Pathology of Rheumatic Fever & Rheumatic Heart Disease

I. Foundational Concepts (Must-Know Before Starting)

Before diving into the complex pathology of Rheumatic Fever (RF) and Rheumatic Heart Disease (RHD), examiners expect you to fully understand the following foundational anatomical and immunological principles.

Foundation 1: Cardiac Anatomy & Hemodynamics

Layers of the Heart: The heart has three layers: the Endocardium (inner lining and valves), the Myocardium (middle muscle), and the Pericardium (outer sac). When a disease affects all three, it is called Pancarditis.
Hemodynamic Stress: The left side of the heart (Left Atrium and Left Ventricle) pumps blood to the entire body, generating massively high pressures. The right side only pumps to the lungs (low pressure). Therefore, the Mitral and Aortic valves experience the most extreme physical trauma and turbulence, making them the most susceptible to structural damage.

Foundation 2: Suppurative vs. Non-Suppurative

Suppurative Infections: Involve live bacteria actively invading tissue and forming pus (e.g., a Staph abscess).
Non-Suppurative Complications: Occur weeks after the bacteria are dead and gone. The damage is caused strictly by the patient's own confused immune system. Rheumatic Fever is entirely non-suppurative and sterile. There are NO live bacteria in the heart!

II. Disease Overview & Definitions

Rheumatic Fever and Rheumatic Heart Disease represent a continuum of the same pathophysiological process, divided into an acute phase and a chronic phase.

Acute Rheumatic Fever (ARF): An acute, immunologically mediated, multisystem inflammatory disease that typically occurs 2 to 4 weeks following an episode of Group A β-hemolytic streptococcal (GAS) pharyngitis. It primarily targets the heart, major joints, central nervous system (CNS), skin, and subcutaneous tissues. While the joint and skin involvement is completely transient and heals without scarring, the cardiac injury can cause permanent structural damage.
Rheumatic Heart Disease (RHD): This is the chronic, long-term cardiac sequela (consequence) of acute rheumatic fever. It is characterized by progressive, fibrotic valvular deformities that lead to permanent mechanical dysfunction of the heart.
Clinical Significance & Epidemiology: RHD is the leading cause of acquired valvular heart disease globally. It most frequently strikes school-age children and adolescents (ages 5 to 15 years), particularly in developing regions and low-resource settings.

III. Etiology & Immunological Triggers

The exact cause of RF involves a deadly triad: a specific environmental infectious agent, a specific time delay, and a genetically susceptible host.

1. The Infectious Agent

Caused exclusively by Group A β-hemolytic Streptococcus (GAS) (Streptococcus pyogenes).

Crucial Exam Point: Only infection of the upper respiratory tract (Streptococcal Pharyngitis / "Strep Throat") triggers ARF. Skin infections (like Impetigo) caused by the exact same bacteria rarely/never trigger ARF (though they can trigger post-streptococcal glomerulonephritis).

2. The Latent Interval

There is a classic, mandatory delay of 2 to 4 weeks between the throat infection and the onset of systemic rheumatic symptoms.

This delay represents the time it takes for the immune system to generate a massive, mature antibody and T-cell response. Live bacteria are not present in the tissue during the acute carditis.

3. Host Susceptibility

Not everyone who gets Strep throat gets RF. It occurs in only ~3% of untreated cases.

Affected individuals are genetically susceptible. They demonstrate hyper-reactive CD4+ T-cell responses and have specific HLA class II allele associations that make their immune system prone to making cross-reactive errors.

IV. Pathogenesis: The Central Concept of Molecular Mimicry

After a long historical controversy, it is now an accepted pathological fact that the lesions in the heart, joints, and CNS are NOT caused by direct bacterial infection. They are caused by the induction of hypersensitivity and autoimmunity via a mechanism called Molecular Mimicry.

Initial Infection: GAS infects the throat.
Immune Activation: The host's immune system appropriately produces antibodies and CD4+ T-cell responses against streptococcal antigens to clear the throat infection.
The Mimicry (The Error): The specific antigens on the Strep bacteria (specifically the Streptococcal M protein and the N-acetylglucosamine capsular antigen) look structurally identical to normal human tissue proteins.
Cross-Reactive Autoimmunity: The immune system gets confused. The antibodies and T-cells begin attacking normal human tissues because they mistake them for the Strep bacteria.
- Target Antigens in the Heart: Epitopes closely mimic human cardiac myosin, tropomyosin, laminin, and vimentin found heavily in heart valves and the endothelium.
Tissue Destruction: This results in a mix of Type II Hypersensitivity (Antibody-mediated cell destruction) and Type IV Hypersensitivity (CD4+ T-cells cross the valvular endothelium, secrete inflammatory cytokines like IFN-γ and TNF-α, leading to macrophage recruitment and granulomatous tissue remodeling).
Pancarditis to Scarring: In the heart, this intense inflammation causes pancarditis. Repeated episodes of inflammation (from repeated strep throat infections) promote rampant fibrosis and permanent valve deformity.

V. Morphology: Acute Rheumatic Fever

Acute RF is characterized by diffuse inflammation. In the heart, this presents as classic Pancarditis (inflammation of the Endocardium, Myocardium, and Pericardium).

1. Microscopic Pathology (The Hallmark Lesions)

The single most important pathognomonic feature of acute rheumatic myocarditis is the Aschoff Body (or Aschoff Nodule). These are distinctive granulomatous focal lesions scattered throughout the cardiac tissue (myocardial interstitium). An Aschoff body has three distinct zones:

Central Zone: A focus of swollen, eosinophilic, fragmented collagen known as Fibrinoid Necrosis.
Middle Zone (The Cells): Contains activated macrophages called Anitschkow Cells.
- Exam Description: These are specialized activated histiocytes featuring abundant cytoplasm and a central vesicular nucleus with slender, wavy, ribbon-like chromatin. When viewed longitudinally, they are called "Caterpillar cells." When viewed in cross-section, they are called "Owl-eye cells."
- Aschoff Giant Cells: Some Anitschkow cells fuse together to form multinucleated giant cells with prominent central nucleoli surrounding the necrotic core.
Outer Zone: A cuff of chronic inflammatory cells, mostly lymphocytes and plasma cells.

2. Gross Morphological Features of Acute Pancarditis

Layer of Heart	Pathological Name	Gross Appearance & Description
Pericardium	Fibrinous Pericarditis	Deposition of a rough, shaggy, fibrinous exudate over the epicardial surface. Classically described as having a "Bread-and-Butter" appearance. Clinically causes a pericardial friction rub.
Myocardium	Acute Myocarditis	Nonspecific, soft, flabby heart chambers. Contains the microscopic Aschoff bodies. Causes conduction delays (prolonged PR interval) and acute heart failure in severe cases.
Endocardium (Valves)	Verrucous Endocarditis	Inflammation causes edema, fibrin deposition, and the formation of small (1-2 mm), bead-like, sterile, friable vegetations (verrucae) arrayed rigidly along the lines of valve closure. There is NO cuspal destruction at this stage.
Endocardium (Atrium)	MacCallum Plaques	Irregular, map-like endocardial thickenings seen in the posterior wall of the left atrium. They form due to the physical trauma of regurgitant jet streams shooting backward from an inflamed, leaky mitral valve.

High-Yield Pathology Differential

Do not confuse Rheumatic Verrucae with other vegetations on exams:

Rheumatic Fever: Small (1-2mm), warty, sterile, arranged in a neat row along the line of closure.
Infective Endocarditis (IE): Large, bulky, infected (full of bacteria), highly destructive to the valve, prone to breaking off (embolizing).
Libman-Sacks Endocarditis (SLE): Small-to-medium, sterile, scattered randomly on both sides of the valve leaflets (upper and lower surfaces).

VI. Clinical Features & Diagnosis (Jones Criteria)

Because there is no single diagnostic lab test for Acute RF, clinicians use the Revised Jones Criteria (first described by Dr. T.D. Jones in 1944, revised by WHO in 2004). Diagnosis requires evidence of a preceding GAS infection PLUS either 2 Major Criteria, OR 1 Major + 2 Minor Criteria.

Major Criteria ("JONES")

J - Joints (Migratory Polyarthritis): Extremely painful inflammation that jumps from one large joint to another (e.g., knees to ankles to wrists). It heals completely without permanent damage.
O - Heart (Carditis): The most critical manifestation because it is the only one that causes permanent death/disability. Presents as new murmurs, cardiomegaly, or pericardial friction rubs.
N - Nodules (Subcutaneous): Small, firm, painless nodules located mostly over bony prominences or extensor tendons.
E - Erythema Marginatum: A characteristic, non-pruritic, evanescent (rapidly appearing and disappearing), pink, ring-like skin rash with a clear center on the trunk.
S - Sydenham Chorea: Also known as "St. Vitus' Dance." Neurological disorder characterized by rapid, involuntary, purposeless, jerky movements due to autoimmune cross-reactivity with the basal ganglia in the CNS.

Minor Criteria

Fever: Usually high during the acute phase.
Arthralgia: Joint pain without clinical swelling/inflammation (cannot be used if Polyarthritis is counted as a major criterion).
Laboratory markers: Elevated Acute Phase Reactants (high ESR, raised C-Reactive Protein), and leukocytosis.
ECG changes: Prolonged PR interval (indicating delayed AV node conduction due to myocarditis).
Previous history: Prior attack of RF or known RHD.

*Supportive Evidence Required: A positive throat culture or elevated anti-streptolysin O (ASO) titer to prove a recent strep infection.

VII. Chronic Rheumatic Heart Disease (Disease Progression & Sequelae)

The transition from acute RF to chronic RHD takes years to decades. The hallmark of Chronic RHD is fibrotic scarring and permanent anatomical deformity of the valves.

1. The Fibrotic Process

During the acute phase, the verrucae cause localized damage. However, patients with one attack of RF are highly susceptible to recurrent attacks with subsequent strep infections. Recurrent attacks trigger cumulative damage. The healing process results in massive scar tissue formation characterized by:

Fibrotic cuspal thickening: The valve leaflets become rigid and heavily calcified.
Commissural fusion: The edges where the valve leaflets meet (commissures) glue themselves together.
Chordal shortening and thickening: The chordae tendineae (the "heart strings" that hold the mitral/tricuspid valves) become thick, fused, and shortened, pulling the valve rigidly downward.
The "Fish-Mouth" or "Buttonhole" Deformity: This cumulative scarring creates a rigid, narrow slit for blood to pass through, primarily resulting in severe Mitral Stenosis.

2. Valvular Involvement Frequency

The distribution of valvular lesions corresponds directly to the hemodynamic stress determinant (the physical mechanical pressure and turbulence experienced by the valve cusps).

Left-Sided Valves (High Pressure)

Mitral Valve Alone: ~65% of chronic cases. (Bears the massive pressure of Left Ventricular systole).
Mitral + Aortic Combined: ~25% of cases.
Aortic Valve Alone: ~8% of cases.

Right-Sided Valves (Low Pressure)

Tricuspid & Pulmonary Valves: Infrequent (<2%). When they are involved, it is almost exclusively seen in cases where the left-sided involvement is so severe that it has caused massive pulmonary hypertension, leading to right-sided pressure overload.

3. Long-Term Sequelae & Complications

The permanent mechanical failure of the valves (stenosis and regurgitation) leads to a cascade of fatal cardiovascular events:

Congestive Heart Failure (CHF): The heart hypertrophies and eventually fails due to the immense pressure needed to push blood through stenotic valves.
Atrial Fibrillation: Mitral stenosis forces blood to back up into the Left Atrium. The massive dilation of the atrium stretches the electrical conduction fibers, leading to chronic, chaotic arrhythmias (A-Fib).
Mural Thrombi & Embolization: The combination of a dilated, fibrillating left atrium means blood pools and stagnates. Massive clots (mural thrombi) form on the walls, break off, and cause severe systemic strokes or infarctions.
Infective Endocarditis (IE): The scarred, deformed valves disrupt smooth blood flow, creating turbulent jets that chronically damage the endocardium. This creates a perfect, sticky landing pad for bacteria in the blood, placing RHD patients at extremely high risk for acquiring fatal bacterial endocarditis.