What conditions can cause pain and stiffness of the left fourth metacarpophalangeal joint?

Conditions Causing Pain and Stiffness of the Left Fourth Metacarpophalangeal Joint

Rheumatoid arthritis is the most likely diagnosis when a single MCP joint presents with pain and stiffness, particularly if morning stiffness exceeds 30-60 minutes or if other small joints are involved symmetrically. 1, 2

Primary Differential Diagnoses

Rheumatoid Arthritis (Most Common)

• RA characteristically targets MCP joints, PIP joints, and wrists in a symmetric pattern, making isolated fourth MCP involvement an early or atypical presentation. 1, 2
• Morning stiffness lasting at least 1 hour before maximal improvement is typical of RA, and its duration correlates with disease activity. 1
• Approximately 70% of RA patients develop hand pathologies, especially of the MCP joints. 3
• Definite clinical synovitis (soft, boggy swelling) in at least one joint not better explained by another disease is the cornerstone of RA diagnosis. 2
• Seronegative RA accounts for 20-30% of cases and has similar prognosis to seropositive disease, so negative RF/ACPA does not exclude the diagnosis. 2

Psoriatic Arthritis

• PsA may target individual digits or affect just one ray (entire digit), distinguishing it from RA's symmetric pattern. 1
• Psoriasis precedes arthritis in 72.7% of cases, so examine carefully for psoriatic skin lesions, nail pitting, or onycholysis. 1, 4
• RF is typically negative in PsA, helping differentiate it from RA. 1
• PsA can present with dactylitis ("sausage digit") affecting the entire finger including the MCP joint. 4

Osteoarthritis (Less Likely for Isolated MCP)

• HOA primarily targets DIP joints, PIP joints, and thumb base (carpometacarpal) joints, making isolated fourth MCP involvement uncommon. 1
• Morning stiffness in HOA is limited to <30 minutes, distinguishing it from inflammatory arthritis. 1
• Produces hard, bony enlargement from osteophytes rather than soft tissue swelling. 4

Adult-Onset Still's Disease

• AOSD can affect MCP joints as part of polyarthritis, with knees, wrists, and ankles most frequently involved. 5
• Look for high-spiking fevers (>39°C, quotidian pattern), salmon-pink evanescent rash, and markedly elevated ferritin (>1,000 ng/mL). 5, 2
• Arthritis occurs in 64-100% of AOSD patients, typically symmetric polyarthritis with joint pain associated with fever spikes. 5

Immune Checkpoint Inhibitor-Induced Arthritis

• ICPi-induced inflammatory arthritis can affect large and/or small joints, with median onset at 38 weeks but ranging from 1-127 weeks after therapy initiation. 5
• Presenting symptoms include joint pain accompanied by joint swelling and/or morning stiffness lasting >30 minutes to 1 hour. 5
• Some patients present with oligoarthritis of large joints, while others develop symmetrical polyarthritis resembling RA. 5
• Inflammatory markers are usually very elevated in ICPi-induced arthritis. 5

Critical Diagnostic Workup

Clinical Examination

• Perform the "squeeze test": gentle compression across MCP joints elicits pain in inflammatory arthritis. 1, 4
• Distinguish soft tissue swelling (inflammatory) from bony enlargement (osteoarthritis): inflammatory arthritis produces soft, boggy joint swelling from synovitis. 4
• Assess morning stiffness duration: >30-60 minutes strongly suggests inflammatory arthritis. 1
• Examine all other joints systematically using 28-joint count (PIPs, MCPs, wrists, elbows, shoulders, knees) to identify symmetric involvement. 2

Essential Laboratory Tests

• RF and ACPA (anti-CCP) should be performed simultaneously at initial presentation, with ACPA having 90% specificity and 60% sensitivity for RA. 1, 2
• CRP is preferred over ESR as it is more reliable and not age-dependent. 1, 2
• Complete blood count with differential, comprehensive metabolic panel including liver and renal function. 2
• Do not dismiss inflammatory arthritis based on normal ESR/CRP, as acute phase reactants can be normal even in active disease. 2, 4

Imaging Strategy

• Baseline bilateral hand and wrist radiographs (posteroanterior view) are mandatory, looking for periarticular osteopenia, uniform joint space narrowing, and marginal erosions. 1, 2, 4
• Presence of erosions on baseline films is highly predictive for RA diagnosis and disease persistence. 2, 4
• Ultrasound with Power Doppler is superior to clinical examination for detecting synovitis and can detect subclinical inflammation that predicts disease progression. 2, 6
• MRI with IV contrast is more sensitive than ultrasound in early stages, detecting bone marrow edema (osteitis) which is the best single predictor of future disease progression. 2

When to Refer and Treat

• Patients presenting with arthritis of more than one joint should be referred to and seen by a rheumatologist within 6 weeks after symptom onset. 1, 2
• Even with isolated MCP involvement, if clinical synovitis is present with morning stiffness >30 minutes, refer urgently as early treatment prevents irreversible joint damage. 2, 4
• Do not delay treatment waiting for positive serology, as seronegative RA is common (20-30% of cases) and has similar prognosis. 2, 4

Common Pitfalls to Avoid

• Do not assume osteoarthritis based on isolated MCP involvement without assessing for inflammatory features (morning stiffness duration, soft tissue swelling, response to NSAIDs). 1
• Do not rely solely on RF/ACPA negativity to exclude RA, as seronegative disease is common. 2
• Do not dismiss the diagnosis based on normal inflammatory markers, as they are poor predictors of inflammatory arthritis. 2, 4
• In patients on immune checkpoint inhibitors, maintain high suspicion for ICPi-induced arthritis even months after therapy initiation. 5