Bilirubin is the end-product of haem catabolism. In newborns:

• ~75% comes from the breakdown of haemoglobin in senescent or haemolysed red blood cells (RBCs).
• ~25% comes from ineffective erythropoiesis and turnover of other haem-containing proteins (myoglobin, cytochromes, catalase).

Key enzymes and transporters: ^[3][4]

1. Haem oxygenase — rate-limiting step; converts haem → biliverdin + CO + iron
2. Biliverdin reductase — converts biliverdin → unconjugated bilirubin (UCB)
3. Albumin — carrier protein in blood; UCB is water-insoluble so must be protein-bound
4. Ligandin (glutathione-S-transferase) — intracellular carrier within hepatocyte
5. UDP-glucuronosyltransferase (UDPGT / UGT1A1) — the key conjugation enzyme; adds glucuronic acid to make bilirubin water-soluble
6. MRP2 (ABCC2) — canalicular transporter excreting conjugated bilirubin into bile
7. β-glucuronidase — intestinal enzyme that deconjugates bilirubin → enables reabsorption (enterohepatic circulation)

Physiological jaundice arises from the convergence of THREE normal neonatal phenomena ^[4][6][7]:

Unlike adults (where TSB > 26 µmol/L is abnormal), neonatal bilirubin levels are interpreted against age-in-hours using a nomogram (e.g., Bhutani nomogram). There is no single "normal" TSB cutoff — it depends on postnatal age and risk zone.

• Term neonate: TSB typically peaks at ~100–170 µmol/L (6–10 mg/dL) around Day 3–5 ^[7].
• Preterm neonate: peaks may be higher (up to ~250 µmol/L) and later (Day 5–7) due to even more immature conjugation.
• Conjugated bilirubin should always be < 17.1 µmol/L (1 mg/dL) or < 15% of TSB — exceeding this defines conjugated hyperbilirubinaemia and is always pathological ^[3].

^[3][4]

Already covered in Section 4.3. To summarise with a mnemonic:

Mnemonic: "PCE" — Production ↑, Conjugation ↓, Enterohepatic circulation ↑

These are frequently confused. They are two distinct entities:

• ABO incompatibility: Mother is blood group O (has naturally occurring anti-A and anti-B IgG) → crosses placenta → attacks fetal A or B RBCs. Usually milder than Rh disease because A and B antigens are expressed on many tissue types (not just RBCs), so antibodies are "mopped up." However, can still cause significant jaundice.
• Rh (anti-D) incompatibility: Rh-negative mother sensitised (previous pregnancy, miscarriage, transfusion) → produces anti-D IgG → crosses placenta → severe haemolysis of Rh-positive fetal RBCs. Can cause hydrops fetalis in severe cases. Prevented by anti-D immunoglobulin prophylaxis.
• G6PD deficiency: X-linked recessive. The enzyme G6PD is essential for the hexose monophosphate shunt (pentose phosphate pathway), which generates NADPH → needed to regenerate reduced glutathione → protects RBCs from oxidative damage. Without it, oxidative stressors (infections, drugs like sulfonamides/antimalarials, naphthalene mothballs, fava beans) cause Heinz body formation (denatured haemoglobin precipitates) → bite cells/blister cells on smear → extravascular (and sometimes intravascular) haemolysis. In neonates, G6PD deficiency can cause severe NNJ even without an identifiable trigger, likely because neonatal RBCs are inherently more susceptible to oxidative stress.

This is the feared complication of unconjugated hyperbilirubinaemia. Conjugated bilirubin does NOT cause kernicterus — instead it signals hepatobiliary disease ^[3].

Mechanism:

1. When UCB levels exceed the albumin-binding capacity, free (unbound) UCB circulates.
2. Free UCB is lipophilic → crosses the blood-brain barrier (BBB).
3. UCB is toxic to neurons, especially in the basal ganglia (globus pallidus, subthalamic nucleus), brainstem auditory nuclei, and hippocampus.
4. Damage occurs via mitochondrial toxicity, excitotoxicity, oxidative stress, and apoptosis.

Factors that ↑ risk of BIND:

• Low serum albumin (preterm infants have lower albumin levels → lower binding capacity)
• Competitive binding to albumin by drugs (e.g., sulfonamides, ceftriaxone) or free fatty acids → displaces bilirubin
• Acidosis (↓ albumin-bilirubin binding affinity)
• Sepsis / infection (opens BBB, ↑ bilirubin production, ↓ albumin)
• Prematurity (immature BBB, lower albumin, greater susceptibility of developing brain)
• Hypoalbuminaemia from any cause
• Haemolysis (rapid ↑ in UCB can overwhelm binding capacity)

Clinical Presentation ^[3]:

The word "kernicterus" literally means "kern" (nucleus, German) + "icterus" (jaundice) — referring to yellow staining of the brain nuclei (especially basal ganglia) seen at autopsy.

Biliary atresia is the single most important cause of conjugated neonatal jaundice to identify early because it is surgically treatable, but outcomes are time-dependent. ^{[3][4][9][10]}

• Epidemiology: Incidence ~1/5,000–15,000 births; higher in East Asians
• Pathophysiology: Unknown aetiology (theories include viral trigger, immune-mediated injury, genetic susceptibility). Progressive inflammatory obliterative cholangiopathy affecting both extrahepatic and intrahepatic bile ducts → complete obstruction of bile flow → cholestasis → progressive hepatic fibrosis → biliary cirrhosis → liver failure and death by age 2 years if untreated ^[3].
• Two forms:
  • Isolated (perinatal) form (~85%): normal anatomy initially, progressive destruction postnatally
  • Syndromic (embryonic/fetal) form (~15%): associated with situs inversus, polysplenia, cardiac defects (laterality defects)

Clinical features of biliary atresia:

• Prolonged conjugated jaundice (the baby looked "well" initially)
• Acholic (clay/pale) stools — the hallmark finding; stool colour card screening used in HK
• Dark (tea-coloured) urine — conjugated bilirubin is water-soluble and spills into urine
• Hepatomegaly → progresses to hepatosplenomegaly (portal hypertension)
• Poor growth / failure to thrive — impaired bile flow → ↓ fat absorption → ↓ absorption of fat-soluble vitamins (A, D, E, K)

• Always plot weight, length, and head circumference on age-appropriate growth charts.
• Poor weight gain may indicate:
  • Inadequate feeding (breastfeeding jaundice)
  • Malabsorption (cholestatic disease — ↓ bile salts → ↓ fat and fat-soluble vitamin absorption)
  • Chronic haemolysis (↑ metabolic demand)
  • Underlying metabolic disease
• Fat-soluble vitamin deficiencies (A, D, E, K) in cholestasis:
  • Vitamin K: coagulopathy → bleeding (can present as haemorrhagic disease of the newborn)
  • Vitamin D: rickets
  • Vitamin A: poor night vision (unlikely to be detected in neonates)
  • Vitamin E: haemolytic anaemia, neuromuscular dysfunction

• Neonatal jaundice is extremely common and often benign — parents need reassurance but also education about warning signs (increasing jaundice, poor feeding, lethargy, pale stools, dark urine).
• In Hong Kong, the stool colour card is given to all parents at discharge to monitor for acholic stools as a screen for biliary atresia.
• Cultural considerations: in Chinese culture, some traditional practices (e.g., keeping the baby wrapped up, avoiding sunlight) may delay recognition of jaundice.
• Consent/assent: In the neonatal period, all consent is obtained from parents/legal guardians. Explain phototherapy and (if needed) exchange transfusion clearly — parents often worry about "the light" and blood transfusion.

Important questions to ask at this stage ^[11]:

• Age of onset ( < 48h, D3–7, after D7)?
• Urine and stool colour (dark urine/pale stool → conjugated)?
• Fever (infection)?
• Weight change and urine output (dehydration / breastfeeding failure)?
• Maternal blood group?
• Birth history: gestation, cord blood screening (ABO/G6PD in HK), mode of delivery, bruising?
• Feeding: breast vs formula, frequency, volume?
• Family history: G6PD, jaundice, anaemia, splenectomy, liver disease?

This is the most classic and important paediatric exam differential ^[3][9][10].

Physiological jaundice is a diagnosis of EXCLUSION ^[3]. The following features make it pathological:

• Jaundice becomes clinically visible when TSB reaches approximately 80–100 µmol/L (≈ 5 mg/dL) ^[3][15].
• Visual assessment alone is unreliable, especially in infants with darker skin tones — always confirm with measurement ^[3][15].

Physiological jaundice is a diagnosis of exclusion — you make it only after ruling out pathological causes ^[3][4]. All of the following must be met:

Conjugated (direct) hyperbilirubinaemia is defined as: ^[3]

Direct bilirubin > 17.1 µmol/L (1 mg/dL) OR > 15% of TSB

This is NEVER physiological and always mandates urgent investigation to exclude surgically treatable causes (especially biliary atresia) ^[3][9].

Any ONE of the following makes jaundice pathological and warrants investigation ^[3][4][15]:

1. Onset < 24 hours
2. Rate of rise > 85 µmol/L/day ( > 5 mg/dL/day)
3. TSB exceeds age-specific phototherapy threshold (Bhutani nomogram / AAP 2022 guidelines)
4. Prolonged: > 2 weeks (term) or > 3 weeks (preterm)
5. Any conjugated bilirubin elevation
6. Signs of bilirubin encephalopathy (lethargy, hypotonia, poor feeding, high-pitched cry)
7. Unwell baby (fever, hepatomegaly, pale stools, dark urine)

This is a separate and urgent investigation pathway triggered whenever conjugated bilirubin is elevated ^[9][15][17].

GC 157 (Paediatric Chemical Pathology) lists "First-line investigations of neonatal cholestasis" — this is high-yield exam content ^[17].

^{[15][17][20][21]}

Why does unconjugated bilirubin damage the brain, but conjugated bilirubin does not?

• Unconjugated bilirubin (UCB) is lipophilic (fat-soluble). It normally circulates tightly bound to albumin, which is too large to cross the blood-brain barrier (BBB). As long as UCB remains albumin-bound, it cannot enter the brain.
• When the albumin-binding capacity is overwhelmed — either because the UCB level is extremely high, or because albumin is low, or because competing substances displace UCB from albumin — free (unbound) UCB enters the circulation.
• Free UCB, being lipophilic, readily crosses the BBB. It is directly toxic to neurons via:
  • Mitochondrial toxicity (uncouples oxidative phosphorylation → energy failure)
  • Excitotoxicity (↑ glutamate release)
  • Oxidative stress (generates reactive oxygen species)
  • Apoptosis of neurons
• The brain regions with the highest metabolic rate and the least myelination are most vulnerable: basal ganglia (globus pallidus, subthalamic nucleus), brainstem auditory nuclei (cochlear nuclei, inferior colliculus), hippocampus, and oculomotor nuclei.
• Conjugated bilirubin, by contrast, is hydrophilic (water-soluble). It does not cross the BBB and therefore does NOT cause kernicterus — instead, elevated conjugated bilirubin reflects underlying hepatobiliary disease ^[3][4].

Severe neonatal hyperbilirubinemia is defined as TSB > 428 µmol/L (25 mg/dL) — this level carries a significantly ↑ risk of BIND ^[4].

ABE is the acute clinical manifestation of bilirubin neurotoxicity. It is potentially reversible if recognised and treated urgently (with exchange transfusion) ^[3][4].

Clinical features progress through three phases:

The word "kernicterus" literally means "kern" (nucleus, German) + "icterus" (jaundice) — referring to the yellow staining of the basal ganglia and brainstem nuclei seen at autopsy.

Kernicterus is the chronic, irreversible form of BIND. If ABE is not treated in time, permanent neurological damage ensues ^[3][4].

The classic kernicterus tetrad:

Additional features may include: intellectual disability (variable severity, may be mild or severe), seizure disorder, and general developmental delay ^[3][4].

Key exam point: Kernicterus causes extrapyramidal (dystonic/choreoathetoid) cerebral palsy, NOT pyramidal (spastic) CP. This is because bilirubin targets the basal ganglia, not the corticospinal tracts.

Even when kernicterus is prevented, there is emerging evidence that moderate-to-severe neonatal hyperbilirubinaemia may be associated with subtle neurodevelopmental effects including:

• Auditory processing difficulties (auditory neuropathy spectrum disorder — ANSD)
• Mild cognitive deficits
• Behavioural problems

This is why all neonates with significant jaundice (especially those requiring phototherapy or exchange transfusion) should be referred for audiology follow-up (ABR testing) and developmental surveillance ^[7].

Why does cholestasis cause fat-soluble vitamin deficiency?

• Bile salts are required for emulsification and absorption of dietary fats in the small intestine.
• In cholestasis, bile salt secretion into the gut is reduced → fat malabsorption → fat-soluble vitamins (A, D, E, K) are not absorbed.
• This leads to specific deficiency syndromes:

• Fat malabsorption → caloric deficit (fat is the most calorie-dense macronutrient)
• Steatorrhoea — fatty, foul-smelling, bulky stools
• ↓ Absorption of essential fatty acids (linoleic, linolenic acid) → can impair brain development
• Chronic liver disease itself increases metabolic demand
• Result: poor weight gain, failure to thrive, growth retardation — a common indication for liver transplantation ^[3]

• In biliary atresia, even after a successful Kasai procedure, the intrahepatic disease typically continues to progress → progressive fibrosis → biliary cirrhosis ^[3][10].
• Majority of biliary atresia patients still develop cirrhosis and portal hypertension even with initial bile clearance after Kasai ^[10].
• Death < 2 years if biliary atresia is untreated ^[3].

As cirrhosis develops, portal pressure rises (resistance to portal blood flow through the fibrotic liver). This leads to:

After Kasai portoenterostomy, the Roux-en-Y jejunal conduit lacks a sphincter of Oddi → intestinal bacteria can ascend into the biliary system → recurrent ascending cholangitis ^[10][23].

• Presents with: fever, worsening jaundice, pale stools, acholic urine
• Can accelerate liver damage and cirrhosis
• Managed with IV antibiotics (e.g., ampicillin + gentamicin ± metronidazole) and prophylactic oral antibiotics (e.g., co-trimoxazole) are given long-term post-Kasai to reduce episodes ^[10]

• Chronic cholestasis and cirrhosis from any cause carry an increased risk of HCC, though this is more relevant in older children/adolescents with long-standing liver disease.

• Choledochal cysts carry a ~2% risk of cholangiocarcinoma ^[3][23]. This is why complete excision (not just drainage) is mandatory.