CHAPTER 25
Paediatric clinical biochemistry
CHAPTER OUTLINE
POSTNATAL INVESTIGATION OF THE SMALL FOR GESTATIONAL AGE NEWBORN
Interpretation of renal function tests
CALCIUM AND PHOSPHORUS METABOLISM
Disorders of calcium and phosphorus metabolism
Unconjugated hyperbilirubinaemia: physiological jaundice
Unconjugated hyperbilirubinaemia: pathological causes
INTRODUCTION
National statistics for the UK record approximately 700 000 live births per year. In England and Wales in 2010, the infant mortality rate was 4.3 deaths per 1000 live births, the lowest ever recorded, which compares with an infant mortality rate of 12 deaths per 1000 live births in 1980. This considerable decrease in infant mortality can be partly explained by improvements in health care and more specifically, in midwifery and neonatal intensive care. The neonatal period, defined as the first four weeks of life, is associated with the highest mortality rate in childhood, with infants of very low (< 1500 g) and low (< 2000 g) birth weight accounting for a significant proportion of deaths. The mortality rate reflects the consequences of congenital malformation and disease and the difficulties of transition from intra- to extrauterine life, particularly in premature infants; such a high rate is not seen again until the seventh decade of life. For these reasons, the majority of this chapter is devoted to neonatal clinical biochemistry. Disorders of older children are only included where there are particular problems with interpretation of clinical biochemistry data.
The most common cause of admission to a neonatal intensive or special care unit is prematurity. ‘Pre-term’ in the practical sense rarely includes neonates of more than 32 weeks of gestation at birth. The problems of prematurity can be related to immaturity of vital systems, for example the lungs, kidneys and central nervous system, or inadequacy of energy stores or of nutrients such as calcium and iron that are accreted during the third trimester of pregnancy. Another high-risk group is those neonates with a birth weight inappropriate for their gestational age. In most cases, these babies are small for gestational age (SGA), defined as birth weight below the 10th centile for newborns of the same gestational age in the same population. However, neonates born to mothers with diabetes may be overweight as a result of hyperglycaemia in utero.
POSTNATAL INVESTIGATION OF THE SMALL FOR GESTATIONAL AGE NEWBORN
Inappropriate birth weight is often readily explicable from the clinical history. Common causes of growth retardation are multiparity, pre-eclampsia, infection and drug addiction. An uncommon example would be a first-born SGA infant with microcephaly, the mother of whom should be investigated for possible hyperphenylalaninaemia by analysis of maternal plasma amino acids.
Intrauterine infections
Antenatal infections can affect the developing fetus, leading to congenital malformations and/or low birth weight. Many viruses have teratogenic effects on the fetus, so maternal infection in the first trimester, when organogenesis is occurring, tends to cause the greatest effect. In the UK, universal screening for syphilis, hepatitis B, HIV and susceptibility to rubella infection is offered to all mothers in the early stages of pregnancy, with follow-up diagnosis and treatment available for both mother and infant as appropriate.
Intrauterine infection is indicated by an increase in the infant’s serum IgM concentration (Table 25.1) and is often associated with prolonged unconjugated hyperbilirubinaemia and abnormal liver function tests. Measurement of the serum IgM concentration is only useful as an indicator of intrauterine infection before six weeks of age. After this time, the normal rise in serum IgM concentration may mask any early increase, and investigation requires specific serological testing.
Maternal drug abuse
Two percent of women in urban areas of the UK have been reported to have had a positive screening test for at least one prohibited substance at the time they first attended for antenatal care. These positive tests included amphetamines, barbiturates, cannabinoids, cocaine metabolites, methadone and opiates.
The consequence of maternal drug abuse is dependent upon the intensity of use. Infants may be born prematurely and/or SGA, and a proportion exhibit withdrawal symptoms during the neonatal period. The appearance of these symptoms depends upon the rates of clearance of the drugs and their active metabolites. For example, the irritability, tremors and convulsions of heroin withdrawal appear during the first 24 h of life, whereas the same symptoms do not generally appear until after 24–48 h when methadone is the substance involved.
A urine drug screen of a symptomatic neonate is often negative. Poor drug penetration across the placenta means that there are higher concentrations in maternal tissues and, consequently, it is more rewarding to screen maternal urine. The likelihood of detecting maternal drug abuse by investigating the urine of the neonate follows a decreasing order of cocaine > methadone > heroin > benzodiazepines.
RESPIRATORY DISORDERS
Respiratory distress
Up to 55% of the fetal cardiac output goes to the placenta. At birth there is an increase in peripheral resistance and a reduction in pulmonary vascular resistance, both of which ensure the closure of the foramen ovale and constriction of the ductus arteriosus. Blood is then diverted through the pulmonary vessels and the adult type of circulation is established. In newborn infants, haemoglobin F (HbF) is the predominant haemoglobin, accounting for about 75% of the total. Haemoglobin F has a greater affinity for oxygen than haemoglobin A (HbA) and its oxygen dissociation curve is shifted to the left, ensuring adequate oxygen exchange at a significantly lower partial pressure of oxygen (PO2). As the partial pressure of arterial oxygen (PaO2) in blood rises with postnatal age, blood HbF concentrations fall, and by six months of age HbF accounts for only about 5% of the total haemoglobin.
Respiratory distress syndrome (RDS) may be caused by a variety of conditions (see Box 25.1). The most common cause in pre-term infants is hyaline membrane disease, primarily attributable to immature surfactant synthesis. Surfactant ensures patency of the alveoli by reducing the surface tension of the alveolar wall, and is made up of phosphatidylcholine (lecithin), phosphatidylglycerol and phosphatidylinositol. Surfactant synthesis begins by the 20th week of gestation. It increases slowly until the 34th week and then more rapidly as the type II alveolar cells mature. The rate of synthesis is sensitive to cold, hypoxia and acidaemia and may be halved by postnatal exposure to temperatures < 35°C or an arterial [H+] > 56 nmol/L (pH < 7.25). High intrauterine glucose concentrations, resulting from poorly controlled maternal diabetes mellitus, can also delay maturation of fetal surfactant synthesis. Fetal type II alveolar cell maturation may be enhanced by maternal steroid therapy. Corticosteroids are often given to women who are at risk of premature delivery at 24–34 weeks of gestation.
The incidence of hyaline membrane disease is inversely related to postconceptional age. The signs of the disease, which occur within four hours of birth, include sternal retraction, intercostal and subcostal recession, expiratory grunt and tachypnoea with a respiratory rate > 60 per min. A characteristic ‘ground glass’ appearance is present on radiological examination of the chest. The appearance of these reticulogranular opacities allows diagnosis of hyaline membrane disease with 90% confidence. Other entities that may produce similar opacities include immature lung, wet lung disease, neonatal pneumonia, idiopathic hypoglycaemia, congestive heart disease, maternal diabetes and early pulmonary haemorrhage.
Group B streptococcal pneumonia may present as early as four hours after infection, during birth, from this common vaginal organism. It occurs more often in the pre-term than the full-term infant. Measurement of serum C-reactive protein and microbiological investigations can be used to distinguish bacterial pneumonia from hyaline membrane disease.
Meconium aspiration and transient tachypnoea of the newborn (TTN) occur most often in full- or post-term infants. The passage of meconium (the contents of the fetal bowel) in utero is associated with fetal hypoxia. In some cases, the contaminated liquor is aspirated. Clinical symptoms usually appear 12–24 h after birth. Transient tachypnoea of the newborn occurs in term neonates, usually those born by caesarean section, and appears to be due to incomplete stimulation of adrenergic mechanisms for lung clearance during birth. Pneumothorax may occur as a complication of either of these conditions or as a result of mechanical ventilation of pre-term infants.
In RDS, the neonate develops hypoxia and a respiratory acidosis, both of which cause an increase in pulmonary vascular resistance and thus pulmonary hypertension with a left-to-right shunt. Hypoxia enhances anaerobic glycolysis and may result in lactic acidosis. Additional complications include brain damage and oedema, and hypotension that may lead to renal failure, paralytic ileus and/or necrotizing enterocolitis (NEC).
Non-pulmonary causes of respiratory distress are usually self-evident and normally improve with treatment of the underlying conditions (see Box 25.1). Up to 20% of infants weighing < 1750 g at birth have a patent ductus arteriosus (PDA). Medical management of PDA includes fluid restriction and stimulation of diuresis. Pharmacological closure may be achieved using indometacin, an inhibitor of prostaglandin synthetase. Contraindications to this treatment include renal insufficiency (plasma creatinine concentration > 160 μmol/L) with or without oliguria, bleeding disorders and NEC.
Management of respiratory distress
This may involve assisted ventilation with oxygen, aiming to maintain the PaO2 in the range 6–12 kPa, PaCO2 5.5–8.0 kPa and arterial O2 saturation (SaO2) 88–95%. Although careful monitoring of blood gases is required, repeated blood sampling may cause anaemia. Anaemia may necessitate transfusion, usually with adult haemoglobin. Increased HbA may compromise oxygen uptake in the lungs in the presence of low alveolar PO2, thereby further aggravating tissue hypoxia. Correction of the metabolic acidosis with sodium bicarbonate can cause oedema and precipitate heart failure, owing to sodium and water overload. Too much oxygen may result in retrolental fibroplasia with retinal detachment and blindness. Long-term ventilation is associated with bronchopulmonary dysplasia, hyperinflated emphysematous lungs with extensive alveolar destruction and widespread fibrosis. The proposed role of free oxygen radicals in the development of these complications has led many paediatricians to administer vitamin E routinely to all infants on ventilators.
Transcutaneous PO2 (TcO2) polarographic electrodes can be used to monitor oxygen treatment in infants who have good skin perfusion; the results correlate well with arterial PO2 measurements. Transcutaneous PO2 is measured at a skin temperature of 44 °C so the electrodes require frequent re-siting with recalibration in order to prevent skin burns. These problems do not occur with pulse oximetry, which measures SaO2 of oxyhaemoglobin and reduced haemoglobin during an arterial pulse, by differential light absorption at 660 and 940 nm. The results correlate well with arterial PaO2 measurements at SaO2 values > 65%, but are falsely low in the presence of methaemoglobin because its molar extinction characteristics imitate reduced haemoglobin.
Surfactant administered through an endotracheal tube significantly reduces the incidence and complications of respiratory distress in newborns at risk of developing hyaline membrane disease.
Apnoea of prematurity
Apnoea of prematurity, defined as a cessation of breathing for > 20 s, with or without bradycardia and cyanosis, occurs in up to 85% of infants weighing < 1000 g at birth. The major cause of apnoea is immaturity of the central respiratory drive, with poor sensitivity to changes in PaCO2. This is compounded by the suppressed respiratory response to hypoxia, which serves to reduce oxygen requirements in utero and which persists in the pre-term infant. Poor coordination of the major respiratory muscles of the chest wall and the upper airways can lead to obstruction, usually at the level of the pharynx. Respiratory effort against a closed airway distorts the chest wall and activates the intercostal phrenic inhibitory reflex. Apnoea is worsened by infection, thermal instability and hypoglycaemia.
Apnoea of prematurity is treated with methylxanthines, which augment central respiratory drive and increase the sensitivity of chemoreceptors to changes in PaCO2.
RENAL FUNCTION
The investigation of renal disorders and the monitoring of fluid and electrolyte replacement in pre-term infants are complicated by immaturity of organ function, and by the difficulty in collecting accurately timed urine samples.
Nephrons develop from about the sixth week of gestation and start producing urine from about the tenth week. The full complement of nephrons is present by the 36th week of gestation. Glomerular function develops more rapidly than tubular function. At term, the tubules are relatively short and underdeveloped. They increase in length and develop increasing absorptive and secretory function during the neonatal period. Functional immaturity is characterized by an inappropriately high urinary sodium excretion for the glomerular filtration rate (GFR) and an impaired response to a sodium load. This is pertinent to the management of pre-term infants, in whom the sodium requirement per kg body weight may be higher than that for term or older infants, with more mature tubular function (Table 25.2). The loops of Henle are principally juxtamedullary in position: they, too, are relatively short compared with those in older infants and adults and do not penetrate deeply into the renal medulla, thereby limiting renal concentrating ability.
In low birth weight infants, glomerular function is adequate for growth but may be inadequate to cope with the increased nitrogenous load during periods of catabolism, starvation, hypoxia, infection and infusion of nitrogen-containing solutions. Tubular function is adequate for the filtered load associated with a reduced GFR and can normally maintain an appropriate excretory function and systemic acid–base status during the anabolic growth phase.
The GFR increases with postconceptional age as renal blood flow increases and renal vascular resistance decreases; full functional maturity is not reached until about the second year of life.
Total body water constitutes about 85% of the body weight of pre-term infants weighing < 1.0 kg at birth, compared with about 75% in term infants and 60% in adults (Fig. 25.1). Relatively more water is in the extracellular compartment than in the intracellular compartment. Total body sodium is about 45 mmol and potassium 75 mmol, and blood volume about 70 mL. During the first few weeks of life, there is a contraction of the extracellular space associated with an increase in urinary sodium loss. This results in an initial 10–15% reduction in body weight. The onset of natriuresis coincides with improvement of lung function and is probably related to a reduction in pulmonary vascular resistance and the release of natriuretic peptides. This contributes to the contraction of the extracellular fluid space. Excess fluid and sodium administration, before the onset of the natriuretic phase, may worsen respiratory distress, delay the closure of the ductus arteriosus and precipitate oedema with hyponatraemia. It is not appropriate to rely on spot urinary sodium concentrations to monitor water and sodium requirements during the first few weeks of life in a premature infant.
FIGURE 25.1 Changes in body water as a function of age for pre-term and term infants. The timescale indicates expected date of birth and postnatal age.
Antidiuretic hormone (ADH) secretion, in response to volume reduction and hyperosmolality, occurs from about the 25th week of gestation. The interstitial osmolality of the renal medulla, which is the main determinant of the urine concentrating ability in the presence of ADH, is dependent on the countercurrent multiplication mechanism in the loops of Henle and on the interstitial concentrations of sodium, chloride and urea. The reduced GFR and urea clearance decrease the tubular reabsorption of urea and the interstitial urea concentration; consequently, the concentrating ability is impaired, despite appropriate ADH output. Even full-term infants have a limited capacity to conserve water, and urine osmolality rarely rises > 700 mmol/kg. Approximate fluid requirements are shown in Table 25.2.
Hyponatraemia
In neonates, hyponatraemia may be caused by maternal fluid retention or overload during labour, or excess neonatal hypotonic fluid administration during the postnatal period. In addition, inappropriate antidiuresis may develop as a consequence of respiratory disease or intraventricular haemorrhage. In older infants, a dilutional hyponatraemia may be seen, caused by a combination of water retention and sodium depletion in response to increased intestinal or renal fluid losses. Signs of hyponatraemia are related to the rate of fall of plasma sodium concentration rather than to its actual value, and may include hypotension, drowsiness and convulsions. Congenital adrenal hyperplasia (CAH) must always be considered as a possible cause of hyponatraemia (see Chapter 21).
Hypernatraemia
As in adult patients, hypernatraemia may be caused either by water depletion or by excess sodium administration or retention. Insensible water loss is significantly greater in pre-term infants compared with children and adults. The reasons include:
• greater surface area to body volume ratio
• increased skin blood flow
• increased metabolic and respiratory rates
• lack of subcutaneous fat
• greater transepidermal fluid loss.
The epidermis of the skin matures by about the 28th week of gestation with keratinization of the stratum corneum. Consequently, infants born before 28 weeks are at greater risk of excess fluid loss and may lose up to 60 mL/kg/24 h of water through the skin, compared with about 10 mL/kg/24 h in term infants. Additional environmental factors, such as temperature and humidity, also affect transepidermal fluid loss.
Impaired responses to changes in blood volume and plasma osmolality make infants particularly vulnerable to developing hypernatraemia. Increased urinary free water loss may be caused by glycosuria secondary to either hyperglycaemia or to the low renal threshold for glucose. In pre-term infants, glycosuria may be present when the plasma glucose concentration is as low as 5.6 mmol/L. Diabetes insipidus, caused, for example, by intracranial injury at birth, may also result in an increase in free water clearance.
Although the major cause of sodium and water imbalance in premature infants is related to the functional immaturity of organs, it is important to be aware that some drugs may exacerbate the situation, e.g. sodium bicarbonate, indometacin and methylxanthine derivatives such as caffeine.
Morbidity and mortality from hypernatraemia are caused by the increased extracellular osmolality and its effects on fluid distribution between fluid compartments. Infants with hypernatraemia may present clinically with irritability and lethargy, convulsions and coma; lesions within the CNS include intracerebral and intraventricular haemorrhages, as well as sinus and small vessel thromboses. The majority of infants who survive suffer no long-term sequelae, although a minority develop persistent neurological abnormalities.
Hydrogen ions
Under normal circumstances, the kidneys can excrete the hydrogen ion load and generate sufficient buffering capacity to maintain normal systemic acid–base status, despite apparent immaturity of renal tubular function. However, if hydrogen ion production is increased, premature infants are prone to develop a metabolic acidosis in addition to the respiratory acidosis of respiratory distress. The proximal tubular threshold for bicarbonate reclamation is reduced, as is the distal tubular response to an ammonium chloride load: thus the generation of buffering capacity is reduced. In addition, urinary phosphate excretion, which is dependent on GFR, phosphorus intake and plasma phosphate concentration, may be significantly reduced in infants with phosphate depletion.
