USMLE Guide: Arthritis

Introduction

Arthritis refers to a group of joint disorders characterized by inflammation, pain, stiffness, and limited range of motion. It can affect people of all ages and is a leading cause of disability worldwide. This USMLE guide will provide an overview of arthritis, its classification, pathophysiology, clinical presentation, diagnosis, and treatment options.

Classification

1. Osteoarthritis (OA): The most common type of arthritis, primarily affecting weight-bearing joints such as the knees, hips, and spine. It is characterized by degenerative changes in the articular cartilage, subchondral bone, and synovial tissue.

2. Rheumatoid Arthritis (RA): A systemic autoimmune disease primarily affecting the small joints of the hands and feet. It is characterized by chronic synovial inflammation, leading to joint destruction and deformity.

3. Gout: A metabolic disorder caused by the deposition of monosodium urate crystals in joints and other tissues. It commonly affects the big toe, causing episodes of severe pain and inflammation.

4. Psoriatic Arthritis (PsA): A chronic inflammatory arthritis associated with psoriasis. It can affect any joint and often involves entheses (attachment sites of tendons and ligaments to bone).

Pathophysiology

1. Osteoarthritis (OA): Mechanical stress and aging lead to the breakdown of articular cartilage, activation of chondrocytes and synovial cells, and release of inflammatory mediators. Subchondral bone changes, osteophyte formation, and synovitis contribute to joint pathology.

2. Rheumatoid Arthritis (RA): Autoimmune processes involve T-cell activation, B-cell production of autoantibodies (e.g., rheumatoid factor, anti-cyclic citrullinated peptide antibodies), and immune complex deposition in joints. Chronic inflammation leads to pannus formation, cartilage destruction, and bone erosion.

3. Gout: Elevated serum uric acid levels (hyperuricemia) result in the deposition of monosodium urate crystals in joints and soft tissues. These crystals trigger an inflammatory response mediated by the NLRP3 inflammasome, causing acute gouty arthritis.

4. Psoriatic Arthritis (PsA): Activation of both innate and adaptive immune responses, including T-cell activation and cytokine production (e.g., tumor necrosis factor-alpha), leads to synovitis and joint inflammation. Enthesitis contributes to characteristic features such as dactylitis and nail changes.

Clinical Presentation

1. Osteoarthritis (OA): Gradual onset of joint pain, stiffness, and limited range of motion. Crepitus and bony enlargement may be present. Commonly affects weight-bearing joints and hands.

2. Rheumatoid Arthritis (RA): Symmetric polyarthritis involving small joints (hands, wrists, feet). Morning stiffness >1 hour, joint swelling, tenderness, and deformities (e.g., swan neck, boutonniere deformities).

3. Gout: Acute episodes of monoarticular arthritis, usually affecting the first metatarsophalangeal joint (podagra). Rapid onset of severe pain, erythema, and swelling. Chronic gout can lead to tophi formation.

4. Psoriatic Arthritis (PsA): Asymmetric oligoarthritis involving peripheral joints (e.g., knees, ankles). Associated with psoriasis, nail pitting, dactylitis (sausage digits), and enthesitis (e.g., Achilles tendonitis).

Diagnosis

1. Osteoarthritis (OA): Clinical diagnosis based on history and physical examination. Radiographs may show joint space narrowing, subchondral sclerosis, and osteophyte formation.

2. Rheumatoid Arthritis (RA): Clinical diagnosis based on the American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) criteria. Positive rheumatoid factor or anti-cyclic citrullinated peptide antibodies, joint involvement, duration of symptoms, and acute phase reactants (e.g., elevated ESR, CRP).

3. Gout: Joint aspiration demonstrating negatively birefringent needle-shaped crystals confirms the diagnosis. Elevated serum uric acid levels may support the diagnosis, but they can be normal during an acute attack.

4. Psoriatic Arthritis (PsA): Clinical diagnosis based on the Classification Criteria for Psoriatic Arthritis (CASPAR). Presence of psoriasis, nail changes, negative rheumatoid factor, and characteristic radiographic findings.

Treatment Options

1. Osteoarthritis (OA): Non-pharmac