Wrist Drop

Etiology

Causes of wrist drop include a stab wound to the shoulder area just below the clavicle as this is the area where the radial nerve is the terminal branch of the posterior cord of the brachial plexus. A knife wound, for example, may easily transect the cord. The radial nerve can also be injured if there is a humeral fracture because the radial nerve runs through the radial groove on the lateral border of the bone.[1] Persistent injury to the nerve from repetitive actions to the radial nerve with prolonged use of crutches or extensive leaning on the elbows can also lead to a wrist drop. Mechanical derangement of the radial nerve need not involve compression. It can also involve enlargement, torsion, and fascicular entwinement.[2] Lead poisoning and thiamin deficiency (beriberi) may also result in wrist drop.[3] Wrist drop may also be a presenting sign of a more systemic neuromuscular disorder, such as limb-onset amyotrophic lateral sclerosis. More exotic etiologies, such as acute upper limb ischemia or excessive injection of muscle-enhancement oil, have also been known to instigate radial nerve palsy.

Epidemiology

Wrist drop is a relatively common condition. "Saturday night palsy" is a commonly seen when the radial nerve is acutely compressed at the spiral groove. The incidence of radial neuropathy in the UK has been reported at 2.97/100,000 for men and 1.42/100,000 for women between the ages 75–84.[4]

Pathophysiology

The radial nerve bifurcates into deep and superficial branches along the lateral border of the cubital fossa, just distal to the elbow. The former, also called the posterior interosseous nerve, courses through the supinator muscle and supplies it as well as the extensor muscles of the wrist and digits. Thus, injury proximal to this bifurcation can be expected to manifest not merely as a radial nerve palsy but also as a radial nerve sensory derangement. The radial nerve appears to be vulnerable to entrapment at the level of the supinator muscle. Radial head entrapment can be proximal to the elbow at the head of the triceps muscle, in which case a deficit in elbow extension may accompany the wrist drop.[5] If wrist drop is the presenting sign of mononeuritis multiplex, then its pathophysiology can be mediated by immunological, infectious, paraneoplastic, or other processes that result in damage to the axon, thereby interfering with nerve conduction.

History and Physical

A typical scenario elicited on history may be that the patient ingested a large amount of alcohol at a party, became intoxicated, and perhaps slept with his or her body weight on the left arm. It is possible that several days elapse before a paresis of the fingers and wrist of the affected hand evolve. In these scenarios, the patient may also complain of pain that runs along the posterior or lateral aspect of the upper arm, travels to the posterior aspect of the forearm, and then to the back of the left hand. The anesthesia may also extend to the posterior aspect of the first three-and-a-half digits.

On physical examination, the individual will have weakness of wrist extension and the inability to extend the fingers. When a lesion to the radial nerve is high above the elbow joint, then the patient may also complain of numbness to the forearm and hand along the radial nerve's dermatome described above.

When testing for wrist-drop deformity, the patient should be asked to hold the affected arm out with the forearm parallel to the floor. The back of the hand should be facing the ceiling and the fingers should be pointed downwards. An individual with a wrist drop will be unable to move the hand from this position to one where the wrist and fingers are straight. There may also be a loss of the triceps muscle reflex, as the radial nerve is responsible for extension of the elbow.

Evaluation

Nerve conduction studies and needle electromyography are required to locate the site of nerve impingement and rule out a more generalized or systemic disorder. Nerve conduction studies reveal axonal damage of the radial nerve. Plain x-rays can look for the presence of a fracture or bone spurs. MRI may also be used to search for nerve compression or damage in the brachial plexus area. Finally, high-resolution ultrasound is a valuable tool to detect lesions of peripheral nerves that can be particularly useful to determine the nature of an injury, such as enlargement, constriction, torsion, or fascicular entwinement, which can be important when planning the surgical intervention.[2]

Treatment / Management

The treatment of wrist drop is with the use of a wrist splint and physical therapy. If there is a mechanical cause such as a bone spur or compression, then surgery may be helpful. The timing of surgery when radial nerve palsy is due to a humeral fracture is not yet known. Some surgeons wait a few months to see if spontaneous recovery occurs before undertaking explorative surgery. In the acute setting of humeral shaft fracture, proper immobilization of the wrist is necessary. This immobilization can be made possible with a modified coaptation splint.[6] Surgical exploration of the radial nerve is appropriate if the radial nerve palsy occurs during splint application and does not resolve with removal of the splint. Approximately 70% of radial nerve palsy cases have been reported to be resolved with conservative treatment. In the case of immunologically mediated wrist drop, as in mixed cryoglobulinemia, drugs such as rituximab may facilitate a rather rapid recovery.[7](B3)

Differential Diagnosis

• CNS

• C7 root

• PIN

• Posterior interosseous neuropathy

• Posterior cord

• Radial Nerve in the spiral groove

• Radial Nerve in the axilla

• Radial neuropathy at the spiral groove