What could be causing pain and mild swelling in the left thumb, especially in the morning, in a 64-year-old?

Thumb Base Osteoarthritis (Most Likely Diagnosis)

The most likely cause of pain and mild morning swelling in the left thumb of a 64-year-old is thumb base osteoarthritis (carpometacarpal joint OA), which can be confidently diagnosed clinically given the patient's age over 40, characteristic symptom pattern, and typical joint involvement. 1

Clinical Reasoning

Why This is Likely Osteoarthritis

• Age and demographics: The patient is 64 years old, and thumb base OA typically occurs in the second half of life, with increasing prevalence after age 40 1, 2
• Symptom pattern: Morning stiffness that is mild and localized is highly characteristic of hand OA rather than inflammatory arthritis 1
• Joint location: The thumb base (carpometacarpal joint) is the second most common site for hand OA, following the distal interphalangeal joints 1, 3
• Clinical presentation: Pain with mild swelling fits the typical presentation of thumb base OA, which causes pain, joint swelling, and reduced strength 2, 3

Key Distinguishing Features from Inflammatory Arthritis

• Duration of morning stiffness: Hand OA causes only mild, limited duration morning stiffness (typically less than 30 minutes), whereas inflammatory arthritis causes prolonged stiffness exceeding 30 minutes 1
• Pattern of involvement: OA symptoms are often intermittent and affect just one or a few joints at a time, rather than multiple joints simultaneously 1
• Pain characteristics: Pain in OA is primarily usage-related and variable over time 1

Differential Diagnosis Considerations

Other Conditions to Consider (Less Likely)

• Psoriatic arthritis: Can target distal joints or affect a single ray, but typically presents with more pronounced inflammatory features and prolonged morning stiffness 1
• Gout: May superimpose on pre-existing OA but usually presents with acute, severe flares rather than chronic mild symptoms 1
• Rheumatoid arthritis: Primarily targets metacarpophalangeal and proximal interphalangeal joints rather than thumb base, with more prominent inflammatory features 1

Clinical Examination Findings to Look For

• Bony enlargement at the thumb base with or without deformity (subluxation, adduction deformity) 1
• Reduced thumb span and grip strength during pinch maneuvers 2, 3
• Pain with grind test (axial compression and rotation of the thumb metacarpal) 3
• Absence of systemic inflammatory signs (no widespread joint involvement, no prolonged stiffness) 1

Recommended Diagnostic Approach

Clinical Diagnosis is Sufficient

• No imaging required initially: With typical features (age >40, mild morning stiffness, thumb base pain), a confident clinical diagnosis can be made without radiographs 1
• When to obtain radiographs: Consider plain radiographs (posteroanterior and lateral views) if considering surgical referral or if diagnosis is uncertain 1, 4
• Radiographic findings: Look for joint space narrowing, osteophytes, and subchondral sclerosis if imaging is performed 1, 4

Initial Management Strategy

First-Line Non-Pharmacological Treatment

• Thumb splinting/orthoses: Custom-made thermoplast or neoprene splints should be used for at least 3 months, as they provide pain relief and functional improvement 1, 4
• Hand exercises: Implement range of motion and strengthening exercises targeting thumb base stability 1, 4
• Patient education: Explain the condition, joint protection strategies, and activity modification 1

First-Line Pharmacological Treatment

• Topical NSAIDs: Preferred initial pharmacological treatment due to favorable safety profile and effectiveness 1
• Oral paracetamol: If topical treatment insufficient, use paracetamol up to 4 g/day as the oral analgesic of first choice 1
• Oral NSAIDs: Reserve for short-term use at lowest effective dose if paracetamol inadequate; use with caution in elderly patients considering cardiovascular and gastrointestinal risks 1

Second-Line Options

• Intra-articular corticosteroid injection: Can be effective for painful flares, particularly in thumb base OA 1
• Heat application: Local heat (paraffin wax, hot packs) before exercises may provide symptomatic benefit 1

Important Caveats

• Avoid premature surgery: The course of thumb base OA is often benign, so invasive treatment should be avoided early except with proven anatomic deformity 5
• Splint compliance: Poorly fitted or non-custom splints lead to poor compliance and reduced effectiveness 4
• Monitor for progression: Watch for development of boutonnière deformity (zigzag collapse pattern), which indicates more advanced disease requiring aggressive management 4
• Functional assessment: Functional impairment in hand OA can be as severe as rheumatoid arthritis and should be carefully monitored 1