What are the possible causes of left thumb swelling in an adult with no known underlying medical conditions?

Left Thumb Swelling: Diagnostic Approach

Begin with plain radiographs in two planes immediately, as this is the mandatory first step for any musculoskeletal swelling to exclude fractures, tumors, foreign bodies, and establish baseline imaging. 1, 2

Initial Imaging and Key Findings

Plain radiographs will reveal critical diagnostic clues:

• Soft tissue swelling alone suggests early infection, inflammatory arthropathy, or soft tissue abscess 1, 2
• Joint effusion indicates possible septic arthritis or inflammatory conditions 3
• Lytic bone changes raise concern for osteomyelitis, tumor (such as giant cell tumor), or tuberculous dactylitis 1, 4, 5
• Gas in soft tissues is a red flag for necrotizing fasciitis requiring emergency surgical consultation 3
• Radiopaque foreign bodies (metal, glass, stone) may be visible and trigger granulomatous infection 1, 3

Common Diagnostic Categories

Infectious Causes

• Septic arthritis: Requires urgent image-guided aspiration with culture for definitive diagnosis, as culture is the reference standard (though negative cultures don't exclude infection if antibiotics were already started) 1, 3
• Soft tissue abscess: Presents with soft tissue swelling, effacement of fat planes, and possible gas on radiographs 1, 2
• Osteomyelitis: MRI has high sensitivity/specificity, and a negative MRI excludes this diagnosis 1, 2, 3
• Retained foreign body with secondary infection: Particularly after puncture wounds; wood and plastic are radiolucent on X-ray 1, 2

Inflammatory/Rheumatologic Causes

• Psoriatic arthritis: Causes characteristic dactylitis ("sausage digit") affecting individual fingers with painful, swollen joints 2
• Gouty tenosynovitis: May mimic infection and can coexist with actual infection, creating diagnostic challenges 2
• Thumb carpometacarpal (CMC) arthritis: Affects 33% of postmenopausal women radiographically; presents with pain, joint swelling, reduced strength, and limited hand function 6, 7, 8

Vascular Causes

• Upper extremity deep vein thrombosis: Unilateral swelling indicates obstruction requiring urgent duplex ultrasound (sensitivity/specificity >80%) 2

Neoplastic Causes

• Giant cell tumor: Can present as painful thumb swelling with lytic changes on radiographs 4
• Tuberculous dactylitis: Rare but causes extensive bone destruction with soft tissue mass, particularly after trauma 5

Advanced Imaging Algorithm

If radiographs show only soft tissue swelling or are normal, proceed immediately to MRI with IV contrast (unless contraindicated). 1, 2, 3

MRI is superior because it:

• Detects inflammation, fasciitis, myositis, and necrosis with high sensitivity 1, 2
• Excludes osteomyelitis definitively if negative 1, 2, 3
• Characterizes soft tissue masses and bone marrow abnormalities 3, 9
• Guides surgical planning for abscesses or debridement 2

Alternative imaging options:

• Ultrasound: Excellent for detecting fluid collections, joint effusions, tenosynovitis, and radiolucent foreign bodies (wood, plastic); particularly useful for guiding aspiration 1, 3
• CT with IV contrast: Use when MRI is contraindicated; evaluates soft tissue extent, detects gas, identifies radiodense foreign bodies, and guides aspiration 1, 2, 3

Critical Next Steps

Perform image-guided aspiration with culture when imaging cannot distinguish infected from non-infected joints or fluid collections. 1, 2, 3

This is essential because:

• Culture provides definitive diagnosis for septic arthritis 1, 3
• Guides antibiotic selection 3
• Distinguishes infection from inflammatory conditions 2

Common Pitfalls

• Do not delay imaging: Radiographs must be obtained before any other intervention 1, 2
• Do not assume negative culture excludes infection: Especially if antibiotics were started before aspiration 1, 3
• Do not rely on ultrasound alone: It can underestimate disease extent and has limited visualization of deeper structures 1
• Do not miss necrotizing fasciitis: Gas on imaging requires emergency surgical consultation 3

Follow-Up Protocol

Reassess within 48-72 hours to evaluate response to initial management and ensure no progression. 2

If upper extremity DVT is confirmed, initiate therapeutic anticoagulation immediately following standard DVT protocols 2