Differential Diagnosis - 74M with Psoriasis, Chronic Left Ankle Monoarthritis

• Extensive psoriasis (spondyloarthropathy substrate)
• Chronic monoarthritis (>1 month, ankle)
• Swelling + erythema without warmth (inflammatory but not hot = subacute/chronic, not acute crystal)
• No tenderness on palpation (unusual - argues strongly against acute gout or septic arthritis)
• No response to naproxen 1000 mg/day x 7 days
• Hyperuricemia (expected in psoriasis: uric acid is elevated in ~71% of PsA patients due to high cell turnover)
• Mildly elevated RF 25 UI/mL (borderline, not strongly positive)
• High ESR, normal CRP (dissociation is clinically meaningful)
• High GGT, normal transaminases (hepatic pattern suggesting alcohol, fatty liver, or drug effect)

1. Psoriatic Arthritis (PsA) - TOP DIAGNOSIS

• The ankle is a classic site for PsA peripheral arthritis
• PsA can present with swelling and erythema that is less "hot" than gout
• RF is typically negative or only mildly elevated (as in this patient - 25 UI/mL is borderline)
• The elevated ESR with normal CRP is consistent with PsA (CRP is less reliably elevated in PsA than in RA)
• Psoriatic arthritis can show enthesitis around the ankle, which may present as periarticular swelling without point tenderness at the joint line itself
• The lack of palpation pain is an important clue: enthesitis (inflammation at tendon/ligament insertions around the ankle) in PsA is often less acutely tender than septic arthritis or acute gout
• NSAIDs can be insufficient in PsA if the disease is established

2. Gout / Tophaceous Gout - Important to exclude

• Gout must be on the differential given hyperuricemia
• The ankle is the second most common joint after the first MTP (Harrison's 22E: "tarsal joints and ankles" follow podagra in frequency)
• However, several features argue against acute gout here:
  • No warmth (acute gout is typically described as "hot" and "pseudocellulitis-like")
  • No pain on palpation is distinctly atypical for gout, which is notoriously exquisitely tender
  • Duration >1 month without spontaneous resolution (acute gout self-resolves in 1-2 weeks untreated; Harrison's 22E)
  • No response to naproxen (NSAIDs are highly effective in acute gout)
• Chronic tophaceous gout is still possible - this can cause chronic synovitis with less dramatic inflammation, and in the elderly the presentation is often atypical
• The high GGT (possible chronic alcohol use) is a risk factor for gout
• Key action: synovial fluid aspiration with crystal analysis is mandatory to confirm or exclude gout, as clinical and lab features overlap significantly with psoriasis patients (who have elevated uric acid regardless)

3. "Psout" - Concurrent PsA + Gout

4. Calcium Pyrophosphate Deposition (CPPD) / Pseudogout

• Affects elderly patients preferentially
• The ankle is a recognized site (less common than knee/wrist but documented)
• Can present as subacute/chronic monoarthritis with swelling and erythema
• Less warmth and less dramatic tenderness compared to acute gout - fits this presentation
• ESR elevation is typical; CRP may be mildly elevated or normal
• Synovial fluid would show rhomboid calcium pyrophosphate crystals (vs. needle-shaped MSU crystals in gout)
• X-ray may show chondrocalcinosis

5. Septic Arthritis - Must exclude urgently

• Cannot be dismissed in any monoarthritis, especially in an elderly patient
• However, several features lower its probability:
  • No warmth
  • Notably: absence of pain on palpation is actually the strongest argument against septic arthritis (septic joints are exquisitely tender)
  • Duration of 1 month without treatment-related improvement is unusual (untreated septic arthritis progresses rapidly)
  • Normal CRP (septic arthritis reliably elevates CRP, not just ESR)
• Still, joint aspiration must be performed and culture sent - this is the single most important next step

6. Reactive Arthritis (ReA)

• Seronegative spondyloarthropathy, classically following urogenital or enteric infection 2-6 weeks prior
• Asymmetric oligoarthritis/monoarthritis; ankles are a classic site
• Lower limb predominance, enthesitis, possible heel pain
• RF negative or mildly positive (consistent with 25 UI/mL)
• ESR elevated, CRP variable
• Should prompt inquiry about recent GI illness, urethritis, eye symptoms, oral ulcers

7. Sarcoid Arthropathy

• Sarcoidosis can cause ankle periarthritis/arthritis (Löfgren syndrome: bilateral ankle arthritis + erythema nodosum + bilateral hilar adenopathy)
• Chronic sarcoid arthritis may be monoarticular, affects large joints
• ESR elevated; RF can be mildly positive (non-specific)
• GGT may be elevated if hepatic sarcoid involvement is present - this fits the lab pattern
• Worth considering in this patient

8. Rheumatoid Arthritis (RA) - Less likely

• Ankle can be affected in RA
• RF of 25 UI/mL is only marginally elevated (upper limit of normal is 20) - this is a weak positive
• RA typically causes polyarthritis; monoarthritis as the sole presentation is uncommon
• Anti-CCP antibody testing would be far more specific than RF alone
• ESR elevation fits; CRP is characteristically elevated in RA (normal here argues against active RA)
• Psoriasis can co-exist with RA, but this is less likely than PsA

9. Osteoarthritis (Erosive/Inflammatory Variant)

• At 74 years, OA must be in the differential
• Typically causes pain on use and on palpation, crepitus, bony enlargement
• However, the lack of palpation pain and the inflammatory markers would be unusual for simple OA
• Erosive inflammatory OA can cause erythema and swelling

10. Pigmented Villonodular Synovitis (PVNS) / Synovial Tumors

• Chronic monoarthritis in a single joint that does not respond to anti-inflammatories should always trigger consideration of synovial pathology
• PVNS presents as painless/minimally painful joint swelling (fits: no pain on palpation)
• Can cause ESR elevation
• More common in younger adults but occurs at any age
• MRI is the key diagnostic test

Summary Table

Recommended Next Steps

1. Synovial fluid aspiration - single most important step: cell count, culture, crystal analysis under polarized microscopy (MSU vs. CPPD)
2. Anti-CCP antibody - to evaluate RA more accurately (more specific than RF alone)
3. HLA-B27 - relevant for spondyloarthropathy spectrum (PsA, reactive arthritis)
4. Ankle X-ray - assess joint space, erosions, chondrocalcinosis, tophaceous deposits
5. Ankle MRI - if X-ray is non-diagnostic; best for enthesitis (PsA), PVNS, cartilage
6. Ultrasound - "double contour sign" for gout; power Doppler for synovitis in PsA; can guide aspiration
7. Chest X-ray or CT - to evaluate for sarcoidosis given GGT elevation
8. ACE level, calcium - if sarcoidosis is suspected
9. Alcohol/medication history - to contextualize elevated GGT (relevant for gout risk and NSAID dosing adequacy)

Best First-Line Treatment - No Access to Further Tests

Clinical Reasoning First

• Extensive psoriasis (the substrate)
• Chronic monoarthritis >1 month without spontaneous resolution
• Swelling and erythema without warmth, without palpation tenderness (not gout, not septic)
• Failed naproxen - 1000 mg/day x 7 days is already an adequate NSAID trial; failure confirms this is not mild PsA and NSAIDs are insufficient
• High ESR, normal CRP (typical PsA dissociation)
• Borderline RF (does not change the management at this level)

First-Line Treatment: csDMARD

Methotrexate (MTX) - PREFERRED

• Start 10-15 mg/week orally (or SC if tolerability is a concern)
• Escalate to 20-25 mg/week after 4-6 weeks if tolerated and no response
• Folic acid 1 mg/day (or 5 mg once weekly, 24-48h after MTX) - mandatory to reduce toxicity
• Takes 6-12 weeks to show full effect - set patient expectations

• Treats both joint disease AND skin psoriasis
• The high GGT warrants attention (see caveat below), but normal transaminases make it still usable
• Elderly patients tolerate lower doses well
• Cost-effective, widely available - important without access to specialized testing

Alternatives if MTX is contraindicated:

Bridge Therapy While Waiting for csDMARD Onset

Intra-articular corticosteroid injection - STRONGLY RECOMMENDED

• Provides rapid, localized anti-inflammatory effect
• Avoids systemic steroid exposure (important: EULAR 2024 explicitly states oral glucocorticoids are NOT recommended in PsA - they are less effective and risk a psoriasis flare on tapering)
• Works regardless of whether the underlying cause is PsA, CPPD, or atypical gout
• Safe in elderly patients
• The ankle is easily accessible for injection (tibiotalar joint)

Important Caveats for This Patient

1. High GGT + normal transaminases: This pattern (isolated GGT elevation) suggests hepatic enzyme induction (alcohol, medications) or fatty liver, rather than hepatocellular damage. Methotrexate is hepatotoxic and requires baseline liver assessment ideally - but even without tests, if this is a resource-limited setting, baseline clinical assessment for alcohol use, obesity, and metabolic syndrome should guide the decision. If alcohol use is significant, sulfasalazine or leflunomide are safer alternatives.
2. Hyperuricemia: Do not start urate-lowering therapy (allopurinol) empirically yet - it can paradoxically trigger acute gout flares if gout is present, and it does not treat PsA. Defer this until the diagnosis is confirmed or the joint inflammation is controlled.
3. Age 74: Avoid high-dose systemic steroids (rebound psoriasis flare, metabolic risk). Start methotrexate at lower end of dose range (10 mg/week). Monitor renal function - MTX is renally cleared and dose adjustment is needed if GFR is reduced.
4. Do NOT continue naproxen alone: This patient has already failed a 7-day NSAID trial for chronic arthritis. Continuing NSAID monotherapy risks GI/renal toxicity without clinical benefit, and delays effective therapy.

Summary Treatment Plan