Introduction
Radial nerve palsy is a high-yield clinical condition because its anatomical course is long and predictable, making localization straightforward when patterns are recognized. The radial nerve travels from the axilla, through the spiral groove, across the elbow, and into the posterior forearm to supply extensor muscles and dorsal hand sensation. This article presents rapid, exam-ready pearls to identify the level of injury based entirely on motor and sensory deficits.
Why Radial Nerve Palsy Localizes So Well
The radial nerve gives off branches at specific, consistent points:
- Proximal motor branches to triceps leave early.
- Spiral groove injuries spare triceps.
- The deep branch becomes the posterior interosseous nerve (PIN), a pure motor nerve.
- Dorsal hand sensation is supplied by branches covered under nerves of dorsum of hand.
These anatomical checkpoints make clinical testing highly reliable.
Pearl 1: Wrist Drop Alone Does Not Localize the Lesion
Wrist drop is a classic finding but occurs at multiple lesion levels. It does not distinguish axillary, spiral groove, or PIN lesions.
Always test:
- Triceps strength
- Finger extension
- Thumb extension
- Dorsal hand sensation
- Brachioradialis reflex
Localization depends on the combination of findings, not wrist drop alone.
Pearl 2: Weak Triceps Means a Very High Lesion
If elbow extension is weak, the lesion is proximal to the spiral groove. This implicates the axilla or proximal arm, where radial branches arise as shown under radial branches in the arm.
Typical causes
- Axillary crutch compression
- Posterior cord lesions
- Prolonged compression while intoxicated
Pattern
- Weak triceps
- Wrist drop
- Sensory loss in posterior arm and forearm
- Weak finger extension
Clinical pearl: Weak triceps = high radial nerve lesion.
Pearl 3: Spiral Groove Lesions Spare Triceps
Midshaft fractures of the humerus commonly injure the radial nerve in the spiral groove.
Pattern
- Triceps preserved
- Wrist drop present
- Finger and thumb extension weak
- Sensory deficit on dorsum of hand
Clinical pearl: Triceps intact + wrist drop = spiral groove lesion.
Pearl 4: PIN Lesions Are Pure Motor (No Sensory Loss)
The PIN is a pure motor branch supplying the posterior compartment of the forearm. Sensation is preserved.
Pattern
- Intact elbow extension
- Intact wrist extension but with radial deviation
- Absent finger extension
- No sensory deficit
Clinical pearl: Finger extension loss + normal sensation = PIN palsy.
Pearl 5: Superficial Radial Nerve Lesions Are Pure Sensory
Lesions to the superficial radial nerve produce sensory loss over the dorsal radial hand. Motor function is normal. The pattern corresponds to territories reviewed under cutaneous innervation of upper limb.
Pattern
- Dorsal radial hand numbness
- Burning or tingling
- Normal wrist and finger strength
Clinical pearl: Normal motor function + dorsal sensory loss = superficial radial nerve injury.
Pearl 6: Brachioradialis Strength Separates High From Low Lesions
Brachioradialis, innervated before the spiral groove, is a key discriminator. If it is weak, the lesion is proximal; if normal, the lesion is in the spiral groove or distal.
Pearl 7: Sensory Loss Pattern Maps the Lesion Level
The distribution of sensory loss provides a stepwise map:
- Posterior arm → high lesion
- Posterior forearm → spiral groove
- Dorsal radial hand → distal radial nerve
- No sensory loss → PIN
Clinical pearl: Sensory mapping is a fast and reliable method of localization.
Pearl 8: Finger Extension Is the Most Useful Motor Test
Finger extensors (see extensor digitorum) are vulnerable to radial compromise.
Interpretation
- Weak in high lesions
- Weak in spiral groove lesions
- Absent in PIN lesions
- Normal in superficial radial nerve lesion
Pearl 9: Thumb Extension Weakens Early
Thumb extensors, including extensor pollicis longus, often weaken before finger extensors, especially in subtle PIN or spiral groove injuries.
Pearl 10: Most Radial Nerve Palsies After Humeral Fracture Recover
Closed humeral shaft fractures usually cause neuropraxia, with high spontaneous recovery rates.
Indications for early exploration
- Open fracture
- Vascular injury
- Progressive neurological deterioration
- No improvement after months
Clinical pearl: Most closed-fracture radial palsies recover without surgery.
Summary
Radial nerve palsy is ideal for rapid localization because of predictable branching patterns. High lesions weaken triceps, spiral groove lesions spare it, PIN palsy causes pure motor loss, and superficial radial lesions cause pure sensory loss. With structured testing of triceps power, wrist extension, finger extension, thumb extension, brachioradialis strength, and dorsal hand sensation, clinicians can localize the lesion accurately and quickly.