Rheumatology and Immunology
Core concepts
Differentiate inflammatory arthritis from degenerative and infectious arthritis, and know the systemic connective tissue diseases: rheumatoid arthritis, systemic lupus erythematosus, Sjögren syndrome, scleroderma, and the vasculitides. Gout and calcium pyrophosphate deposition are crystal arthritides diagnosed by joint aspiration. RA is symmetric, small-joint, with prolonged morning stiffness and anti-CCP antibodies. SLE is multisystem with ANA as a sensitive but nonspecific screen; anti-dsDNA and anti-Smith are specific. Before starting biologics or immunosuppressants, screen for latent tuberculosis and hepatitis B.
Traps
- Diagnosing gout without aspirating a hot joint, thereby missing septic arthritis.
- Treating a positive ANA as lupus.
- Missing renal involvement in SLE; biopsy guides therapy.
- Starting allopurinol without checking renal function and thiopurine interactions.
- Forgetting that hydroxychloroquine requires periodic eye screening.
- Overlooking giant cell arteritis in an older patient with headache and jaw claudication; give steroids before biopsy.
Worked example
A 55-year-old man has an acutely swollen first MTP joint. Aspiration shows needle-shaped, negatively birefringent crystals. That confirms gout, but a simultaneous cell count and Gram stain are mandatory because septic arthritis can coexist. Treat the flare with NSAIDs, colchicine, or steroids, and consider urate-lowering therapy after the flare. Rule of thumb: hot joint equals aspiration before assumption.