Radial Nerve Lesion (C5-8)

There are 3 posterior divisions of the brachial plexus that form the posterior cord. The largest and most frequently injured part of both the posterior cord and the brachial plexus is the radial nerve. The segmental origin is C5 to C8 but there is also a sensory component from T1.

Radial nerve compression or injury may occur at any point along the course of the nerve. The most frequent site of compression is in the proximal forearm in the area of the supinator muscle and involves the posterior interosseous branch. Problems may also occur proximally as a result of fractures of the humerus at the junction of the middle and proximal thirds, as well as distally on the radial aspect of the wrist.¹

• In the axilla the nerve gives off the posterior cutaneous nerve of the arm, a branch to the long and medial heads of triceps and then to the lateral head of triceps. It may travel in or near the spiral groove of the humerus.
• The nerve passes between the brachialis and brachioradialis and about 10cm above the lateral epicondyle, on the anterior side of the arm, it gives branches to brachioradialis and extensor carpi radialis longus.
• It divides in front of the radial head and gives the posterior interosseous nerve and passes backwards through supinator to supply the extensor muscles of the forearm.
• The nerve also supplies sensory fibres:
  • A sensory branch passes into the forearm deep to brachioradialis and about 8cm from the radial head it emerges between the tendons of brachioradialis and extensor carpi radialis longus.
  • It supplies sensation to the dorsum of the thumb with the exception of the subungual region that is supplied by the median nerve.
  • It also supplies the dorsum of the fingers as far distal as the proximal interphalangeal joint and as far medial as the middle of the ring finger.

• The radial nerve may be damaged by trauma or entrapped, especially between the heads of muscles.
• In the axilla, with features of weak triceps, wrist drop and possibly median and ulnar nerve involvement too, the commonest cause is compression.
• The radial nerve may be damaged in the axilla by fracture or dislocation of the head of the humerus.
• Saturday night syndrome:
  • Is due to compression of the lower part of the brachial plexus.
  • It is so named because the typical way it as acquired was by sleeping with the arm over the back of a chair whilst in a drunken stupor, so as to compress the plexus.
  • It may also be compressed by the use of shoulder crutches.
  • This is really a brachial plexus injury and the median and ulnar nerves may also be involved.
  • Nerve function usually fully recovers within a few weeks.
• In the upper arm, with triceps and brachioradialis often spared, there may be a compression lesion but fracture is the usual cause. If small babies are given injections in the arm it can damage the radial nerve.
• As the nerve often passes down in the spiral groove of the radius it may be injured in fracture of the shaft of humerus.
• The radial nerve may be entrapped at the elbow at a number of sites but the commonest is the proximal border of tendon of supinator called the arcade of Froshe. Check for tenderness over the radial tunnel. There may be pain when the fingers are extended against resistance. Supination from a pronated position along with flexion of the wrist may reproduce the symptoms.
• Lesions at the wrist cause finger drop with a normal wrist and intact sensation. Fracture of the radius, elbow deformity, soft tissue masses and compression by extensor carpi radialis brevis are all causes.
• Lesions of the superficial nerves cause pain and sensory loss but no motor loss:
  • At the elbow, ruptured synovial effusion is the commonest cause. In the forearm there may be an aberrant course through the muscles.
  • At the wrist, causes include compression from plaster casts, wristbands or handcuffs, especially the type than get tighter with struggling. Other causes are surgery, injections and nerve tumours.

A thorough neurological history and examination is required.

• The extent of loss of muscle power will depend upon the level of the lesion.
• Gowers described the typical posture that accompanies radial nerve lesions, especially Saturday night syndrome. There is wrist drop with slight flaccid flexion of the wrist and the hand is pronated with the thumb adducted.
• The following table lists the muscles supplied by the radial nerve and how to test each:
  • C7,8: triceps - ask patient to extend elbow against resistance.
  • C5,6: brachioradialis - ask patient to flex elbow with forearm half way between pronation and supination.
  • C6,7: extensor carpi radialis longus - ask patient to extend wrist to radial side with fingers extended.
  • C5,6: supinator - with arm by side, ask patient to resist hand pronation.
  • C7,8: extensor digitorum - ask patient to keep fingers extended at MCP joint.
  • C7,8: extensor carpi ulnaris - ask patient to extend wrist to ulnar side.
  • C7,8: abductor pollicis longus - ask patient to abduct thumb at 90° to palm.
  • C7,8: extensor pollicis brevis - ask patient to extend thumb at MCP joint.
  • C7,8: extensor pollicis longus - ask patient to resist thumb flexion at IP joint.
• Sensation:
  • The cutaneous branches of the radial nerve supply the dorsal aspect of the forearm from below the elbow down over the lateral part of the hand to include the thumb to the interphalangeal joint and the fingers to the distal interphalangeal joint.
  • This includes the index and middle fingers but not the little finger. It usually includes the lateral side of the ring finger but may include all or none of it.
• From examination of power and sensation, it is possible to determine the site of the lesion:
  • Compression of the brachial plexus gives the typical picture seen with the lower segmental levels affected. Paralysis affects the forearm extensor muscles whilst triceps is unaffected. Impaired sensation is limited to the dorsum of the hand.
  • In the upper arm the triceps is supplied above the spiral groove and below the groove originate branches to brachioradialis, extensor carpi radialis longus and brevis and the posterior cutaneous nerves of the arm and forearm. The last supply the lateral and dorsolateral arm and forearm.
  • Around the elbow at the Arcade of Froshe, the superficial radial nerve originates to supply the dorso-lateral part of the hand and the first 3 digits. The posterior interosseus nerve also arises to supply extensor carpi radialis brevis and supinator.
  • Beyond the arcade of Froshe, the posterior interosseous nerve gives its terminal branches to supply supinator, extensors of fingers and thumb, extensor carpi ulnaris, abductor pollicis longus and branches to the wrist joint.

A department of neurophysiology will be able to arrange nerve conduction studies.

• Lesions from compression such as Saturday night syndrome and simple fractures usually recover spontaneously.
• Complex trauma needs exploration with a view to surgical repair. Entrapment requires surgical decompression.