Superficial radial nerve lesion

Anatomy of Superficial Radial Nerve

• Pure sensory nerve
• Radial nerve bifurcates in the proximal forearm to give rise to superficial radial nerve (SRN) and posterior interosseous nerve (PIN)
• SRN travels deep to the brachioradialis and lies on the undersurface of the muscle
• It emerges between brachioradialis and ECRL in the distal third forearm, to become superficial (under the skin)
• It bifurcates further proximal to the wrist:
  • Dorsal branch to supply the 1st (autonomous dermatome) and 2nd web spaces
  • Palmar branch to supply dorsoradial thumb 

Nerve Injury

Because of the superficial position of the nerve around the radial border of the wrist, it is vulnerable to injury from wrist trauma (fractures, contusions, open injuries) and wrist surgery (for instance, it is a well-recognised risk of de Quervain's release).

Key Message

• SRN is notorious in developing into a painful neuroma following even seemingly innocuous injury.
• Entrapment (non-traumatic) neuropathy of the SRN is called Wartenberg's syndrome.

Symptoms and Signs

• Evidence of trauma/surgery to the radial border of the wrist 
• Shooting pain into the dorsoradial aspect of the hand with radiation into thumb, index and long fingers
• Tingling and altered sensation over the 1st and 2nd webspaces of the dorsum of the hand
• Allodynia (Pain due to a stimulus, which normally does not provoke pain. For instance, light stroking the skin would cause extremely unpleasant sensation.)
• Tinel's sign (the most important finding to localise the site of nerve irritation) 

Investigations

• Due to its superficial position, SRN is readily amenable to nerve conduction studies which can be used to confirm a focal conduction block. However the clinical presentation is often apparent.

• Diagnostic local anaesthetic injection is the most useful test: 

  • Injection of 1ml of fast-acting local anaesthetic (1 or 2% lidocaine) at the Tinel's spot followed by resolution of the above symptoms is regarded as a positive test for a nerve lesion. Should the symptoms return predictably as the local anaesthetic wears off (after 4 hours), that is taken as a further level of confirmation.

Treatment

Nonoperative

• Hand therapy 
• Wound desensitisation
• Painkillers

Operative 

If the symptoms are overly intrusive and nonoperative measures have failed to improve the situation, surgery is recommended. At the time of exploration, there are two potential scenarios:

• Neurolysis: the nerve is intact but scarred or tethered. In this case, preoperative sensibility of SRN is likely to have been preserved. Neurolysis of the scarred nerve followed by a nerve wrap is performed.

• Proximal burial: The nerve has been cut and the proximal nerve end has formed a neuroma (swelling). The preoperative sensibility is likely to have been impaired or lost already. Attempts to restore the sensation with nerve repair or grafting are fraught with problems of further neuroma formation and persistent sensitivity. A more reliable way of relieving the allodynia is to translocate the proximal nerve end and bury it in the brachioradialis muscle belly. The sensibility however would not be restored.

PIN Neurectomy as a Supplementary Procedure

The posterior interosseous nerve (PIN) has been shown to be responsible for at least some of the discomfort when the SRN is irritated. This may be confirmed with a separate local anaesthetic injection test targeted at the distal PIN at the wrist. Resecting the short distal segment of PIN through a separate incision over the back of the wrist has thus been shown to alleviate the allodynia. As this part of the nerve provides sensory fibres to the wrist capsule only, there will be no functional loss.