Hepatitis

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Viral Hepatitis - Pathophysiology & Mechanisms

Overview

Five hepatotropic viruses - A, B, C, D, and E - account for 90-95% of all acute viral hepatitis cases. They are a genetically diverse group linked by their tropism for the liver (their primary site of replication), not by molecular structure. Hepatitis B, C, D, and in some circumstances E can progress to chronic hepatitis; HAV is always self-limited.

Hepatitis A Virus (HAV)

Virology: A small, nonenveloped, positive-strand RNA picornavirus in the genus Hepatovirus, 27 nm in diameter (icosahedral capsid). Discovered in 1973.

Receptor: HAVcr-1 (TIM-1), a glycoprotein on hepatocytes.

Pathogenesis:

HAV is not cytopathic - it does not directly destroy cells
Hepatocellular injury is inflicted by cytotoxic T lymphocytes (CTL) and NK cells that recognize and kill infected hepatocytes
Spread is fecal-oral; the virus sheds in stool for 2-3 weeks before and 1 week after jaundice onset
Viremia is transient, making bloodborne transmission rare

Outcome: Always self-limited. No chronic hepatitis, no carrier state. Case fatality rate < 0.5% in most populations, but severe in the elderly and those with pre-existing liver disease.

Hepatitis B Virus (HBV)

Virology: A small, enveloped, partially double-stranded DNA virus (hepadnavirus). The genome encodes four proteins:

Protein	Function
HBsAg (surface antigen)	Envelope protein; released in excess as non-infectious particles; basis for vaccination
HBcAg (core antigen)	Nucleocapsid protein; assists viral assembly
HBeAg	Pre-core/core polypeptide; marker of active replication & infectivity
HBx protein	Transcriptional activator of viral and host genes; implicated in HCC pathogenesis
HBV polymerase	Has both DNA polymerase and reverse transcriptase activity

Replication cycle:

Large HBsAg binds to the bile salt transporter NTCP (sodium taurocholate cotransporting polypeptide) on hepatocytes
Viral genome enters the nucleus and forms covalently closed circular DNA (cccDNA) - the transcriptional template
Replication proceeds through reverse transcription via an RNA intermediate (like retroviruses)
cccDNA persists in the nucleus as a minichromosome, making complete cure difficult

Immune Pathogenesis:

High-level viral replication can be directly cytopathic, but most hepatocyte injury is immune-mediated
CD8+ cytotoxic T cells attack infected hepatocytes
A strong CD4+ and CD8+ IFN-γ response = resolution of acute infection
A weak or dysregulated immune response = chronic infection and progressive liver damage

Serologic markers and their meaning:

Marker	Meaning
HBsAg	Active infection (acute or chronic)
Anti-HBs	Recovery or vaccine immunity
IgM anti-HBc	Recent/acute infection ("window period" marker)
HBeAg	Active viral replication, high infectivity
Anti-HBe	Waning acute infection or mutation preventing HBeAg production
HBV DNA	Direct measure of viral load

Chronicity: ~2/3 of acute infections are mild or subclinical. Chronic infection occurs in ~5-10% of adults but rises to >90% in neonates (immature immune system). Chronic HBV significantly raises the risk of cirrhosis and hepatocellular carcinoma (HCC) - partly via HBx-mediated transcriptional activation.

Hepatitis C Virus (HCV)

Virology: A small, enveloped, single-stranded positive-sense RNA virus in the family Flaviviridae. Discovered in 1989.

Key structural proteins:

E1 & E2 - envelope glycoproteins; E2 is the primary target for neutralizing antibodies
NS3/4A - serine protease (target of current antivirals)
NS5A - replication complex protein (target of DAAs)
NS5B - RNA-dependent RNA polymerase (target of sofosbuvir)

Pathogenesis and immune evasion - the key to HCV's success:

High mutation rate: The NS5B RNA polymerase has low fidelity, generating vast genetic diversity. This creates 7 genotypes and numerous subtypes
Quasispecies: Within each infected person, multiple genetic variants (quasispecies) emerge over time
Antibody escape: Neutralizing antibodies target E2 - but emerging variants with altered E2 epitopes evade them
Innate immune sabotage: NS3/NS4A protease cleaves MAVS and TRIF - adaptor proteins critical for interferon signaling - blunting the host antiviral response

Chronicity: Spontaneous clearance occurs in only ~15-20% of people. Chronic infection develops in 80-90%. Of those with chronic HCV:

~20% progress to cirrhosis over 20-30 years
Those with cirrhosis have substantial risk of HCC

Progression is accelerated by: older age, male sex, alcohol use, immunosuppression, HBV/HIV co-infection, obesity, type 2 diabetes, and metabolic syndrome.

Hepatitis D Virus (HDV)

Virology: A unique satellite virus - it is a defective RNA virus that can only replicate in cells simultaneously infected with HBV. It uses HBsAg as its own outer envelope.

Two patterns of infection:

Pattern	Description	Outcome
Co-infection	Simultaneous HDV + HBV infection	Usually self-limited; fulminant hepatitis possible
Superinfection	HDV infects a chronic HBV carrier	>80% develop chronic HDV; severe exacerbation common

Pathogenesis: HDV RNA is detectable before symptoms. In superinfection with chronic HDV there are two phases - an acute phase with active HDV replication and suppression of HBV replication (high transaminases), and a chronic phase where HDV replication falls and HBV replication rebounds.

Key point: HBV vaccination prevents HDV by eliminating the host requirement.

Hepatitis E Virus (HEV)

Virology: An unenveloped, positive-stranded RNA virus in the genus Orthohepevirus (family Hepeviridae). Genome is 7.3 kb with 4 open reading frames encoding viral protease and RNA polymerase.

Transmission: Fecal-oral, waterborne (zoonosis - reservoirs include pigs, monkeys, dogs, cats).

Pathogenesis:

HEV is not cytopathic - hepatic damage stems from the host immune response to infected cells
Incubation period: 4-5 weeks
Virions shed in stool during acute illness
HEV RNA and virions detectable in stool and serum before symptoms
IgM anti-HEV appears simultaneously with clinical illness, then replaced by persistent IgG

Special populations:

Pregnant women: Mortality rate approaching 20% - the mechanism is thought to involve immune dysregulation of pregnancy
Immunocompromised patients (AIDS, transplant recipients): Chronic HEV infection can occur
Immunocompetent patients: Always self-limited, no chronic carrier state

Common Pathologic Mechanisms Across All Hepatitis Viruses

Direct cytopathic injury - high-level viral replication (mainly HBV at peak viremia)
T cell-mediated hepatocyte killing - the dominant mechanism for HAV, HBV, and HCV; CD8+ CTLs recognize MHC-I-presented viral peptides and destroy infected hepatocytes
Immune complex-mediated extrahepatic disease - particularly HBV (glomerulonephritis, polyarteritis nodosa)
Fibrogenesis and cirrhosis - chronic inflammation activates hepatic stellate cells (Ito cells) → collagen deposition → portal fibrosis → cirrhosis
Hepatocellular carcinoma - HBV via HBx protein (transcriptional activation, genomic integration); HCV via chronic inflammation and oxidative stress

Summary Comparison Table

Feature	HAV	HBV	HCV	HDV	HEV
Virus family	Picornaviridae	Hepadnaviridae	Flaviviridae	Satellite (deltaviridae)	Hepeviridae
Genome	+ssRNA	Partial dsDNA	+ssRNA	-ssRNA	+ssRNA
Transmission	Fecal-oral	Parenteral/sexual/vertical	Parenteral	Parenteral (requires HBV)	Fecal-oral
Chronicity	Never	Yes (~5-10% adults)	Yes (~80-90%)	Yes (>80% superinfection)	Rarely (immunocompromised)
Cytopathic?	No (T cell mediated)	Partial	No (immune mediated)	No	No (host immune response)
HCC risk	No	Yes (via HBx)	Yes (via cirrhosis)	Yes (additive to HBV)	No
Vaccine available	Yes	Yes	No	Prevented by HBV vaccine	Limited (China only)

Sources: Robbins, Cotran & Kumar Pathologic Basis of Disease, 10e; Yamada's Textbook of Gastroenterology, 7e

Portal hypertension

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Here is a comprehensive breakdown of portal hypertension from pathophysiology to clinical complications.

Portal Hypertension

Definition and Measurement

Portal hypertension is defined as elevation of the hepatic venous pressure gradient (HVPG) above 5 mmHg. HVPG is the gold standard measurement - it represents the perfusion pressure of the liver with portal blood (measured by hepatic vein catheterization).

Key HVPG thresholds:

>5 mmHg - portal hypertension (by definition)
≥6 mmHg - clinically measurable elevation
≥10 mmHg - clinically significant portal hypertension (CSPH); substantially increased decompensation risk
≥12 mmHg - variceal hemorrhage risk begins; target for TIPS reduction

Normal portal pressure is 5-10 mmHg. Once HVPG reaches ≥10 mmHg, the risk of decompensation (variceal bleeding, ascites, encephalopathy) rises sharply, and median survival after decompensation is less than 2 years.

Anatomy of the Portal Venous System

The portal vein carries blood from the entire GI tract (except upper esophagus and distal rectum), spleen, pancreas, and gallbladder into the liver. The superior mesenteric vein + splenic vein merge behind the neck of the pancreas to form the portal vein. The portal vein and hepatic artery drain into hepatic sinusoids, which drain into the hepatic veins and then the inferior vena cava (IVC).

The liver receives ~25-30% of cardiac output. Portal venous blood accounts for 75% of hepatic blood flow and ~50% of oxygen delivery. Sinusoids are highly permeable (fenestrated endothelium, no basement membrane), with the space of Disse containing Kupffer cells and hepatic stellate cells (HSCs) that regulate hepatic hemodynamics.

Pathophysiology

Portal venous pressure follows Ohm's law: P = Q × R (pressure = flow × resistance)

Therefore, portal hypertension results from:

Increased resistance to portal blood flow
Increased portal blood inflow (hyperdynamic circulation)
In advanced disease: neurohumoral activation and systemic inflammation

Mechanism 1 - Increased Intrahepatic Resistance

In cirrhosis, two components raise intrahepatic resistance:

A. Structural (passive/fixed) component:

Progressive collagen deposition and nodule formation distort sinusoidal architecture
Hepatic stellate cells (HSCs) become activated into myofibroblasts and develop contractile properties
This architectural distortion reduces the cross-sectional area of sinusoidal channels

B. Dynamic (functional/reversible) component (~30-40% of resistance): This is therapeutically targetable:

Nitric oxide (NO) deficiency: Despite excess NO production in the systemic/splanchnic circulation, intrahepatic eNOS activity is reduced in cirrhosis - the opposite of what occurs systemically. This leads to intrahepatic vasoconstriction
Endothelin-1 (ET-1) excess: ET-1 binds ET-A receptors on HSCs → vasoconstriction
Activated HSCs behave as pericytes - they contract around sinusoids and amplify resistance
Imbalance between vasodilators (NO, prostacyclin) and vasoconstrictors (ET-1, angiotensin II, sympathetic tone) favors constriction in the portal bed

Mechanism 2 - Increased Portal Blood Inflow (Hyperdynamic Circulation)

As portal pressure rises, vasodilator substances (especially NO, prostacyclin, glucagon, substance P, endocannabinoids) produced in excess spill into the systemic/splanchnic circulation, causing:

Splanchnic arterial vasodilation - markedly reduced splanchnic vascular resistance
Increased splanchnic blood flow → augments portal venous inflow
Systemic hyperdynamic state: high cardiac output, low systemic vascular resistance, low mean arterial pressure

This is a vicious cycle - the attempt to decompress the portal system by forming collaterals paradoxically sustains and amplifies it.

Mechanism 3 - Neurohumoral Activation and Systemic Inflammation

Splanchnic vasodilation reduces effective arterial blood volume (despite overall volume expansion). This triggers compensatory systems:

Pathophysiology of portal hypertension complications in cirrhosis

As shown above:

RAAS activation → aldosterone → renal sodium and water retention → ascites
Sympathetic nervous system (SNS) activation → renal vasoconstriction
ADH (vasopressin) release → solute-free water retention → dilutional hyponatremia
Bacterial translocation → gut flora traverse the intestine into mesenteric lymph nodes → PAMPs enter the circulation → pattern recognition receptors (PRRs) activate innate immunity → proinflammatory cytokines and ROS → further impair circulatory function
In advanced disease: decreased cardiac output (cirrhotic cardiomyopathy) compounds effective hypovolemia

Classification by Anatomical Site

Intrahepatic causes account for >95% of all portal hypertension.

Category	Subtype	Examples
Prehepatic	-	Portal vein thrombosis (PVT), splenic vein thrombosis, Banti's syndrome (massive splenomegaly)
Intrahepatic	Presinusoidal	Schistosomiasis, congenital hepatic fibrosis, primary biliary cholangitis (early)
	Sinusoidal	Cirrhosis (all causes), alcoholic hepatitis
	Postsinusoidal	Sinusoidal obstruction syndrome (veno-occlusive disease)
Posthepatic	-	Budd-Chiari syndrome (BCS), IVC webs, constrictive pericarditis, restrictive cardiomyopathy, right heart failure

Clinical Complications

1. Gastroesophageal Varices

As portal pressure rises above the critical 12 mmHg threshold, portosystemic collaterals dilate. The gastroesophageal junction (GEJ) is the most clinically dangerous site because:

The palisade zone (2-3 cm above the GEJ) is where portal and azygos venous systems communicate
Incompetent valves in perforating veins allow retrograde flow, dilating intrinsic veins
Variceal rupture in this zone causes massive upper GI hemorrhage

Variceal hemorrhage mortality: ~15-25% per episode. Rebleeding risk is 60-70% without prophylaxis.

2. Ascites

Mechanism: splanchnic vasodilation → reduced effective arterial volume → RAAS/SNS/ADH activation → renal sodium and water retention → fluid accumulates in the peritoneal cavity (hydrostatic pressure + reduced oncotic pressure from low albumin).

Diagnosed/categorized by the Serum-Ascites Albumin Gradient (SAAG):

SAAG ≥1.1 g/dL (high gradient) = portal hypertension-related (cirrhosis, BCS, cardiac ascites, portal vein thrombosis)
SAAG <1.1 g/dL (low gradient) = non-portal hypertension cause (malignancy, tuberculosis, pancreatitis)

3. Spontaneous Bacterial Peritonitis (SBP)

Occurs in up to 30% of hospitalized cirrhotic patients with ascites
Mechanism: bacterial translocation from gut (E. coli and other Gram-negatives most common)
Diagnosis: ascitic fluid neutrophil count >250/μL
Treatment: IV 3rd-generation cephalosporin × 5 days + IV albumin (1.5 g/kg day 1, 1.0 g/kg day 3) to prevent HRS
Quinolone prophylaxis recommended after a first episode

4. Hepatorenal Syndrome (HRS)

Functional renal failure in ~10% of advanced cirrhosis or acute liver failure patients. No intrinsic renal pathology - it is driven by extreme renal vasoconstriction secondary to splanchnic vasodilation and RAAS/SNS activation.

Two patterns:

HRS-AKI (formerly Type 1): Rapid progressive renal impairment (creatinine rise ≥0.3 mg/dL in 48h); high short-term mortality
HRS-CKD (formerly Type 2): Stable, moderate renal impairment; better short-term outlook

Treatment: vasoconstrictors (terlipressin - preferred; norepinephrine in ICU; midodrine + octreotide as 3rd line) + IV albumin. Best treatment: liver transplantation.

5. Hepatic Encephalopathy (HE)

Neuropsychiatric complication from failure of the cirrhotic liver (and portosystemic shunting) to clear gut-derived neurotoxins, primarily ammonia. Ammonia metabolism by astrocytes produces glutamine, causing astrocyte swelling (cerebral edema). Additional contributors include manganese deposition in basal ganglia and neuroinflammation.

6. Splenomegaly and Hypersplenism

Congestion of the splenic vein → splenomegaly → sequestration of blood cells → thrombocytopenia, leukopenia, and anemia (hypersplenism). Thrombocytopenia also results from reduced thrombopoietin production by the diseased liver.

7. Hepatopulmonary Syndrome / Portopulmonary Hypertension

Hepatopulmonary syndrome: Intrapulmonary vascular dilation → V/Q mismatch → hypoxemia. Characteristic: platypnea-orthodeoxia (desaturation on standing, relief on lying)
Portopulmonary hypertension: Elevated mean pulmonary arterial pressure (>25 mmHg) due to portal hypertension-induced mediators reaching the pulmonary vasculature

Summary: The Dual-Hit Model

CIRRHOSIS
   │
   ├─► Architectural distortion + HSC activation
   │       → ↑ Intrahepatic resistance (structural + dynamic)
   │       → ↑ Portal pressure
   │
   └─► Portal pressure → Splanchnic vasodilation (NO, prostacyclin)
           → ↑ Portal inflow (hyperdynamic circulation)
           → ↓ Effective arterial volume
           → RAAS / SNS / ADH activation
                   ↓              ↓             ↓
               Ascites    Dilutional HypoNa   HRS

Sources: Harrison's Principles of Internal Medicine 22e; Yamada's Textbook of Gastroenterology 7e; Sleisenger & Fordtran's Gastrointestinal and Liver Disease

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