Internal Medicine: Comprehensive Study Guide on Acute Rheumatic Fever (ARF)

Introduction and Definition of Acute Rheumatic Fever

Acute Rheumatic Fever (ARF) is defined as a multisystem disease resulting from an autoimmune reaction following an infection with Group A Streptococcus (GAS), specifically the group A beta-hemolytic streptococcus rheumatic strain. This condition primarily arises from an upper respiratory tract infection. While the majority of clinical manifestations associated with ARF resolve completely over time, a significant exception is the potential for permanent cardiac valvular damage. This condition serves as the precursor to Rheumatic Heart Disease (RHD), which remains a substantial global health concern.

Global Considerations and Epidemiology

Acute Rheumatic Fever is fundamentally characterized as a disease of poverty. While it was common across all countries until the early 20th century, its incidence has seen a sharp decline in industrialized nations. This reduction is attributed to improved living conditions, such as less crowded housing and better hygiene, which reduced the transmission of Group A Streptococcus, alongside the introduction of antibiotics and improved systems of medical care. In contrast, RHD remains the most common cause of heart disease in children in developing countries and continues to be a major cause of mortality and morbidity in adults. Globally, approximately $15-19\text{ million}$ people are affected by RHD, resulting in roughly $\frac{1}{4}$ of a million (250,000) deaths annually.

In terms of demographics, ARF is primarily a disease of children aged $5-14\text{ years}$. Initial episodes of the disease become less common in older adolescents and young adults, and they are considered rare in individuals over the age of $30\text{ years}$. However, recurrent episodes of ARF remain relatively common in adolescents and young adults. This age distribution contrasts with the prevalence of RHD, which peaks later in life, typically between the ages of $25$ and $40\text{ years}$. While there is no clear gender association for the initial episodes of ARF, RHD affects females more commonly than males, with a ratio of approximately $2:1$. The main contributing factors to the prevalence of the disease include overcrowded living conditions, socioeconomic deprivation, an increased incidence of pharyngitis with GAS, and decreased access to effective healthcare.

Pathogenesis and the Immune Response

The pathogenesis of ARF involves both organism-specific and host-specific factors. The disease is exclusively caused by GAS infection of the upper respiratory tract. Specific M-serotypes are particularly associated with rheumatogenic potential, including types $1$, $3$, $5$, $6$, $14$ , $18$, $19$, $24$, $27$, and $29$. Scientists are also investigating the potential roles of skin infections and groups C and G streptococcus. Host susceptibility is also a factor, with approximately $3-6\%\text{ of the population}$ showing a predisposition. Familial clustering suggests that susceptibility may be inherited, and there is a strong association with HLA class II alleles.

The immune response in ARF is an autoimmune reaction driven by molecular mimicry. Specific components of the GAS bacteria, namely the M protein and the N-acetylglucosamine of the GAS carbohydrate, are immunologically similar to various host tissues. These similarities include proteins such as myosin, tropomyosin, keratin, actin, laminin, vimentin, and N-acetylglucosamine within human tissues. This similarity leads to the production of cross-reactive antibodies that attach to the cardiac valve endothelium. This attachment triggers a T-cell-mediated inflammatory response. The pathogenetic pathway follows a sequence starting with environmental factors like overcrowding, followed by a precipitating infection with a virulent strain of GAS. This leads to molecular mimicry and an exaggerated T-cell response in a susceptible host. Repeated GAS infections can drive ongoing valvular inflammatory responses, eventually leading to RHD.

Clinical Features and Temporal Patterns

There is typically a latent period of approximately $3\text{ weeks}$ (ranging from $1-5\text{ weeks}$) between the precipitating GAS infection and the clinical features of ARF. Notable exceptions to this timeframe include chorea and indolent carditis, which can manifest up to $6\text{ months}$ later. While patients may report a prior sore throat, it is commonly subclinical and must be confirmed using streptococcal antibody testing. The most common clinical manifestations include polyarthritis, which occurs in $60-75\%$ of cases, and carditis, seen in $50-60\%$. Sydenham's chorea occurs in varying rates between populations, ranging from less than $2\%$ to $30\%$. Rarer manifestations, occurring in less than $5\%$ of cases, include erythema marginatum and subcutaneous nodules.

Pathology of the Heart and Valvular Involvement

The pathognomonic lesion of ARF is the Aschoff body. This is a localized area of inflammation characterized by a central deposit of amorphous fibrinoid material, which is surrounded by an inflammatory infiltrate of mesenchymal cells. When fully developed, these bodies are diagnostic of rheumatic fever. In the context of chronic rheumatic scarring, the mitral valve often demonstrates a characteristic "fish mouth" shape when viewed from the left atrium. The mitral valve is the most frequently affected valve in RHD. This is followed in frequency by combined mitral and aortic valve involvement, then isolated aortic valve involvement, and finally the triple involvement of the mitral, aortic, and tricuspid valves.

Valvular damage is the hallmark of cardiac involvement in ARF. Pancarditis (inflammation of the endocardium, myocardium, and pericardium) can occur, with heart lesions present in $30-45\%$ of rheumatic fever episodes. Early valvular damage typically results in regurgitation. However, recurrent episodes lead to leaflet thickening, scarring, calcification, and eventually stenosis. Myocarditis associated with ARF may also affect electrical conduction, most commonly manifesting as a prolonged P-R interval (first-degree AV block), though higher-level blocks are rare.

Musculoskeletal and Neurological Manifestations

Joint involvement in ARF must present as actual arthritis, meaning the joints are hot, swollen, red, and/or tender. The classic presentation is a migratory polyarthritis, where the inflammation moves from one joint to another over a period of hours. It typically affects large joints such as the knees, ankles, hips, and elbows in an asymmetric fashion. The pain associated with this arthritis is severe and usually disabling. In some cases, patients may experience arthralgia (joint pain without objective inflammation). These joint symptoms are highly responsive to salicylates and other non-steroidal anti-inflammatory drugs (NSAIDs).

Sydenham's chorea is a neurological manifestation that is more common in females. It involves involuntary, purposeless movements, particularly of the head (such as darting movements of the tongue) and the upper limbs. Chorea can be generalized or localized to one side of the body (hemi-chorea). The severity varies; in severe cases, patients may be unable to perform activities of daily living and are at risk of injuring themselves. Unlike cardiac damage, chorea usually resolves completely, typically within a period of $6\text{ weeks}$.

Dermatological and Other Clinical Features

Dermatological signs of ARF include erythema marginatum and subcutaneous nodules. Erythema marginatum presents as pink macules that clear centrally, leaving a serpiginous, spreading edge. These lesions can appear and disappear rapidly, often appearing on the trunk and occasionally the limbs, but never on the face. Subcutaneous nodules are painless, small ($0.5-2\text{ cm}$), mobile lumps located over bony prominences such as the hands, feet, elbows, occiput, and vertebrae. These are a delayed manifestation, appearing $2-3\text{ weeks}$ after the onset of the disease, lasting from a few days up to $3\text{ weeks}$, and are commonly associated with carditis.

Other clinical features include a high-grade fever, typically reaching $39^\circ\text{C}$. Laboratory findings often show dramatically elevated acute phase reactants, specifically C-reactive protein (CRP) and Erythrocyte Sedimentation Rate (ESR). The leukocyte count may be mildly elevated. Evidence of a preceding GAS infection is essential for diagnosis. Since most patients do not have a positive throat swab culture or rapid antigen test by the time ARF manifests, antibody titers such as Anti-streptolysin O (ASO) and anti-DNase B (ADB) are the common tests used. Exceptions for needing immediate evidence of infection include chorea and low-grade carditis, which may manifest months after the original infection.

Diagnostic Criteria: Modified Jones Criteria

The diagnosis of ARF is guided by the Modified Jones Criteria. The Major Criteria include Carditis, Migratory Arthritis, Chorea, Erythema marginatum, and Subcutaneous nodules. The Minor Criteria include Arthralgia, Fever, and laboratory findings of elevated acute phase reactants (ESR, CRP) or a prolonged PR interval on an ECG. For a primary episode of Rheumatic Fever or a recurrence without established RHD, the diagnosis requires either two major manifestations or one major and two minor manifestations, plus evidence of a preceding GAS infection.

In cases of a recurrent attack where RHD is already established, two minor manifestations may suffice. Specific exceptions exist where neither evidence of preceding streptococcal infection nor other major manifestations are required: these include Rheumatic chorea and insidious onset carditis. However, the Jones Criteria have limitations; indolent carditis may only come to medical attention months after the attack began, chorea is often the last sign to appear, and late in the disease course, antibody titers may have decreased while acute phase reactants have normalized.

Treatment, Prognosis, and Prevention

The management of ARF focuses on confirming the diagnosis, treating symptoms such as heart failure (HF), and initiating preventive measures. These measures include secondary prophylaxis, inclusion on an ARF registry, and health education. An echocardiogram (Echo) should be performed for all possible cases to assist in diagnosis and establish baseline severity. Untreated, ARF lasts an average of $12\text{ weeks}$. With treatment, patients are usually discharged within $1-2\text{ weeks}$. Inflammatory markers must be monitored every $1-2\text{ weeks}$ until they normalize, which usually occurs within $4-6\text{ weeks}$. Patients with severe carditis require long-term clinical and Echo monitoring. Adherence to secondary prophylaxis is critical, and for those with carditis, antibiotic prophylaxis against endocarditis is required for dental and surgical procedures.

Prevention is divided into primary and secondary strategies. Primary prevention involves the elimination of major risk factors, such as overcrowded housing, although this is difficult to achieve. The mainstay of primary prevention is the timely and complete treatment of GAS sore throats with antibiotics. If penicillin is commenced within $9\text{ days}$ of the onset of a sore throat, it will prevent almost all cases of ARF. Secondary prevention focuses on preventing recurrent GAS infections in individuals who have already had ARF, primarily through long-term antibiotic prophylaxis.