Radial nerve palsy (Saturday night palsy) is one of the common clinical presentations. The radial nerve is one of the three major peripheral nerves of the upper limb. It supplies the extensor aspect of both arm and forearm. It innervates triceps and all the muscles in the posterior aspect of the forearm which are responsible for extension of the elbow and the wrist, fingers, and supination of the forearm.
It originates in the axilla as the terminal continuation of the posterior cord of the brachial plexus carrying its all five nerve root values from C5 to T1.
Then it passes posterior to the axillary artery to enter the anterior compartment of the arm crossing the inferior border of the teres major muscle. After a short course, radial nerve leaves the anterior compartment of the arm accompanying the profunda brachii artery to enter its posterior compartment. Here the nerve directly lies on a diagonal groove on the shaft of the mid humerus (Spiral Groove) running from medial to lateral.
Reaching the lateral side of the arm, it pierces the lateral intramuscular septum to re-enter the anterior compartment of the arm in between brachialis and brachioradialis muscles. Then it enters the forearm passing anterior to the lateral epicondyle of the humerus and immediately divides into two branches, namely deep branch and superficial branch.
The deep branch of the radial nerve enters the posterior compartment of the forearm piercing the supinator muscle where it is termed posterior interosseus nerve for the remainder of its course.
Out of the four sensory branches of the radial nerve, three of them arise in the upper arm descends downwards. They are namely, lower lateral cutaneous nerve of the arm, posterior cutaneous nerve of the arm and posterior cutaneous nerve of the forearm. The fourth branch is the terminal continuation of the radial nerve itself as the superficial branch of the radial nerve which is the only branch of radial nerve crosses which the wrist joint.
The radial nerve itself innervates all three heads of the triceps in the arm and extensor carpi radialis longus and brachioradialis muscles. It supplies the supinator muscle while piercing through it and the remainder of the muscles are innervated by the posterior interosseous nerve.
Cutaneous branches of the radial nerve supply the skin of the lower lateral arm, dorsal aspect of the arm, forearm and the lateral three and a half fingers. However, there is a significant overlap of sensory innervation from the adjacent nerves except in the dorsal aspect of 1st webspace of hand.
The clinical presentation of radial nerve palsy is different depending on the site of the lesion. It is susceptible to injury commonly at four sites.
Due to its close relationship to proximal humerus and shoulder joint, the radial nerve is liable for injury in axilla by shoulder dislocation and fractures involving the proximal humerus. It may also be damaged falling asleep one’s hand hanging over the arm of a chair compressing the radial nerve at axilla for a prolonged duration (Saturday night palsy) or by a badly fitting crutch (Crutch palsy).
The extension of the forearm at the elbow is totally lost as the triceps is denervated completely when the nerve is damaged at the axilla. Obviously, the wrist extensors in the forearm are also paralyzed resulting in wrist drop due to unopposed activity of intact flexor muscles.
As all four of the sensory branches are affected in this case, there will be a widespread sensory loss affecting the lateral aspect of the arm, posterior aspect of arm and forearm and the dorsal aspect of lateral three and a half fingers of the hand and palm.
Here the nerve lies directly on the humerus as it winds around it and susceptible for damage in fractures involving midshaft of the humerus.
The forearm extension is only weakened (in contrast to total paralysis) due to triceps’ partially intact innervation. The branches for long head & lateral head of the muscle are given off proximal to the spiral groove resulting in incompletely denervated muscle. Only its medial head loses the nerve supply. There will be wrist drop deformity owing to loss of motor supply to wrist extensors as the deep branch of the radial nerve is affected.
Since the sensory branches to arm & forearm are given off in the upper arm, they remain intact. The only branch to be affected is the superficial branch of the radial nerve which in turn results in sensory loss over the dorsal surface of the lateral three and a half fingers which is more marked over the 1st webspace.
Here the nerve lies close proximity with the radial neck and can be easily damaged in posterior dislocation of elbow joint and fractures involving the head & neck of the radius.
Even though most of the muscles in the posterior compartment of the forearm are affected which are innervated by the deep branch of the radial nerve, surprisingly the wrist drop deformity does not occur. It is due to intact extensor carpi radialis longus which is innervated by the main branch of the radial nerve itself before its division hence unaffected in the injury. This muscle alone can maintain the wrist in extension though there is some degree of weakness.
As the superficial branch of the radial nerve is unaffected there will be no sensory loss.
The superficial branch of the radial nerve can be damaged due to direct trauma causing soft tissue injuries in the forearm such as stabs or lacerations.
The only branch may be involved is the superficial branch of the radial nerve which is purely sensory. The patient will have no motor weakness and there will be sensory loss over the lateral three and a half finger of the affected hand