the dam so the effects of this distressing experience were also studied. When the brains of these animals were assayed at adulthood, air exposure was found to have increased dopamine and reduced 5-hydroxytryptamine levels in the frontal cortex. Norepinephrine levels were reduced in both midbrain and pons-medulla but the 5-hydroxytryptamine level was increased in midbrain. Hypoxia, induced with carbon monoxide, prevented changes in 5-hydroxytryptamine levels in cortex and brain stem and modified effects on norepinephrine. However, hypoxia resulted in significant decreases in striatal dopamine levels accompanied in females by increased norepinephrine and 5-hydroxytryptamine levels. The data reported here indicate that neonatal distress can have considerable impact on developing neurotransmitter systems. Furthermore, brain region and sex influence the outcome. Carbon monoxide-induced hypoxia, under the same conditions, produced a different pattern of change with particular predeliction for the striatum. Again there were regional and sex differences in the neurotransmitter effects. It is concluded that this hypoxic model may be valuable in the study of developmental disorders.

Neuropathology in the chronic fetal lamb preparation: structure-function correlates under different environmental conditions

In this study we examined the relationship between fetal metabolism, cardiovascular function, brain function, and eventual neuropathological outcome in the last third of gestation in the chronic fetal lamb preparation under a variety of environmental conditions. We concluded that the progressive hypoxia and metabolic acidosis secondary to acute placental insufficiency result in cerebral damage and functional deficit. Growth retardation and its attendant chronic hypoxemia, secondary to chronic placental insufficiency, do not have these results. However, intermittent interference with the umbilical circulation is associated with both functional and structural evidence of cerebral damage without systemic metabolic abnormality. The experimental, physiologic, and clinical implications of these observations are discussed.

The consistent acceleration of lifted objects: implications for kinesthetic illusions and the perception of weight

In examining films of lifting movements in a study of the size-weight illusion (Davis & Roberts, 1976) a consistency was noted in the values obtained for the maximum accelerations of the objects lifted. While at first surprising, this finding can be embedded significantly in theories relating to kinesthetic illusions and the perception of weight and to theories on the control of general physical movement. This study was designed to confirm its existence. Twenty-four subjects were filmed lifting four objects differing in size, shape, substance, color, and weight. The film was analyzed frame-by-frame and the data were subjected to a two-way analysis of variance. Subjects, while differing from one another, were consistent in the maximum accelerations they applied to the three heaviest of the four objects. The accelerations of the lightest object differed significantly from the accelerations of the other three, but it seems likely that this was due to the experimental task itself.

Clinical characteristics of pregnancies complicated by intrapartum fetal asphyxia

The clinical characteristics of 124 pregnancies complicated by intrapartum fetal asphyxia have been reviewed. The evidence of fetal asphyxia tends to appear earlier in patients with maternal medical and obstetric complications than in those with labor complications. Evidence of clinical fetal distress was present in 36 per cent and was not related to the severity of the asphyxia. Low Apgar scores occurred in 40 per cent of infants with moderate asphyxia and in 80 per cent of infants with severe asphyxia at delivery. In the newborn infants, clinical evidence of cerebral abnormality was observed in 3 per cent, and evidence of the respiratory distress syndrome was seen in 3 per cent of the study group.

Development behaviors: delayed appearance in monkeys asphyxiated at birth

Developmental behaviors were studied in monkeys subjected to asphyxia at birth. Visual depth perception, visual pla ing, and locomotion appeared significantly later than in nonasphyxiated monkeys. After these behaviors had been established in asphyxiates, however, there was little difference from those observed in normal monkeys. These results were compared with reports of permanent learning deficits that occur in monkeys asphyxiated at birth for similar periods of time. Such comparison suggests that the neural structures responsible for the developmental behaviors studied are not damaged by asphyxia to the same extent as those for acquisition. Delay in development may be an early indication of brain damage with subsequent mental retardation.

Cerebral hemorrhage in relation to birth asphyxia

The brains of monke guinea pigs asphyxiated at birth pletely resuscitated, and killed a ous times thereafter revealed no chial hemorrhages. However, postnatal distress and other factoring to a moribund state occurro brains revealed petechial hemor.