What motor deficits are expected in a patient with a radial nerve injury at the mid‑humeral level presenting with paresthesia over the extensor forearm and dorsal hand?

Radial Nerve Injury at Mid-Humeral Level: Expected Motor Deficits

A patient struck along the lower posterior upper arm with paresthesia over the extensor forearm and dorsal hand will present with wrist drop (inability to extend the wrist), finger drop (inability to extend the fingers at the metacarpophalangeal joints), and loss of thumb extension, while triceps function remains intact.

Primary Motor Deficits

The radial nerve injury at the mid-humeral (spiral groove) level produces a characteristic pattern of motor loss:

• Wrist extension weakness/paralysis - Complete inability to extend the wrist, resulting in the classic "wrist drop" deformity 1, 2
• Finger extension weakness - Loss of extension at the metacarpophalangeal (MCP) joints, particularly affecting the extensor digitorum communis (EDC) and extensor indicis proprius (EIP) 2, 3
• Thumb extension loss - Inability to extend the thumb (extensor pollicis longus involvement) 3
• Brachioradialis weakness - Loss of forearm flexion and supination strength 2, 3

Preserved Motor Function

Critically, triceps function should be preserved because the nerve branches to the triceps originate proximal to the spiral groove injury site 1, 2. This distinguishes mid-humeral injuries from more proximal radial nerve lesions. However, be aware that 22-49% of arm-level lesions may show some triceps involvement, particularly in traumatic injuries 2.

Clinical Examination Findings

When examining this patient, specifically assess:

• Wrist extension against gravity - Patient cannot lift hand dorsally with forearm pronated
• Finger MCP extension - Patient cannot extend fingers at knuckles (interphalangeal extension via intrinsics may be preserved)
• Thumb extension and abduction - Loss of "hitchhiker's thumb" position
• Brachioradialis strength - Weakness of forearm flexion with forearm in neutral position
• Triceps strength - Should be 5/5 (elbow extension preserved) 1

Mechanism and Anatomical Considerations

The injury location at the "lower posterior part of the upper arm" corresponds to the spiral groove where the radial nerve wraps around the humerus. At this level, the nerve is vulnerable to:

• Direct trauma from blunt force
• Compression against the humeral shaft
• Entrapment between fracture fragments if associated with humeral fracture 1, 4

Important anatomical variant: Approximately 20% of patients may have auxiliary branches originating from the radial cord at higher levels, which can affect the clinical presentation 1.

Common Pitfalls to Avoid

1. Don't assume all radial nerve injuries are distal - While Holstein-Lewis fractures (distal third spiral fractures) are classically associated with radial nerve palsy, mid-shaft injuries with nerve entrapment do occur and require different management 1

2. Don't delay surgical exploration in traumatic injuries - If associated with humeral fracture or penetrating trauma, early exploration within 24 hours allows for nerve decompression and optimal recovery 4. All patients in one series who underwent early surgical exploration achieved good nerve recovery (mean 6.2 months) 4

3. Don't miss triceps involvement - While typically preserved, 22-49% of arm-level lesions show some triceps weakness, which doesn't exclude a spiral groove compression 2

Sensory Findings

The paresthesia over the extensor forearm and dorsal hand corresponds to the superficial sensory branch of the radial nerve, affecting:

• Dorsal first web space (most reliable area)
• Dorsal radial hand
• Dorsal thumb, index, and middle fingers (proximal phalanges)