Different problems occur when low birth weight is due to prematurity or intrauterine growth restriction.

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The best guide to gestation is the menstrual history and ultrasound estimate of fetal size made before 20 weeks’ gestation. Physical and neurological features of the newborn change with gestation. As gestation increases, skin becomes keratinized, thicker, and has less lanugo (fine hair). Ear cartilage and the breast nodule develop during the third trimester. In boys, the testes descend into the scrotum at around 36 weeks. In girls, the labia majora cover the labia minora towards term. As gestation increases, muscle tone and ligamentous laxity increase.

8.2 The preterm baby

8.2.1 Respiratory distress syndrome (RDS)

RDS, also known as hyaline membrane disease, is virtually confined to preterm babies and is common below 32 weeks.

PRACTICE POINT

Antenatal maternal steroids lead to a 60% reduction in the likelihood of RDS, and should be used whenever possible.

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Surfactant lowers surface tension in the liquid that lines the respiratory tract. Without surfactant, the alveoli cannot be inflated and tend to collapse. The result is atelectasis (poor aeration) and the appearance on X-ray (Figure 8.2). Increased work of breathing (respiratory distress) occurs, leading to poor oxygenation and carbon dioxide retention (respiratory failure), and often to apnoeic spells.

Figure 8.2 Respiratory distress syndrome: note the characteristic ground glass appearance in all lung fields. In this picture, the tracheal tube is too far down, and was pulled back afterwards.

Respiratory distress

• Tachypnoea (>60 bpm)
  
• Intercostal recession
  
• Subcostal recession
  
• Grunting
  
• Nasal flare
  
• Cyanosis.

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Surfactant replacement therapy can be given through the endotracheal tube soon after birth, and leads to reduced mortality, less risk of pneumothorax and reduced lung damage. RDS resolves over a period of 3–7 days as surfactant production increases. The key to management is to keep the baby alive and undamaged during this period. Infants with significant RDS need assisted ventilation (mechanical ventilation or continuous positive airway pressure). Careful monitoring is crucial.

8.2.2 Bronchopulmonary dysplasia (BPD)

This chronic lung disease occurs in some preterm babies who have been ventilated for severe RDS. It is due to lung damage by high oxygen concentration and high positive pressures. Steroid therapy is helpful in ventilator-dependent infants, but has side effects. Some babies with BPD require prolonged oxygen therapy for months.

PRACTICE POINT Blood gas stability is essential

	Blood oxygen	Blood carbon dioxide
Low	Tissue or brain damage	Brain ischaemia and PVL
High	Retinopathy of prematurity	Intracranial haemorrhage

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8.2.3 Recurrent apnoea

Preterm infants do not breathe regularly (Figure 8.3). If periods of shallow breathing or apnoea exceed 10–20s, they are associated with bradycardia and cyanosis. Apnoeas may be the first sign of neonatal illness but commonly are due to immaturity. Caffeine is given to stimulate regular breathing. Apnoea monitors are in regular use on neonatal units! (see Figure 28.2f).

Figure 8.3 Irregular breathing in a preterm infant with periods of apnoea or shallow breathing.

8.2.3.1 Temperature control

Hypothermia increases oxygen consumption and predisposes to RDS and infection. It must be avoided. Incubators and radiant heaters are used so that the baby uses little energy keeping warm or cooling down.

PRACTICE POINT

Tiny preterm infants are placed in a polythene bag at birth to prevent evaporative loss of fluid and heat.

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8.2.4 Fluid and electrolyte balance

Renal function is relatively poor in the preterm baby. Coupled with a high (but largely unmeasurable) water-loss through the very permeable skin, this can lead rapidly to dehydration and electrolyte disturbance.

8.2.5 Nutrition

If the preterm newborn is to equal the fetal growth rate, requirements are high. If the baby is not ill, milk is given. Frequent, small volume nasogastric feeds are used. The best milk is the mother’s expressed breast milk – it is better tolerated, induces gut maturation and reduces risk of necrotizing enterocolitis. In VLBW infants, protein and calories are added to breast milk. Nutritive sucking and swallowing develops at about 34 weeks. For tiny sick babies, parenteral nutrition is often used.

8.2.6 Patent ductus arteriosus (PDA)

The ductus arteriosus (Figure 8.4) of the preterm infant often fails to close, especially when RDS is present. Left-to-right shunting may lead to heart failure. Most close with time. The PDA can be closed by giving a prostaglandin synthetase inhibitor (e.g. ibuprofen), but sometimes surgical closure is required.

Figure 8.4 Patent ductus arteriosus.

8.2.7 Intraventricular haemorrhage and other forms of brain damage

In the walls of the lateral ventricles of the preterm baby’s brain are fragile capillaries (the germinal matrix), where bleeding may occur. Haemorrhage, easily seen on cranial ultrasound through the fontanelle, may be local or extend into the ventricles or brain tissue. The more extensive the haemorrhage, the more likely are brain damage and hydrocephalus. It is estimated that some degree of haemorrhage occurs in 40% of very small babies, although it causes serious damage in only a small minority.

Periventricular leukomalacia (PVL) is the most important pattern of brain damage in the preterm. It is seen in 5–10% under 32 weeks. White matter beside the ventricle is damaged by hypoxia/ischaemia and also inflammation and low carbon dioxide levels. It characteristically leads to spastic diplegia.

8.2.8 Anaemia

Anaemia is common because of red cell breakdown, and ‘bleeding into the laboratory’. Transfusion is often used. Extra iron is given with breast feeding when the preterm infant is 6 weeks old. Preterm formula milk is supplemented with iron.

8.2.9 Necrotizing enterocolitis (NEC)

NEC is the most common surgical emergency in neonatal medicine. Surgery is often needed. Mortality is 20%. The infant becomes sick and acidotic with a distended abdomen due to gut necrosis. NEC is almost restricted to the preterm. Aetiology is unknown, but immaturity, infection, gut ischaemia and enteral milk feeding all contribute to pathogenesis. Breast milk feeds reduce the risk of NEC considerably.

8.2.10 Management of the preterm infant

Preterm infants who are unwell are looked after in neonatal units or special care baby units (SCBUs) (Figure 8.5).

Figure 8.5 Neonatal intensive care. The hi-tech set-up is daunting for the parents.

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