What are the next steps for sudden numbness in the thumb and index finger with inability to dorsiflex the wrist without any injury?

Acute Radial Nerve Palsy: Immediate Evaluation and Management

This presentation of sudden numbness in the thumb and index finger with inability to dorsiflex the wrist (wrist drop) is classic for radial nerve palsy and requires urgent evaluation to determine the underlying cause, with imaging starting with plain radiographs to exclude compressive lesions or fractures, followed by MRI if no bony abnormality is identified.

Clinical Recognition

• The combination of wrist drop (inability to dorsiflex the wrist) with numbness in the radial distribution (thumb and index finger) localizes the lesion to the radial nerve, most commonly at the spiral groove of the humerus or in the forearm 1
• Despite the absence of reported trauma, occult fractures, nerve compression from masses, or spontaneous nerve injury must be excluded urgently 2

Initial Diagnostic Approach

Immediate Imaging Protocol

• Obtain plain radiographs of the entire forearm and elbow (minimum 3 views: AP, lateral, and oblique) as the first imaging study to rule out occult fracture, bone lesions, or other bony abnormalities that could compress the radial nerve 2, 3
• The American College of Radiology establishes that radiographs must be obtained first even in atraumatic presentations to exclude underlying pathology before proceeding to soft tissue imaging 4, 3

Advanced Imaging When Radiographs Are Normal

• If radiographs are negative or equivocal, proceed immediately to MRI without IV contrast to evaluate for nerve compression, soft tissue masses, inflammatory changes, or occult nerve injury 2, 3
• MRI with dedicated neurography sequences (including diffusion-weighted imaging) provides improved visualization of injured nerves and can identify the exact site and cause of radial nerve compromise 2
• MRI sensitivity for detecting nerve pathology and associated soft tissue abnormalities is superior to other modalities in this clinical context 3

Critical Differential Diagnoses to Exclude

Compressive Lesions

• Lipomas, ganglion cysts, or other soft tissue masses compressing the radial nerve along its course require MRI identification 2, 4
• Occult fractures of the humerus (spiral groove region) or radius that were missed on initial examination must be ruled out, as worsening symptoms despite negative initial assessment demands advanced imaging 3

Nerve Injury Patterns

• Radial nerve compression can occur at multiple sites: spiral groove (most common), radial tunnel (forearm), or posterior interosseous nerve syndrome 1
• The pattern of motor and sensory loss helps localize the lesion, but imaging confirms the anatomic site and underlying cause 1

Urgent Referral Indications

• Refer immediately to hand surgery or neurology if MRI reveals a compressive mass, nerve discontinuity, or other surgical pathology requiring intervention 3
• Even without identified structural lesion, radial nerve palsy warrants specialist evaluation within 24-48 hours to prevent permanent motor deficit 1

Common Pitfalls to Avoid

• Do not assume "no injury" means no fracture—occult fractures can present without recalled trauma, and progressive neurologic symptoms mandate imaging to exclude this 3
• Do not skip radiographs and proceed directly to MRI—bony pathology must be excluded first per ACR guidelines, and radiographs may reveal the diagnosis immediately 2, 4, 3
• Do not delay imaging pending specialist consultation—obtain radiographs immediately and MRI urgently if radiographs are normal, as early diagnosis significantly impacts outcomes 2, 3
• Radial nerve palsy can be confused with carpal tunnel syndrome, but carpal tunnel affects the median nerve distribution (thumb, index, middle, and radial half of ring finger) without wrist drop 5, 1

Management Pending Imaging Results

• Immobilize the wrist in a neutral position with a volar splint to prevent contracture and protect the nerve from further injury 5
• Avoid any activities requiring wrist extension or forceful gripping until the underlying cause is identified 3
• Document baseline motor function (finger extension, thumb extension, wrist extension strength) and sensory distribution to track progression 1