Evaluation of Wrist Drop
Begin with a focused neurological examination to distinguish between peripheral radial nerve injury (most common) and central nervous system lesions, followed by targeted imaging based on clinical localization.
Initial Clinical Assessment
Key Examination Findings to Localize the Lesion
Peripheral radial nerve injury (most common):
- Inability to extend wrist and fingers at the metacarpophalangeal joints 1
- Sensory loss over the dorsal first web space (if present, confirms peripheral) 1
- Preserved finger flexion and intrinsic hand muscle function 1
- Check for Wartenberg sign (thumb abduction weakness) - if POSITIVE, suggests central lesion 1
Central nervous system lesion (rare but critical):
- Bilateral wrist drop suggests cortical involvement 1
- Positive Wartenberg sign indicates central motor dysfunction 1
- Associated upper motor neuron signs (hyperreflexia, Babinski sign) 1, 2
- Absence of sensory loss despite motor weakness 2, 3
History Elements That Guide Diagnosis
- Trauma or compression history: Recent surgery with blood pressure cuff placement 4, prolonged arm positioning ("Saturday night palsy"), direct trauma 5
- Acute onset without trauma: Consider stroke, especially in patients with vascular risk factors 2, 3
- Bilateral presentation: Strongly suggests central etiology requiring urgent brain imaging 1
Diagnostic Algorithm
Step 1: Electrodiagnostic Studies
Nerve conduction studies (NCS) and electromyography (EMG) of the radial nerve:
- Perform early to confirm peripheral radial neuropathy and localize the lesion site 4, 5
- Normal NCS/EMG with clinical wrist drop = central lesion until proven otherwise 2, 3
- Axonal lesion pattern on EMG confirms peripheral nerve injury 5
Step 2: Imaging Based on Clinical Localization
If peripheral radial nerve injury suspected (abnormal NCS/EMG):
- High-resolution ultrasound of the radial nerve: First-line imaging to visualize nerve constriction, torsion, or compression 5
- Ultrasound can identify surgical candidates (nerve torsion, mass compression) requiring early intervention 5
- Standard radiographs of the arm/wrist if trauma history to exclude fractures 6
If central lesion suspected (normal NCS/EMG, positive Wartenberg sign, bilateral presentation, or acute onset with vascular risk factors):
- Urgent MRI brain without contrast: Essential to detect acute infarction in motor cortex (hand knob area of precentral gyrus), cerebral peduncle, or corpus callosum 1, 2, 3
- MRI superior to CT for detecting acute ischemic stroke in these locations 1, 3
- If MRI unavailable, obtain non-contrast CT brain immediately, though it may miss hyperacute infarcts 1
- MR angiography: Evaluate for vascular occlusion or stenosis 2, 3
Step 3: Additional Workup for Central Lesions
When stroke confirmed:
- Echocardiography to evaluate for cardioembolic source 2
- 24-hour Holter monitoring for paroxysmal atrial fibrillation 2
- Consider thrombolysis if within therapeutic window 3
Common Pitfalls to Avoid
- Assuming all wrist drops are peripheral: Bilateral wrist drop or normal NCS/EMG should immediately trigger brain imaging 1, 2
- Delaying brain imaging in acute presentations: Isolated hand/wrist weakness can be the sole manifestation of acute stroke requiring urgent thrombolysis 3
- Missing iatrogenic causes: Automated blood pressure cuffs cycling frequently can cause radial nerve palsy 4
- Inadequate ultrasound evaluation: High-resolution nerve ultrasound can identify rare surgical causes like nerve torsion that benefit from early intervention 5
- Ignoring the Wartenberg sign: This simple bedside test distinguishes central from peripheral pathology 1
Management Implications
Peripheral radial nerve injury:
- Most cases managed conservatively with splinting and physical therapy 4
- Surgical exploration indicated if nerve torsion or structural compression identified on ultrasound 5
- Prognosis generally favorable with complete recovery in 3 months 4
Central lesions: