ChemPath: Paediatric Clinical Chemistry
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What is the average birthweight of male babies born at term?
3.3 kg
List some common problems in LBW babies.
• Respiratory distress syndrome • Retinopathy of prematurity • Intraventricular haemorrhage • Patent ductus arteriosus • Necrotising enterocolitis
What is necrotising enterocolitis?
• Inflammation of the bowel wall progressing to necrosis and perforation • Characterised by bloody stools, abdominal distension and intramural air (pneumatosis intestinalis)
In the developing fetus, when do: • Nephrons develop • Start producing urine • Have fully competent nephrons • Achieve functional maturity of glomerular function
• Nephrons develop = week 6 • Start producing urine = week 10 • Have fully competent nephrons = week 36 • Achieve functional maturity of glomerular function = 2 years
What are the implications of the large SA:V ratio of babies?
• Low GFR for surface area • Results in slow excretion of solute load • Limited Na+ available for H+ exchange
List some key differences of the neonatal kdineys compared to adult kidneys and their implications.
• Short proximal tubule so lower reabsorptive capability • Reduce reabsorption of bicarbonate leading to a propensity to acidosis • Loop of Henle and distal collecting ducts are short and juxtaglomerular leading to reduced concentrating ability (maximum urine osmolality of 700 mmol/kg) • Distal tubule is relatively unresponsive to aldosterone leading to persisten sodium loss and reduced potassium excretion (sodium loss of 1.8 mmol/kg/day, and upper limit of K+ of 6 mmol/L in neonates)
Why does glycosuria occur at a lower plasma glucose level in neonates?
Short proximal tubule means that they have a lower ability to reabsorb
Describe how body water content is different in neonates compared to adults.
Term neonates are 75% water compared to 60% in adults (and 85% in preterm infants)
What happens to the body water content in the first week of life?
• Pulmonary resistance drops and you get release of ANP leading to redistribution of fluid • This can lead to up to 10% weight loss within the first week of life • Roughly 40 mL/kg loss in preterm infants
How are the daily fluid and electrolyte requirements different in neonates compared to adults?
• Sodium, potassium and water requirements are higher NOTE: sodium requirements are particularly high in preterm neonates (<30 weeks), so plasma Na+ should be measured daily in these patients. K+ supplements should be given once urine output > 1mL/kg/hr has been achieved
Why do babies have higher insensible water loss?
• High surface area • Increased skin blood flow • High respiratory rate and metabolic rate • Increased transdermal fluid loss NOTE: skin is not keratinised in premature infants
Drugs can cause electrolyte disturbances in neonates. Give examples of drugs that an do this and briefly describe the mechanism.
• Bicarbonate for acidosis (contains high Na+) • Antibiotics (usually sodium salts) • Caffeine/theophylline (for apnoea) - increases renal Na+ loss • Indomethacin (for PDA) - causes oliguria NOTE: growth can also cause electrolyte disturbance
What is hypernatraemia usually caused by in neonates?
• Dehydration NOTE: usually uncommon after 2 weeks NOTE: food poisoning and osmoregulatory dysfunction are differentials
What is hyponatraemia usually caused by in neonates?
Congenital adrenal hyperplasia
Outline the pathophysiology of congenital adrenal hyperplasia.
• Most commonly caused by 21-hydroxylase deficiency • Leads to reduced cortisol and aldosterone production and shunting of 17-OH progesterone and 17-OH pregnenelone which goes towards androgen synthesis
Outline the clinical features of congenital adrenal hyperplasia.
• Hyponatraemia/hyperkalaemia • Hypoglycaemia • Ambiguous genitalia in female neonates • Growth acceleration
List three reasons for neonatal hyperbilirubinaemia.
• High level of bilirubin synthesis • Low rate of transport into the liver • Enhanced enterohepatic circulation
How much bilirubin can 1 g/L of albumin bind?
10 µM/L
How much albumin does the average term neonate have? How much bilirubin can this albumin bind?
34 g/L of albumin 340 µM/L of bilirubin
What is the issue with free bilirubin?
It can cross the blood-brain barrier leading to kernicterus
What are the three bilirubin thresholds in neonates?
• No treatment • Phototherapy • Exchange transfusion
List some causes of neonatal jaundice.
• G6PD deficiency • haemolytic anaemia (ABO, rhesus) • Crigler-Najjar syndrome
What is prolonged jaundice?
Jaundice that lasts >14 days in term babies or >21 days in preterm babies
List some causes of prolonged jaundice.
• Prenatal infection/sepsis • Hypothyroidism • Breast milk jaundice