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?
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?