Neonatal Medicine

Definitions and Epidemiology

A neonate is a newborn in the first 28 days of life. Preterm birth is defined as delivery before 37 completed weeks of gestation; subcategories of clinical importance:

• Moderate-to-late preterm: 32–36⁺⁶ weeks — largest group numerically, relatively lower risk
• Very preterm (VPT): <32 weeks — substantially higher risk of respiratory, neurological, and infectious complications
• Extremely preterm (EPT): <28 weeks — at the limits of viability (~22–24 weeks in high-resource settings); survival with major morbidity common

Birth weight classification: Low birth weight (LBW) <2,500 g; Very low birth weight (VLBW) <1,500 g; Extremely low birth weight (ELBW) <1,000 g. LBW encompasses both prematurity and intrauterine growth restriction (IUGR). Globally, ~15 million preterm births occur each year (WHO, 2023); prematurity complications are the leading cause of death in children under 5, accounting for approximately 47% of deaths in the neonatal period (first 28 days).

Transition at Birth — Circulatory and Respiratory

Birth triggers the most abrupt physiological transition in human life. In the fetal circulation, the lungs receive only ~8% of combined ventricular output (high pulmonary vascular resistance, low oxygen tension), with oxygenated blood reaching the fetus via the placenta. Three fetal shunts direct blood away from the lungs and liver:

• Ductus venosus: bypasses hepatic sinusoids; oxygenated umbilical venous blood → inferior vena cava
• Foramen ovale: interatrial shunt (right → left); preferentially streams well-oxygenated IVC blood to the coronary arteries and brain
• Ductus arteriosus (DA): connects pulmonary artery trunk to the descending aorta; maintained patent by prostaglandin E2 (PGE2, produced by the placenta and ductal wall) and low PO₂; carries ~60% of right ventricular output away from the lungs

At birth: Clamping of the umbilical cord removes the placental PGE2 source + low-resistance placental circulation. The first breath delivers oxygen → pulmonary arteriolar vasodilatation (PO₂ rise + pH rise → ↓hypoxic vasoconstriction) → pulmonary vascular resistance falls → pulmonary blood flow ↑10-fold within minutes. Increased pulmonary venous return → left atrial pressure rises → foramen ovale functional closure (flap valve). Oxygen exposure + reduced circulating PGE2 → ductal smooth muscle contraction → functional DA closure within 24–72 hours in term infants; anatomical closure follows over weeks. Failure of DA closure (patent ductus arteriosus, PDA) is common in preterm infants and causes a left-to-right shunt with pulmonary overcirculation.

The Apgar score, assessed at 1 minute and 5 minutes after birth, provides a rapid standardized assessment of neonatal transition:

Respiratory Distress Syndrome (RDS)

RDS (historically called Hyaline Membrane Disease) results from pulmonary surfactant deficiency in preterm infants. Surfactant synthesis begins in type II pneumocytes around 22–24 weeks' gestation but reaches physiologically adequate levels only around 34–36 weeks. Below ~32 weeks, surfactant quantity and composition (particularly DPPC and SP-B) are insufficient to prevent alveolar collapse at end-expiration.

Pathophysiology: Insufficient surfactant → alveolar surface tension remains high → alveoli collapse at end-expiration (atelectasis) → increasing work of breathing → hypoxia → anaerobic metabolism → lactic acidosis → pulmonary vasoconstriction → further ischemia to type II pneumocytes → further surfactant synthesis impairment (vicious cycle). Protein-rich exudate from damaged epithelium lines alveolar ducts forming hyaline membranes (the histological hallmark). Incidence ~65% in infants <1,500 g.

Management — Antenatal Corticosteroids

Antenatal corticosteroids (ACS) are the single most effective intervention for preterm birth. Betamethasone 11.4 mg IM ×2 doses 24 hours apart (or dexamethasone 6 mg IM ×4 doses 12 hours apart) given to mothers at 24–34⁺⁶ weeks at risk of preterm delivery within 7 days accelerates fetal lung maturation via glucocorticoid receptor-mediated upregulation of surfactant protein gene expression (SP-A, SP-B, SP-C, SP-D) and phospholipid synthesis enzymes. The landmark 1972 trial by Liggins and Howie (New Zealand) established this benefit; meta-analyses confirm ~50% reduction in RDS incidence and significant reductions in IVH, NEC, and neonatal mortality.

Management — Exogenous Surfactant Replacement

Natural surfactant preparations derived from animal lungs are instilled via endotracheal tube:

• Beractant (Survanta): bovine lipid extract; contains SP-B and SP-C analogues; 100 mg/kg dose
• Poractant alfa (Curosurf): porcine minced lung extract; higher phospholipid concentration; 100–200 mg/kg dose
• Calfactant (Infasurf): calf lung lavage extract

Delivery strategies: INSURE (INtubate-SURfactant-Extubate to CPAP) — brief intubation for surfactant delivery then immediate extubation to CPAP; LISA (Less Invasive Surfactant Administration) — thin catheter passed through vocal cords during CPAP breathing, surfactant instilled without intubation; reduces mechanical ventilation exposure. Both strategies are superior to intubation-ventilation alone (CURPAP, COIN trials).

Respiratory support: CPAP (continuous positive airway pressure, 5–8 cmH₂O) maintains end-expiratory lung volume, preventing alveolar collapse; first-line for spontaneously breathing preterm infants. HFNC (high-flow nasal cannula) used as step-down or mild RDS. Intubation and conventional mechanical ventilation (or high-frequency oscillatory ventilation, HFOV) for severe RDS or CPAP failure.

Neonatal Sepsis

Neonatal sepsis is a clinical syndrome of systemic infection in the first 28 days of life; a major cause of neonatal mortality in low-income settings (~680,000 deaths/year globally, WHO).

Early-onset sepsis (EOS, <72 hours)

Results from vertical transmission — organisms colonizing the maternal genital tract ascend to infect the fetus/neonate before or during delivery. Principal pathogens:

• Group B Streptococcus (GBS, Streptococcus agalactiae): leading cause in high-income settings; maternal rectovaginal colonization ~15–30%; prevention: intrapartum GBS prophylaxis (IV penicillin G) for GBS-positive mothers
• Escherichia coli: second most common; ESBL-producing strains increasingly prevalent; especially in preterm ELBW infants
• Listeria monocytogenes: from contaminated food; causes granulomatosis infantiseptica (disseminated abscesses)

Empirical treatment: ampicillin + gentamicin (covers GBS, E. coli, Listeria; synergistic bactericidal activity). Duration 10–14 days for confirmed bacteremia/meningitis.

Late-onset sepsis (LOS, >72 hours)

Predominantly nosocomial (hospital-acquired), especially in NICU settings. Principal pathogens:

• Coagulase-negative staphylococci (CoNS, esp. S. epidermidis): most common in VLBW infants; associated with central venous catheters (biofilm formation)
• Staphylococcus aureus including MRSA; Candida species in ELBW infants receiving broad-spectrum antibiotics/steroids; Gram-negative rods (Klebsiella, Pseudomonas, Enterobacter)

Empirical treatment: vancomycin ± aminoglycoside (covers CoNS and MRSA); add antifungal (fluconazole or micafungin) if Candida risk factors. Laboratory: CBC with differential, CRP, blood culture (minimum 1 mL); procalcitonin useful after 48h of life. Lumbar puncture mandatory if blood culture positive or clinical meningitis suspected. NEC must be considered in septic preterm infants.

Neonatal Hyperbilirubinemia

Unconjugated hyperbilirubinemia is extremely common in neonates; it becomes clinically significant — jaundice visible when serum bilirubin >5–7 mg/dL — in ~60% of term and ~80% of preterm infants in the first week of life.

Physiological basis: Fetal hemoglobin (HbF) is replaced by adult hemoglobin postnatally → accelerated red cell turnover → increased bilirubin load. Hepatic UDP-glucuronosyltransferase 1A1 (UGT1A1) is immature in neonates → reduced conjugating capacity → unconjugated bilirubin accumulates. Intestinal bacteria that convert bilirubin to urobilin are absent → enterohepatic circulation of bilirubin (β-glucuronidase in gut deconjugates bilirubin → reabsorption). Peak: day 3–5 in term infants, day 5–7 in preterm.

Pathological causes require earlier and more aggressive management:

• ABO incompatibility: maternal O blood type + infant A or B → maternal IgG anti-A/B crosses placenta → hemolysis; most common cause of severe jaundice requiring exchange transfusion in high-income settings
• Rh (D) incompatibility: Rh-negative mother previously sensitized → anti-D IgG → severe hemolytic disease of the fetus and newborn (HDFN); prevented by antenatal and postpartum anti-D immunoglobulin (RhoGAM)
• G6PD deficiency: X-linked; oxidative stress → episodic hemolysis; common in populations from malaria-endemic regions; triggers: naphthalene (mothballs), fava beans, infections, certain drugs
• Cephalohematoma: subperiosteal hemorrhage → hematoma resorption → bilirubin load

Treatment — Phototherapy: Blue-spectrum light (peak 460–490 nm, matching peak bilirubin absorption) converts unconjugated bilirubin in skin capillaries via two photochemical reactions: (1) configurational photoisomerization: 4Z,15Z-bilirubin → 4Z,15E-bilirubin (lumibilirubin) — water-soluble, excreted in bile without conjugation; (2) structural photoisomerization → lumirubin — excreted in urine. Intensive phototherapy (irradiance ≥30 µW/cm²/nm) accelerates conversion; double-surface LED phototherapy units are standard. Exchange transfusion (double-volume, removing ~85% of circulating bilirubin and sensitized RBCs) is reserved for severe hyperbilirubinemia at threshold levels per nomograms (Bhutani) or when bilirubin rises despite phototherapy.

Bilirubin neurotoxicity (kernicterus): Unconjugated bilirubin is lipophilic and crosses the blood-brain barrier when albumin binding capacity is saturated. Preferential deposition in the basal ganglia, subthalamic nuclei, and auditory nuclei → acute bilirubin encephalopathy (hypotonia → hypertonia → seizures → opisthotonus) → chronic kernicterus: athetoid cerebral palsy, sensorineural hearing loss, upward gaze palsy. Kernicterus is preventable with timely phototherapy.

Key Interventions Summary

Cross-Atlas Connections

• Related organPlacenta — feto-maternal physiology, preeclampsia, intrauterine growth restriction, IgG transfer
• Related organLung Alveolus — surfactant biology and RDS pathophysiology
• PreventiveVaccines — neonatal immunization schedule; maternal vaccination (TdaP, influenza) to protect newborns
• Co-morbidityCKD — prematurity-associated nephron deficit increases lifetime CKD risk; aminoglycoside nephrotoxicity in neonatal sepsis