Conjugated Hyperbilirubinemia

Definition

Bilirubin Metabolism - Quick Review

Causes - Classified by Mechanism

1. Hereditary / Isolated (Normal other LFTs)

• Defect: Mutation in ABCC2 gene encoding the canalicular export pump MRP2
• Consequence: Conjugated bilirubin cannot enter the bile canaliculus; it regurgitates back into blood
• Serum bilirubin: typically 2-5 mg/dL (can reach 20-25 mg/dL during illness, OCP use, or pregnancy)
• Histology: Black coarsely granular pigment (epinephrine metabolites) in centrilobular hepatocyte lysosomes - the liver may appear grossly black
• Urine coproporphyrin: Total normal, but coproporphyrin I = 80% (normally coproporphyrin III predominates)
• All other LFTs: Normal
• Prognosis: Benign

• Defect: Mutations in SLCO1B1 and SLCO1B3 (encoding OATP1B1 and OATP1B3) - impaired hepatic re-uptake of conjugated bilirubin secreted by MRP3 into plasma
• No black pigment on biopsy
• Urine coproporphyrin: Total elevated 2-5x; both I and III elevated
• Similarly benign

2. Hepatocellular Disease (Mixed / Predominantly Conjugated)

• Viral hepatitis (HAV, HBV, HCV, HDV, HEV) - HAV/HEV are enteral and self-limited; HBV/HCV/HDV are parenteral and may become chronic
• Drug-induced liver injury (DILI) - acetaminophen overdose is most common toxic cause; idiosyncratic reactions with many other drugs
• Alcoholic hepatitis / cirrhosis
• Autoimmune hepatitis
• Ischemic hepatitis (shock liver)

3. Cholestatic Disorders

• Primary biliary cholangitis (PBC) - autoimmune destruction of small bile ducts; anti-mitochondrial antibody positive
• Primary sclerosing cholangitis (PSC) - fibro-obliterative disease of large ducts; associated with IBD
• Drug-induced cholestasis (e.g., estrogens, cyclosporine, chlorpromazine)
• Intrahepatic cholestasis of pregnancy
• Progressive familial intrahepatic cholestasis (PFIC) - mutations in bile transport genes (FIC1, BSEP, MDR3)
• Infiltrative disorders (sarcoidosis, lymphoma, amyloid, metastases)
• Sepsis-associated cholestasis

• Choledocholithiasis (gallstone in common bile duct) - most common cause
• Cancer of the head of pancreas - classic painless obstructive jaundice
• Cholangiocarcinoma
• Periampullary carcinoma
• Biliary strictures (post-surgical, post-inflammatory)

4. Neonatal / Pediatric Causes

1. Biliary atresia - progressive fibro-obliteration of bile ducts; requires Kasai portoenterostomy (surgical); leading cause of pediatric liver transplant
2. Neonatal hepatitis - a syndromic diagnosis encompassing toxic, metabolic, genetic, and infectious causes; >85% have identifiable etiology
   • Infectious: CMV, rubella, syphilis, toxoplasmosis, HSV
   • Metabolic: alpha-1-antitrypsin deficiency, galactosemia, tyrosinemia, Niemann-Pick
   • Idiopathic (~10-15%)

Clinical Features of Conjugated Hyperbilirubinemia

Key Lab Pattern Comparison

Pathology of Cholestasis

• Enlarged hepatocytes with dilated canalicular spaces
• Bile plugs in expanded bile canaliculi
• Kupffer cells loaded with regurgitated bile pigment
• Hepatocyte apoptosis
• In chronic obstruction: biliary cirrhosis

Important Clinical Points

• Bilirubinuria always = conjugated hyperbilirubinemia (unconjugated bilirubin is albumin-bound and not filtered by kidneys)
• In prolonged conjugated hyperbilirubinemia, bilirubin can become covalently bound to albumin (delta bilirubin). This has a long half-life (~18 days like albumin) and persists after resolution of the underlying disease - patients may remain mildly jaundiced even as cholestasis resolves
• DJS and Rotor syndrome are the only causes of isolated conjugated hyperbilirubinemia with completely normal all other liver function tests
• Distinguishing intrahepatic from extrahepatic cholestasis by bilirubin fractions alone is unreliable - imaging (ultrasound, MRCP) is necessary

Hyperuricemia

Definition

Biochemical Basis - Purine Metabolism

AMP / GMP
    ↓ (5'-nucleotidase)
Adenosine / Guanosine / Inosine
    ↓ (purine nucleoside phosphorylase)
Hypoxanthine / Guanine
    ↓ (xanthine oxidase)
Xanthine
    ↓ (xanthine oxidase)
Uric acid

• Xanthine oxidase (in liver + intestine) catalyzes the final two steps - the drug target for allopurinol and febuxostat
• Most mammals possess uricase which converts uric acid → allantoin (highly soluble). Humans and other great apes lost uricase ~18 million years ago, making urate accumulation possible
• ~2/3 of daily urate turnover is excreted by kidneys; ~1/3 by gut (intestinal uricolysis)

Renal Handling of Urate

Causes of Hyperuricemia

Mechanism-Based Classification

A. Underexcretion (Renal)

• Mutations in URAT1 (SLC22A12) - gain-of-function → increased reabsorption → familial gout
• GLUT-9 (SLC2A9) polymorphisms - the single most significant genetic determinant of serum urate
• ABCG2 mutations - intestinal/renal urate exporter; loss-of-function → up to 10% of gout in White Europeans
• Familial juvenile hyperuricemic nephropathy (UMOD mutations) - early-onset gout + progressive CKD

B. Overproduction

• Lesch-Nyhan Syndrome - X-linked recessive; complete deficiency of HPRT (hypoxanthine-guanine phosphoribosyl transferase); no purine salvage → PRPP accumulates → de novo synthesis floods → marked hyperuricemia + self-mutilation + choreoathetosis + intellectual disability
• Kelley-Seegmiller Syndrome - partial HPRT deficiency → milder disease, gout without neurologic features
• PRPP Synthetase Overactivity - X-linked; superactive enzyme with increased Vmax, decreased Km for ribose-5-phosphate, or decreased feedback sensitivity to purine nucleotides → excess PRPP → excess de novo synthesis
• Von Gierke Disease (GSD type Ia) - G6Pase deficiency; shunts glucose-6-phosphate into pentose phosphate pathway → excess PRPP + lactic acidosis (blocks renal excretion)
• Hereditary fructose intolerance - fructose-1-phosphate accumulation traps phosphate → AMP degradation

Monosodium Urate Crystal Formation and Gout

• Supersaturated urate concentration (sustained)
• Low pH and low temperature (favors peripheral joints - first MTP, Achilles)
• Seed nuclei (cartilage debris, collagen, chondroitin)
• Immunoglobulins IgG and IgM (serve as crystal nucleation platforms)
• Intra-articular dehydration
• Osteoarthritic cartilage degradation products (lower urate solubility)

1. MSU crystals in synovial space coated by IgG → phagocytosis by neutrophils/macrophages
2. Activation of NLRP3 inflammasome → caspase-1 activation → IL-1β processing and secretion
3. IL-1β → neutrophil recruitment, endothelial activation (E-selectin, ICAM-1), cytokine/chemokine cascade
4. Apolipoprotein B/E coating of crystals inhibits this response - explains why crystals can exist without inflammation (intercritical gout)

Clinical Spectrum

1. Asymptomatic Hyperuricemia

• No symptoms; not a disease in itself
• Risk of clinical gout rises steeply with urate level: incidence 4.9%/year at SU >9 mg/dL vs. 0.5%/year at 7-8.9 mg/dL
• 50% of those with SU >10 mg/dL eventually develop clinical gout
• Comorbidities: metabolic syndrome, obesity, dyslipidemia, hypertension, DM, CKD, congestive heart failure

2. Acute Gout Flare

• Explosive onset of severe joint pain, reaching maximum intensity in 12-24 hours
• Classic: first metatarsophalangeal joint (podagra), ankle, midfoot, knee
• Erythema, warmth, exquisite tenderness ("worst pain ever")
• Spontaneously resolves in days-weeks if untreated
• Triggered by: rapid change in urate level (initiation of ULT, dehydration, illness, alcohol, surgery)

3. Intercritical Gout

• Asymptomatic periods between flares
• MSU crystals may still be detectable in joints on DECT/ultrasound even asymptomatically

4. Chronic Tophaceous Gout

• Nodular MSU crystal deposits (tophi) in soft tissues: pinnae of ears, olecranon bursa, Achilles tendon, finger pads
• Chronic joint destruction and deformity
• Can also develop in axial skeleton, heart valves, kidneys

5. Uric Acid Nephrolithiasis

• MSU / uric acid crystals in collecting system
• Associated with hyperuricemia, low urine pH, concentrated urine
• Produces radiolucent stones on plain X-ray (uric acid stones are not radio-opaque)

6. Urate Nephropathy

• Chronic: MSU crystal deposition in renal interstitium → tubular damage + CKD
• Acute: massive uric acid precipitation in tubules (tumor lysis syndrome) → oliguric AKI

Diagnosis

Management

Non-pharmacologic

• Dietary: reduce red meat, organ meats, shellfish, high-fructose corn syrup; limit alcohol (especially beer/spirits)
• Increase low-fat dairy, cherry consumption, hydration
• Weight loss, manage metabolic syndrome
• Avoid precipitating drugs where possible (thiazides → switch to losartan which is uricosuric; aspirin → avoid low-dose in gouty patients)

Acute Flare Treatment (Targets: pain and inflammation)

Urate-Lowering Therapy (ULT) - Targets: serum urate <6 mg/dL (or <5 mg/dL in tophaceous gout)

Key Associations and Comorbidities

• Hypertension (bidirectional - urate promotes endothelial dysfunction and renin-angiotensin activation)
• Metabolic syndrome (insulin resistance reduces renal urate clearance)
• CKD (reduced GFR → hyperuricemia; hyperuricemia → tubular damage → worsens CKD)
• Cardiovascular disease (hyperuricemia is an independent risk factor; controversial if causative)
• Lead exposure (saturnine gout) - occupational or environmental