Systemic lupus erythematosus Study Guide

📖 Core Concepts Systemic Lupus Erythematosus (SLE) – a multisystem autoimmune disease where auto‑antibodies attack healthy tissues, causing flares and remission. Autoantibodies – most important are antinuclear antibodies (ANA) (high sensitivity) and anti‑double‑stranded DNA (anti‑dsDNA) (high specificity). Immune‑complex (type III) hypersensitivity – circulating immune complexes deposit in organs, activate complement, and drive inflammation. Complement consumption – low serum C3/C4 indicates active disease because complexes use up complement proteins. Classification vs. Diagnosis – ACR/1997 and 2019 EULAR/ACR criteria (≥4/11 items) are research tools; clinicians also rely on ANA, anti‑dsDNA, clinical picture. Hydroxychloroquine (HCQ) – cornerstone DMARD; reduces mortality (54 %), flares, and long‑term organ damage. Organ‑specific risk – kidney (lupus nephritis), heart (Libman‑Sacks endocarditis, accelerated atherosclerosis), CNS (neuropsychiatric SLE), skin (malar rash, discoid lesions). --- 📌 Must Remember Epidemiology: 9 × higher in women (child‑bearing age); prevalence 20–70/100 k. Key lab markers: ANA – sensitive (>95 %) but not specific. Anti‑dsDNA – specific (70 % of pts); levels parallel disease activity. Low C3/C4 – sign of active immune‑complex disease. Antiphospholipid antibodies – ↑ thrombosis, false‑positive VDRL. ACR 1997 criteria: 4 of 11 items (malar rash, discoid rash, serositis, oral ulcers, arthritis, photosensitivity, hematologic disorder, renal disorder, ANA, immunologic disorder). First‑line medication: HCQ for all patients unless contraindicated. Steroid stewardship: Use lowest effective dose; chronic >5 mg pred equivalent ↑ CVD, osteoporosis, cataracts. Renal protocol: Mycophenolate mofetil (MMF) preferred for lupus nephritis; cyclophosphamide for severe/induction. Pregnancy safety: HCQ safe; avoid cyclophosphamide, methotrexate, high‑dose steroids when possible. Mortality: Cardiovascular disease is leading cause; >90 % survive >10 yr, but overall mortality remains ↑ vs general population. --- 🔄 Key Processes Autoantibody Generation Genetic predisposition (HLA, C4 copy number) + environmental trigger → loss of tolerance → B‑cell activation → ANA → anti‑dsDNA, antiphospholipid, anti‑Smith, etc. Immune‑Complex Deposition Circulating antibodies bind nuclear antigens → complexes → deposit in glomeruli, skin, serosa → complement activation → inflammation. Complement Consumption Classical pathway → C3/C4 ↓ → lab marker of active disease. Renal Involvement Pathway Immune‑complexes in glomeruli → “wire‑loop” deposits → proteinuria/hematuria → progressive fibrosis if untreated. Drug‑Induced Lupus (DIL) Chronic exposure to hydralazine, procainamide, minocycline, etc. → lupus‑compatible symptoms + positive ANA, anti‑histone; resolves after drug withdrawal. --- 🔍 Key Comparisons ANA vs. Anti‑dsDNA ANA: highly sensitive, low specificity. Anti‑dsDNA: lower sensitivity, high specificity; tracks disease activity. Hydroxychloroquine vs. Corticosteroids HCQ: long‑term disease‑modifying, mortality benefit, minimal acute toxicity. Steroids: rapid control of flares, high short‑ and long‑term side‑effect burden. Mycophenolate vs. Cyclophosphamide (nephritis induction) MMF: oral, fewer gonadal toxicities, comparable efficacy in many protocols. Cyclophosphamide: IV, higher infection & infertility risk; reserved for severe/ refractory disease. Drug‑Induced Lupus vs. Idiopathic SLE DIL: usually anti‑histone antibodies, resolves on cessation, less organ damage. Idiopathic: broader autoantibody profile, chronic course, multi‑organ involvement. --- ⚠️ Common Misunderstandings “ANA negative rules out SLE.” – Rare ANA‑negative cases exist; diagnosis relies on clinical criteria plus other serologies. “All rashes in SLE are malar.” – SLE skin disease includes discoid, subacute, and photosensitive rashes; not all are butterfly‑shaped. “Hydroxychloroquine is harmless.” – Retinal toxicity (2 % after 10 yr) mandates baseline and annual eye exams. “Pregnancy cures lupus.” – Pregnancy can worsen disease activity and increase fetal risks; careful monitoring is essential. “Low complement always means active disease.” – Complement can be low chronically in some patients; interpret in clinical context. --- 🧠 Mental Models / Intuition “Immune‑complex → complement → organ damage” – Visualize a “sticky net” (immune complexes) that drags complement proteins, leaving the net depleted (low C3/C4) and the organ inflamed. “Fire‑fighter analogy” – HCQ is the pre‑emptive patrol (prevents flares), steroids are the fire‑truck (rushes in for an active blaze). “Gender bias = estrogen + X‑chromosome escape” – Think of extra estrogen as fuel and extra X‑linked immune genes as extra ignition switches in females. --- 🚩 Exceptions & Edge Cases Seronegative Lupus: Rare patients meet clinical criteria but lack ANA/anti‑dsDNA; consider skin biopsy (lupus band test) or repeat testing. Drug‑Induced Lupus with anti‑dsDNA: Occasionally DIL can produce anti‑dsDNA; distinguish by rapid resolution after drug stop. Low complement with infection: Acute infections can also lower C3/C4; rule out infection before intensifying immunosuppression. Pregnancy‑related nephritis flare: Distinguish from pre‑eclampsia by checking anti‑dsDNA, complement, and proteinuria patterns. --- 📍 When to Use Which Mild‑to‑moderate disease (skin, arthritis, fatigue): Start HCQ + NSAIDs; add low‑dose steroids only for flare control. Severe organ involvement (nephritis, CNS, serositis): High‑dose steroids + immunosuppressant (MMF or cyclophosphamide). Refractory disease despite standard therapy: Consider belimumab or rituximab; assess infection risk. Pregnant patient: Continue HCQ; switch to azathioprine if immunosuppression needed; avoid cyclophosphamide & methotrexate. Drug‑induced lupus suspicion: Stop offending drug → monitor; if symptoms persist, treat as idiopathic SLE. --- 👀 Patterns to Recognize “Malar rash + positive ANA + low complement” → classic SLE presentation. “Proteinuria + anti‑dsDNA rise + C3 drop” → active lupus nephritis flare. “Thrombosis + anticardiolipin / lupus anticoagulant” → antiphospholipid syndrome; start low‑dose aspirin or warfarin. “Fatigue + anemia + depression” → multifactorial lupus fatigue; screen for thyroid, treat anemia, address mood. “New joint pain + non‑erosive arthritis” → lupus arthritis (distinguish from rheumatoid). --- 🗂️ Exam Traps Distractor: “Anti‑Smith antibody is the most sensitive test.” – False. It’s highly specific but low sensitivity. Distractor: “Hydroxychloroquine is contraindicated in pregnancy.” – False. It is safe and recommended. Distractor: “Low complement always indicates renal disease.” – False. Low C3/C4 can occur with any active immune‑complex disease. Distractor: “All patients with SLE develop renal involvement.” – False. Only 30‑50 % develop lupus nephritis. Distractor: “Cyclophosphamide is first‑line for all lupus patients.” – False. Reserved for severe organ disease; MMF is preferred for most nephritis. ---