Section 4.1: Rheumatologic & Autoimmune MSK Conditions (RA, Lupus, Gout, OA)

Rheumatologic & Autoimmune MSK Conditions

PANCE High-Yield Focus: Rheumatologic and autoimmune conditions represent highly tested topics on the PANCE. Key areas of focus include distinguishing inflammatory arthropathies (RA, SLE) from degenerative joint diseases (OA), interpreting specific autoantibody panels (anti-dsDNA, anti-CCP), analyzing synovial fluid crystals (needle-shaped vs. rhomboid-shaped), and knowing critical medication safety profiles (e.g., DMARD screening and HLA-B*5801 testing).

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Pathophysiology and Clinical Comparison

Rheumatoid Arthritis (RA)

Rheumatoid arthritis is a chronic, systemic autoimmune inflammatory disease characterized by symmetric polyarthritis. It is driven by an autoimmune attack against the synovial membrane. T-helper cells release proinflammatory cytokines (such as tumor necrosis factor [TNF]-alpha, interleukin [IL]-1, and IL-6) that activate macrophages and synovial fibroblasts. This leads to marked hyperplasia of the synovium, forming a hypertrophic, vascularized tissue called a pannus. The pannus invades and destroys the adjacent articular cartilage and subchondral bone, mediated by osteoclast activation and collagenase release, leading to joint space narrowing, marginal bone erosions, and joint deformity.

Systemic Lupus Erythematosus (SLE)

Systemic lupus erythematosus is a multi-system autoimmune disease driven by a type III (immune complex) and type II (antibody-mediated) hypersensitivity reaction. Autoantibodies target nuclear antigens (antinuclear antibodies, or ANA), leading to the formation of immune complexes that deposit in various organs, including the skin, joints, kidneys, and blood vessels. This deposition activates the complement cascade, triggering inflammation, tissue damage, and vasculitis.

Gout

Gout is an inflammatory arthritis characterized by the deposition of monosodium urate (MSU) crystals in joints and soft tissues, resulting from chronic hyperuricemia (serum uric acid > 6.8 mg/dL). Hyperuricemia occurs due to either renal underexcretion of uric acid (90% of cases, often worsened by diuretics, renal insufficiency, or low-dose aspirin) or purine overproduction (10% of cases, seen in myeloproliferative disorders or tumor lysis). MSU crystals precipitate in the joint, phagocytosed by macrophages, which triggers an inflammatory cascade and recruits neutrophils, causing acute synovitis.

Osteoarthritis (OA)

Osteoarthritis is a chronic, progressive joint disorder characterized by the mechanical degeneration of articular cartilage, subchondral bone remodeling, osteophyte formation, and joint space narrowing. It is primarily a degenerative, non-inflammatory disease. Mechanical stress on weight-bearing joints combined with a loss of chondrocyte function leads to cartilage fibrillation, flaking, and erosion. The underlying subchondral bone thickens (subchondral sclerosis) and develops fluid-filled subchondral cysts. Marginally, new bone formation produces bony projections known as osteophytes.

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Clinical Presentation and Diagnostic Criteria

Rheumatoid Arthritis

• Joint Distribution: Symmetric polyarthritis primarily affecting the small joints of the hands and feet: metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints, wrists, and metatarsophalangeal (MTP) joints. The distal interphalangeal (DIP) joints and the lumbar/thoracic spine are spared, though the cervical spine can be involved (risk of atlantoaxial subluxation).
• Key Symptoms: Morning stiffness lasting greater than 1 hour that improves with physical activity or warm showers.
• Deformities: Swan-neck deformity (hyperextension of PIP, flexion of DIP), Boutonniere deformity (flexion of PIP, hyperextension of DIP), and ulnar deviation/drift at the MCP joints.
• Extra-articular Signs: Subcutaneous rheumatoid nodules over extensor surfaces, pleural/pericardial effusions, and Felty Syndrome (triad of RA, splenomegaly, and neutropenia).
• Diagnostics:
  • Anti-Cyclic Citrullinated Peptide (Anti-CCP): Most specific marker (>95% specificity).
  • Rheumatoid Factor (RF): Sensitive but non-specific.
  • Radiography: Shows symmetric joint space narrowing, periarticular osteopenia, and marginal bone erosions.

Systemic Lupus Erythematosus

• Criteria (SOAP BRAIN MD):
  • Serositis (pleuritis, pericarditis)
  • Oral ulcers (painless nasopharyngeal ulcers)
  • Arthritis (non-erosive, involving two or more peripheral joints)
  • Photosensitivity (skin rash from sun exposure)
  • Blood disorders (hemolytic anemia, leukopenia, lymphopenia, thrombocytopenia)
  • Renal disorder (proteinuria > 0.5 g/day or cellular casts; lupus nephritis)
  • ANA positive (present in >95% of patients; best initial screening test)
  • Immunologic disorder (anti-dsDNA, anti-Smith, or anti-phospholipid antibodies)
  • Neurologic disorder (seizures or psychosis in the absence of other causes)
  • Malar rash (fixed erythema over the malar eminences, sparing the nasolabial folds)
  • Discoid rash (erythematous raised patches with adherent keratotic scaling)
• Diagnostics:
  • ANA: Highly sensitive, best initial test.
  • Anti-dsDNA: Highly specific; titers correlate with disease activity and risk of lupus nephritis.
  • Anti-Smith (Anti-Sm): Highly specific, but does not correlate with disease activity.
  • Complement Levels (C3, C4): Decreased during active flares.

Gout

• Clinical Signs: Sudden, excruciating onset of warm, erythematous, swollen monoarthritis. The first metatarsophalangeal (MTP) joint is affected in >50% of cases (podagra). Chronic gout features tophi (firm deposits of urate crystals) in soft tissues (e.g., Achilles tendon, helix of the ear).
• Diagnostics:
  • Arthrocentesis (Gold Standard): Shows needle-shaped, strongly negatively birefringent crystals (appear yellow when parallel to the polarized light axis).
  • Radiography: Shows 'rat-bite' punched-out bone erosions with overhanging margins in chronic disease.
  • Serum Uric Acid: Can be normal during an acute flare.

Osteoarthritis

• Clinical Signs: Deep, aching joint pain that worsens with weight-bearing or joint use and is relieved by rest. Morning stiffness is brief, lasting less than 30 minutes. Bony enlargements include Heberden's nodes (at the DIP joints) and Bouchard's nodes (at the PIP joints). Crepitus is felt during range of motion.
• Diagnostics:
  • Radiography: Shows asymmetric joint space narrowing, subchondral sclerosis, osteophytes, and subchondral cysts.
  • Synovial Fluid: Non-inflammatory (clear fluid, WBC count < 2,000 cells/mcL).

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Management and Clinical Safety Rules

Rheumatoid Arthritis

• First-line Therapy: Disease-Modifying Antirheumatic Drugs (DMARDs) should be started immediately. Methotrexate is the non-biologic drug of choice.
  • Safety Rules: Methotrexate requires regular monitoring of complete blood count (CBC) and liver function tests (LFTs) due to risks of bone marrow suppression and hepatotoxicity. Folic acid (1 mg daily) must be co-prescribed to minimize GI side effects, stomatitis, and cytopenias. It is strictly contraindicated in pregnancy.
• Biologic Therapy: TNF-alpha inhibitors (e.g., Adalimumab, Etanercept, Infliximab).
  • Safety Rules: Must screen for latent Tuberculosis (PPD or QuantiFERON) and Hepatitis B/C prior to initiation to prevent reactivation.
• Symptomatic Bridge: NSAIDs or systemic corticosteroids are used briefly for acute flares or as a bridge while waiting for DMARDs to take effect (6-12 weeks).

Systemic Lupus Erythematosus

• Mainstay Pharmacotherapy: Hydroxychloroquine is prescribed to all SLE patients to reduce flares and prolong survival.
  • Safety Rules: Hydroxychloroquine can cause retinal toxicity (macular degeneration). Requires baseline and annual comprehensive ophthalmologic exams.
• Flares & Severe Organ Disease: NSAIDs for mild arthritis. Systemic corticosteroids for acute flares. Immunosuppressants (e.g., Mycophenolate Mofetil, Cyclophosphamide) are indicated for lupus nephritis.

Gout

• Acute Flare Management: Focuses on reducing inflammation. First-line options include NSAIDs (e.g., Indomethacin, Naproxen), Colchicine (best within 36 hours; side effects include severe diarrhea), or Corticosteroids (oral or intra-articular).
  • Safety Rules: Avoid NSAIDs and Colchicine in patients with moderate-to-severe Chronic Kidney Disease (CKD) or active peptic ulcer disease; use corticosteroids instead. Do NOT initiate or stop urate-lowering therapy (Allopurinol) during an acute flare, as sudden shifts in uric acid can prolong inflammation.
• Chronic Management (Urate-Lowering Therapy): Indicated for patients with >= 2 flares/year, tophi, or uric acid stones. Allopurinol is first-line.
  • Safety Rules: Co-prescribe low-dose Colchicine or NSAIDs for the first 3-6 months of Allopurinol initiation to prevent mobilization flares. Screen for the HLA-B*5801 allele in patients of Asian or African descent before starting Allopurinol due to a high risk of life-threatening Stevens-Johnson syndrome/hypersensitivity.

Osteoarthritis

• Management: Weight loss and low-impact exercise are primary. First-line pharmacotherapy is topical NSAIDs (e.g., Diclofenac gel) for superficial joints (hand, knee), which minimize systemic absorption. Oral NSAIDs (e.g., Ibuprofen, Celecoxib) are used with caution in elderly patients, often co-prescribed with a PPI for gastroprotection. Intra-articular corticosteroid injections provide temporary relief. Surgical joint replacement is the definitive treatment for severe functional impairment.

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Comparison Table: Autoimmune & Inflammatory MSK Conditions