6.4 Musculoskeletal System & Adult Rheumatology
Key Takeaways
Rheumatoid arthritis (RA) is a symmetric, small-joint-predominant inflammatory polyarthritis; diagnosis combines ≥6 ACR/EULAR 2010 points (joint involvement, serology, acute-phase, duration) and DMARDs (methotrexate first-line) are started early to prevent erosions.
Osteoarthritis is a mechanical, wear-and-tear disease affecting knees, hips and the DIP/PIP (Heberden/Bouchard nodes) with non-inflammatory synovial fluid and osteophytes on X-ray; management is weight loss, exercise, analgesia and joint replacement for end-stage disease.
Ankylosing spondylitis (HLA-B27, inflammatory back pain, sacroiliitis, bamboo spine) is the prototype seronegative spondyloarthropathy; NSAIDs and TNF-inhibitors are mainstay and physiotherapy prevents fusion deformity.
Gout is caused by monosodium urate crystal deposition; acute attacks (first MTP, hot red joint) are treated with NSAIDs/colchicine/steroids and chronic hyperuricaemia with allopurinol targeting serum urate <6 mg/dL.
Osteoporosis (T-score ≤ −2.5 on DEXA) is silent until fragility fracture; calcium, vitamin D, weight-bearing exercise, bisphosphonates and denosumab reduce vertebral/hip fracture risk.
6.4 Musculoskeletal System & Adult Rheumatology
High-Yield Core Concept: The General Medicine syllabus explicitly lists the Musculoskeletal System as a tested domain. CMS questions reward pattern recognition — inflammatory vs mechanical joint disease, a synovial-fluid profile that narrows the differential, and the first-line disease-modifying drug for each chronic rheumatological condition.
1. Approach to the Painful Joint
A focused history separates inflammatory from mechanical joint pain, the single most useful branching point:
| Feature | Inflammatory (e.g. RA) | Mechanical (e.g. OA) |
|---|---|---|
| Morning stiffness | >1 hour, improves with activity | <30 min, worse with activity |
| Rest vs use | Rest worsens; activity relieves | Activity worsens; rest relieves |
| Joint pattern | Symmetric, small joints, polyarticular | Weight-bearing, asymmetric, oligo |
| Synovial fluid | Inflammatory (WBC 2,000–50,000) | Non-inflammatory (WBC <2,000) |
| Systemic features | Fatigue, weight loss, fever possible | Absent |
Synovial fluid analysis classifies effusions: non-inflammatory (<2,000 WBC), inflammatory (2,000–75,000), septic (>50,000 with ≥75% neutrophils and positive culture) and haemorrhagic (trauma, haemophilia, pigmented villonodular synovitis). A Gram stain and culture are mandatory whenever septic arthritis is possible, because a septic joint can destroy cartilage within 24–48 hours.
2. Rheumatoid Arthritis (RA)
RA is a chronic, symmetric, autoimmune polyarthritis targeting synovial joints. Hand involvement (MCP, PIP, wrists) with sparing of the DIP is classic; extra-articular features include rheumatoid nodules, vasculitis, interstitial lung disease, anaemia of chronic disease and secondary amyloidosis.
Diagnosis — ACR/EULAR 2010 criteria score points from joint involvement (1 small joint = 1 pt, large = 0.5), serology (RF and/or anti-CCP), acute-phase (CRP/ESR) and symptom duration ≥6 weeks. ≥6 points = definite RA. Anti-CCP is more specific than RF.
Management:
- DMARDs started early — methotrexate is first-line (folate supplementation, monitor LFTs).
- Add biologicals (TNF-inhibitors e.g. etanercept, or anti-IL-6 tocilizumab) if inadequate response.
- NSAIDs and low-dose prednisolone bridge inflammation control while DMARDs take effect (6–12 weeks).
- Treat-to-target aiming for remission or low disease activity.
3. Osteoarthritis (OA)
The commonest arthritis — degenerative cartilage loss with subchondral sclerosis, cysts and osteophytes. Knees, hips, spine and the DIP (Heberden nodes) / PIP (Bouchard nodes) are typical. Pain is mechanical, stiffness brief, and systemic features absent.
X-ray: joint-space narrowing, osteophytes, subchondral sclerosis.
Management: weight reduction, muscle-strengthening exercise, paracetamol/topical NSAIDs first, intra-articular steroid for flares, and total joint replacement for end-stage disease.
4. Seronegative Spondyloarthropathies
Group unified by negative RF, axial skeleton involvement, HLA-B27 association and enthesitis.
| Disease | Hallmarks | Key test |
|---|---|---|
| Ankylosing spondylitis | Inflammatory back pain, morning stiffness, sacroiliitis, bamboo spine | HLA-B27, X-ray/MRI sacroiliac joints |
| Reactive arthritis | Urethritis/cervicitis + conjunctivitis + arthritis ("can't see, can't pee, can't climb a tree") post-STI/dysentery | Trigger history |
| Psoriatic arthritis | Nail pitting, psoriasis, DIP involvement, pencil-in-cup X-ray | Skin/nail exam |
| Enteropathic arthritis | IBD-associated, mirrors bowel activity | Colonoscopy |
Ankylosing spondylitis management: intensive NSAIDs (first-line), physiotherapy/postural exercises to prevent fusion deformity, TNF-inhibitors for refractory disease.
5. Crystal Arthropathies
Gout — monosodium urate crystals (needle-shaped, negatively birefringent). Acute attack: exquisitely tender, hot, red first MTP (podagra). Triggers: alcohol, purine-rich food, diuretics.
- Acute: NSAIDs, colchicine (microtubule inhibitor) or oral/intra-articular steroids.
- Chronic: allopurinol (xanthine-oxidase inhibitor) — start only after inflammation settles; target serum urate <6 mg/dL.
Pseudogout — calcium pyrophosphate crystals (rhomboid, positively birefringent); knee/wrist; X-ray chondrocalcinosis. Acute treatment mirrors gout.
6. Metabolic Bone Disease & Selected Connective-Tissue Disease
Osteoporosis — low bone mass, fragility fractures. T-score ≤ −2.5 on DEXA (hip/spine). Risk: post-menopausal, steroids, low BMI, smoking, hyperthyroidism. FRAX estimates 10-year fracture risk. Treatment: calcium + vitamin D, bisphosphonates (alendronate, with a drug holiday after 3–5 years), denosumab, teriparatide for severe disease.
SLE — multi-system autoimmunity (ANA screening, anti-dsDNA/anti-Sm specific). ACR/EULAR classification uses weighted criteria. Therapy ranges from hydroxychloroquine (skin/joint, and reduces flares) to steroids and immunosuppressants for nephritis (mycophenolate/cyclophosphamide).
7. Septic Arthritis — the Don't-Miss Diagnosis
Acute monoarthritis with fever, severe pain and restricted movement is septic until proven otherwise. Staph aureus is commonest in adults; gonococcal in young sexually active patients. Investigate with synovial fluid (cell count, Gram stain, culture) before antibiotics. Treatment: joint washout/debridement plus empirical IV antibiotics — delay destroys cartilage. Consider atypical pathogens (TB, fungal) in immunocompromised hosts.
Key Takeaways for the CMS Candidate
- Inflammatory vs mechanical joint history is the highest-yield branching question.
- Memorise ACR/EULAR RA criteria threshold (≥6 points) and that anti-CCP is more specific than RF.
- Know the HLA-B27 seronegative cluster and the clinical triad of reactive arthritis.
- Gout crystal shape/birefringence and allopurinol target (<6 mg/dL) are favourite MCQ facts.
- For any hot, red, monoarticular joint, exclude septic arthritis with synovial fluid before anything else.
A 42-year-old woman has symmetric swelling of the MCP, PIP and wrists for 8 weeks with 90 minutes of morning stiffness. Anti-CCP is positive. Which is the most appropriate first-line disease-modifying therapy?
Intra-articular corticosteroid injections to all involved joints
Methotrexate with folic acid supplementation
Allopurinol titrated to serum urate below 6 mg/dL
Total joint replacement of the wrists
A 28-year-old man has 2 hours of morning low-back stiffness improving with exercise, alternating buttock pain and reduced chest expansion. Which investigation finding best supports the diagnosis of ankylosing spondylitis?
Rheumatoid factor positive in high titre
Bilateral sacroiliitis on X-ray/MRI with HLA-B27 positivity
Needle-shaped negatively birefringent crystals in synovial fluid
Chondrocalcinosis on knee X-ray
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