Radial Nerve Injury (Wrist Drop)
Radial nerve injury causes wrist drop and finger drop due to paralysis of the extensor muscles. The radial nerve is vulnerable to injury in the arm as it spirals around the humerus in the spiral (radial) groove, commonly occurring with humeral shaft fractures or "Saturday night palsy."
Anatomical Basis
The radial nerve (C5-T1) runs posterior to the humerus in the spiral (radial) groove, where it is in direct contact with bone and vulnerable to compression or fracture injury. It innervates all the extensors of the elbow, wrist, and fingers. High lesions affect triceps; mid-arm lesions spare triceps.
Relevant Structures
Mechanism
Humeral shaft fracture (especially middle third), compression from prolonged pressure ("Saturday night palsy" from sleeping with arm over chair), improper crutch use, tight tourniquet, or injection injury.
Clinical Presentation
- âĒWrist drop: inability to extend wrist
- âĒFinger drop: inability to extend fingers at MCP joints
- âĒWeakness of thumb extension and abduction
- âĒWeakness of elbow extension (if injured proximal to triceps branches)
- âĒSensory loss over posterior arm, forearm, and dorsal hand (lateral 3.5 digits)
- âĒUnable to extend wrist against gravity
Physical Examination
- âWrist drop: Wrist hangs flexed, cannot extend against gravity
- âTest wrist extension, finger extension at MCPs, thumb extension
- âTest triceps function (elbow extension) to localize lesion
- âTest sensation over first dorsal web space (superficial radial nerve)
- âTest brachioradialis reflex (may be decreased)
- âGrip strength decreased (wrist extension required for power grip)
Treatment
- âConservative: Most compression injuries (Saturday night palsy) recover spontaneously
- âWrist splint in extension (cock-up splint) to prevent contracture and maintain function
- âPhysical/occupational therapy for range of motion and strengthening
- âSurgical: Exploration if associated with fracture with no recovery, nerve grafting for lacerations
- âTendon transfers if no recovery after 6-12 months
Prognosis
Neuropraxia (compression): Excellent, recovery in weeks to months. Axonotmesis: Good, regeneration at 1mm/day. Neurotmesis (complete transection): Poor without surgical repair. Saturday night palsy typically recovers fully within 6-8 weeks.
Study Tips
- ðĄRadial nerve = "Saturday night palsy" (sleeping drunk with arm over chair back)
- ðĄLOST mnemonic: Wrist extension Lost in radial nerve injury
- ðĄPIN syndrome spares wrist extension (ECRL intact) and has no sensory loss
- ðĄLesion localization: Triceps weakness = proximal to spiral groove; Triceps intact = spiral groove or distal
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Common questions about this condition
Effective grip requires wrist stabilization in extension. When the wrist drops into flexion, the finger flexors are mechanically disadvantaged and cannot generate full power. This is why a cock-up splint dramatically improves function.
PIN syndrome is compression of the deep branch of the radial nerve at the arcade of Frohse (supinator muscle). It causes finger and thumb drop but SPARES wrist extension (ECRL branches off proximally) and has NO sensory loss (PIN is pure motor).