Glucose, like oxygen, is of essential and fundamental importance for brain metabolism. Indeed, because oxygen consumption is relatively low in the neonatal human brain and minimal in such areas as cerebral white matter (see Chapter 13 ), glucose supply to the brain may be even more important. The major source of brain glucose is the blood supply; thus it is readily understood that serious encephalopathy may ensue when the glucose content of blood becomes deficient.
In this chapter, the normal aspects of glucose metabolism in the brain are discussed, followed by a review of the biochemical derangements that occur with hypoglycemia. The neuropathology of hypoglycemia is described next and, on the background of the biochemical and neuropathological derangements, the clinical aspects are reviewed. To begin the discussion, an attempt to define hypoglycemia is presented with an explanation of why this attempt is so difficult.
Definition
Definition of a blood glucose level that should be considered too low is difficult, in part because the newborn does not have the neural capacity to demonstrate when the critical lower limit has been passed, consistently and by overt symptoms. Thus the critical limit of blood glucose level for the maintenance of neonatal neuronal integrity in various clinical circumstances is unknown. Relevant clinical circumstances include those with (1) reduced availability of glucose or alternative fuels (ketone bodies, lactate), because of deficient glycogen and fat stores or impaired hepatic gluconeogenesis or both (e.g., intrauterine growth restriction, prematurity); (2) increased systemic glucose utilization (e.g., infant of a diabetic mother, hyperinsulinism, cold stress, sepsis); (3) increased cerebral glucose utilization (e.g., hypoxic-ischemic states, seizures); or (4) decreased cerebral utilization (e.g., hypotension). Moreover, and perhaps most important, the lower limits of blood glucose are likely higher when there are also concomitant insults that increase cerebral demand for glucose and that are deleterious to the brain (e.g., hypoxemia, ischemia, repetitive seizures). Indeed, the concept of an additive and potentiating role of hypoglycemia in the production of brain injury in the sick newborn infant is a critical neurological aspect of neonatal hypoglycemia.
Definition of a blood glucose level below which hypoglycemia should be designated is complex and cannot be based on a single number that can be uniformly applied to all infants. Attempts at such definitions have been based on statistical thresholds derived from the study of serial changes of blood glucose in normal infants; on operational thresholds based on blood glucose levels and the presence or absence of symptoms or risk factors; on neurophysiological thresholds based on changes in brain stem auditory evoked responses (BSAERs), cerebral blood flow (CBF), or cerebral glucose metabolism; and on neurological outcome thresholds based on neurodevelopmental outcome as a function of different blood glucose levels (see next). The results of seven large-scale studies concerning the incidence of hypoglycemia in various neonatal populations are shown in Table 25.1 and noted in relevant sections that follow.
| REFERENCE | GLUCOSE CONCENTRATION (mg/dL) | INCIDENCE (%) | HOURS FOLLOWING DELIVERY |
|---|---|---|---|
| Srinivasan and colleagues a | <40 (plasma) | 13 | 3 |
| Heck and Erenberg b | <30 (plasma) | 8 | 48 |
| Hawdon a | <47 (blood) | 12 | 96 |
| Hoseth et al a | <47 (blood) | 14 | 96 |
| <40 (blood) | 4 | ||
| <30 (blood) | 0.4 | ||
| Lucas et al c | <45 (blood) | 66 | 120 |
| <30 (blood) | 28 | ||
| <10 (blood) | 10 | ||
| Harris et al d | <47 (blood) | 51 | ≤48 |
| <40 (blood) | 19 | ||
| Kaiser et al e | <45 (plasma) | 19.3 | 3 |
| <30 (plasma) | 10.3 | ||
| <10 (plasma) | 6.4 |
a Healthy, appropriate for gestational age babies.
b Healthy, small or large for gestational age, term babies.
c Infants <1850 g birthweight (mean, 1337 g).
d >35 Weeks, high-risk infants (small or large for gestational age, infant of diabetic mothers, late preterm).
Postnatal Changes
During the first 2 hours of postnatal life, plasma glucose levels decline to a nadir followed by a rise, reaching a steady-state glucose concentration by 3 to 4 hours after birth. Glucose levels then increase to higher and relatively more stable concentrations, generally above 60 mg/dL by 12 hours after birth ( Fig. 25.1 ). This adaptation is associated in part with the hepatic release of glucose at the rate of 4 to 6 mg/min per kg (see later). These neonatal glucose levels are less than those in the adult and rise slowly to achieve the latter levels by the third or fourth day.
Statistical Thresholds
Historically, the designation of hypoglycemia has usually been based on statistical measures (i.e., marked deviation from normal blood glucose levels). Previous determinations of such blood glucose levels in the newborn were derived from infants generally not fed in the first hours of life. Such determinations led to the definition of significant hypoglycemia in the newborn as a whole blood glucose concentration lower than 30 mg/dL in the term infant and lower than 20 mg/dL in the preterm infant. Subsequent reports in healthy infants, initially breast-fed within the first 3 hours, show that blood glucose concentrations as low as 30 mg/dL are observed in some infants within 1 to 2 hours after birth and are usually transient, asymptomatic, and considered to be part of normal adaptation to postnatal life.
Operational Thresholds
Cornblath and co-workers suggested that the term hypoglycemia is not readily defined for individual patients and that operational thresholds (i.e., “a concentration of blood or plasma glucose at which clinicians should consider intervention”) should be established. An operational threshold by itself is an indication for action and is not diagnostic of disease or predictive of adverse neurological sequelae. These thresholds were defined as less than 45 mg/dL (2.5 mmol/L) for “the infant with abnormal clinical signs” and less than 36 mg/dL (2.0 mmol/L) for the asymptomatic infant and the infant “at risk for hypoglycemia” (without regard to other influencing factors mentioned previously). Using this threshold, Harris and colleagues recently studied infants above 35 weeks of gestational age (GA) and noted hypoglycemia (blood sugar < 47 mg/dL [2.6 mmol/dL]) in 51% of high-risk infants (small or large for GA infants, infants of diabetic mothers, or late preterm infants) and in 19% of infants with a blood sugar below 36 mg/dL (2 mmol/L) (see Table 25.1 ). Clearly the incidence of hypoglycemia will vary substantially depending on the applied operational threshold (see later; also see Table 25.1 ). The operational threshold still focuses, however, on individual glucose concentrations and does not address whether the threshold level of blood glucose represents the threshold level for neuronal injury. Hawdon makes the plausible argument that hypoglycemia should perhaps be defined as a persistently low blood glucose level in a baby at risk for impaired metabolic adaptation but with no abnormal clinical signs or a single low blood glucose level in a baby presenting with abnormal clinical signs.
Neurophysiological Threshold
The complexity of defining hypoglycemia is further illustrated by the lack of consistency in a particular threshold value and outcome (see next). Thus determinations of BSAERs in a small series of term infants showed prolonged latencies at levels lower than approximately 47 mg/dL ( Fig. 25.2 ) (see later discussion). A later report of a single infant did not detect prolongation of latency to wave V until blood glucose fell to 25 mg/dL.
Other relevant physiological measures include CBF and cerebral glucose metabolism. Thus two studies of human infants showed that CBF increases at glucose values lower than 30 mg/dL and that transport becomes limiting for cerebral glucose utilization at a glucose level lower than approximately 54 mg/dL. In addition, the level of blood glucose required for brain homeostasis is different in the infant with impaired CBF , as with hypotension, or with increased cerebral glucose utilization , as with seizures, or with the anaerobic glycolytic metabolism of hypoxia-ischemia or asphyxia (see later discussion). Moreover, the ability to compensate for low cerebral fuel availability during hypoglycemia includes the capacity to use nonglucose cerebral fuels such as lactate and ketones. However, regarding the latter, recent evidence suggests that neonatal hypoglycemia within the first 48 hours evolves in the context of a hypoketotic state, with varying levels of lactate and detectable insulin levels in many infants.
Because the rate of ketone utilization by the brain is directly proportional to plasma concentrations, the contribution of ketones to neonatal brain metabolism is less than one tenth that of older children with similar degrees of hypoglycemia.
Neurological Outcome Thresholds
A careful epidemiological study suggested a deleterious effect on subsequent cognitive development in infants whose plasma glucose levels were less than approximately 47 mg/dL on at least one occasion on 3 or more separate days (see Table 25.1 and Fig. 25.3 ). Abnormalities in arithmetic and motor scores persisted at 7.5 to 8 years. Conversely, in a subsequent study that attempted to duplicate these findings, 47 of 566 infants of GA below 32 weeks with a blood glucose level below 47 mg/dL on at least 3 days were matched with hypoglycemia-free infants and followed up through 15 years. The investigators found no difference in physical disability or developmental progress at 2 years or in psychometric assessment at 15 years. In a recent retrospective review of 1943 infants (23 to 42 weeks GA), early transient newborn hypoglycemia was noted in 19.3% using a value of 45 mg/dL (<2.6 mmol/L), 10.3% using a value of 40 mg/dL (<2.3 mmol/L), and in 6.4% using a value of <35 mg/dL (<2.0 mmol/L) (see Table 25.1 ). In this study, in assessing for multiple confounding variables, transient hypoglycemia was associated with a decreased probability of proficiency on literacy and mathematics fourth-grade achievement tests at 8-year testing (see later). As will be discussed, the preponderance of evidence suggests that duration and degree of depression of blood glucose are both important.
In summary, defining hypoglycemia is highly complex and must be individualized according to the infant’s clinical situation. The definition requires consideration of factors that influence the vulnerability of specific cells and regions in the brain, the status of brain energy reserves, hepatic glycogen reserves, and gluconeogenic capacity, such as the GA of the infant, status of intrauterine nutrition, prior or concomitant hypoxic-ischemic insults, and seizures, among others. These and related issues are discussed subsequently.
Normal Metabolic Aspects
Brain as the Primary Determinant of Glucose Production
The pathophysiology of neonatal hypoglycemic encephalopathy has as its basis the importance of glucose as the primary metabolic fuel for the brain. Glucose for normal brain metabolism is derived from the blood, and glucose production in mammals is primarily a function of the liver. The postnatal induction of hepatic glycogenolysis and gluconeogenesis and the interplay of insulin, glucagon, catecholamines, corticosteroids, and other hormones in the regulation of hepatic glucose metabolism have been reviewed in detail by others. It need only be emphasized here that the brain appears to be the major determinant of (hepatic) glucose production. Thus glucose production was measured in a series of infants and children from 1 to 25 kg in body weight by a continuous 3- to 4-hour infusion of the nonradioactive tracer, 6,6-dideuteroglucose. Glucose production on a body-weight basis was found to be twofold to threefold greater in newborns than in older patients. The infants clearly had disproportionately higher rates of glucose production as compared with adult subjects. This observation becomes understandable when glucose production is plotted as a function of estimated brain weight ( Fig. 25.4 ). The linear relationship suggests that the disproportionately high rates of glucose production in the neonatal period relate to the disproportionately large neonatal brain. Because central nervous system consumption of glucose accounts for 30% or more of total hepatic glucose output, at least in the premature infant, this relationship between glucose production rate and brain weight seems reasonable.
The mechanisms by which utilization of glucose by the brain may regulate hepatic glucose output are unknown. It is possible that the effect is mediated by subtle changes in blood glucose levels acting directly on pancreatic insulin secretion or on hepatic glucose output. More provocative is the possibility that the brain mediates control over hepatic glucose production by neural or hormonal effectors originating within the central nervous system. This possibility leads to the interesting logical extension that disturbances of the brain may lead to disturbances in glucose output by the liver and result in hypoglycemia or hyperglycemia (see later discussion). Moreover, the size of the brain per se may also possibly lead to disturbances in glucose output secondary to changes in glucose utilization. At any rate, in the normal human, from the newborn period to adulthood, it is now clear that a very close relationship exists between brain mass and glucose production.
Glucose Metabolism in the Brain
Glucose metabolism in the brain is depicted in a simplified fashion in Fig. 25.5 . Those aspects particularly relevant to this chapter are shown; a further review of cerebral glucose and energy metabolism is contained in Chapter 13 .
Glucose Uptake
Glucose uptake from the blood into the brain occurs by a process that is not energy-dependent but that proceeds faster than expected by simple diffusion (i.e., carrier-mediated facilitated diffusion). The transport is mediated by a specific protein, a glucose transporter.
The brain glucose transporter is concentrated in the capillaries and the concentration of the transporter increases with development. In the rat, the lower apparent blood-brain glucose permeability in the newborn (≈25% of adult values) is related to a lower concentration of the glucose transporter (not to a lower affinity of the transporter for glucose). Studies of human premature infants also suggest that the number of available endothelial transporters is approximately one third to one half the value for the adult human brain. The importance of the transporter for brain function and structure is illustrated by the occurrence of seizures and developmental delay in infants with partial deficiency of the transporter (see Chapters 12 and 29 ). The glucose concentration normally present in blood in the newborn rat is approximately one fourth that required for glucose uptake to proceed at maximal velocity. Studies of the human premature infant by positron emission tomography (PET) indicate that at a plasma glucose level of approximately 3 µmol/mL (i.e., ≈54 mg/dL), transport becomes limiting for cerebral glucose utilization . Thus uptake is one potential site for regulation of glucose metabolism in the brain, and this regulation is particularly dependent on changes in blood glucose concentrations .
Hexokinase
The initial step in glucose utilization in the brain is phosphorylation to glucose-6-phosphate by hexokinase (see Fig. 25.5 ). This enzyme is inhibited not only by its product but also by adenosine triphosphate (ATP). Under certain circumstances, hexokinase is an important control point in glycolysis.
Major Fates of Glucose-6-Phosphate
The product of the hexokinase reaction, glucose-6-phosphate, is at an important branch point in glucose metabolism (see Fig. 25.5 ). From glucose-6-phosphate originate pathways to the formation of glycogen, to the pentose monophosphate shunt, and through glycolysis to pyruvate. Glycogen is important as a readily available store of glucose in the brain; glycogenolysis is an actively regulated process that is called into play during periods of glucose lack (i.e., hypoglycemia) or accelerated glucose utilization (e.g., oxygen deprivation [with associated anaerobic glycolysis] or seizures). Glycogen is concentrated in astrocytes, and with low brain glucose, astrocytic glycogenolysis is activated to produce glucose-6-phosphate. The latter is converted to lactate, which then enters the neuron for use as an energy source (see later). The pentose monophosphate shunt provides reducing equivalents, important for lipid synthesis, and ribose units, important for nucleic acid synthesis. These two synthetic processes are of particular importance in the developing brain. The generation of reducing equivalents is also critical for the generation of reduced glutathione, which is crucial for defense against free radicals and thereby hypoglycemic cellular injury (see later discussion).
The major fate of glucose-6-phosphate in the brain is entrance into the glycolytic pathway , principally for the ultimate production of chemical energy in the form of high-energy phosphate bonds (i.e., ATP and its storage form, phosphocreatine). When oxidized aerobically, each molecule of glucose generates 38 molecules of high-energy phosphate compounds. The next several sections describe the utilization of glucose for energy production.
Phosphofructokinase
The most critical step in the glycolytic pathway is the conversion of fructose-6-phosphate to fructose-1,6-diphosphate; the enzyme involved, phosphofructokinase, is a major regulatory, rate-limiting step in glycolysis (see Fig. 25.5 ). The enzyme is inhibited by ATP and is activated by adenosine diphosphate (ADP). The ammonium ion (NH 4 + ), generated by amino acid transamination, is also a potent activator of this complex.
Pyruvate
The glycolytic pathway ultimately results in the formation of pyruvate, most of which enters the mitochondrion and is converted to acetyl-coenzyme A (acetyl-CoA) (see Fig. 25.5 ). However, pyruvate can also result in the formation of lactate when the cytosolic redox state is shifted toward reduction. Conversely, under the conditions of hypoglycemia (i.e., [1] available lactate and deficient pyruvate, [2] a cytosolic redox state that is normal or shifted toward oxidation, and [3] the action of lactate dehydrogenase), lactate can lead to formation of pyruvate and can become an energy source (see later discussion). Finally, alanine may be converted to pyruvate by transamination and can therefore become a source of glucose or acetyl-CoA.
Acetyl-Coenzyme A
The formation of acetyl-CoA by pyruvate dehydrogenase is the major starting point for the citric acid cycle (see Fig. 25.5 ). This step is an important rate-limiting process in glucose utilization in the neonatal brain. Acetyl-CoA is also the major starting point for the synthesis of brain lipids and acetylcholine. Moreover, ketone bodies are converted to acetyl-CoA to become an energy source.
Citric Acid Cycle
The citric acid cycle (with the linked electron transport system) ultimately results in the complete oxidation of the carbon of glucose to carbon dioxide and the generation of nearly all the ATP derived from this sugar ( Fig. 25.6 ). Transamination reactions interface this segment of glucose utilization with certain amino acids, which thereby can be used for energy production.
Glucose as the Primary Metabolic Fuel for Brain
The role of glucose as the primary fuel for the production of chemical energy and the maintenance of normal function in the mature brain are supported by three main facts. First, the respiratory quotient (i.e., carbon dioxide output/oxygen uptake) of the brain is approximately 1, a finding indicating that carbohydrate is the major substrate oxidized by neural tissue. Glucose is the only carbohydrate extracted by the brain in any significant quantity. Second, cerebral glucose uptake is almost completely accounted for by cerebral oxygen uptake. Third, central nervous system function is rapidly and seriously disturbed by hypoglycemia.
Current data support a similar preeminence for glucose in the immature brain . Thus studies in the newborn dog indicate that glucose consumption in the brain accounts for 95% of cerebral energy supply. Moreover, studies in term fetal sheep demonstrated that, under aerobic conditions, glucose is the main substrate metabolized for energy production. Glucose/oxygen quotients of approximately 1.1 were obtained in two different laboratories. The glucose/oxygen quotient is equivalent to the arteriovenous difference of glucose (×6) divided by the arteriovenous difference of oxygen and represents the fraction of cerebral oxygen consumption required for the aerobic metabolism of cerebral glucose. Although the data demonstrate that glucose is the primary substrate metabolized by the brain, the finding that the values for glucose/oxygen quotients are slightly but consistently in excess of 1 suggests that a portion of the glucose is used for purposes other than complete oxidation to generate high-energy phosphate bonds. Other data, based on the fate of labeled glucose in the brain, indicate that glucose is also used for the synthesis of other materials (e.g., amino acids via transaminations and lipids via appropriate biosynthetic pathways; see Fig. 25.5 ). Syntheses of membrane lipids and proteins, of course, are critical events in the developing brain and probably account for a relatively larger proportion of cerebral glucose utilization than in the mature brain.
Important regional and developmental changes in cerebral glucose utilization have been defined primarily in animals but also in human infants. a
a References .
Thus, early in development, regional differences are relatively few, and brain stem structures generally exhibit the highest rates of glucose utilization. With development, increases in cerebral glucose utilization are most prominent, particularly in cerebral cortical regions. In the human infant, the developmental progression in the first year of life occurs first in the sensorimotor cortex and thalamus, next in the parietal, temporal, and occipital cortices, and last in the frontal cortex and association areas. Careful studies in animals, focused primarily on electrophysiological maturation of the brain stem and diencephalic structures, showed a close correlation between increases in rates of glucose utilization and the acquisition of neuronal function.Additional compelling evidence for the obligatory role of glucose in the developing brain emanates from studies of human newborns by PET. Thus values for cerebral metabolic rate for oxygen in the brain of premature and term infants are only 3% and 28%, respectively, of the adult values. One reasonable conclusion from these data is that glucose is critical for energy production in the brain, especially in the premature infant. The data raise the possibility that anaerobic glucose utilization is important in the neonatal brain, and because energy production is markedly less with anaerobic versus aerobic metabolism (see Chapter 13 ), glucose delivery to the brain is critical for energy production in the neonatal brain, especially in the premature infant.
Alternative Substrates for Glucose in Brain Metabolism
Overview
Although glucose is the primary metabolic fuel for the brain, it is apparent that certain other substrates can also be used for energy production and other metabolic purposes. Under normal circumstances, such alternative substrates are probably not of major importance for energy production. However, under conditions in which glucose is limited (e.g., hypoglycemia), alternative substrates may spare brain function and structure. Substances such as lactate, pyruvate, free fatty acids, glycerol, a variety of ketoacids (i.e., ketone bodies), and certain amino acids have been shown to be capable of partially or wholly supporting respiration of brain tissue slices and related in vitro systems. a
a References .
Certain of these substrates are produced in the brain during hypoglycemia (e.g., amino acids from the degradation of protein and fatty acids from the degradation of phospholipid) and are potentially utilizable as alternative energy sources (see later discussion). Clearly, however, these latter alternative substrates are not optimal because their sources (i.e., proteins and phospholipids) are largely structural components, and conservation of energy production at the cost of brain structure is not a desirable adaptive response. Moreover, because most of the systemically produced alternative substrates noted either do not appear in appreciable quantities in blood or are not capable of crossing the blood-brain barrier to a major extent, they can contribute relatively little to brain energy levels in hypoglycemia. The two substrates most often considered to be useful as primarily blood-borne, alternative sources of brain energy with hypoglycemia are ketone bodies and lactate; considerable data show evidence of their value for the support of oxidative metabolism in the neonatal brain.Ketone Bodies
Appreciable data have accumulated to suggest that ketone bodies may be used as alternative substrates for brain metabolism in the neonatal period. Ketone bodies are taken up by the brain by a carrier-mediated transport system and are subsequently used according to the reactions outlined in Fig. 25.6 .
Energy Production.
Studies of newborn infants have demonstrated that the cerebral extraction of ketone bodies from blood is markedly greater in the newborn than it is in older infants and adults. Associated with this finding is an enhanced rate of ketone body utilization in the newborn brain. Thus it was shown that ketone bodies account for approximately 12% of total cerebral oxygen consumption in newborns subjected to 6-hour fasts. An enhanced capacity to use ketone bodies was also demonstrated in the human fetal brain . These data indicate relevance for animal studies that demonstrate relatively high activities for the enzymes involved in ketone body utilization in the immature versus the mature brain. These enzymatic activities have also been demonstrated in the human fetal brain.
Thus the newborn brain, at least under conditions of brief fasting, normally satisfies a small portion of its energy demands by the conversion of ketone bodies to acetyl-CoA, which then proceeds through the citric acid cycle (see Fig. 25.5 ). Whether ketone bodies satisfy a greater portion of cerebral energy demands when glucose is deficient is a separate issue and not so readily demonstrated (see later). As noted previously, recent evidence suggests that neonatal hypoglycemia within the first 48 hours appears to be a hypoketotic state, with varying levels of lactate and detectable insulin levels in many infants. This suggests that during this early time frame lactate may be more important than ketones as alternative energy source to glucose. Moreover, experimental data in the newborn dog do not support an important role for ketone bodies in this context (see later discussion).
Limitations of Hepatic Ketone Synthesis.
Utilization of ketone bodies as alternative substrates for glucose in brain energy production under conditions of glucose deprivation depends on the capacity of the liver to deliver these compounds to the blood. Data obtained in human newborn infants suggest that hepatic ketone synthesis is restricted during the early neonatal period. The findings demonstrate (1) low levels of ketone bodies, (2) failure of ketone bodies to rise with fasting (in contrast to fasting in older children), and (3) failure of ketone bodies to rise with hypoglycemia ( Table 25.2 ). In a subsequent study, relatively low plasma concentrations of ketone bodies were also documented with formula feeding. Because cerebral utilization of ketone bodies linearly depends on plasma concentrations, these data from studies of human infants suggest that limitations of hepatic ketone synthesis prevent a major role for these materials as alternative metabolic substrates in the brains of human infants with hypoglycemia. However, these data do not rule out the possibility that exogenous administration of ketone bodies or of exogenous sources of ketone bodies (e . g., fatty acids) could serve as alternative metabolic substrates . One report demonstrated cerebral uptake of exogenously administered beta-hydroxybutyrate for the management of hypoglycemic infants in the first year of life.
| KETONE BODIES (mmol/L) | ||
|---|---|---|
| INFANTS | BETA-HYDROXYBUTYRATE | ACETOACETATE |
| Normoglycemic, term, AGA | 0.31 ± 0.04 | 0.06 ± 0.01 |
| Hypoglycemic, term, AGA | 0.16 ± 0.03 | 0.02 ± 0.01 |
| Hypoglycemic, SGA | 0.24 ± 0.07 | 0.03 ± 0.01 |
Lactate
Lactate as an important energy source in neonatal hypoglycemia was suggested by elegant experiments in the newborn dog. a
a References .
Thus determinations of cerebral metabolic rates for oxygen, glucose, lactate, and beta-hydroxybutyrate were accomplished by measurements of CBF and cerebral arteriovenous differences of these compounds. These data were then used to determine the relative proportions of cerebral energy requirements derived from glucose, lactate, and beta-hydroxybutyrate under conditions of normoglycemia and insulin-induced hypoglycemia ( Table 25.3 ). During normoglycemia, the newborn dog obtained 95% of its cerebral energy requirements from glucose and only a small fraction from lactate (4%) and beta-hydroxybutyrate (<1%). With hypoglycemia, in concert with the expected decline in cerebral utilization of glucose, a striking increase in lactate use was observed (see Table 25.3 ). (No appreciable change in the contribution of ketone body utilization was noted.) In subsequent experiments, no significant decrease in brain high-energy phosphate levels occurred under these conditions. Thus the data indicate that increased utilization of lactate spared brain energy levels under conditions of severe hypoglycemia .| SOURCE OF CEREBRAL ENERGY REQUIREMENTS | |||
|---|---|---|---|
| BLOOD GLUCOSE a | GLUCOSE | LACTATE | BETA-HYDROXYBUTYRATE |
| Normoglycemia | 95% | 4% | <1% |
| Hypoglycemia (13 mg/dL) | 48% | 52% | <1% |
| Hypoglycemia (5 mg/dL) | 42% | 56% | 2% |
The mechanisms by which blood lactate leads to energy production in the brain probably include enhanced lactate uptake by the brain from blood and active oxidation to pyruvate by lactate dehydrogenase (see Fig. 25.5 ). Indeed, available data indicate that lactate uptake in newborn dogs occurs at a rate that exceeds that of adult dogs, even when arterial lactate concentrations are within or near the physiological range. b
b References .
Concerning conversion of lactate to pyruvate, the activity of lactate dehydrogenase in the brain of the perinatal animal has been shown to be relatively high. Moreover, other data suggest that the neonatal brain may have a particular ability to use lactate as a brain energy source as an adaptation to the relative lactic acidemia in the first hours and days after birth. Lactic acidemia related to the hypoxic stress of normal vaginal delivery has been documented in newborn rats and lambs. These data also bear on the relative resistance of the neonatal versus the adult brain to hypoglycemic injury (see later). The sparing role of lactate in neonatal hypoglycemia requires further elucidation, but the data from studies of the newborn dog suggest that this role is considerable.Biochemical Aspects of Hypoglycemia
The pathophysiological aspects of the encephalopathy caused by hypoglycemia are best considered in terms of the initial biochemical effects on brain metabolism, the later effects, and the combined effects of hypoglycemia with hypoxemia, ischemia, or seizures. These combined effects may be of major clinical relevance because hypoglycemia rarely occurs as an isolated neonatal event and also because hypoglycemia not severe enough to cause brain injury alone may attain that capacity when combined with certain other deleterious insults to brain metabolism.
Major Initial Biochemical Effects of Hypoglycemia on Brain Metabolism
Major Biochemical Changes
At the outset, it is crucial to recognize that no biochemical effects of hypoglycemia occur as long as the initial physiological response of increased CBF supplies sufficient glucose to the brain. This initial hyperemic response, first described in adult models of hypoglycemia, was documented in neonatal animal models and human infants. The marked increase in CBF begins in the human infant when blood glucose declines to less than approximately 30 mg/dL ( Fig. 25.7 ). Studies of changes in cerebral blood volume, measured continuously in preterm infants by near-infrared spectroscopy (see Chapter 11 ), suggested that previously unperfused capillaries are recruited to maintain glucose levels in the brain with hypoglycemia. However, clearly with marked decreases in cerebral glucose delivery (e.g., because of marked decreases in blood glucose or impaired CBF, or both) or with marked increases in cerebral glucose utilization (e.g., because of seizure), biochemical derangements begin.
The major initial biochemical effects of hypoglycemia on brain metabolism are summarized in Table 25.4 . a
a References .
The principal consequences involve cerebral glucose metabolism and the metabolic attempts to preserve cerebral energy status by use of alternatives to glucose. Sharp falls in brain glucose concentrations are expected. Glycogenolysis responds in an attempt to restore some of the brain’s supply of glucose. Nevertheless, the result is a sharp decrease in the cerebral metabolic rate for glucose. However, an important feature of hypoglycemia, noted in 1948 in humans and subsequently studied in more detail in experimental animals, is a disproportionately smaller disturbance of the cerebral metabolic rate for oxygen. Indeed, in neonatal animals, cerebral metabolic rates of oxygen tend to be unchanged. This discrepancy between the cerebral utilization of glucose and that of oxygen implies that the brain’s energy needs are being met by alternative substrates to glucose (see next paragraph). Indeed, except in very severe hypoglycemia, significant declines in high-energy phosphate levels in the brain are not consistent initial features.
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The preservation of oxidative metabolism and high-energy phosphate levels in the brain despite the decrease in cerebral glucose metabolic rate presumably relates to the use of alternative substrates (see Table 25.4 ). The major substrates considered are lactate, ketone bodies, and amino acids. As noted previously, lactate is the most likely candidate as the major alternative substrate for maintenance of the brain’s oxidative metabolism during hypoglycemia, and the increase in its rate of utilization in the newborn dog with hypoglycemia is sufficient to account for preservation of the cerebral metabolic rate for oxygen and of tissue levels of high-energy phosphate compounds (see Table 25.3 ). The possibility of increased ketone body utilization as an additional alternative energy source is suggested by data in adult and young (not newborn) animals. However, as described earlier, direct measurements of ketone body utilization in the hypoglycemic newborn dog did not suggest that ketone bodies are important alternative substrates, at least in that insulin-induced model. Other alternative substrates for glucose or energy production or both are amino acids. Indeed, a sharp decrease in brain concentrations of most, although not all, amino acids occurs, with a consequent increase in brain ammonia levels (through transamination and deamination reactions; see Table 25.4 and subsequent discussion).
Dissociation of Impaired Brain Function and Energy Metabolism
Changes in brain energy metabolism do not clearly explain the striking changes in clinical signs and the electroencephalographic (EEG) activity of the brain during the initial phases of hypoglycemia. Thus, although findings differ qualitatively if newborn or adult animals are studied, in general the evolution with hypoglycemia of clinical changes from an alert state to a depressed level of consciousness (and even to seizures) and of EEG changes from normal activity to slowing (and even to burst-suppression patterns and seizure discharges) occurs with no definite change in ATP levels in the whole brain or the cerebral cortex and several other brain regions. a
a References .
In many respects, this dissociative phenomenon is similar to that observed in the initial phases of hypoxic–ischemic insults (see Chapter 13 ). It is unclear whether this occurrence is an adaptive phenomenon (i.e., when faced with an imminent power failure, the brain curtails neuronal activity), perhaps to conserve energy stores.The mechanism by which the dissociation of functional and electrical activity of the brain from brain energy levels occurs may relate to certain of the metabolic concomitants of hypoglycemia (see Table 25.4 ). Such concomitants could include alterations in relative amounts of excitatory and inhibitory amino acids, in tissue levels of ammonia, or in neurotransmitter metabolism. Indeed, the degradation of amino acids described earlier results in an increase in levels of brain ammonia that are considered potentially sufficient to produce stupor in adult hypoglycemic animals. Whether such levels are generated in newborn animals is unknown. Data in mature rats rendered hypoglycemic demonstrated that impaired synthesis of acetylcholine (see Table 25.4 ) occurs in the first minutes after onset of hypoglycemia. Indeed, only modest decreases in plasma glucose levels caused 20% to 45% decreases in concentrations of acetylcholine and 40% to 60% decreases in the synthesis of this neurotransmitter in the cortex and striatum. The likely mechanism of this disturbance relates to a decrease in synthesis of acetyl-CoA because of the drastic decrease in brain glucose and, as a consequence, glycolysis. Even with modest hypoglycemia, pyruvate concentrations in the brain decrease by 50% in minutes. Data concerning acetylcholine synthesis in hypoglycemic newborn animals are lacking, but similar sharp decreases in brain glucose and pyruvate levels suggest that the same impairment of acetylcholine synthesis is likely.
Major Later Biochemical Effects of Hypoglycemia on Brain Metabolism
Glucose and Energy Metabolism
The major later biochemical effects of hypoglycemia are summarized in Table 25.5 . Because most of the available data have been derived from studies of mature animals and the limited data in newborn animals suggest some differences (although also many similarities) compared with mature animals, in the following discussion I review the major effects on the mature and immature nervous systems separately.
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Biochemical Changes in the Mature Animal
Glucose and Energy Metabolism.
The biochemical effects of severe or prolonged hypoglycemia include an accentuation of the changes described in the previous section as well as the addition of other effects (see Table 25.5 ). Thus the decreases in brain glucose level become very marked, the cerebral metabolic rate of glucose falls drastically, and a distinct decrease in cerebral oxidative metabolism and in the synthesis of high-energy phosphate compounds becomes apparent. a
a References .
Metabolic Responses to Preserve Brain Energy Levels.
To preserve brain energy levels, the use of endogenous amino acids—derived from protein degradation, glycolytic intermediates, and lactate—continues as described earlier for initial biochemical effects (glycogen is essentially exhausted by this time). Indeed, as a consequence of the use of amino acids, ammonia levels in the brain increase markedly (i.e., 10- to 15-fold). An additional metabolic response (i.e., phospholipid degradation with the generation of free fatty acids) becomes apparent. The free fatty acids become an energy source in severe hypoglycemia. However, the responses are insufficient in severe and prolonged hypoglycemia to prevent the onset of declines in levels of high-energy phosphate compounds in the brain and the occurrences of coma and an isoelectric EEG pattern. At this point, an additional series of events develops.
Intracellular Calcium and Cell Injury With Hypoglycemia.
At approximately the time of onset of EEG isoelectricity, striking changes in intracellular calcium (Ca 2+ ) and extracellular potassium (K + ) occur and appear to initiate a series of events that result in cell death. Thus at this time the capacity of the neuron to maintain normal energy-dependent ionic gradients is lost, extracellular Ca 2+ levels decrease abruptly by approximately 6-fold, and extracellular K + levels increase by approximately 14-fold. Movements of Ca 2+ into the cell and of K + out of the cell account for these observations. The initiating event is probably failure of the energy-dependent sodium-K + (Na + /K + ) pump, which extrudes Na + and retains K + . With failure of this system, sodium accumulates intracellularly, K + is extruded, and sustained membrane depolarization occurs; the intracellular increase of Na + then leads to activation of the Na + /Ca 2+ exchange system and movement of Ca 2+ intracellularly in exchange for Na + . Additional crucial effects of this membrane depolarization are excessive release of excitatory amino acids from synaptic nerve endings and reduced reuptake secondary to failure of glutamate transport; the resulting extracellular accumulation of these excitatory neurotransmitters and consequent activation of glutamate receptors result in a variety of deleterious effects, including influx of Ca 2+ (see later). Ca 2+ also may accumulate intracellularly because of failure of energy-dependent Ca 2+ transport mechanisms designed to maintain low cytosolic Ca 2+ levels (see Chapter 13 ). The metabolic consequences of these ionic changes appear to be similar to those described in Chapter 13 concerning the mechanisms of cell death with oxygen deprivation. The importance of cytosolic Ca 2+ -induced phospholipase activation (see Chapter 13 ) is emphasized by the observation of an abrupt decline in phospholipid concentration in the brain and a further elevation in free fatty acid concentration. The deleterious effects of arachidonate (e.g., generation of free radicals and harmful vasoactive compounds) are summarized in Chapter 13 . Thus the final common pathway to neuronal injury in hypoglycemia may be very similar to that in oxygen deprivation and relates especially to the accumulation of cytosolic Ca 2+ . Moreover, in hypoglycemia as in hypoxia-ischemia, massive depletion of high-energy phosphate compounds does not appear to be an obligatory event in producing cell death.
Role of Excitotoxic Amino Acids in Hypoglycemic Neuronal Death.
As discussed in Chapter 13 , considerable data indicate that the mechanism of cell death with hypoxia-ischemia is mediated by the extracellular accumulation of excitatory amino acids, which are toxic in high concentrations. It now appears likely that excitatory amino acids play a major role in the mediation of neuronal death with hypoglycemia. Evidence in support of this conclusion includes the demonstration of a rise in extracellular concentrations of excitatory amino acids (aspartate and glutamate) in advanced hypoglycemia and an attenuation of neuronal injury by simultaneous administration of antagonists of the N -methyl- d -aspartate (NMDA) type of glutamate receptor, both in in vivo models in cultured neurons. These observations suggest that the Ca 2+ accumulation and Ca 2+ -mediated deleterious events, noted in the previous section and described in detail in Chapter 13 , including especially the generation of reactive oxygen and nitrogen species, are intertwined with and provoked in considerable part by activation of the NMDA receptor, with the resulting influx of Ca 2+ through the NMDA channel (as well as through voltage-dependent Ca 2+ channels). The free radicals generated result in DNA damage and, as a consequence, the DNA repair enzyme poly(ADP-ribose) polymerase-1 (PARP). With excessive activation of PARP and, as a consequence, adenosine depletion, energy failure and activation of apoptosis occur. PARP inhibitors have been shown to protect neurons from hypoglycemia in in vitro and in vivo experimental models. Thus the data concerning excitotoxicity and the prevention thereof raise interesting new therapeutic possibilities for the prevention or amelioration of hypoglycemic neuronal death—possibilities that exhibit analogies with potential therapies for ischemic neuronal death (see Chapter 13 ).
Biochemical Changes in the Newborn Animal
Similarities and Differences in Changes in the Newborn and Adult Brain.
Many of the biochemical effects of hypoglycemia described earlier in the adult brain can be documented in the neonatal brain, such as sharp decreases in levels of glucose, diminished cerebral utilization of glucose, and diminished concentrations of glycolytic intermediates (see Table 25.5 ). a
a References .
However, certain metabolic differences from the changes observed in the adult brain are prominent (e.g., preservation of phosphocreatine and ATP levels despite severe decreases in glucose levels and markedly greater utilization of lactate as an alternative substrate for energy metabolism). The data contained in Table 25.6 show that hypoglycemia severe enough to deplete brain of glucose almost entirely is accompanied by some preservation of such glycolytic intermediates as pyruvate and lactate and, most strikingly, by complete preservation of phosphocreatine and ATP levels. Indeed, hypoglycemia of comparable severity in the adult animal causes marked reductions in the levels of phosphocreatine and ATP in the brain.| CEREBRAL METABOLITE (PERCENTAGE OF CONTROL) a | |||||
|---|---|---|---|---|---|
| BLOOD GLUCOSE (mg/dL) b | GLUCOSE (%) | PYRUVATE (%) | LACTATE (%) | PHOSPHOCREATINE (%) | ATP (%) |
| 20–30 | 9 | 86 | 69 | 91 | 93 |
| 10–20 | 1 | 35 | 45 | 98 | 100 |
| <10 | 1 | 36 | 26 | 91 | 97 |
a All values for glucose, pyruvate, and lactate, but none for phosphocreatine and ATP, were statistically significant from control values.
The relative preservation of the energy status of a brain with severe hypoglycemia in the newborn animal is accompanied by a similar preservation of neurological function and electrical activity. Thus, in the adult rat, insulin-induced hypoglycemia to plasma glucose values of approximately 35 mg/dL resulted in prominent slowing on the EEG tracing, and plasma glucose values of approximately 30 mg/dL resulted in lethargy and markedly slow activity on the EEG tracing. Prolongation of this degree of hypoglycemia for approximately 1 hour resulted in coma and an isoelectric EEG pattern. These latter states were attained in less time in the adult animals when plasma glucose levels were reduced to 10 to 15 mg/dL. In contrast, in the newborn rat rendered hypoglycemic to a plasma glucose level of approximately 15 mg/dL, no change in neurological function could be observed over 2 hours. In the newborn dog, prominent slow activity on the EEG tracing was observed only at plasma glucose levels lower than approximately 20 mg/dL. Moreover, at plasma glucose values of approximately 10 to 15 mg/dL (i.e., levels sufficient to cause an isoelectric EEG pattern in the adult dog), considerable electrical activity, albeit slow, was apparent in the newborn dog. Indeed, at this level, seizure discharges often became apparent, and the accompanying respiratory failure and cardiovascular collapse could result in death of the animal.
Reasons for the Relative Resistance of the Newborn Brain to Hypoglycemia.
The data reviewed demonstrate clearly a relative resistance of the newborn versus the adult animal to the deleterious effects of hypoglycemia. The major reasons for this relative resistance are shown in Table 25.7 . Of particular importance is the lower cerebral energy requirement in the immature brain with the consequently lower rate of energy utilization. This situation, discussed in Chapter 13 concerning the relative resistance of the perinatal brain to hypoxic injury, presumably relates first to the less developed dendritic-axonal ramifications and synaptic connections and, as a consequence, energy-dependent ion pumping and neurotransmitter synthesis. However, the relatively advanced dendritic development and synaptogenesis in the occipital cortex of the human newborn may explain the predominance of occipital involvement observed on magnetic resonance imaging (MRI) of hypoglycemic newborns (see later). The second reason for the relative resistance of the newborn animal and human to hypoglycemia relates to the marked increase in CBF provoked by even moderate hypoglycemia. As noted earlier, blood glucose levels lower than 30 mg/dL in the human newborn are associated with prominent increases in CBF. In mature animals, severe hypoglycemia is required to lead to increases in CBF. The third reason for the relative resistance to hypoglycemia presumably relates to an increased capacity for both cerebral uptake and utilization of lactate for brain energy production (see previous discussion of alternative substrates). Fourth, severe hypoglycemia does not have as profound an effect on cardiovascular function in the newborn as in the adult animal. The relative resistance of the immature heart relates to its rich endogenous carbohydrate stores (glucose and glycogen), which can be mobilized for energy during hypoglycemia, and the capacity of the immature heart to use fuels other than glucose for energy. Thus, although it is clear that more data are needed concerning the impact of hypoglycemia on the neonatal brain, current information suggests that cerebral and myocardial metabolic capacities provide remarkable degrees of resistance.
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Role for Excitotoxic Amino Acids in Hypoglycemic Neuronal Death.
A possible role for excitatory amino acids in hypoglycemic neuronal death with severe hypoglycemia in neonatal animals, as in mature animals (see earlier discussion), is suggested by studies of severe insulin-induced hypoglycemia in 7-day-old rats. Thus insulin-induced hypoglycemia caused an increase in striatal extracellular glutamate, measured by microdialysis, with the onset of the increase at blood glucose levels of 20 mg/dL ( Fig. 25.8 ). After 
hours of hypoglycemia (terminal glucose level of < 5 mg/dL), striatal glutamate was approximately 2.4-fold higher than baseline levels. The increased extracellular glutamate may be caused by failure of high-affinity glutamate uptake mechanisms or by increased release (secondary to synaptic release provoked by membrane depolarization [resulting from Na + or Ca 2+ influx] or by reversal of the Na + -dependent glutamate transport system [resulting from increased intracellular Na + ], or by both mechanisms). The potential consequence would be excitotoxic neuronal death by the mechanisms described in Chapter 13 . Prevention of neuronal death in organotypic hippocampal cultures derived from newborn rat brains and maintained in the absence of glucose by the NMDA receptor antagonist MK-801 also illustrates the importance of excitotoxic mechanisms in hypoglycemic neuronal death. The prevention of neuronal death by addition of the antagonist 30 minutes after the insult may have important therapeutic implications. The increase in apparent affinity of the NMDA receptor observed in the hypoglycemic piglet suggests that the excitotoxic potential of glutamate may be enhanced by hypoglycemia.
Glutathione Depletion and Oxidative Stress.
A role for oxidative stress in hypoglycemic cell death was suggested by studies of cultured neural cells and a newborn piglet model. In cultured cells, glucose deprivation caused a decrease in glutathione levels and then cell death. Glucose is involved in the production of reduced glutathione by generating reducing equivalents and providing a carbon source required for biosynthesis of this critical antioxidant. That the cell death in the studies of cultured glial cells was mediated by oxidative stress and free radical attack was shown by the demonstration of protection by free radical scavengers. Because Ca 2+ influx and glutamate receptor activation in neurons may lead to the generation of free radicals (see Chapter 13 ), the deleterious effect of reduced glutathione levels could be critical in the final common pathway to cell death with hypoglycemia. Studies of the hypoglycemic newborn piglet showed markedly elevated mitochondrial production of reactive oxygen species. The demonstration that brain-derived neurotrophic factor (BDNF) protected cultured neurons from hypoglycemic injury further suggests a role for oxidative stress because BDNF induces antioxidant systems.
Hypoglycemia and Hypoxemia or Asphyxia
Hypoglycemia and Hypoxemia
The vulnerability of the immature brain to hypoxemic injury is enhanced by concomitant hypoglycemia, an observation first made in 1942. Studies of cerebral carbohydrate metabolism during hypoxemia and hypoglycemia in newborn rats have provided further insight into the mechanism of this effect. Thus newborn rats subjected to hypoxemia by breathing 100% nitrogen exhibited greater mortality rates when they were also subjected to insulin-induced hypoglycemia ( Fig. 25.9 ). Indeed, animals rendered hypoglycemic for 1 hour experienced a fivefold reduction in survival ability, and those hypoglycemic for 2 hours did even worse. Supplementation of hypoglycemic animals with glucose before anoxia improved outcome (see Fig. 25.9 ). Animals rendered hypoglycemic as well as hypoxemic exhibited less accumulation of lactate in the brain and a faster decline in cerebral energy reserves (ATP and phosphocreatine) than those rendered hypoxemic alone. Moreover, glucose supplementation ameliorated the adverse metabolic effects. The mechanism for the enhanced deleterious effect of hypoxemia when hypoglycemia was associated appeared to relate to a diminution in brain glucose reserves and thus retarded glycolytic flux. The improvement with glucose supplementation supports this notion.
