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Radial Nerve – Anatomy, Function and Palsy

The radial nerve is the main nerve of the extensor side of the arm, supplying muscles and skin of the upper arm, forearm, and hand with both motor and sensory fibers.

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Things worth knowing about "Radial nerve"

The radial nerve is the main nerve of the extensor side of the arm, supplying muscles and skin of the upper arm, forearm, and hand with both motor and sensory fibers.

What Is the Radial Nerve?

The radial nerve (Latin: Nervus radialis) is a peripheral nerve of the upper limb and the largest branch of the brachial plexus, the nerve network arising from cervical spine segments C5 to T1. It runs along the extensor (dorsal) side of the arm and is primarily responsible for extension movements of the elbow, wrist, and fingers, as well as for sensation on the back of the hand.

Anatomical Course

The radial nerve branches off from the brachial plexus in the axilla (armpit) and spirals around the posterior surface of the humerus (upper arm bone) through the radial groove (sulcus nervi radialis). At the level of the elbow, it divides into two main branches:

Superficial branch (ramus superficialis): A purely sensory branch that supplies the skin on the dorsal (extensor) side of the thumb, index finger, middle finger, and the radial half of the ring finger, as well as the dorsum of the hand.
Deep branch (ramus profundus): A motor branch that passes through the supinator muscle and continues as the posterior interosseous nerve, innervating the extensor muscles of the forearm and hand.

Motor Function

The radial nerve innervates all major extensor muscles of the arm. Key muscles supplied include:

Triceps brachii: Extension of the elbow
Brachioradialis: Flexion of the elbow in neutral forearm position
Extensor carpi radialis longus and brevis: Extension and radial deviation of the wrist
Extensor digitorum: Extension of the fingers
Extensor pollicis longus and brevis: Extension of the thumb
Abductor pollicis longus: Abduction of the thumb

Sensory Function

Sensory innervation covers the skin of the extensor surface of the upper and forearm and the radial (thumb-side) dorsum of the hand. A characteristic sensory area is the anatomical snuffbox, located between the extensor tendons of the thumb and index finger.

Common Injuries and Palsies

The radial nerve can be damaged at various points along its course. The most common causes and sites of injury include:

High Radial Nerve Palsy (Axillary Lesion)

Damage at the level of the axilla can result from prolonged pressure on the armpit -- for example, from crutches or sleeping with the arm draped over a surface (so-called Saturday night palsy). This leads to paralysis of all muscles innervated by the radial nerve, including the triceps brachii.

Injury in the Radial Groove (Humeral Shaft Fracture)

The most common cause of radial nerve palsy is a fracture of the humeral shaft, where the nerve is injured as it passes through the radial groove. The classic sign is wrist drop: the patient is unable to actively extend the wrist and fingers, causing them to hang passively. The triceps muscle is usually spared, as its nerve branches arise above the injury site.

Injury at the Elbow Level

Compression within the supinator canal (arcade of Frohse) can damage the deep branch of the radial nerve and results in purely motor weakness of finger and thumb extension without significant sensory loss. This condition is known as posterior interosseous nerve syndrome or radial tunnel syndrome.

Diagnosis

Diagnosis of radial nerve palsy is based on clinical examination of motor function (wrist and finger extension) and sensory testing (radial dorsum of the hand). Supplementary investigations include:

Electromyography (EMG): Measures electrical activity of muscles to assess nerve damage
Electroneurography (ENG): Measures nerve conduction velocity
Imaging: Ultrasound or MRI to visualize the nerve and identify possible causes of compression

Treatment

Treatment depends on the cause and severity of the nerve injury:

Conservative treatment: In mild cases such as compression palsy, the nerve often recovers spontaneously over weeks to months. Physiotherapy and occupational therapy support regeneration. A cock-up wrist splint prevents wrist drop and helps with daily activities.
Surgical treatment: In severe injuries, such as complete nerve transection following a humeral fracture, surgical nerve reconstruction (neurolysis, suture repair, or nerve grafting) may be required.

Prognosis

The prognosis for radial nerve palsy is generally favorable in compression injuries. Complete nerve transections require surgical treatment and may involve a lengthy rehabilitation period of several months to one year before full recovery is achieved.

References

Drake R, Vogl AW, Mitchell AWM: Gray's Anatomy for Students. 4th edition. Elsevier, Philadelphia 2019.
Spinner RJ, Amadio PC: Compressive neuropathies of the upper extremity. Clinical Orthopaedics and Related Research, 2003; 409: 1-7.
Shao YC, Harwood P, Grotz MR, et al.: Radial nerve palsy associated with fractures of the shaft of the humerus: a systematic review. Journal of Bone and Joint Surgery (British Volume), 2005; 87(12): 1647-1652.

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