Differential Diagnosis: Fever + Ankle Swelling + Joint Pain

Diagnostic Algorithm

Fig. 102.6 — ROSEN's Emergency Medicine

1. INFECTIOUS (Highest Priority — Must Exclude)

Septic Arthritis (Bacterial)

• Fever, acute joint pain, effusion — most often a single large joint (knee > ankle > hip)
• Up to 20% can be polyarticular
• Common organisms: S. aureus, Streptococcus spp. (all ages); N. gonorrhoeae (young, sexually active)
• Fever may be blunted in elderly or immunocompromised patients
• Emergency — joint destruction occurs rapidly if untreated

• ROSEN's Emergency Medicine, p. 2333–2336

Gonococcal (Disseminated GC) Arthritis

• Most common form of septic arthritis in young, sexually active adults
• Classic triad: migratory polyarthritis → tenosynovitis → dermatitis (pustular rash)
• Ankles, wrists, and knees most commonly involved

• ROSEN's Emergency Medicine, p. 2336

Lyme Disease

• Tick exposure history; migratory arthritis pattern
• Knee > ankle involvement; associated with erythema migrans rash

• ROSEN's Emergency Medicine, p. 2407

Viral Arthritis

ROSEN's Emergency Medicine, Table 102.9

Infective Endocarditis

• Joint pain and fever with murmur; embolic phenomena
• Should be considered when fever + arthritis + cardiac risk factors

2. REACTIVE ARTHRITIS (Post-Infectious)

• Arthritis 1–4 weeks after GI infection (Salmonella, Shigella, Campylobacter, Yersinia) or STI (Chlamydia)
• Asymmetric oligoarthritis — lower limb predominant (knee, ankle)
• Classic triad: urethritis + conjunctivitis + arthritis
• Associated with HLA-B27
• Enthesitis (Achilles, plantar fascia) is a hallmark

• Goldman-Cecil Medicine, p. 2802

3. ACUTE RHEUMATIC FEVER (ARF)

• Occurs 1–5 weeks after Group A Streptococcal pharyngitis
• Arthritis is the most common manifestation — migratory polyarthritis of large joints: knee, ankle, elbow, wrist
• Fever is universally present in the acute phase
• Other features: carditis, Sydenham's chorea, subcutaneous nodules, erythema marginatum
• Diagnosed by Jones Criteria: requires 2 major or 1 major + 2 minor criteria + evidence of streptococcal infection

• ROSEN's Emergency Medicine, p. 1132

4. CRYSTAL ARTHROPATHIES

Gout

• Hyperuricemia; often monoarticular (first MTP, ankle, knee)
• Presents with acute severe pain, swelling, erythema, and warmth
• Fever can occur during acute flares; may mimic septic arthritis

Pseudogout (CPPD)

• Calcium pyrophosphate crystals; knee and wrist predominate
• Acute flares simulate septic arthritis with fever + hot, swollen joint

5. SERONEGATIVE SPONDYLOARTHROPATHIES

• Ankylosing Spondylitis: peripheral oligoarthritis in 30%, lower limbs, asymmetric; heel pain/enthesitis
• Psoriatic Arthritis: asymmetric, axial + peripheral; associated psoriatic skin/nail changes
• Enteropathic Arthritis: associated with Crohn's disease or ulcerative colitis
• All are HLA-B27 associated; ankle involvement is common

• Goldman-Cecil Medicine, Table 244-2

6. RHEUMATOID ARTHRITIS (RA)

• Usually symmetric small joint involvement (PIPs, MCPs of hands), but can affect ankles
• ~20% present acutely; constitutional symptoms (fatigue, malaise, low-grade fever, weight loss)
• Morning stiffness >1 hour

• ROSEN's Emergency Medicine, p. 2340

7. SYSTEMIC RHEUMATOLOGIC DISEASES

8. OTHER CAUSES

• Hyperlipoproteinemia — Self-limited mono/oligoarticular arthritis involving knee or ankle
• Carcinomatous polyarthritis — Paraneoplastic; asymmetric, large joints
• Sickle cell disease — Vaso-occlusive crisis with periarticular pain and swelling; fever from infection

Key Diagnostic Steps

Bottom line: Fever + ankle swelling + joint pain is septic arthritis until proven otherwise. Arthrocentesis (joint aspiration) is the single most important diagnostic step. Once septic arthritis is excluded, work through the infectious, reactive, crystal, and rheumatologic differentials based on clinical pattern.