The posterior cord of the brachial plexus gives the axillary and radial nerves. Radial nerve compression or injury may occur at any point along the course of the nerve. All motor and sensory function below the axilla will be affected with injury at this level. With injury to the radial nerve at the axilla, there will be loss of function to the triceps and weak elbow extension. The patient will experience wrist drop due to loss of function associated with the extensor carpi radialis longus and extensor carpi radialis brevis muscles. There will also be a loss of finger extension. Put the wrist in extension and ask the patient to extend the fingers. Sensory loss will also be seen in the distribution of the superficial branch of the radial nerve.
Saturday Night Palsy is a common cause of compression or injury to the radial nerve at the axilla. This condition is named due to the position a person may fall asleep in while drinking, with the back of their arm compressed by a chair back, or bar edge, etc. Another common condition is referred to as Honeymoon Palsy, which occurs from another individual sleeping on one’s arm overnight, compressing the nerve. Crutch Palsy occurs from the compression on the nerve from walking with crutches.
Additionally, there are several compressions or fractures that may cause injury to the radial nerve within the spiral groove. For example, a fracture of the distal third of the humerus may entrap the radial nerve, causing all motor and sensory function below the level of injury to be affected. The radial nerve is vulnerable, usually due to a fracture of the humerus at the spiral groove. Injury may also occur below the spinal groove when there is a fracture in the distal third of the humeral shaft. If a Holstein-Lewis Fracture occurs, injury to the nerve at this level will cause the condition known as wrist drop, as well as weakness of finger extension. Sensory loss will also be seen in the distribution of the superficial branch of the radial nerve.
Entrapment of the posterior interosseous nerve at the “Arcade of Frohse” is a low radial nerve palsy; occurring below the elbow. Motor function below this area will be affected by the injury. The posterior interosseous nerve is purely motor and the patient will have no sensory loss. The Arcade of Frohse is a site of radial nerve entrapment which may cause paralysis of the posterior interosseous nerve.
With injury to the posterior interosseous nerve, the patient will experience difficulty with extension of the fingers only. It is important for the physician to ask the patient to extend the wrist first when examining the finger extension. A Monteggia Fracture, or a fracture of the proximal third of the ulna/radius with dislocation of the radial head may cause compression of the radial nerve secondary to the fracture. A neurovascular examination is important for these fractures. A nerve injury, especially involving the posterior interosseous nerve, is not uncommon (observe). The patient will have a loss of finger extension. Wartenberg’s Syndrome is a compression of the superficial branch of the radial nerve. Pain associated with this condition is located 8cm proximal to the radial styloid; wearing a wristwatch may irritate this area. Pain and paresthesia will be experienced on the dorsum of the hand and the patient will have a positive Tinel’s sign.