Why is my thumb thickened despite normal sensory and motor function after an upper arm injury?

Thumb Thickening with Preserved Sensory and Motor Function After Upper Arm Injury

Your thumb thickening with intact sensation and movement most likely represents soft tissue swelling from altered venous or lymphatic drainage following your upper arm injury, rather than a nerve or tendon problem. 1

Understanding the Mechanism

Soft tissue swelling occurs when fluid shifts from blood vessels into surrounding tissues due to several possible mechanisms after upper extremity trauma 1:

• Increased capillary hydrostatic pressure from venous obstruction or compression at the injury site 1
• Lymphatic obstruction preventing normal fluid clearance 1, 2
• Post-traumatic inflammation increasing capillary permeability 1
• Reflex sympathetic changes altering local blood flow patterns 1

The preservation of your sensory and motor function effectively rules out significant nerve injury or tendon rupture, which would cause weakness or numbness 3, 4.

Critical Diagnostic Considerations

Rule Out Deep Vein Thrombosis First

Upper extremity deep vein thrombosis (UEDVT) must be excluded, as it commonly presents with unilateral arm swelling after trauma and can lead to serious complications if untreated. 1

• UEDVT accounts for up to 10% of all DVTs and frequently follows upper extremity trauma 1
• Patients typically present with ipsilateral upper-extremity edema, pain, and sometimes paresthesias 1
• Ultrasound duplex Doppler of the upper extremity is the appropriate initial imaging study to evaluate for venous thrombosis 1
• US can assess the jugular, axillary, basilic, cephalic, and brachial veins for thrombus 1

Evaluate for Occult Fracture or Joint Injury

Even with normal motor and sensory function, underlying bony or joint pathology may cause persistent swelling 1, 3:

• Obtain standard three-view radiographs (PA, lateral, and oblique) of the thumb and hand as initial imaging, since two views alone miss fractures 1, 3
• Specify "hand" or "thumb" on imaging orders to ensure complete first ray coverage, as ordering just "finger" may miss carpometacarpal pathology 3
• If radiographs are negative but clinical suspicion remains high, MRI without IV contrast can detect occult fractures and soft tissue injuries that explain persistent swelling 1

Consider Post-Traumatic Arthritis

If your injury occurred weeks to months ago, post-traumatic thumb base osteoarthritis may develop 1, 5:

• Thumb base (carpometacarpal) osteoarthritis commonly causes thumb thickening and swelling 1
• Unlike inflammatory arthritis, osteoarthritis typically preserves basic motor function until advanced stages 1
• Erosive osteoarthritis presents with abrupt onset, marked pain, inflammatory signs (swelling, erythema), and mildly elevated CRP levels 1

Differential Diagnosis to Exclude

Do not assume all thumb swelling is simple edema—several conditions mimic post-traumatic swelling but require different treatments 1, 3:

• Psoriatic arthritis may target a single digit and cause swelling ("sausage digit") 1
• Gout can superimpose on pre-existing trauma or osteoarthritis 1
• Rheumatoid arthritis typically targets MCP and PIP joints more than the thumb base, but can cause thumb swelling 1, 6
• Infection must be considered if there was any skin break or penetrating injury 1

Management Algorithm

Immediate Steps (First 2 Weeks)

1. Obtain ultrasound duplex Doppler of the upper extremity to exclude UEDVT 1
2. Obtain three-view radiographs of the thumb and hand to exclude fracture or dislocation 1, 3
3. Elevate the affected extremity above heart level to promote venous and lymphatic drainage 2
4. Apply compression if UEDVT is excluded 2

If Initial Workup is Negative

• If swelling persists beyond 10-14 days with negative initial imaging, obtain MRI without IV contrast to detect occult fractures, ligament injuries, or soft tissue pathology 1
• Consider referral to hand therapy for custom orthoses and exercise prescription, which optimize functional outcomes 7

Chronic Management (Beyond 6 Weeks)

If swelling persists chronically after excluding acute pathology 2:

• Compression garments and physical therapy remain the mainstay for lymphedema management 2
• Topical NSAIDs are first-line pharmacological treatment due to favorable safety profile if post-traumatic arthritis develops 3
• Reserve surgery for severe cases when conservative treatments fail 3

Common Pitfalls to Avoid

• Delaying imaging in equivocal cases leads to functional impairment—proceed to advanced imaging rather than waiting weeks in a cast 3
• Relying on two-view radiographs alone is inadequate for thumb fracture detection 1, 3
• Assuming all thumb swelling is benign edema can miss UEDVT, which requires anticoagulation 1
• Ordering "finger" instead of "hand" or "thumb" radiographs may miss carpometacarpal pathology 3