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Hypoglycaemia: principles of diagnosis and treatment in children
3 Hypoglycaemia: principles of diagnosis and treatment in children J. F E R N A N D E S R. BERGER
INTRODUCTION Glucose is the main fuel for the brain and the brain is the main consumer of glucose. The relatively higher proportion of brain mass to body size in infants and children places them at higher risk of hypoglycaemia than adults. This higher risk for the youngest ages may to some extent be counterbalanced by a higher availability of ketone bodies, the second brain fuel. The concentration of ketone bodies in blood plasma is inversely related to the age of the patient, provided that fatty acid oxidation and ketone body production are intact (Saudubray et al, 1981; Wolsdorf et al, 1982; Lamers et al, 1985). Thus, the age of the patient is one of the factors that should be taken into account in the diagnostic approach to hypoglycaemia. The definition of hypoglycaemia is the subject of much debate and this was discussed in Chapter 2. For the sake of clarity and in agreement with the best available evidence, the cut-off point for the existence of hypoglycaemia is set at a plasma glucose concentration of < 2.6 mmol/1 (47 mg/dl) (Koh et al, 1988; Cornblath et al, 1990; Aynsley-Green, 1991). The main clinical symptoms that might indicate hypoglycaemia are given in Table 1. If one or more of these symptoms are present, blood should be taken immediately for bedside testing with glucose strips; blood should also be taken for an accurate glucose assay in the laboratory. Extra blood should be taken and the plasma frozen for further biochemical and hormonal investigations later. Storing plasma (2 ml) in the freezer is an important measure to prevent diagnostic wandering once the hypoglycaemia has been treated. As hypoglycaemia is an emergency, treatment should not wait for Table 1. Clinical symptoms of hypoglycaemia. Adrenergic
Sweating, tachycardia, pallor
Cerebral
Headache, irritability, weakness, confusion, seizures, coma
Newborns
Feeding difficulties, tremor, lethargy, seizures, tachypnoea, apnoea, cyanosis, hypothermia
Bailli~re's Clinical Endocrinology and Metabolism--
Vol. 7, No. 3, July 1993 ISBN 0-7020-1700-0
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Copyright O 1993, by Baillitre Tindall All rights of reproduction in any form reserved
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the result of the glucose assay, but should start immediately after the taking of the blood sample. IMMEDIATE TREATMENT
Circulatory arrest. Blood glucose concentrations are frequently zero following cardiac arrest in children. Because cardiac massage is relatively inefficient, the cardiac output is likely to be only 15% of normal; relatively large doses of glucose should be given, i g/kg of 20% solution (5 ml/kg). Further glucose should be given if blood glucose concentrations fall below 4 mmol/1. Symptomatic hypoglycaemia. Once the appropriate specimens have been collected, glucose should be given intravenously, 0.2 g/kg followed by continuous infusion of 5-8 mg/kg per min. If after 3-5 min there has been no improvement, the blood glucose concentration should be checked. Only if the concentration is less than 4 mmol/1 should further glucose be given. It must be emphasized that giving too much glucose can be dangerous (Shah et al, 1992).
Asymptomatic hypoglycaemia. If the child has no symptoms, appropriate specimens should be collected and the child should be given oral feed appropriate for its age: milk for a baby; a drink or something to eat for an older child. THE CLINICAL APPROACH TO DIAGNOSIS The clinical approach to the patient is the first step in elucidating the aetiology of the hypoglycaemia. Several schemes exist in which the pathogenesis of hypoglycaemia is differentiated from a physiological point of view (Haymond, 1989; Senior and Sadeghi-Nejad, 1989; de Parsceau and Gibaud, 1990). We prefer a clinical approach starting from two main aetiologies: 1. 2.
Hypoglycaemia as a fuel deficit, without accompanying symptoms of intoxication. Hypoglycaemia amidst symptoms of intoxication.
This clinical differentiation is used for the presentation of the most important hypoglycaemic disorders in Table 2. Symptoms of intoxication vary from acute to chronic. Acute intoxication may express itself as a metabolic derangement, such as lactic acidosis, some organic acidaemias. In chronic intoxication an excess of normal or abnormal metabolites may gradually damage an organ and impair its functions: liver failure, cataract, malformations, developmental delay. A transition between acute and chronic intoxication occurs, for instance, in galactose intolerance. The differentiation between the two main aetiologies should be kept in mind while taking the medical history of the patient and performing
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Table 2. Aetiologies of hypoglycaemia. Disorders characterized by a fuel deficit
1. Disorders of hormonal regulation Hyperinsulinism Hypopituitarism Hypocortisolism Glucagon deficiency? Adrenaline deficiency? 2. Disorders of fatty acid oxidation or transport* 3. Disorders of ket01ysis 4. Disorders of glycogen degradation or synthesis Hepatic glycogen storage diseases? Glycogen synthase deficiency 5. Miscellaneous Malnutrition, low-birth-weight infants Liver failure Renal failure Disorders characterized by a fuel deficit and intoxication
1. Disorders of gluconeogenesis Glucose-6-phosphatase deficiencyt Fructose-1,6-diphosphatase deficiency Glycerol intolerance Phosphoenolpyruvate carboxykinase deficiency Pyruvate earboxylase deficiency 2. 3. 4. 5. 6.
Disorders of fatty acid oxidation or transport* Organic acidaemias Galactose intolerance Fructose intolerance Reye and Reye-like syndromes
* Two subgroups exist, with a fuel deficit only and with a fuel deficit plus intoxication (see Table 4). t Glucose-6-phosphatase deficiencyis a defect of both glycogen degradation and gluconeogenesis. It is placed under the latter group of intoxications because of its tendency to lactic acidosis. the physical examination. This a p p r o a c h is similar to that for inherited m e t a b o l i c disorders in general, as put f o r w a r d by S a u d u b r a y ( S a u d u b r a y and Ogier, 1990).
THE MEDICAL HISTORY T h e age of the patient at which h y p o g l y c a e m i a starts rarely gives a clue to the diagnosis. Exceptions in the i m m e d i a t e postnatal period are hyperinsulina e m i a that is characterized by excessive overutilization of glucose outside the brain, and s o m e organic acidaemias triggered by the initiation of nutrition after birth. T h e r e are a large n u m b e r of causes of h y p o g l y c a e m i a presenting during later infancy or early childhood. S y m p t o m s m a y d e v e l o p
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because of longer intervals between meals, implying longer periods of fasting, and because of metabolic sequelae triggered by infections. Hypoglycaemia during adolescence or at adult age is more often secondary to a tumour producing insulin or insulin-like growth factors (Polonski, 1992). Diet. Hypoglycaemia may be precipitated by components of the diet and any
disorders in this group belong to the category of intoxications, including galactose intolerance, fructose intolerance, some organic acidaemias and maple syrup urine disease. The hypoglycaemia is caused by complex reduction of activity of enzymes of gluconeogenesis and glycogenolysis or interference with transmitochondrial shuttles essential for gluconeogenesis (Van den Berghe, 1991). THE PHYSICAL EXAMINATION First, it should be emphasized that the rapid and 'mysterious' deterioration of an infant after a normal initial period is the most important signal of intoxication (Saudubray and Ogier, 1990). In energy deficiencies there is usually no free interval, although some milder diseases need provocative triggers such as anorexia and insufficient food intake to become manifest. Some features in the newborn infant or in the early postnatal period come to immediate attention. Marked obesity is observed in hyperinsulinaemia and related syndromes (Beckwith-Wiedemann syndrome). Cholestasis, micropenis and hypoglycaemia occur in hypopituitarism (Sheehan et al, 1992). Cataract, though not specific for diseases of galactose metabolism, should, none the less, arouse suspicion in that direction. Hepatomegaly is an important manifestation of both intoxication and storage without intoxication. The time schedule for its appearance is variable. Hepatomegaly due to galactose intolerance and some defects of gluconeogenesis develops early. Hepatomegaly due to glycogen storage disease becomes gradually more pronounced, whereas its appearance in fructose intolerance is linked to the introduction of fruit juices and sucrose. Muscle hypotonia may be a prominent feature of disorders of fatty acid oxidation and the glycogen storage diseases. Gross abnormalities such as congenital malformations and microcephaly are of widely different origin, but they occur also in some inborn errors of gluconeogenesis. LABORATORY INVESTIGATION The outline of the laboratory investigation of the hypoglycaemic infant should be in keeping with the aetiological differentiation into diseases with a fuel deficit only and those with a fuel deficit and intoxication (Table 2). The laboratory investigation is not the same for all ages, for two reasons. First, diagnostic approaches differ according to the age of onset of a disorder. Second, the young infant is more vulnerable to 'aggressive' tolerance tests that usually include a period of fasting. The different approaches for the two
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Table 3. Laboratory investigation. Fuel deficit without intoxication
Plasma
Urine
Infants Older
Glucoseprofile around the clock Acid-base status KB, FFA*, carnitine (total and free) Complete hormone profile Glucose, other sugars, lactate KB, dicarboxylicacids Carnitine (total and free) Organic acids, amino acids Calculateglucoserequirement Oral glucose tolerance test Fasting test or 3-phenylpropionicacid loading test
Fuel deficit and intoxication
Plasma
Urine Infants Older Always
Glucose,other sugars Acid-base status KB, FFA*, carnitine (total and free) Lactate, pyruvate, ammonia Organic acids, amino acids See above No tolerance tests Oral glucose tolerance test Other tolerance tests if indicated Conservationof plasma, urine (cerebrospinalfluid) and fibroblasts Diagnostic confirmationby enzyme assays or probes
* KB = ketone bodies; FFA = free fatty acids. aetiologically distinct groups and for infants against older children are detailed in Table 3. In the situation of acute fuel deficit without recognizable signs of intoxication (Table 3, upper part) it is important to determine the plasma concentrations of the three main fuels: glucose, free fatty acids and ketone bodies. It makes a great difference whether a low plasma glucose concentration is accompanied by increased levels of free fatty acids and ketone bodies, or the levels of all three fuels are too low. The latter is the case in hyperinsulinaemia. Disorders of hormonal regulation
The overconsumption of glucose by non-cerebral tissues, which is almost pathognomic for hyperinsulinaemia, can be substantiated by calculating the glucose requirement of the infant. The amount of glucose needed to keep the plasma glucose concentration at 1>2.6 mmol/1 then appears to exceed the normal requirement in infancy of 6-7 mg glucose per kg body weight per minute (Bier et al, 1977). In cases with a less excessive glucose demand, glucose homeostasis can be observed around the clock while determining the plasma glucose profile be~fore and after feeding and assaying plasma insulin concentrations at glucose nadirs. Plasma insulin concentration > 1 0 U / m l with a concomitant plasma glucose concentration < 5 0 m g / d l (2.Smmol/1) at any time suggests the existence of hyperinsulinaemia ( H a y m o n d , 1989). Simultaneous measurement of C-peptide might reinforce
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the diagnosis as soon as a cut-off point between normal and elevated values is known for this age. For older children basal C-peptide amounts to 0.250.50 nmol/1 (Bennish et al, 1990). For the differentiation of different aetiologies of hyperinsulinism, see Baker and Stanley (1991). Treatment of hyperinsulinaemia
If a discrete pancreatic tumour is responsible for the hyperinsulinaemia, then this should be removed surgically. Other patients will require medical therapy. The mainstay of this is diazoxide, in doses up to 20 mg/kg per day, which should be combined with chlorothiazide as this not only potentiates the diazoxide but also helps to reduce the fluid overload that is a frequent complication. A supplement of potassium chloride should also be given. Alternatively, the patient may be given the long-acting somatostatin analogue, octreatide, starting in a dose of 1 p~g/kg every 4-8 h (Kirk et al, 1988). The dose may need to be increased, but there is a greater chance of complications including diarrhoea. The addition of glucagon may improve control of the hypoglycaemia. If these measures fail then it may be necessary to proceed to subtotal pancreatectomy. With respect to hypoglycaemia of other hormonal aetiology (Table 2), the fuel profile is more variable and less characteristic than that of hyperinsulinaemia. For instance, hypoglycaemia due to growth hormone deficiency is accompanied by hypoketonaemia (Wolsdorf et al, 1983) and hypoglycaemia due to cortisol deficiency by hyperketonaemia (Soltesz et al, 1985). Thus, in case of doubt the hormonal profile should encompass the assay of all hormones: insulin, C-peptide, growth hormone, cortisol, pancreatic glucagon and adrenaline. Some hormone disregulations are ambiguous. It is uncertain whether adrenaline deficiency is a separate entity. It has been observed as such (Hansen et al, 1983) and associated with growth hormone deficiency (Voorhess and MacGillivray, 1984). Glucagon deficiency has been described only twice (Vidnes and Oyasaeter, 1977; Koll6e et al, 1978). As more recent literature has not appeared, its existence might be doubted. Patients with adrenal failure should be treated with cortisol in a dose of 20 mg/m 2 in divided doses. Similar doses should be used in patients with hypopituitarism, but in the young patient in particular, hypoglycaemia may remain a problem. Treatment with growth hormone may be necessary in a dose of 15 U/m 2 per week divided into daily doses. All patients on cortisol must also have a careful plan of what to do during periods of metabolic stress, such as intercurrent illness or anaesthesia. During mild infections the dose of cortisol should be doubled and during more severe illness the dose should be quadrupled. Disorders of fatty acid oxidation or transport
This group consists of disorders in which hypoglycaemia is primarily the result of a fuel deficit, and disorders characterized by both a fuel deficit and intoxication (Tables 2 and 4). Patients from either group usually have a 'silent' clinical course until a metabolic stress such as fasting, an intercurrent
PRINCIPLES OF DIAGNOSISAND TREATMENTIN CHILDREN Table
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4. Disordersof fatty acidoxidationor transport.
Fuel deficit
Primarycarnitinedeficiencyor carnitinetransport deficiency Carnitine palmityltransferaseI deficiency Hydroxymethylglutaryl-CoAlyasedeficiency Fuel deficit and intoxication
Medium-chainacyl-CoAdehydrogenasedeficiency Long-chainacyl-CoAdehydrogenasedeficiency Short-chainacyl-CoAdehydrogenasedeficiency 3-Hydroxyacyl-CoAdehydrogenasedeficiency Multiple acyl-CoAdehydrogenasedeficiencies
infection or anaesthesia elicits a shortage of energy. Even the sudden infant death syndrome and Reye syndrome have been described. According to Hale and Bennet (1992), common features shared by all fatty acid disorders are episodes of hypoketotic hypoglycaemia associated with fasting, chronic involvement of fatty acid-dependent tissues (cardiac and skeletal muscle), and alteration in the degree of esterification of (plasma) carnitine. The mechanisms by which hypoglycaemia is induced are in part explained by a shortage of glucose-replacing energy substrates (ketone bodies), by lack of stimulation of gluconeogenesis through limited production of reducing equivalents, and by a limited availability of acetyl-CoA, a positive effector of pyruvate carboxylase (Sovik, 1989). All disorders of fatty acid oxidation and transport have one characteristic metabolic derangement in common. During fasting the plasma concentration of free fatty acids increases normally, whereas ketone body production fails. This dissociation between high plasma levels of free fatty acids and (very) low plasma levels of ketone bodies can be used as a screening method. However, before trying to interpret the free fatty acid/ketone body ratio, the influence of the age of the patient on fasting ketone body concentration should be assessed. Figure 1 shows the correlation between [3-hydroxybutyrate, quantitatively the most important ketone body, and the age of the child, after a 24-h fast (Wolsdorf et al, 1982). The advantage of this nomogram is the fact that it also shows the normal 95% confidence limits. The next step is to compare the ketone body level with that of glucose. Among existing nomograms we prefer to use that of Teyema et al (1980) because it specifes the age ranges of the reference children and also presents the 95% confidence limits (Figure 2). If the patient appears to be hypoketotic both with respect to age (Figure 1) and fasting glucose concentration (Figure 2), the last screening step should be to correlate plasma ketone body concentration and plasma free fatty acid concentration during fasting (Figure 3). A dissociation as mentioned above would point to a disorder of fatty acid oxidation or transport. The most informative procedure for diagnosing and differentiating fatty acid disorders is the fasting test (see Laboratory Procedures). However, as almost all patients suffering from one of these defects are fasting-intolerant, the test can be dangerous. If properly performed the majority of fatty acid disorders can be differentiated by
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considering the relationship of glucose, ketone bodies and free fatty acids in plasma, and by measuring total and flee carnitine in plasma and organic (dicarboxylic) acids and their glycine and carnitine derivatives in urine. Detection of specific metabolites, especially in urine, can be suggestive for a particular disorder, e.g. hexanoyl- and suberylglycine in medium-chain acyl-CoA dehydrogenase deficiency. Likewise, loading tests with substrates (long-chain fatty acids, 3-phenylpropionic acid) are informative for the exact site of the defect and have the advantage of safety. The ultimate diagnosis is made by enzyme investigations in cultured skin fibroblasts, leukocytes or lymphocytes (Duran et al, 1992). In the case of systemic carnitine deficiency, the carnitine clearance protocol developed by Engel et al (1981) should be performed.
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Treatment of disorders of fatty acid oxidation Acute symptoms should be treated as outlined earlier and in the longer term the patient should have a careful plan (Dixon and Leonard, 1992) to prevent complications, being given a high carbohydrate intake orally or intravenously. Patients with long-chain fatty acid disorders may be very unstable and require frequent feeds and a continuous nasogastric infusion at night, similar to that used in patients with a glycogen storage disease. Older patients may be controlled with uncooked corn starch. In contrast to disorders of fatty acid oxidation and transport are a few rare ketolysis defects in which hyperketosis is associated with hypoglycaemia, the latter symptom not being obligatory (Saudubray and Specola, 1990). It is important to differentiate these disorders from organic acidaemias. Disorders of glycogen degradation There is much overlap between the two hypoglycaemic categories of disorders with attendant hepatomegaly--without and with intoxication, respectively. This applies in the frst place to the glycogen storage diseases in which the liver enlargement is the most conspicuous clinical feature. The tendency to fasting hypoglycaemia and its severity depend on the site of the enzyme defect in the glycogenolytic cascade. In deficiencies of debranching enzyme or the phosphorylase system, gluconeogenesis is not affected and fasting hypoglycaemia is accompanied by ketosis without intoxication. In deficiency of the glucose-6-phosphatase unit gluconeogenesis, too, is blocked and fasting hypoglycaemia is accompanied by a toxic lactic acidosis, as is the case in the other enzymopathies of gluconeogenesis. For all glycogen storage diseases a laboratory screening should precede the biochemical investigation, as a preliminary diagnosis from the screening can be of help in choosing the tissue for the enzyme assay: liver tissue for glucose-6-phosphatase deficiency only, leukocytes or fibroblasts for the other glycogenoses. The screening consists of an oral glucose tolerance test (see Laboratory Procedures). In glucose-6-phosphatase deficiency, glucose ingestion suppresses hepatic lactate overproduction. This is reflected in the plasma glucose and lactate curves. While glucose rises, lactate falls. This result is typical for glucose-6-phosphatase deficiency. In the other glycogenoses lactate shows a small upward deflection that is not significantly different from that in normal children. For a firmer allocation of the other glycogenoses an oral galactose tolerance test is very helpful. In this test again the lactate curve is the most important. An abnormal increase of lactate t>3mmol/l is suspect for one of the other glycogenoses (Fernandes and van de Kamer, 1969). If the result is abnormal, enzyme assays should be performed in order to differentiate between deficiency of debranching enzyme and that of the phosphorylase system. Other screening procedures have been described, which encompass an intravenous galactose test or a glucagon test as an inital procedure. This may be dangerous, as both tests, in fact, continue the fasting state in the case of glucose-6-phosphatase deficiency.
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Glycogen synthase deficiency is a very rare disorder in which hepatic glycogen synthesis from glucose is blocked (Table 2) (Aynsley-Green et al, 1977). Fasting hypoglycaemia is accompanied by hyperketonaemia but normal lactate; the liver is not enlarged. This disorder can be suspected when plasma lactate shows an exaggerated increase after a meal (glucose effect?). In glucose-6-phosphatase deficiency the production of free glucose in the liver is reduced and the only way of maintaining normal blood glucose concentrations is to give exogenous glucose. In young babies it is often sufficient to give frequent feeds, but once the child starts sleeping through the night then additional measures will be needed. The most effective treatment is regular high-carbohydrate drinks or meals during the day (Fernandes et al, 1988). The frequency of the drinks is adjusted according to the child's response to the treatment. Most children will require 2-hourly feeds giving 0.5 g carbohydrate/kg per hour. At night the babies should have continuous nasogastric infusion of a similar rate. In young babies this is usually given as milk, but once children are older the feeds can be changed to soluble glucose polymer. The quantity of glucose decreases as the children grow older. The interval between drinks can be extended by giving uncooked corn starch. In older children and adults corn starch may maintain normoglycaemia for longer periods, so that this alone may be used to maintain good metabolic control (Smit et al, 1984). In debranching enzyme deficiency free glucose can only be synthesized via gluconeogenesis. It is usual therefore to give a high-protein diet and this may be sufficient to control hypoglycaemia (Fernandes et al, 1988). However, some patients require treatment similar to that used in patients with type I glycogen storage disease. For phosphorylase kinase and phosphorylase deficiency treatment is not often needed, but if needed it should be similar to that used in GSD type III. Miscellaneous
The last category in which the infant or child is at risk for a fuel deficit without intoxication is a miscellaneous group (Table 2). The hypoglycaemia is of multifactorial origin: limited fuel stores, organ failure, inappropriate hormone regulation. Examples are low-birth-weight infant, the diarrhoeamalnutrition syndrome (Bennish et al, 1990), hepatic failure, and renal failure in which renal gluconeogenesis falls short and insulin is insufficiently cleared by the kidneys (Arem, 1989). Disorders characterized by a fuel deficit and intoxication
Disorders characterized by a fuel deficit and intoxication are as heterogeneous as the first category with a fuel deficit only. Usually the fuel deficit is overshadowed by the severity of the intoxication. This applies particularly for disorders of gluconeogenesis and some organic acidaemias (Table 2). In disorders of gluconeogenesis, fasting hypoglycaemia is generally more
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pronounced the closer the enzyme defect is located to glucose 6-phosphate (Van den Berghe, 1991). In pyruvate carboxylase deficiency, hypoglycaemia is not an obligatory symptom, whereas in glucose-6-phosphatase deficiency it is permanently imminent and life-threatening during fasting. The reverse order might be defined for signs of intoxication, which are more severe and irreversibly damaging the closer the enzyme defect is located to pyruvate. Sites of intoxication are mainly the brain and the liver (Clayton et al, 1986; Wong et al, 1986; Biihrdel et al, 1990). The laboratory work-up is an urgent issue in order to detect those disorders that are amenable to treatment. All disorders of gluconeogenesis are characterized by hyperlactacidaemia, hyperpyruvicaemia and hyperalaninaemia, substrates at the origin of gluconeogenesis. The combination of a low plasma glucose concentration and an elevated fasting lactate concentration is particularly key information indicating that something is wrong with gluconeogenesis, either primarily or secondarily, provided that hypoxaemia is absent. The increased plasma lactate/pyruvate ratio found in pyruvate carboxylase deficiency (Robinson et al, 1984) is not a constant finding and it is doubted whether it really reflects a disturbed redox state (Blass, 1979). In pyruvate carboxylase deficiency and phosphoenolpyruvate carboxykinase deficiency, the urea cycle may be secondarily compromised. This results in increased plasma levels of ammonia, citrulline and lysine. In fructose-l,6diphosphatase deficiency hyperexcretion of glycerol (Dremsek et al, 1985) and glycerol 3-phosphate (Krywawych et al, 1986) are typical findings, shared only by glycerol intolerance, another rare disorder of gluconeogenesis (Fort et al, 1985). Next to the assay of metabolities, which are almost continuously excreted in excess in plasma and urine and therefore reflect chronic intoxication, gluconeogenesis might be explored by a tolerance test. The classic approach entails the administration of a substrate for gluconeogenesis orally or intravenously, whilst assaying metabolites at the origin of gluconeogenesis (lactate, pyruvate, alanine) and at the end of it (glucose). Many substrates have been used: alanine, lactate, glycerol, dihydroxyacetone, fructose. It should be emphasized here that one should never use a substrate that is already present in excess in the fasting state (alanine, lactate, glycerol). This limits the usefulness of this diagnostic approach. One should also be aware of the potential danger of using fructose, as a fatal outcome of fructose loading has been described (Dremsek et al, 1985). A good candidate for explorative use as regards the 'proximal' part of gluconeogenesis is dihydroxyacetone. It has been used as an oral tolerance test by Baerlocher et al (1971). A new elegant approach is the intravenous administration of a stable isotope of glucose, the use of which might disclose whether glucose cycling is disrupted and gluconeogenesis compromised. The method is both specific and safe as it involves the loading of the patient with a tracer dose of an exogenous substrate, which behaves like its endogenous analogue and is nevertheless discernible from it. In principle the patient is given a priming dose of a particular metabolite followed by a continuous infusion of the same tracer into the blood to achieve an isotopic enrichment of the metabolite in
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the blood cdmpartment 1-2% above the natural abundance. After reaching a steady state, blood samples are drawn at regular intervals for the measurement of isotopic enrichment. Together with the isotopic enrichment of the tracer in the infusate, the known rate of tracer infusion into the blood compartment and the correction for inflow of non-labelled metabolite, the measurements yield the rate of appearance of a particular metabolite into the blood compartment. In a steady state this rate equals the rate of metabolite production or disappearance. Glucose production rates can be measured by using, e.g., [2H2]glucose. Provided that glycogen stores in the liver are depleted by fasting (12-14 h), glucose production rate approaches the rate of gluconeogenesis. This test has been applied in a variety of clinical conditions, it is relatively easy to perform, and most clinical laboratories equipped with mass-spectrometric facilities are able to measure isotopic enrichment by standard gas chromatographic-mass spectrometric techniques (Bier, 1987). Using [2H2]glucose, the rate of gluconeogenesis can at least be estimated. The administration of specific C3-precursors such as [13C]alanine (Frazer et al, 1981), [13C]lactate or [13C]glycerol (Bougn6res et al, 1982) might give a better estimation of the rate of gtuconeogenesis from that precursor. However, the limited experience with the use of these substrates and technical difficulties are drawbacks in their application. A protocol for the test with [2H2]glucose is given under Laboratory Procedures. Although patients with fructose 1,6 diphosphatase deficiency may present with hypoglycaemia and lactic acidosis in the newborn period, patients usually tolerate a longer fast than patients with glucose-6-phosphatase deficiency. Indeed many patients do not present until they are older. For many patients it is sufficient to feed them regularly and give a high-carbohydrate snack before they go to sleep. When well, patients may be allowed a normal diet, but as soon as they are unwell they should exclude sucrose and all fructose and take regular high-carbohydrate drinks. If oral drinks are not tolerated, glucose should be given intravenously. Occasionally patients may require more intensive treatment similar to that used in glucose-6phosphatase deficiency. Gluconeogenesis can also be secondarily involved in some organic acidaemias, in which the overproduction of normal or abnormal compounds may cause severe intoxication (Table 2). Ketotic hypoglycaemia is observed in some organic acidaemias, notably in methylmalonic acidaemia, propionic acidaemia (late onset), maple syrup urine disease, [3-methylcrotonylcarboxylase deficiency and multiple carboxylase deficiency (Sovik, 1989). The mechanism by which hypoglycaemia occurs may in part be due to direct inhibition of a gluconeogenic enzyme by the CoA-derivative of the organic acid or by sequestration of CoA impairing the activation of pyruvate carboxylase by acetyl-CoA. In two diseases hypoglycaemia and intoxication are elicited by normal nutrients: galactose and fructose. Galactose intolerance is caused by deficiency of galactose-l-phosphate uridyltransferase. Galactose and galactose 1-phosphate accumulate before the enzyme defect, as soon as the child consumes its mother's milk or a lactose-containing formula. Galactose
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1-phosphate is the more toxic of the two substrates. It not only suppresses gluconeogenesis and glycogenolysis by inhibition of various enzymes, it also sequestrates phosphate donated by adenosine triphosphate (ATP). The phosphorylysis of ATP induces abnormalities of nucleotide metabolism and a severe energy deficit in the cells of many organs, mainly the brain, liver, kidneys and intestines. A description of the wide range of clinical symptoms is outside the scope of this chapter. Some laboratory abnormalities may lead to appropriate alarm: e.g. the presence of reducing substances in the urine, a finding that must be confirmed by sugar chromatography. A plasma sample, taken before stopping lactose-containing nutrition, should also contain galactose (normally only present in traces). Lactic acidaemia, hyperbilirubinaemia and abnormal liver function tests are less specific. In cases of suspicion, a tolerance test with galactose or lactose should be avoided (Gitzelmann, 1990) because of the risk of severe intoxication. Instead an enzyme assay of galactose-l-phosphate transferase in erythrocytes is the only proof of the diagnosis. The treatment of galactosaemia is immediate and total exclusion of all galactose for life. Fructose intolerance is caused by deficiency of fructose-l-phosphate aldolase B, which catalyses the conversion of fructose 1-phosphate into two trioses. The toxicity of fructose 1-phosphate bears some resemblance to that of galactose 1-phosphate in the case of galactose intolerance. Exposure of affected infants or children to fructose or sucrose causes hypoglycaemia, hypophosphataemia, hyperuricaemia and hypermagnesaemia (the latter two abnormalities reflecting the abnormal nucleotide metabolism). The clinical symptoms show a wide spectrum, related to the localization of the enzyme defect in liver, kidneys and intestines, not the brain. Symptoms of acute intoxication in fructose intolerance are usually more serious than in galactose metabolism, and for that reason chronic intoxication occurs less frequently in the former because of early development of a strong aversion against all fructose-containing foods and effective elimination of them. An intravenous tolerance test (not oral because of severe intestinal side-effects) is carried out if the diagnosis is suspected (see Laboratory Procedures). The results of this test in fructose-aldolase-B deficiency and fructose-l,6diphosphatase deficiency show strong resemblance (Steinmann and Gitzelmann, 1981). Thus, the diagnosis should be confirmed in liver tissue, biochemically or genetically with a probe (Cox, 1990). Treatment is by exclusion of all fructose, sucrose and sorbitol from the diet.
Reye and Reye-like syndromes. In this heterogeneous group acute or subacute metabolic derailment is elicited by a drug or a viral infection (Table 2) (Osterloh et al, 1989). Otherwise, the drug or viral infection may be only a trigger that induces a latent metabolic disorder to become manifest. Examples of drug toxicities result from ingestion of salicylate, hypoglycin (Jamaican vomiting disease) or valproate. Hypoglycaemia, present in 70% of cases, is due to impaired fatty acid [3-oxidation and impaired gluconeogenesis. Other metabolic abnormalities are, therefore, lactic acidosis, dicarboxylic acidaemia and hyperammonaemia. Accumulated products may injure cerebral and hepatic function.
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LABORATORY PROCEDURES The oral glucose tolerance test
Glucose dose: 2 g/kg body weight (max. 50 g) as a 20% solution in water Duration of test: 3 h. Previous fasting,: 3-8 h, depending on the meal interval Substrates in plasma: • Every 30 rain (two separate samples at zero time): glucose, lactate • At the beginning and the end of the test a complete profile of: glucose, lactate, pyruvate, acid-base status; free fatty acids, [3-hydroxybutyrate, acetoacetate; carnitine (total and esterified); aminoacids, organic acids, dicarboxylic acids; insulin, glucagon, adrenaline; cortisol, growth hormone Indications: glycogen storage diseases; disorders of gluconeogenesis; hyperinsulinaemia. The oral galactose tolerance test
Galactose dose: 2 g/kg body weight (max. 50 g) as a 20% solution in water. Duration of test: 3 h. Previous fasting: as for glucose test. Substrates in plasma • Every 30 rain (two separate samples at zero time): glucose, galactose, lactate. Indication: glycogen storage diseases (glucose-6-phosphatase deficiency excepted). The intravenous fructose tolerance test
Fructose dose: 0.2 g/kg body weight as a 10% solution in water. Duration of test: 3 h. Previous fasting: as for glucose test. Substrates in plasma: • Every 30 min (two separate samples at zero time): glucose, fructose, lactate, urate, phosphate. Indications: fructose intolerance; fructose-l,6-diphosphatase deficiency. Exploration of fasting tolerance
• Start fasting at 22:00, determine blood glucose concentration by a rapid bedside method after 6, 8, 9, 10 h, thereafter every 30 rain. Encourage the drinking of water. • Stop at noon or earlier, as soon as blood glucose < 3.3 mmol/l (60 mg/dl); do not determine substrates other than glucose.
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• The duration of the fasting period tolerated by the child allows one to fix the earliest time at which the extended fasting test can start 2-3 days later, without endangering the child at night.
The extended fasting test Do not perform an extended fasting test before having explored the fasting tolerance of the child (see above). Duration of the test is 20 hours (max. 24); the drinking of water is encouraged. Start the test around 18:00 (see above). Substrates in plasma: • Glucose after 12h (or earlier, see Exploration of Fasting Tolerance), thereafter hourly until glucose <~3.3 retool/l, then every 30 min; stop at glucose 2.6 mmol/l. • Acid-base status, free fatty acids, ketone bodies, lactate as soon as glucose ~<3.3 mmol/1; continue thereafter simultaneously with glucose. • Complete profile at the end of the test (see Oral Glucose Tolerance Test). Substrates in urine (6-h periods, starting at 18:00): glucose, other sugars, lactate; ketone bodies, dicarboxylic acids; carnitine (total and free); organic acids, amino acids. Indications: disorders of fatty acid oxidation.
Stable isotope test using [2H2]glucose Infusion of [2H2]glucose 5 rag/ks for 10 rain followed by a rate of 45 ~Lg/kg per min using a syringe infusion pump. After achieving steady-state conditions (usually 60 rain after starting the test) blood samples are drawn at 30-rain intervals for the measurement of glucose concentration and isotopic enrichment. Duration of test: 5-6 h. Previous fasting: 12-14 h. Substrates in plasma: • Every 30 rain (one sample prior to the test): glucose and [2H2]glucose. Indications: disorders of gluconeogenesis, disorders of fatty acid oxidation or transport, glycogen storage diseases.
SUMMARY The diagnostic approach to children with suspected hypoglycaemia is facilitated by the fact that two clinically distinct groups exist with little overlap: 1. 2.
Hypoglycaemias characterized by a fuel deficit only. Hypoglycaemias in which the fuel deficit is overshadowed by symptoms of intoxication.
This differentiation is used when taking the medical history, performing the physical examination, and planning the laboratory investigations. The latter
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may encompass tolerance tests, a fasting test or the use of a non-radioactive glucose isotope. Protocols for such tests are given.
REFERENCES Arem R (1989) Hypoglycemia associated with renal failure. Endocrinology and Metabolism Clinics of North America 18: 103-121. Aynsley-Green A (1991) Glucose: a fuel for thought! Journal of Paediatric Child Health 27: 21-30. Aynsley-Green A, Williamson DH & Gitzelmann R (1977) Hepatic glycogen synthase deficiency. Archives of Disease in Childhood 52: 573-579. Baerlocher K, Gitzelmann R, Ntissli R & Dummermuth G (1971) Infantile lactic acidosis due to hereditary fructose-l,6-diphosphatase deficiency. Helvetica Paediatrica Acta 26: 489-506. Baker L & Stanley CA (1991) Management of hyperinsulinismin infants. Editor's column. The Journal of Pediatrics 119: 755-757. Bennish ML, Azad AK, Rahman O & Phillips RE (1990) Hypoglycemia during diarrhea in childhood. Prevalence, pathophysiology, and outcome. New England Journal of Medicine 322: 1357-1363. Bier DM, Leake RD, Haymond MW et al (1977) Measurement of 'true' glucose production rates in infancy and childhood with 6,6-dideuteroglucose. Diabetes 26: 1016-1023. Bier DM (1987) The use of stable isotopes in metabolic investigations. Baillidre's Clinical Endocrinology and Metabolism 1: 817-836. Blass JP (1979) Disorders of pyruvate metabolism. Clinical review. Neurology 29: 280-286. Bonefont JP, Specola NB, Vassault A e t al (1990) The fasting test in paediatrics: application to the diagnosis of pathological hypo- and hyperketotic states. European Journal of Pediatrics 150: 80-85. Bougn~res PF, Karl IE, Hillmann LS & Bier DM (1982) Lipid transport in the human newborn: palmitate and glycerol turnover and the contribution of glycerol to neonatal hepatic glucose output. Journal of Clinical Investigation 70: 262-270. Bfihrdel P, B6hme HJ & Didt L (1990) Biochemical and clinical observations in four patients with fructose-1,6-diphosphatase deficiency. European Journal of Pediatrics 149: 574--576. Clayton PT, Hyland K, Brand M & Leonard JV (1986) Mitochondrial phosphoenolpyruvate carboxykinase deficiency. European Journal of Pediatrics 1986: 46-50. Cornblath M, Schwartz R, Aynsley-Green A & Lloyd JK (1990) Hypoglycemia in infancy: the need for a rational definition. Pediatrics 85: 834-837. Cox TM (1990) Hereditary fructose intolerance Baillidre's Clinical Gastroenterology 4: 61-78. Dixon MA & Leonard JV (1992) Intercurrent illness in inborn errors of intermediary metabolism. Archives of Disease in Childhood 67: 1387-1391. Duran M, Cleutjens CBJM, Ketting D et al (1992) Diagnosis of medium-chain acyl-CoA dehydrogenase deficiency in lymphocytes and liver by a gas chromatographic method: the effect of oral riboflavin supplementation. Pediatric Research 31: 39-42. Dremsek PA, Sacher M, St6gmann W e t al (1985) Fructose-l,6-diphosphatase deficiency: glycerol excretion during fasting test. European Journal of Pediatrics 144: 203-204. Engel AG, Rebouche CJ, Wilson DM et al (1981) Primary systemic carnitine deficiency. II Renal handling of carnitine. Neurology 31: 819-825. Fernandes J & van de Kamer JH (1969) A screening method for liver glycogen diseases. Archives of Disease in Childhood 44: 311-317. Fernandes J, Leonard JV, Moses SW et al (1988) Glycogen storage disease-Recommendations for treatment. European Journal of Pediatrics 147: 226-228. Fort P, Wapnir RA, de Rosas F & Lifshitz F (1985) Long-term evolution of glycerol intolerance syndrome. Journal of Pediatrics 106: 453-456. Frazer TE, Karl IE, Hillman LS & Bier DM (1981) Direct measurement of gluconeogenesis from L-[2,3-13Cz]alanine in human neonate during the first eight hours of life. American Journal of Physiology 240: E615-E621. Gitzelmann R (1990) Disorders of gatactose metabolism. In Fernandes J, Saudubray J-M & Tada K (eds) Inborn Metabolic Diseases, pp 95-105. Berlin: Springer-Verlag.
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Hale DE & Bennett MJ (1992) Fatty acid oxidation d/sorders: A new class of metabolic disorders. Journal of Pediatrics 121: 1-11. Hansen IL, Levy MM & Kerr DS (1983) Differential diagnosis of hypoglycemia in children by responses to fasting and 2-deoxyglucose. Metabolism 32: 960-970. Haymond MW (1989) Hypoglycemia in infants and children. Endocrinology and Metabolism Clinics of North America 18: 211-252. Kirk JM, Di Silvio L, Hindmarsh PC & Brook CGD (1988) Somatostatin analogue in short term management of hyperinsulinism. Archives of Disease in Childhood 63: 1493-1494. Koh THHG, Aynsley-Green A, Tarbit M & Eyre JA (1988) Neural dysfunction during hypoglycaemia. Archives of Disease in Childhood 63: 1353-1358. Koll6e LA, Monnens LA, Cejka V & Wilms RH (1978) Persistent neonatal'hypoglycemia due to glucagon deficiency. Archives of Disease in Childhood 53: 422-424. Krywawych S, Katz G, Lawson AM et al (1986) Glycerol-3-phosphate excretion in fructose-l,6diphosphatase deficiency. Journal of lnherited Metabolic Diseases 9: 388-392. Lamers KJB, Doesburg WH, Gabre61s FJM et al (1985) The concentration of blood components related to fuel metabolism during prolonged fasting in children. CIinica Chimica Acta 152: 155-163. Osterloh J, Cunningham W, Dixon A & Combest D (1989) Biochemical relationships between Reye's and Reye's-like metabolic and toxicological syndromes. Medical Toxicology and Adverse Drug Experience 4: 272-294. de Parscau L & Guibaud P (1990) Diagnostic 6tiologique des hypoglyc6mies de l'enfant. P~diatrie 45: 165-171. Polonski KS (1992) A practical approach to fasting hypoglycemia. New England Journal of Medicine 326: 1020-1021. Robinson BH, Oei J, Sherwood WG et al (1984) The molecular basis for the two different clinical presentations of classical pyruvate carboxylase deficiency. American Journal of Human Genetics 36: 283-294. Saudubray J-M & Ogier H (1990) Clinical approach to inherited metabolic disorders. In Fernandes J, Saudubray J-M & Tada K (eds) Inborn Metabolic Diseases, pp 3-25. Berlin: Springer-Verlag. Saudubray J-M & Specola N (1990) Ketolysis defects. In Fernandes J, Saudubray J-M & Tada K (eds) Inborn Metabolic Diseases', pp 411-418. Berlin: Springer-Verlag. Saudubray J-M, Marsac C, Limal J-M et al (1981) Variation in plasma ketone bodies during a 24 hour fast in normal and in hypoglycemic children: relationship to age. Journal of Pediatrics 98: 904-908. Senior B & Sadeghi-Nejad A (1989) Hypoglycemia: a pathophysiologic approach. Acta Paediatrica Scandinavica (supplement 325): 1-27. Shah A, Stanhope R & Matthew D (1992) Hazards of pharmacological tests of growth hormone secretion in childhood. British Medical Journal 304: 173-174. Sheehan AG, Martin SR, Stephure D & Scott RB (1992) Neonatal cholestasis, hypoglycemia, and congenital hypopituitarism. Journal of Pediatric Gastroenterology and Nutrition 14: 426-430. Smit GPA, Berger R, Potasnick R et al (1984) Starch therapy in type I glycogen storage disease. Paediatric Research 18: 879-881. Soltesz G, Dillon MJ, Jenkins PA et al (1985) Isolated glucocorticoid deficiency: metabolic and endocrine studies in a 5-year-old boy. European Journal of Pediatrics 143: 297-300. Sovik O (1989) Inborn errors of amino acids and fatty acid metabolism with hypoglycemia as a major clinical manifestation. Acta Paediatrica Scandinavica 78: 161-170. Stanley CA (1990) Disorders of fatty acid oxidation. In Fernandes J, Saudubray J-M & Tada K (eds) Inborn Metabolic Diseases, pp 395-410. Berlin: Springer-Verlag. Steinmann B & Gitzelmann R (1981) The diagnosis of hereditary fructose intolerance. Helvetica Paediatrica Acta 36: 297-316. Teyema HL, van Gelderen HH & Giesberts AH (1980) Hypoketosis as a cause of symptoms in childhood hypoglycemia. European Journal of Pediatrics 134: 51-55. Van den Berghe G (1991) The role of the liver in metabolic homeostasis: implications for inborn errors of metabolism. Journal of Inherited Metabolic Diseases 14: 407--420. Vidnes J & Oyasaeter S (1977) Glucagon deficiency causing severe neonatal hypoglycemia in a patient with normal insulin secretion. Pediatric Research 11: 943-949. Voorhess ML & MacGillivray MH (1984) Low plasma norepinephrine responses to acute
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hypoglycemia in children with isolated growth hormone deficiency. Journal of Clinical Endocrinology and Metabolism 59: 790-793. Wolsdorf JI, Sadeghi-Nejad A & Senior B (1982) Fat-derived fuels during a 24-hour fast in children. European Journal of Pediatrics 138: 141-144. Wolsdorf JI, Sadeghi-Nejad A & Senior B (1983) Hypoketonemia and age-related fasting hypoglycemia in growth hormone deficiency. Metabolism 32: 457-462. Wong LTK, Davidson AGF, Applegarth DA et al (1986) Biochemical and histologic pathology in an infant with cross-reacting material (negative) pyruvate carboxylase deficiency. Pediatric Research 20: 274-279.
INTRODUCTION Glucose is the main fuel for the brain and the brain is the main consumer of glucose. The relatively higher proportion of brain mass to body size in infants and children places them at higher risk of hypoglycaemia than adults. This higher risk for the youngest ages may to some extent be counterbalanced by a higher availability of ketone bodies, the second brain fuel. The concentration of ketone bodies in blood plasma is inversely related to the age of the patient, provided that fatty acid oxidation and ketone body production are intact (Saudubray et al, 1981; Wolsdorf et al, 1982; Lamers et al, 1985). Thus, the age of the patient is one of the factors that should be taken into account in the diagnostic approach to hypoglycaemia. The definition of hypoglycaemia is the subject of much debate and this was discussed in Chapter 2. For the sake of clarity and in agreement with the best available evidence, the cut-off point for the existence of hypoglycaemia is set at a plasma glucose concentration of < 2.6 mmol/1 (47 mg/dl) (Koh et al, 1988; Cornblath et al, 1990; Aynsley-Green, 1991). The main clinical symptoms that might indicate hypoglycaemia are given in Table 1. If one or more of these symptoms are present, blood should be taken immediately for bedside testing with glucose strips; blood should also be taken for an accurate glucose assay in the laboratory. Extra blood should be taken and the plasma frozen for further biochemical and hormonal investigations later. Storing plasma (2 ml) in the freezer is an important measure to prevent diagnostic wandering once the hypoglycaemia has been treated. As hypoglycaemia is an emergency, treatment should not wait for Table 1. Clinical symptoms of hypoglycaemia. Adrenergic
Sweating, tachycardia, pallor
Cerebral
Headache, irritability, weakness, confusion, seizures, coma
Newborns
Feeding difficulties, tremor, lethargy, seizures, tachypnoea, apnoea, cyanosis, hypothermia
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the result of the glucose assay, but should start immediately after the taking of the blood sample. IMMEDIATE TREATMENT
Circulatory arrest. Blood glucose concentrations are frequently zero following cardiac arrest in children. Because cardiac massage is relatively inefficient, the cardiac output is likely to be only 15% of normal; relatively large doses of glucose should be given, i g/kg of 20% solution (5 ml/kg). Further glucose should be given if blood glucose concentrations fall below 4 mmol/1. Symptomatic hypoglycaemia. Once the appropriate specimens have been collected, glucose should be given intravenously, 0.2 g/kg followed by continuous infusion of 5-8 mg/kg per min. If after 3-5 min there has been no improvement, the blood glucose concentration should be checked. Only if the concentration is less than 4 mmol/1 should further glucose be given. It must be emphasized that giving too much glucose can be dangerous (Shah et al, 1992).
Asymptomatic hypoglycaemia. If the child has no symptoms, appropriate specimens should be collected and the child should be given oral feed appropriate for its age: milk for a baby; a drink or something to eat for an older child. THE CLINICAL APPROACH TO DIAGNOSIS The clinical approach to the patient is the first step in elucidating the aetiology of the hypoglycaemia. Several schemes exist in which the pathogenesis of hypoglycaemia is differentiated from a physiological point of view (Haymond, 1989; Senior and Sadeghi-Nejad, 1989; de Parsceau and Gibaud, 1990). We prefer a clinical approach starting from two main aetiologies: 1. 2.
Hypoglycaemia as a fuel deficit, without accompanying symptoms of intoxication. Hypoglycaemia amidst symptoms of intoxication.
This clinical differentiation is used for the presentation of the most important hypoglycaemic disorders in Table 2. Symptoms of intoxication vary from acute to chronic. Acute intoxication may express itself as a metabolic derangement, such as lactic acidosis, some organic acidaemias. In chronic intoxication an excess of normal or abnormal metabolites may gradually damage an organ and impair its functions: liver failure, cataract, malformations, developmental delay. A transition between acute and chronic intoxication occurs, for instance, in galactose intolerance. The differentiation between the two main aetiologies should be kept in mind while taking the medical history of the patient and performing
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Table 2. Aetiologies of hypoglycaemia. Disorders characterized by a fuel deficit
1. Disorders of hormonal regulation Hyperinsulinism Hypopituitarism Hypocortisolism Glucagon deficiency? Adrenaline deficiency? 2. Disorders of fatty acid oxidation or transport* 3. Disorders of ket01ysis 4. Disorders of glycogen degradation or synthesis Hepatic glycogen storage diseases? Glycogen synthase deficiency 5. Miscellaneous Malnutrition, low-birth-weight infants Liver failure Renal failure Disorders characterized by a fuel deficit and intoxication
1. Disorders of gluconeogenesis Glucose-6-phosphatase deficiencyt Fructose-1,6-diphosphatase deficiency Glycerol intolerance Phosphoenolpyruvate carboxykinase deficiency Pyruvate earboxylase deficiency 2. 3. 4. 5. 6.
Disorders of fatty acid oxidation or transport* Organic acidaemias Galactose intolerance Fructose intolerance Reye and Reye-like syndromes
* Two subgroups exist, with a fuel deficit only and with a fuel deficit plus intoxication (see Table 4). t Glucose-6-phosphatase deficiencyis a defect of both glycogen degradation and gluconeogenesis. It is placed under the latter group of intoxications because of its tendency to lactic acidosis. the physical examination. This a p p r o a c h is similar to that for inherited m e t a b o l i c disorders in general, as put f o r w a r d by S a u d u b r a y ( S a u d u b r a y and Ogier, 1990).
THE MEDICAL HISTORY T h e age of the patient at which h y p o g l y c a e m i a starts rarely gives a clue to the diagnosis. Exceptions in the i m m e d i a t e postnatal period are hyperinsulina e m i a that is characterized by excessive overutilization of glucose outside the brain, and s o m e organic acidaemias triggered by the initiation of nutrition after birth. T h e r e are a large n u m b e r of causes of h y p o g l y c a e m i a presenting during later infancy or early childhood. S y m p t o m s m a y d e v e l o p
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because of longer intervals between meals, implying longer periods of fasting, and because of metabolic sequelae triggered by infections. Hypoglycaemia during adolescence or at adult age is more often secondary to a tumour producing insulin or insulin-like growth factors (Polonski, 1992). Diet. Hypoglycaemia may be precipitated by components of the diet and any
disorders in this group belong to the category of intoxications, including galactose intolerance, fructose intolerance, some organic acidaemias and maple syrup urine disease. The hypoglycaemia is caused by complex reduction of activity of enzymes of gluconeogenesis and glycogenolysis or interference with transmitochondrial shuttles essential for gluconeogenesis (Van den Berghe, 1991). THE PHYSICAL EXAMINATION First, it should be emphasized that the rapid and 'mysterious' deterioration of an infant after a normal initial period is the most important signal of intoxication (Saudubray and Ogier, 1990). In energy deficiencies there is usually no free interval, although some milder diseases need provocative triggers such as anorexia and insufficient food intake to become manifest. Some features in the newborn infant or in the early postnatal period come to immediate attention. Marked obesity is observed in hyperinsulinaemia and related syndromes (Beckwith-Wiedemann syndrome). Cholestasis, micropenis and hypoglycaemia occur in hypopituitarism (Sheehan et al, 1992). Cataract, though not specific for diseases of galactose metabolism, should, none the less, arouse suspicion in that direction. Hepatomegaly is an important manifestation of both intoxication and storage without intoxication. The time schedule for its appearance is variable. Hepatomegaly due to galactose intolerance and some defects of gluconeogenesis develops early. Hepatomegaly due to glycogen storage disease becomes gradually more pronounced, whereas its appearance in fructose intolerance is linked to the introduction of fruit juices and sucrose. Muscle hypotonia may be a prominent feature of disorders of fatty acid oxidation and the glycogen storage diseases. Gross abnormalities such as congenital malformations and microcephaly are of widely different origin, but they occur also in some inborn errors of gluconeogenesis. LABORATORY INVESTIGATION The outline of the laboratory investigation of the hypoglycaemic infant should be in keeping with the aetiological differentiation into diseases with a fuel deficit only and those with a fuel deficit and intoxication (Table 2). The laboratory investigation is not the same for all ages, for two reasons. First, diagnostic approaches differ according to the age of onset of a disorder. Second, the young infant is more vulnerable to 'aggressive' tolerance tests that usually include a period of fasting. The different approaches for the two
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Table 3. Laboratory investigation. Fuel deficit without intoxication
Plasma
Urine
Infants Older
Glucoseprofile around the clock Acid-base status KB, FFA*, carnitine (total and free) Complete hormone profile Glucose, other sugars, lactate KB, dicarboxylicacids Carnitine (total and free) Organic acids, amino acids Calculateglucoserequirement Oral glucose tolerance test Fasting test or 3-phenylpropionicacid loading test
Fuel deficit and intoxication
Plasma
Urine Infants Older Always
Glucose,other sugars Acid-base status KB, FFA*, carnitine (total and free) Lactate, pyruvate, ammonia Organic acids, amino acids See above No tolerance tests Oral glucose tolerance test Other tolerance tests if indicated Conservationof plasma, urine (cerebrospinalfluid) and fibroblasts Diagnostic confirmationby enzyme assays or probes
* KB = ketone bodies; FFA = free fatty acids. aetiologically distinct groups and for infants against older children are detailed in Table 3. In the situation of acute fuel deficit without recognizable signs of intoxication (Table 3, upper part) it is important to determine the plasma concentrations of the three main fuels: glucose, free fatty acids and ketone bodies. It makes a great difference whether a low plasma glucose concentration is accompanied by increased levels of free fatty acids and ketone bodies, or the levels of all three fuels are too low. The latter is the case in hyperinsulinaemia. Disorders of hormonal regulation
The overconsumption of glucose by non-cerebral tissues, which is almost pathognomic for hyperinsulinaemia, can be substantiated by calculating the glucose requirement of the infant. The amount of glucose needed to keep the plasma glucose concentration at 1>2.6 mmol/1 then appears to exceed the normal requirement in infancy of 6-7 mg glucose per kg body weight per minute (Bier et al, 1977). In cases with a less excessive glucose demand, glucose homeostasis can be observed around the clock while determining the plasma glucose profile be~fore and after feeding and assaying plasma insulin concentrations at glucose nadirs. Plasma insulin concentration > 1 0 U / m l with a concomitant plasma glucose concentration < 5 0 m g / d l (2.Smmol/1) at any time suggests the existence of hyperinsulinaemia ( H a y m o n d , 1989). Simultaneous measurement of C-peptide might reinforce
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the diagnosis as soon as a cut-off point between normal and elevated values is known for this age. For older children basal C-peptide amounts to 0.250.50 nmol/1 (Bennish et al, 1990). For the differentiation of different aetiologies of hyperinsulinism, see Baker and Stanley (1991). Treatment of hyperinsulinaemia
If a discrete pancreatic tumour is responsible for the hyperinsulinaemia, then this should be removed surgically. Other patients will require medical therapy. The mainstay of this is diazoxide, in doses up to 20 mg/kg per day, which should be combined with chlorothiazide as this not only potentiates the diazoxide but also helps to reduce the fluid overload that is a frequent complication. A supplement of potassium chloride should also be given. Alternatively, the patient may be given the long-acting somatostatin analogue, octreatide, starting in a dose of 1 p~g/kg every 4-8 h (Kirk et al, 1988). The dose may need to be increased, but there is a greater chance of complications including diarrhoea. The addition of glucagon may improve control of the hypoglycaemia. If these measures fail then it may be necessary to proceed to subtotal pancreatectomy. With respect to hypoglycaemia of other hormonal aetiology (Table 2), the fuel profile is more variable and less characteristic than that of hyperinsulinaemia. For instance, hypoglycaemia due to growth hormone deficiency is accompanied by hypoketonaemia (Wolsdorf et al, 1983) and hypoglycaemia due to cortisol deficiency by hyperketonaemia (Soltesz et al, 1985). Thus, in case of doubt the hormonal profile should encompass the assay of all hormones: insulin, C-peptide, growth hormone, cortisol, pancreatic glucagon and adrenaline. Some hormone disregulations are ambiguous. It is uncertain whether adrenaline deficiency is a separate entity. It has been observed as such (Hansen et al, 1983) and associated with growth hormone deficiency (Voorhess and MacGillivray, 1984). Glucagon deficiency has been described only twice (Vidnes and Oyasaeter, 1977; Koll6e et al, 1978). As more recent literature has not appeared, its existence might be doubted. Patients with adrenal failure should be treated with cortisol in a dose of 20 mg/m 2 in divided doses. Similar doses should be used in patients with hypopituitarism, but in the young patient in particular, hypoglycaemia may remain a problem. Treatment with growth hormone may be necessary in a dose of 15 U/m 2 per week divided into daily doses. All patients on cortisol must also have a careful plan of what to do during periods of metabolic stress, such as intercurrent illness or anaesthesia. During mild infections the dose of cortisol should be doubled and during more severe illness the dose should be quadrupled. Disorders of fatty acid oxidation or transport
This group consists of disorders in which hypoglycaemia is primarily the result of a fuel deficit, and disorders characterized by both a fuel deficit and intoxication (Tables 2 and 4). Patients from either group usually have a 'silent' clinical course until a metabolic stress such as fasting, an intercurrent
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4. Disordersof fatty acidoxidationor transport.
Fuel deficit
Primarycarnitinedeficiencyor carnitinetransport deficiency Carnitine palmityltransferaseI deficiency Hydroxymethylglutaryl-CoAlyasedeficiency Fuel deficit and intoxication
Medium-chainacyl-CoAdehydrogenasedeficiency Long-chainacyl-CoAdehydrogenasedeficiency Short-chainacyl-CoAdehydrogenasedeficiency 3-Hydroxyacyl-CoAdehydrogenasedeficiency Multiple acyl-CoAdehydrogenasedeficiencies
infection or anaesthesia elicits a shortage of energy. Even the sudden infant death syndrome and Reye syndrome have been described. According to Hale and Bennet (1992), common features shared by all fatty acid disorders are episodes of hypoketotic hypoglycaemia associated with fasting, chronic involvement of fatty acid-dependent tissues (cardiac and skeletal muscle), and alteration in the degree of esterification of (plasma) carnitine. The mechanisms by which hypoglycaemia is induced are in part explained by a shortage of glucose-replacing energy substrates (ketone bodies), by lack of stimulation of gluconeogenesis through limited production of reducing equivalents, and by a limited availability of acetyl-CoA, a positive effector of pyruvate carboxylase (Sovik, 1989). All disorders of fatty acid oxidation and transport have one characteristic metabolic derangement in common. During fasting the plasma concentration of free fatty acids increases normally, whereas ketone body production fails. This dissociation between high plasma levels of free fatty acids and (very) low plasma levels of ketone bodies can be used as a screening method. However, before trying to interpret the free fatty acid/ketone body ratio, the influence of the age of the patient on fasting ketone body concentration should be assessed. Figure 1 shows the correlation between [3-hydroxybutyrate, quantitatively the most important ketone body, and the age of the child, after a 24-h fast (Wolsdorf et al, 1982). The advantage of this nomogram is the fact that it also shows the normal 95% confidence limits. The next step is to compare the ketone body level with that of glucose. Among existing nomograms we prefer to use that of Teyema et al (1980) because it specifes the age ranges of the reference children and also presents the 95% confidence limits (Figure 2). If the patient appears to be hypoketotic both with respect to age (Figure 1) and fasting glucose concentration (Figure 2), the last screening step should be to correlate plasma ketone body concentration and plasma free fatty acid concentration during fasting (Figure 3). A dissociation as mentioned above would point to a disorder of fatty acid oxidation or transport. The most informative procedure for diagnosing and differentiating fatty acid disorders is the fasting test (see Laboratory Procedures). However, as almost all patients suffering from one of these defects are fasting-intolerant, the test can be dangerous. If properly performed the majority of fatty acid disorders can be differentiated by
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considering the relationship of glucose, ketone bodies and free fatty acids in plasma, and by measuring total and flee carnitine in plasma and organic (dicarboxylic) acids and their glycine and carnitine derivatives in urine. Detection of specific metabolites, especially in urine, can be suggestive for a particular disorder, e.g. hexanoyl- and suberylglycine in medium-chain acyl-CoA dehydrogenase deficiency. Likewise, loading tests with substrates (long-chain fatty acids, 3-phenylpropionic acid) are informative for the exact site of the defect and have the advantage of safety. The ultimate diagnosis is made by enzyme investigations in cultured skin fibroblasts, leukocytes or lymphocytes (Duran et al, 1992). In the case of systemic carnitine deficiency, the carnitine clearance protocol developed by Engel et al (1981) should be performed.
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Treatment of disorders of fatty acid oxidation Acute symptoms should be treated as outlined earlier and in the longer term the patient should have a careful plan (Dixon and Leonard, 1992) to prevent complications, being given a high carbohydrate intake orally or intravenously. Patients with long-chain fatty acid disorders may be very unstable and require frequent feeds and a continuous nasogastric infusion at night, similar to that used in patients with a glycogen storage disease. Older patients may be controlled with uncooked corn starch. In contrast to disorders of fatty acid oxidation and transport are a few rare ketolysis defects in which hyperketosis is associated with hypoglycaemia, the latter symptom not being obligatory (Saudubray and Specola, 1990). It is important to differentiate these disorders from organic acidaemias. Disorders of glycogen degradation There is much overlap between the two hypoglycaemic categories of disorders with attendant hepatomegaly--without and with intoxication, respectively. This applies in the frst place to the glycogen storage diseases in which the liver enlargement is the most conspicuous clinical feature. The tendency to fasting hypoglycaemia and its severity depend on the site of the enzyme defect in the glycogenolytic cascade. In deficiencies of debranching enzyme or the phosphorylase system, gluconeogenesis is not affected and fasting hypoglycaemia is accompanied by ketosis without intoxication. In deficiency of the glucose-6-phosphatase unit gluconeogenesis, too, is blocked and fasting hypoglycaemia is accompanied by a toxic lactic acidosis, as is the case in the other enzymopathies of gluconeogenesis. For all glycogen storage diseases a laboratory screening should precede the biochemical investigation, as a preliminary diagnosis from the screening can be of help in choosing the tissue for the enzyme assay: liver tissue for glucose-6-phosphatase deficiency only, leukocytes or fibroblasts for the other glycogenoses. The screening consists of an oral glucose tolerance test (see Laboratory Procedures). In glucose-6-phosphatase deficiency, glucose ingestion suppresses hepatic lactate overproduction. This is reflected in the plasma glucose and lactate curves. While glucose rises, lactate falls. This result is typical for glucose-6-phosphatase deficiency. In the other glycogenoses lactate shows a small upward deflection that is not significantly different from that in normal children. For a firmer allocation of the other glycogenoses an oral galactose tolerance test is very helpful. In this test again the lactate curve is the most important. An abnormal increase of lactate t>3mmol/l is suspect for one of the other glycogenoses (Fernandes and van de Kamer, 1969). If the result is abnormal, enzyme assays should be performed in order to differentiate between deficiency of debranching enzyme and that of the phosphorylase system. Other screening procedures have been described, which encompass an intravenous galactose test or a glucagon test as an inital procedure. This may be dangerous, as both tests, in fact, continue the fasting state in the case of glucose-6-phosphatase deficiency.
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Glycogen synthase deficiency is a very rare disorder in which hepatic glycogen synthesis from glucose is blocked (Table 2) (Aynsley-Green et al, 1977). Fasting hypoglycaemia is accompanied by hyperketonaemia but normal lactate; the liver is not enlarged. This disorder can be suspected when plasma lactate shows an exaggerated increase after a meal (glucose effect?). In glucose-6-phosphatase deficiency the production of free glucose in the liver is reduced and the only way of maintaining normal blood glucose concentrations is to give exogenous glucose. In young babies it is often sufficient to give frequent feeds, but once the child starts sleeping through the night then additional measures will be needed. The most effective treatment is regular high-carbohydrate drinks or meals during the day (Fernandes et al, 1988). The frequency of the drinks is adjusted according to the child's response to the treatment. Most children will require 2-hourly feeds giving 0.5 g carbohydrate/kg per hour. At night the babies should have continuous nasogastric infusion of a similar rate. In young babies this is usually given as milk, but once children are older the feeds can be changed to soluble glucose polymer. The quantity of glucose decreases as the children grow older. The interval between drinks can be extended by giving uncooked corn starch. In older children and adults corn starch may maintain normoglycaemia for longer periods, so that this alone may be used to maintain good metabolic control (Smit et al, 1984). In debranching enzyme deficiency free glucose can only be synthesized via gluconeogenesis. It is usual therefore to give a high-protein diet and this may be sufficient to control hypoglycaemia (Fernandes et al, 1988). However, some patients require treatment similar to that used in patients with type I glycogen storage disease. For phosphorylase kinase and phosphorylase deficiency treatment is not often needed, but if needed it should be similar to that used in GSD type III. Miscellaneous
The last category in which the infant or child is at risk for a fuel deficit without intoxication is a miscellaneous group (Table 2). The hypoglycaemia is of multifactorial origin: limited fuel stores, organ failure, inappropriate hormone regulation. Examples are low-birth-weight infant, the diarrhoeamalnutrition syndrome (Bennish et al, 1990), hepatic failure, and renal failure in which renal gluconeogenesis falls short and insulin is insufficiently cleared by the kidneys (Arem, 1989). Disorders characterized by a fuel deficit and intoxication
Disorders characterized by a fuel deficit and intoxication are as heterogeneous as the first category with a fuel deficit only. Usually the fuel deficit is overshadowed by the severity of the intoxication. This applies particularly for disorders of gluconeogenesis and some organic acidaemias (Table 2). In disorders of gluconeogenesis, fasting hypoglycaemia is generally more
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pronounced the closer the enzyme defect is located to glucose 6-phosphate (Van den Berghe, 1991). In pyruvate carboxylase deficiency, hypoglycaemia is not an obligatory symptom, whereas in glucose-6-phosphatase deficiency it is permanently imminent and life-threatening during fasting. The reverse order might be defined for signs of intoxication, which are more severe and irreversibly damaging the closer the enzyme defect is located to pyruvate. Sites of intoxication are mainly the brain and the liver (Clayton et al, 1986; Wong et al, 1986; Biihrdel et al, 1990). The laboratory work-up is an urgent issue in order to detect those disorders that are amenable to treatment. All disorders of gluconeogenesis are characterized by hyperlactacidaemia, hyperpyruvicaemia and hyperalaninaemia, substrates at the origin of gluconeogenesis. The combination of a low plasma glucose concentration and an elevated fasting lactate concentration is particularly key information indicating that something is wrong with gluconeogenesis, either primarily or secondarily, provided that hypoxaemia is absent. The increased plasma lactate/pyruvate ratio found in pyruvate carboxylase deficiency (Robinson et al, 1984) is not a constant finding and it is doubted whether it really reflects a disturbed redox state (Blass, 1979). In pyruvate carboxylase deficiency and phosphoenolpyruvate carboxykinase deficiency, the urea cycle may be secondarily compromised. This results in increased plasma levels of ammonia, citrulline and lysine. In fructose-l,6diphosphatase deficiency hyperexcretion of glycerol (Dremsek et al, 1985) and glycerol 3-phosphate (Krywawych et al, 1986) are typical findings, shared only by glycerol intolerance, another rare disorder of gluconeogenesis (Fort et al, 1985). Next to the assay of metabolities, which are almost continuously excreted in excess in plasma and urine and therefore reflect chronic intoxication, gluconeogenesis might be explored by a tolerance test. The classic approach entails the administration of a substrate for gluconeogenesis orally or intravenously, whilst assaying metabolites at the origin of gluconeogenesis (lactate, pyruvate, alanine) and at the end of it (glucose). Many substrates have been used: alanine, lactate, glycerol, dihydroxyacetone, fructose. It should be emphasized here that one should never use a substrate that is already present in excess in the fasting state (alanine, lactate, glycerol). This limits the usefulness of this diagnostic approach. One should also be aware of the potential danger of using fructose, as a fatal outcome of fructose loading has been described (Dremsek et al, 1985). A good candidate for explorative use as regards the 'proximal' part of gluconeogenesis is dihydroxyacetone. It has been used as an oral tolerance test by Baerlocher et al (1971). A new elegant approach is the intravenous administration of a stable isotope of glucose, the use of which might disclose whether glucose cycling is disrupted and gluconeogenesis compromised. The method is both specific and safe as it involves the loading of the patient with a tracer dose of an exogenous substrate, which behaves like its endogenous analogue and is nevertheless discernible from it. In principle the patient is given a priming dose of a particular metabolite followed by a continuous infusion of the same tracer into the blood to achieve an isotopic enrichment of the metabolite in
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the blood cdmpartment 1-2% above the natural abundance. After reaching a steady state, blood samples are drawn at regular intervals for the measurement of isotopic enrichment. Together with the isotopic enrichment of the tracer in the infusate, the known rate of tracer infusion into the blood compartment and the correction for inflow of non-labelled metabolite, the measurements yield the rate of appearance of a particular metabolite into the blood compartment. In a steady state this rate equals the rate of metabolite production or disappearance. Glucose production rates can be measured by using, e.g., [2H2]glucose. Provided that glycogen stores in the liver are depleted by fasting (12-14 h), glucose production rate approaches the rate of gluconeogenesis. This test has been applied in a variety of clinical conditions, it is relatively easy to perform, and most clinical laboratories equipped with mass-spectrometric facilities are able to measure isotopic enrichment by standard gas chromatographic-mass spectrometric techniques (Bier, 1987). Using [2H2]glucose, the rate of gluconeogenesis can at least be estimated. The administration of specific C3-precursors such as [13C]alanine (Frazer et al, 1981), [13C]lactate or [13C]glycerol (Bougn6res et al, 1982) might give a better estimation of the rate of gtuconeogenesis from that precursor. However, the limited experience with the use of these substrates and technical difficulties are drawbacks in their application. A protocol for the test with [2H2]glucose is given under Laboratory Procedures. Although patients with fructose 1,6 diphosphatase deficiency may present with hypoglycaemia and lactic acidosis in the newborn period, patients usually tolerate a longer fast than patients with glucose-6-phosphatase deficiency. Indeed many patients do not present until they are older. For many patients it is sufficient to feed them regularly and give a high-carbohydrate snack before they go to sleep. When well, patients may be allowed a normal diet, but as soon as they are unwell they should exclude sucrose and all fructose and take regular high-carbohydrate drinks. If oral drinks are not tolerated, glucose should be given intravenously. Occasionally patients may require more intensive treatment similar to that used in glucose-6phosphatase deficiency. Gluconeogenesis can also be secondarily involved in some organic acidaemias, in which the overproduction of normal or abnormal compounds may cause severe intoxication (Table 2). Ketotic hypoglycaemia is observed in some organic acidaemias, notably in methylmalonic acidaemia, propionic acidaemia (late onset), maple syrup urine disease, [3-methylcrotonylcarboxylase deficiency and multiple carboxylase deficiency (Sovik, 1989). The mechanism by which hypoglycaemia occurs may in part be due to direct inhibition of a gluconeogenic enzyme by the CoA-derivative of the organic acid or by sequestration of CoA impairing the activation of pyruvate carboxylase by acetyl-CoA. In two diseases hypoglycaemia and intoxication are elicited by normal nutrients: galactose and fructose. Galactose intolerance is caused by deficiency of galactose-l-phosphate uridyltransferase. Galactose and galactose 1-phosphate accumulate before the enzyme defect, as soon as the child consumes its mother's milk or a lactose-containing formula. Galactose
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1-phosphate is the more toxic of the two substrates. It not only suppresses gluconeogenesis and glycogenolysis by inhibition of various enzymes, it also sequestrates phosphate donated by adenosine triphosphate (ATP). The phosphorylysis of ATP induces abnormalities of nucleotide metabolism and a severe energy deficit in the cells of many organs, mainly the brain, liver, kidneys and intestines. A description of the wide range of clinical symptoms is outside the scope of this chapter. Some laboratory abnormalities may lead to appropriate alarm: e.g. the presence of reducing substances in the urine, a finding that must be confirmed by sugar chromatography. A plasma sample, taken before stopping lactose-containing nutrition, should also contain galactose (normally only present in traces). Lactic acidaemia, hyperbilirubinaemia and abnormal liver function tests are less specific. In cases of suspicion, a tolerance test with galactose or lactose should be avoided (Gitzelmann, 1990) because of the risk of severe intoxication. Instead an enzyme assay of galactose-l-phosphate transferase in erythrocytes is the only proof of the diagnosis. The treatment of galactosaemia is immediate and total exclusion of all galactose for life. Fructose intolerance is caused by deficiency of fructose-l-phosphate aldolase B, which catalyses the conversion of fructose 1-phosphate into two trioses. The toxicity of fructose 1-phosphate bears some resemblance to that of galactose 1-phosphate in the case of galactose intolerance. Exposure of affected infants or children to fructose or sucrose causes hypoglycaemia, hypophosphataemia, hyperuricaemia and hypermagnesaemia (the latter two abnormalities reflecting the abnormal nucleotide metabolism). The clinical symptoms show a wide spectrum, related to the localization of the enzyme defect in liver, kidneys and intestines, not the brain. Symptoms of acute intoxication in fructose intolerance are usually more serious than in galactose metabolism, and for that reason chronic intoxication occurs less frequently in the former because of early development of a strong aversion against all fructose-containing foods and effective elimination of them. An intravenous tolerance test (not oral because of severe intestinal side-effects) is carried out if the diagnosis is suspected (see Laboratory Procedures). The results of this test in fructose-aldolase-B deficiency and fructose-l,6diphosphatase deficiency show strong resemblance (Steinmann and Gitzelmann, 1981). Thus, the diagnosis should be confirmed in liver tissue, biochemically or genetically with a probe (Cox, 1990). Treatment is by exclusion of all fructose, sucrose and sorbitol from the diet.
Reye and Reye-like syndromes. In this heterogeneous group acute or subacute metabolic derailment is elicited by a drug or a viral infection (Table 2) (Osterloh et al, 1989). Otherwise, the drug or viral infection may be only a trigger that induces a latent metabolic disorder to become manifest. Examples of drug toxicities result from ingestion of salicylate, hypoglycin (Jamaican vomiting disease) or valproate. Hypoglycaemia, present in 70% of cases, is due to impaired fatty acid [3-oxidation and impaired gluconeogenesis. Other metabolic abnormalities are, therefore, lactic acidosis, dicarboxylic acidaemia and hyperammonaemia. Accumulated products may injure cerebral and hepatic function.
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LABORATORY PROCEDURES The oral glucose tolerance test
Glucose dose: 2 g/kg body weight (max. 50 g) as a 20% solution in water Duration of test: 3 h. Previous fasting,: 3-8 h, depending on the meal interval Substrates in plasma: • Every 30 rain (two separate samples at zero time): glucose, lactate • At the beginning and the end of the test a complete profile of: glucose, lactate, pyruvate, acid-base status; free fatty acids, [3-hydroxybutyrate, acetoacetate; carnitine (total and esterified); aminoacids, organic acids, dicarboxylic acids; insulin, glucagon, adrenaline; cortisol, growth hormone Indications: glycogen storage diseases; disorders of gluconeogenesis; hyperinsulinaemia. The oral galactose tolerance test
Galactose dose: 2 g/kg body weight (max. 50 g) as a 20% solution in water. Duration of test: 3 h. Previous fasting: as for glucose test. Substrates in plasma • Every 30 rain (two separate samples at zero time): glucose, galactose, lactate. Indication: glycogen storage diseases (glucose-6-phosphatase deficiency excepted). The intravenous fructose tolerance test
Fructose dose: 0.2 g/kg body weight as a 10% solution in water. Duration of test: 3 h. Previous fasting: as for glucose test. Substrates in plasma: • Every 30 min (two separate samples at zero time): glucose, fructose, lactate, urate, phosphate. Indications: fructose intolerance; fructose-l,6-diphosphatase deficiency. Exploration of fasting tolerance
• Start fasting at 22:00, determine blood glucose concentration by a rapid bedside method after 6, 8, 9, 10 h, thereafter every 30 rain. Encourage the drinking of water. • Stop at noon or earlier, as soon as blood glucose < 3.3 mmol/l (60 mg/dl); do not determine substrates other than glucose.
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• The duration of the fasting period tolerated by the child allows one to fix the earliest time at which the extended fasting test can start 2-3 days later, without endangering the child at night.
The extended fasting test Do not perform an extended fasting test before having explored the fasting tolerance of the child (see above). Duration of the test is 20 hours (max. 24); the drinking of water is encouraged. Start the test around 18:00 (see above). Substrates in plasma: • Glucose after 12h (or earlier, see Exploration of Fasting Tolerance), thereafter hourly until glucose <~3.3 retool/l, then every 30 min; stop at glucose 2.6 mmol/l. • Acid-base status, free fatty acids, ketone bodies, lactate as soon as glucose ~<3.3 mmol/1; continue thereafter simultaneously with glucose. • Complete profile at the end of the test (see Oral Glucose Tolerance Test). Substrates in urine (6-h periods, starting at 18:00): glucose, other sugars, lactate; ketone bodies, dicarboxylic acids; carnitine (total and free); organic acids, amino acids. Indications: disorders of fatty acid oxidation.
Stable isotope test using [2H2]glucose Infusion of [2H2]glucose 5 rag/ks for 10 rain followed by a rate of 45 ~Lg/kg per min using a syringe infusion pump. After achieving steady-state conditions (usually 60 rain after starting the test) blood samples are drawn at 30-rain intervals for the measurement of glucose concentration and isotopic enrichment. Duration of test: 5-6 h. Previous fasting: 12-14 h. Substrates in plasma: • Every 30 rain (one sample prior to the test): glucose and [2H2]glucose. Indications: disorders of gluconeogenesis, disorders of fatty acid oxidation or transport, glycogen storage diseases.
SUMMARY The diagnostic approach to children with suspected hypoglycaemia is facilitated by the fact that two clinically distinct groups exist with little overlap: 1. 2.
Hypoglycaemias characterized by a fuel deficit only. Hypoglycaemias in which the fuel deficit is overshadowed by symptoms of intoxication.
This differentiation is used when taking the medical history, performing the physical examination, and planning the laboratory investigations. The latter
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may encompass tolerance tests, a fasting test or the use of a non-radioactive glucose isotope. Protocols for such tests are given.
REFERENCES Arem R (1989) Hypoglycemia associated with renal failure. Endocrinology and Metabolism Clinics of North America 18: 103-121. Aynsley-Green A (1991) Glucose: a fuel for thought! Journal of Paediatric Child Health 27: 21-30. Aynsley-Green A, Williamson DH & Gitzelmann R (1977) Hepatic glycogen synthase deficiency. Archives of Disease in Childhood 52: 573-579. Baerlocher K, Gitzelmann R, Ntissli R & Dummermuth G (1971) Infantile lactic acidosis due to hereditary fructose-l,6-diphosphatase deficiency. Helvetica Paediatrica Acta 26: 489-506. Baker L & Stanley CA (1991) Management of hyperinsulinismin infants. Editor's column. The Journal of Pediatrics 119: 755-757. Bennish ML, Azad AK, Rahman O & Phillips RE (1990) Hypoglycemia during diarrhea in childhood. Prevalence, pathophysiology, and outcome. New England Journal of Medicine 322: 1357-1363. Bier DM, Leake RD, Haymond MW et al (1977) Measurement of 'true' glucose production rates in infancy and childhood with 6,6-dideuteroglucose. Diabetes 26: 1016-1023. Bier DM (1987) The use of stable isotopes in metabolic investigations. Baillidre's Clinical Endocrinology and Metabolism 1: 817-836. Blass JP (1979) Disorders of pyruvate metabolism. Clinical review. Neurology 29: 280-286. Bonefont JP, Specola NB, Vassault A e t al (1990) The fasting test in paediatrics: application to the diagnosis of pathological hypo- and hyperketotic states. European Journal of Pediatrics 150: 80-85. Bougn~res PF, Karl IE, Hillmann LS & Bier DM (1982) Lipid transport in the human newborn: palmitate and glycerol turnover and the contribution of glycerol to neonatal hepatic glucose output. Journal of Clinical Investigation 70: 262-270. Bfihrdel P, B6hme HJ & Didt L (1990) Biochemical and clinical observations in four patients with fructose-1,6-diphosphatase deficiency. European Journal of Pediatrics 149: 574--576. Clayton PT, Hyland K, Brand M & Leonard JV (1986) Mitochondrial phosphoenolpyruvate carboxykinase deficiency. European Journal of Pediatrics 1986: 46-50. Cornblath M, Schwartz R, Aynsley-Green A & Lloyd JK (1990) Hypoglycemia in infancy: the need for a rational definition. Pediatrics 85: 834-837. Cox TM (1990) Hereditary fructose intolerance Baillidre's Clinical Gastroenterology 4: 61-78. Dixon MA & Leonard JV (1992) Intercurrent illness in inborn errors of intermediary metabolism. Archives of Disease in Childhood 67: 1387-1391. Duran M, Cleutjens CBJM, Ketting D et al (1992) Diagnosis of medium-chain acyl-CoA dehydrogenase deficiency in lymphocytes and liver by a gas chromatographic method: the effect of oral riboflavin supplementation. Pediatric Research 31: 39-42. Dremsek PA, Sacher M, St6gmann W e t al (1985) Fructose-l,6-diphosphatase deficiency: glycerol excretion during fasting test. European Journal of Pediatrics 144: 203-204. Engel AG, Rebouche CJ, Wilson DM et al (1981) Primary systemic carnitine deficiency. II Renal handling of carnitine. Neurology 31: 819-825. Fernandes J & van de Kamer JH (1969) A screening method for liver glycogen diseases. Archives of Disease in Childhood 44: 311-317. Fernandes J, Leonard JV, Moses SW et al (1988) Glycogen storage disease-Recommendations for treatment. European Journal of Pediatrics 147: 226-228. Fort P, Wapnir RA, de Rosas F & Lifshitz F (1985) Long-term evolution of glycerol intolerance syndrome. Journal of Pediatrics 106: 453-456. Frazer TE, Karl IE, Hillman LS & Bier DM (1981) Direct measurement of gluconeogenesis from L-[2,3-13Cz]alanine in human neonate during the first eight hours of life. American Journal of Physiology 240: E615-E621. Gitzelmann R (1990) Disorders of gatactose metabolism. In Fernandes J, Saudubray J-M & Tada K (eds) Inborn Metabolic Diseases, pp 95-105. Berlin: Springer-Verlag.
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Hale DE & Bennett MJ (1992) Fatty acid oxidation d/sorders: A new class of metabolic disorders. Journal of Pediatrics 121: 1-11. Hansen IL, Levy MM & Kerr DS (1983) Differential diagnosis of hypoglycemia in children by responses to fasting and 2-deoxyglucose. Metabolism 32: 960-970. Haymond MW (1989) Hypoglycemia in infants and children. Endocrinology and Metabolism Clinics of North America 18: 211-252. Kirk JM, Di Silvio L, Hindmarsh PC & Brook CGD (1988) Somatostatin analogue in short term management of hyperinsulinism. Archives of Disease in Childhood 63: 1493-1494. Koh THHG, Aynsley-Green A, Tarbit M & Eyre JA (1988) Neural dysfunction during hypoglycaemia. Archives of Disease in Childhood 63: 1353-1358. Koll6e LA, Monnens LA, Cejka V & Wilms RH (1978) Persistent neonatal'hypoglycemia due to glucagon deficiency. Archives of Disease in Childhood 53: 422-424. Krywawych S, Katz G, Lawson AM et al (1986) Glycerol-3-phosphate excretion in fructose-l,6diphosphatase deficiency. Journal of lnherited Metabolic Diseases 9: 388-392. Lamers KJB, Doesburg WH, Gabre61s FJM et al (1985) The concentration of blood components related to fuel metabolism during prolonged fasting in children. CIinica Chimica Acta 152: 155-163. Osterloh J, Cunningham W, Dixon A & Combest D (1989) Biochemical relationships between Reye's and Reye's-like metabolic and toxicological syndromes. Medical Toxicology and Adverse Drug Experience 4: 272-294. de Parscau L & Guibaud P (1990) Diagnostic 6tiologique des hypoglyc6mies de l'enfant. P~diatrie 45: 165-171. Polonski KS (1992) A practical approach to fasting hypoglycemia. New England Journal of Medicine 326: 1020-1021. Robinson BH, Oei J, Sherwood WG et al (1984) The molecular basis for the two different clinical presentations of classical pyruvate carboxylase deficiency. American Journal of Human Genetics 36: 283-294. Saudubray J-M & Ogier H (1990) Clinical approach to inherited metabolic disorders. In Fernandes J, Saudubray J-M & Tada K (eds) Inborn Metabolic Diseases, pp 3-25. Berlin: Springer-Verlag. Saudubray J-M & Specola N (1990) Ketolysis defects. In Fernandes J, Saudubray J-M & Tada K (eds) Inborn Metabolic Diseases', pp 411-418. Berlin: Springer-Verlag. Saudubray J-M, Marsac C, Limal J-M et al (1981) Variation in plasma ketone bodies during a 24 hour fast in normal and in hypoglycemic children: relationship to age. Journal of Pediatrics 98: 904-908. Senior B & Sadeghi-Nejad A (1989) Hypoglycemia: a pathophysiologic approach. Acta Paediatrica Scandinavica (supplement 325): 1-27. Shah A, Stanhope R & Matthew D (1992) Hazards of pharmacological tests of growth hormone secretion in childhood. British Medical Journal 304: 173-174. Sheehan AG, Martin SR, Stephure D & Scott RB (1992) Neonatal cholestasis, hypoglycemia, and congenital hypopituitarism. Journal of Pediatric Gastroenterology and Nutrition 14: 426-430. Smit GPA, Berger R, Potasnick R et al (1984) Starch therapy in type I glycogen storage disease. Paediatric Research 18: 879-881. Soltesz G, Dillon MJ, Jenkins PA et al (1985) Isolated glucocorticoid deficiency: metabolic and endocrine studies in a 5-year-old boy. European Journal of Pediatrics 143: 297-300. Sovik O (1989) Inborn errors of amino acids and fatty acid metabolism with hypoglycemia as a major clinical manifestation. Acta Paediatrica Scandinavica 78: 161-170. Stanley CA (1990) Disorders of fatty acid oxidation. In Fernandes J, Saudubray J-M & Tada K (eds) Inborn Metabolic Diseases, pp 395-410. Berlin: Springer-Verlag. Steinmann B & Gitzelmann R (1981) The diagnosis of hereditary fructose intolerance. Helvetica Paediatrica Acta 36: 297-316. Teyema HL, van Gelderen HH & Giesberts AH (1980) Hypoketosis as a cause of symptoms in childhood hypoglycemia. European Journal of Pediatrics 134: 51-55. Van den Berghe G (1991) The role of the liver in metabolic homeostasis: implications for inborn errors of metabolism. Journal of Inherited Metabolic Diseases 14: 407--420. Vidnes J & Oyasaeter S (1977) Glucagon deficiency causing severe neonatal hypoglycemia in a patient with normal insulin secretion. Pediatric Research 11: 943-949. Voorhess ML & MacGillivray MH (1984) Low plasma norepinephrine responses to acute
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