Pathophysiology
Clinical meaning
Neonatal jaundice results from an imbalance between bilirubin production and conjugation/excretion that is physiologically exaggerated in the newborn period. Neonates produce bilirubin at approximately twice the adult rate because of higher red blood cell mass (hematocrit 50-65%), shorter RBC lifespan (70-90 days versus 120 days in adults), and greater enterohepatic circulation of bilirubin. Simultaneously, the neonatal liver has limited conjugation capacity: UDP-glucuronosyltransferase (UGT1A1) activity is only 1% of adult levels at birth, reaching mature capacity by 14 weeks of age. Unconjugated (indirect) bilirubin is lipophilic and circulates bound to albumin; when the bilirubin-albumin binding capacity is exceeded (or albumin is displaced by drugs or acidosis), free unconjugated bilirubin crosses the blood-brain barrier and deposits in the basal ganglia, hippocampus, and cranial nerve nuclei, causing acute bilirubin encephalopathy (lethargy, hypotonia, high-pitched cry, opisthotonus) and potentially chronic bilirubin encephalopathy (kernicterus โ permanent choreoathetoid cerebral palsy, sensorineural hearing loss, upward gaze palsy). The Bhutani hour-specific nomogram plots total serum bilirubin (TSB) against the neonate's age in hours and assigns a risk zone (low, low-intermediate, high-intermediate, or high risk) that guides the intensity of...