How to differentiate and manage Rheumatoid Arthritis (RA), Psoriatic Arthritis (PsA), and Posterior Tibial Tendon Dysfunction (PTTD) in a patient with these potential diagnoses?

Differentiating RA, Psoriatic Arthritis, and PTTD

Begin by examining for skin and nail findings—the presence of psoriatic plaques or nail psoriasis establishes PsA, while their absence with rheumatoid nodules, extra-articular involvement, or high-titer rheumatoid factor confirms RA. 1

Key Distinguishing Features

Clinical Examination Priorities

Skin and Nail Assessment:

• Look specifically for psoriatic plaques or nail changes (pitting, onycholysis, oil spots) which strongly favor PsA over RA 1
• Psoriasis typically precedes joint symptoms by an average of 12 years in 84% of PsA patients 1

Joint Distribution Pattern:

• RA: Symmetric polyarticular involvement, typically sparing DIP joints; commonly affects MCPs, PIPs (2nd-5th), wrists, elbows, shoulders, and knees bilaterally 1
• PsA: Less symmetric distribution, joints are usually less tender and swollen than RA; however, 20% of PsA patients (especially females) present with symmetric polyarthritis 1
• DIP joint involvement strongly suggests PsA as this is uncommon in RA 1

Pathognomonic Features for PsA:

• Dactylitis ("sausage digit"): Combination of enthesitis and synovitis involving an entire digit—uncommon in RA 1
• Enthesitis: Inflammation at tendon insertion sites (Achilles, plantar fascia)—uncommon in RA 1
• These two features are critical differentiators when present 1

Features Favoring RA Over PsA:

• Rheumatoid nodules 1
• Extra-articular manifestations (pulmonary, cardiac, ocular) 1
• High titers of rheumatoid factor 1
• Patients displaying these should not be diagnosed with PsA 1

Serologic Testing

For RA Diagnosis:

• Rheumatoid factor (RF) and anti-citrullinated protein antibody (ACPA) 1
• High positive RF or ACPA (>3 times upper limit of normal) adds 3 points to ACR/EULAR classification 1
• Critical caveat: Seronegative RA with coincidental psoriasis must be differentiated from true PsA using clinical features 1

For PsA:

• No specific serologic tests confirm PsA 1
• Rheumatoid factor should be negative 1
• Elevated acute phase reactants (ESR, CRP) support inflammatory arthritis but are nonspecific 1

Synovial Tissue Biomarkers (when biopsy available):

• CD117+ cells are significantly elevated in PsA compared to seronegative RA 2
• CD138+ cells are significantly elevated in seronegative RA compared to PsA 2
• These can help resolve diagnostic uncertainty between the two conditions 2

PTTD Differentiation

PTTD is fundamentally different—it is a mechanical/degenerative condition, not a primary inflammatory arthritis 3, 4

Key Clinical Features:

• Loss of longitudinal arch (acquired flatfoot deformity) 5
• Inability to perform single-leg heel rise 5
• Lack of palpable posterior tibial tendon 5
• "Too many toes" sign on posterior view
• Pain and swelling along the medial ankle and hindfoot, not polyarticular 3

Critical Distinction:

• PTTD can occur secondary to RA due to inflammatory joint destruction causing hindfoot instability, which then renders the posterior tibial tendon dysfunctional 5
• Between 11-64% of RA patients may develop secondary PTTD depending on diagnostic criteria used 5
• Primary PTTD presents without systemic inflammatory features or polyarticular involvement 3, 4

Imaging Studies

Radiographs:

• PsA: Look for pencil-in-cup deformity, joint space widening, gross osteolysis, ankylosis, and new bone formation at entheses 6
• RA: Periarticular osteopenia, symmetric joint space narrowing, marginal erosions 1
• PTTD: Flatfoot deformity, talar head uncovering, loss of longitudinal arch on weight-bearing films 3

Advanced Imaging:

• MRI or CT can detect asymptomatic sacroiliitis or spinal involvement in PsA 1
• MRI is valuable for assessing posterior tibial tendon integrity and inflammation in suspected PTTD 3

Diagnostic Algorithm

1. Examine skin and nails first - presence of psoriasis strongly suggests PsA 1
2. Assess joint pattern - DIP involvement, dactylitis, or enthesitis favor PsA; symmetric small joint involvement favors RA 1
3. Check RF and ACPA - high titers favor RA; negativity doesn't exclude either diagnosis 1
4. Look for RA-specific features - nodules and extra-articular disease exclude PsA 1
5. Evaluate for PTTD - if unilateral hindfoot/ankle pain with flatfoot deformity and no polyarticular involvement, consider primary PTTD 3, 5
6. If diagnostic uncertainty persists between PsA and seronegative RA, consider synovial biopsy for CD117+/CD138+ analysis 2

Management Implications

Early diagnosis is critical as both RA and PsA can cause progressive joint damage within the first year, leading to severe physical limitations and disability if untreated 1

For PsA:

• Mild disease responds to NSAIDs 1
• Moderate disease requires DMARDs or TNF blockers 1
• Severe disease requires DMARDs plus TNF blockers or other biologics 1
• Etanercept 25mg SC twice weekly is effective for peripheral PsA with DIP involvement 6

For RA:

• Early DMARD therapy is essential 1
• Methotrexate is typically first-line 1

For PTTD:

• Treatment depends on stage and whether primary or secondary to inflammatory arthritis 3, 4
• If secondary to RA, treating the underlying inflammatory disease is essential 5

Common Pitfall: Misdiagnosing generalized OA with entheseal involvement as PsA can lead to apparent biologic therapy failure, as the pathology is degenerative rather than inflammatory 7