Wrist drop is caused by damage to the radial nerve, the nerve responsible for extending your wrist, fingers, and hand. When this nerve is injured or compressed anywhere along its path from your neck to your forearm, the muscles that lift your wrist stop working, and your hand hangs limply. The most common cause is prolonged pressure on the nerve in the upper arm, but fractures, systemic diseases, and rarer neurological conditions can also be responsible.
How the Radial Nerve Controls Your Wrist
The radial nerve originates from the spinal cord in your neck, travels through your armpit, and wraps around the humerus (your upper arm bone) in a channel called the spiral groove. From there, it passes through a narrow tunnel of muscle and ligament at the outside of your elbow, then branches into your forearm and hand. Along this route, it controls the triceps muscle at the back of your upper arm, the muscles that rotate your forearm, and the muscles that extend your wrist and open your fingers from a clenched fist.
The nerve is especially vulnerable at two points: the spiral groove of the humerus, where it sits directly against bone, and the radial tunnel at the elbow, where it threads between muscles and ligaments. Damage at either location can knock out the signals that keep your wrist lifted.
Compression From Prolonged Pressure
The most widely recognized cause of wrist drop is external pressure on the radial nerve at the spiral groove. This often happens when someone falls asleep with their arm draped over a chair back or another hard surface, cutting off the nerve’s blood supply and function for hours. It’s sometimes called “Saturday night palsy” because it classically occurs after someone falls asleep in an awkward position while intoxicated. The same mechanism can happen from improper crutch use, where pressure in the armpit compresses the nerve higher up.
In most of these compression cases, the nerve sheath is damaged but the nerve fibers themselves remain intact. This type of injury, called a demyelinating lesion, tends to recover on its own over weeks as the protective coating around the nerve repairs itself. Full recovery within two to three months is typical when no deeper damage has occurred.
Fractures of the Upper Arm Bone
A broken humerus is one of the most serious causes of wrist drop. Because the radial nerve wraps directly around the middle of this bone, a fracture through the shaft can stretch, bruise, or even sever the nerve. Bone fragments may trap the nerve, or swelling around the fracture site can compress it.
Research published in the Journal of Hand Surgery found that about 90% of patients with radial nerve palsy from a surgically treated humerus fracture showed the first signs of nerve recovery within six months. By 12 months, roughly 84% had regained full hand and wrist function, and by 18 months that number climbed to 94%. The median time to full motor recovery was 36 weeks, though some patients took over a year and a half.
The current clinical approach is to wait approximately three months for signs of recovery before considering surgical exploration, unless the fracture is open (meaning bone has broken through the skin). Early surgery is reserved for those open fractures or cases where the nerve has a clean-cut injury. If no electrical signs of recovery appear on testing after three months, surgeons typically intervene, with the goal of repairing the nerve no later than six months after the original injury.
Lead Poisoning and Other Systemic Causes
Certain diseases that damage nerves throughout the body can show up first as wrist drop. Lead poisoning is a classic example. Chronic lead exposure preferentially attacks motor nerves, and the radial nerve is often affected earliest, producing wrist drop even before other symptoms of lead toxicity become obvious. This pattern is most relevant for people with occupational exposure to lead, such as in battery manufacturing or old paint removal.
Diabetes, which damages nerves by disrupting their blood supply, can occasionally cause isolated radial nerve palsy. Other systemic conditions that inflame or damage peripheral nerves, including certain autoimmune disorders and infections, can produce the same result, though these are less common starting points.
Parsonage-Turner Syndrome
A rarer but important cause is Parsonage-Turner syndrome (also called neuralgic amyotrophy), an inflammatory condition that attacks specific nerves in the shoulder and arm. It typically begins with sudden, severe pain in the shoulder or upper arm that often starts at night. The pain may last days to weeks and then subsides, but weakness and muscle wasting follow within the first five weeks. When the radial nerve is involved, wrist drop develops after the pain fades.
The condition has been linked to infections, surgery, strenuous exercise, and vaccinations, though the exact trigger often remains unclear. Diagnosis is made primarily from the distinctive pattern of intense pain followed by weakness, confirmed with nerve conduction testing. Recognizing this pattern matters because the treatment approach and expected timeline differ from a simple compression injury.
How Wrist Drop Is Diagnosed
A physical exam can often narrow down where the radial nerve is injured based on which muscles are affected and where sensation is altered. If the triceps muscle (which straightens your elbow) still works but wrist extension is lost, the injury is likely at or below the spiral groove. Numbness on the back of the hand between the thumb and index finger points to the radial nerve specifically, while numbness in the middle finger suggests a cervical spine problem at the C7 nerve root instead.
Distinguishing a radial nerve injury from a pinched nerve in the neck is one of the key diagnostic challenges. A C7 nerve root problem from a herniated disc in the neck can also weaken wrist extension, but it will also affect wrist flexors, the pronator muscles in the forearm, and certain shoulder movements. The triceps reflex may be diminished. Radial nerve damage, by contrast, spares those muscles and instead weakens the forearm muscle on the thumb side (the brachioradialis) and may reduce its reflex.
Electrical Testing
Nerve conduction studies and needle EMG are the main tools for confirming the diagnosis and assessing severity. In a nerve conduction study, electrical impulses are sent along the nerve to find where the signal slows or stops. Stimulation above and below the spiral groove can pinpoint a compression injury there by revealing a “conduction block,” where the signal fails to pass through the damaged segment.
These tests also help predict recovery. In one study of traumatic radial nerve injuries, patients who still had a detectable motor signal on nerve conduction testing had an 85% rate of partial or full recovery, compared to 65% when no signal was present. EMG of the brachioradialis muscle was even more predictive: 92% of patients with at least some muscle recruitment on EMG had good outcomes, while only 33% of those with no recruitment recovered well. Testing is most informative about four weeks after the injury, when the distinction between a nerve that’s bruised versus one with deeper fiber damage becomes clearest.
Splinting and Recovery
While waiting for the nerve to heal, a wrist splint is the standard first step. The traditional rigid “cock-up” splint holds your wrist in a slightly extended position, preventing the paralyzed muscles from being overstretched by gravity. This protects the muscles and keeps the wrist joint from stiffening into a fixed, flexed position.
An elastic or dynamic version of the splint offers an advantage: it holds the wrist up while still allowing your unaffected muscles to move. This keeps the flexor muscles active and strong, prevents joint stiffness and muscle wasting from disuse, and generally produces better functional results when nerve recovery does occur. Consistent splint use throughout the recovery period makes a meaningful difference in how well the hand works once the nerve comes back online.
Physical therapy focused on maintaining joint range of motion and muscle conditioning complements splinting. The combination keeps your hand and wrist ready to work normally again as the nerve gradually regenerates, which in many cases it does on its own given enough time.