What are the possible causes and diagnostic approaches for finger swelling at the ulnar side, considering conditions like rheumatoid arthritis or gout?

Ulnar-Sided Finger Swelling: Causes and Diagnostic Approach

Ulnar-sided finger swelling most commonly indicates rheumatoid arthritis (RA), particularly when involving the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints, though psoriatic arthritis, gout superimposed on osteoarthritis, and erosive hand osteoarthritis must be systematically excluded. 1, 2

Primary Differential Diagnoses

Rheumatoid Arthritis (Most Likely)

RA characteristically targets the ulnar-sided joints with symmetric involvement of MCPs and PIPs, while typically sparing the distal interphalangeal (DIP) joints. 1

• Classic early radiographic finding: Soft tissue swelling at the ulnar styloid is pathognomonic for early RA, appearing before bone destruction 3
• Joint distribution pattern: Symmetric involvement of small joints, particularly the 4th and 5th MCP joints on the ulnar side, with sparing of DIP joints and first carpometacarpal joints 1
• Clinical hallmarks: Morning stiffness >30 minutes, symmetric joint swelling, and tenderness detected by the scissor technique for MCPs (74% sensitivity for 2nd MCP, 70% for 3rd MCP) 4
• Earliest bony change: Loss of cortical white line on the radial aspect of 4th and 5th metacarpal heads precedes erosions 3

Psoriatic Arthritis

PsA presents with dactylitis ("sausage digit") affecting entire digits including ulnar-sided fingers, with characteristic warmth and functional impairment. 5

• Distinguishing features: Asymmetric involvement, nail changes (pitting, onycholysis), and enthesitis at tendon insertion sites 5
• Pattern: Can affect single rays or DIP joints (unlike RA), with swelling extending along entire digit into palm 5
• Serology: Rheumatoid factor typically negative 5
• Timing: Psoriasis precedes arthritis in 72.7% of cases 5

Gout Superimposed on Pre-existing Arthritis

Gout can superimpose on osteoarthritis, particularly affecting the ulnar-sided MCPs and wrists, presenting with acute inflammatory episodes. 2

• Target sites: MCPs and wrists are common gout locations, though less typical than first metatarsophalangeal joint 2
• Diagnostic imaging: Ultrasound showing double-contour sign or tophi, or DECT demonstrating monosodium urate crystals 2
• Clinical probability: Rapidly painful, swollen tender joint has only 0.63% probability of gout in general population, increasing to 32.3% with podagra pattern 2

Erosive Hand Osteoarthritis

Erosive OA targets IPJs with abrupt onset, marked pain, inflammatory signs (stiffness, soft tissue swelling, erythema), and mildly elevated CRP. 2

• Characteristic features: Heberden and Bouchard nodes with bony enlargement, lateral deviation of IPJs 2
• Radiographic hallmark: Subchondral erosion progressing to bone attrition and potential bony ankylosis 2
• Distribution: Primarily affects DIP and PIP joints, with index and middle MCPs also involved 2

Systematic Diagnostic Approach

Clinical Examination (Perform First)

Conduct standardized musculoskeletal examination using validated techniques with 66-80% sensitivity for detecting arthritis. 4

1. Inspection: Document swelling pattern (symmetric vs asymmetric), erythema, deformities, and skin changes 4
2. Screening maneuvers:
   • Scissor technique for MCPs: 74% sensitivity for detecting swelling in 2nd MCP, 70% in 3rd MCP 4
   • MCP squeeze test: 66% sensitivity for tenderness 4
   • 4-finger technique for PIPs: 69% sensitivity for 3rd PIP 4
   • 2-thumb technique for wrist: 80% sensitivity 4
3. Specific assessments:
   • Morning stiffness duration (>30 minutes suggests inflammatory arthritis) 1
   • Ulnar styloid tenderness (classic for early RA) 3
   • Nail examination for psoriatic changes 5
   • Skin examination for psoriatic plaques, tophi, or rheumatoid nodules 2, 5

Laboratory Testing (Order Immediately)

Obtain comprehensive autoimmune and inflammatory panel to differentiate causes. 1

Essential first-line tests:

• RF and anti-CCP antibodies: High specificity (90%) for RA, though seronegative RA accounts for 20-30% of cases 1
• CRP (preferred over ESR): More reliable, not age-dependent; markedly elevated in inflammatory arthritis 1
• Complete blood count with differential: Assess for cytopenias before treatment 1
• Comprehensive metabolic panel: Including liver function, renal function, glucose, urate levels 1
• Serum uric acid: Normal level effectively excludes gout as primary diagnosis 2

Conditional testing based on clinical presentation:

• ANA and extractable nuclear antigens: If diagnosis uncertain or to screen for lupus/connective tissue disease 1
• HLA-B27: If spondyloarthropathy suspected (axial or entheseal involvement) 2, 1

Imaging Studies (Algorithmic Approach)

Step 1: Plain radiographs (obtain at baseline)

• Bilateral hand, wrist, and foot X-rays: Gold standard for morphological assessment, detecting erosions that predict RA diagnosis and disease persistence 2, 1
• Classic RA features: Periarticular osteopenia, uniform joint space narrowing, soft tissue swelling at ulnar styloid, loss of cortical white line on 4th-5th metacarpal heads 2, 3
• Erosive OA features: Subchondral erosion with joint space narrowing and osteophytes 2

Step 2: Ultrasound with Power Doppler (if diagnosis uncertain)

• Indications: When clinical examination shows no definite synovitis but suspicion remains high, or when early disease suspected 2, 1
• Diagnostic findings: Double-contour sign and tophi for gout; synovial hypertrophy and hyperemia for inflammatory arthritis 2
• Sensitivity advantage: Superior to clinical examination for detecting subclinical synovitis that predicts disease progression 2, 1

Step 3: MRI with IV contrast (if ultrasound inconclusive)

• Highest sensitivity: Detects bone marrow edema (osteitis), the best single predictor of future disease progression and functional deterioration 2, 1
• Specific findings: Synovitis, erosions, and flexor tenosynovitis (sensitivity 60%, specificity 73% for early RA) 1

Step 4: DECT (for suspected gout)

• Indications: When crystal deposition suspected but synovial fluid analysis not feasible 2
• Performance: 87% sensitivity, 84% specificity for monosodium urate deposition 2
• Limitation: Inaccurate for shoulder and hip; less useful than ultrasound for early disease 2

Critical Diagnostic Pitfalls to Avoid

Do not delay diagnosis or treatment based solely on negative serology or normal inflammatory markers. 1

• Seronegative RA is common (20-30%) and has similar prognosis to seropositive disease 1
• Normal ESR/CRP occurs in active RA: Acute phase reactants are poor predictors with only 50% sensitivity 1
• ACR/EULAR criteria assign only 1 point for abnormal CRP/ESR versus up to 5 points for joint involvement, emphasizing clinical synovitis over laboratory values 1

Do not dismiss psoriatic arthritis without thorough skin and nail examination. 5

• Psoriasis may be subtle (scalp, gluteal cleft, umbilicus) 5
• Nail changes are highly suggestive even without obvious skin lesions 5

Do not assume osteoarthritis based on age alone when inflammatory features present. 2

• Erosive OA presents with inflammatory symptoms (stiffness, swelling, erythema) and elevated CRP 2
• Requires different management than non-erosive OA 2

Urgent Referral Criteria

Refer to rheumatology within 6 weeks of symptom onset if inflammatory arthritis suspected, as early treatment prevents irreversible joint damage. 1

• Definite indications: Clinical synovitis in ≥1 joint, morning stiffness >30 minutes, symmetric small joint involvement 1
• Imaging-detected synovitis: Even without clinical findings, if ultrasound or MRI shows active inflammation 2, 1