"Finger" drop
Summary
Finger drop refers to the inability to extend the fingers at the metacarpophalangeal (MCP) joints while wrist extension remains relatively preserved. This is a hallmark finding of posterior interosseous nerve (PIN) syndrome, a pure motor branch of the radial nerve that can be compromised distal to the elbow, as opposed to wrist drop seen in proximal radial nerve injury.
Detail
The radial nerve bifurcates near the elbow into the posterior interosseous nerve (PIN, deep motor branch) and the superficial radial nerve (sensory branch). The PIN innervates the extensor digitorum, extensor indicis, extensor digiti minimi, extensor pollicis longus/brevis, and abductor pollicis longus, controlling finger and thumb extension. It passes through the supinator muscle via the arcade of Frohse, a common site of compression (e.g., from lipomas, ganglion cysts, radial head fractures, or repetitive pronation-supination as in 'radial tunnel syndrome' or after Monteggia fracture-dislocation).
Because the PIN branches off after the extensor carpi radialis longus and brevis (ECRL/ECRB) are already innervated, wrist extension is preserved, though it may deviate radially due to weakness of the extensor carpi ulnaris (ECU), which IS innervated by the PIN. Since the PIN is a pure motor nerve, there is no associated sensory deficit—this distinguishes it from a more proximal radial nerve lesion (e.g., at the spinal groove from 'Saturday night palsy' or humeral shaft fracture), which causes classic wrist drop PLUS finger drop PLUS sensory loss over the dorsal first web space.
Clinical testing: ask the patient to extend fingers at the MCP joints with the wrist stabilized in neutral position; interphalangeal joint extension may still occur via intact intrinsic hand muscles (ulnar/median innervated lumbricals and interossei), so testing MCP extension specifically is key.
Etiologies include compressive neuropathies, trauma (radial head dislocation/fracture, Monteggia fracture), rheumatoid synovitis, and tumors. Management ranges from conservative splinting and NSAIDs to surgical decompression if refractory.
Sources
- Gray's Anatomy for Students
- Kaplan USMLE Step 1 Neurology
- First Aid for the USMLE Step 1
- Hoppenfeld's Physical Examination of the Spine and Extremities
- UpToDate: Radial nerve injury and posterior interosseous nerve syndrome
Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.