When the Hand Suddenly Drops – Radial Nerve Palsy at the Upper Arm

A sudden loss of strength when trying to extend the wrist or fingers – possibly combined with numbness on the back of the hand? This may be caused by a compression of the radial nerve in the upper arm. The condition is often referred to as “Saturday night palsy” or “park bench palsy”, named after the typical situation in which it arises. In many cases, the cause can be identified with modern, non-invasive diagnostic tools – especially high-resolution nerve ultrasound.

Schematic view of the radial nerve running through the spiral groove on the posterior side of the humerus. — The radial nerve runs along the posterior humerus in a bony groove called the spiral canal. This area is particularly vulnerable to compression injuries, such as prolonged arm pressure.

What Is Radial Nerve Palsy (Park Bench Palsy)?

This term describes a pressure-related injury to the radial nerve that typically occurs after lying with the upper arm against a hard surface – such as the edge of a bench – for an extended period. People who are deeply relaxed or unconscious (e.g. after alcohol consumption or sleep medication) are particularly at risk.

Cartoon-style illustration of a man resting his upper arm on the hard backrest of a bench – a typical position for Saturday night palsy or called Radial Nerve Palsy. — In Saturday night palsy, the radial nerve is compressed at the upper arm by prolonged pressure on a hard surface such as a bench backrest.

Similar nerve compressions may also happen during prolonged surgical procedures, after falls, or from tightly applied splints or casts.

The Radial Nerve – Movement and Sensation

The radial nerve is a major nerve of the arm that controls the extension of the elbow, wrist, and fingers. It also transmits sensory information from the back of the hand, particularly the area between the thumb and index finger.

At the upper arm, the nerve runs through a bony groove on the back of the humerus (upper arm bone) called the spiral groove. At this point, the nerve lies very close to the bone – making it vulnerable to pressure injuries.

What Are the Symptoms?

Typical signs include a sudden inability or weakness when attempting to extend the wrist and fingers. The hand may hang limply – often referred to as a “wrist drop”.

Schematic illustration of wrist drop as a typical sign of radial nerve injury — Wrist drop: Radial nerve injury can lead to weakness or loss of active extension in the wrist and fingers, causing the hand to droop.

Many individuals also experience tingling or numbness on the back of the hand, especially in the area between the thumb and index finger. Depending on the severity, the nerve’s motor or sensory function may be affected either separately or in combination.

Schematic view of the dorsum of the right hand showing the area of numbness in Saturday night palsy: Involves the metacarpal region of the thumb, index, and middle fingers. — Saturday night palsy may cause numbness over the dorsum of the hand – typically involving the metacarpal region of the thumb, index, and middle fingers.

How Does the Damage Occur?

The most common cause is external pressure on the nerve at the upper arm. Typical triggers include:

Prolonged lying on a hard surface (e.g. park bench)
Deep sleep under the influence of alcohol or sedatives
Improper arm positioning during surgery
Tight casts or splints
Upper arm fractures with swelling or bruising

Schematic illustration of a broken humerus with radial nerve injury at the fracture site. — Radial nerve injury in humeral fracture: The nerve runs along the middle third of the humerus and may be affected in fractures at this level.

Less commonly, a space-occupying lesion (e.g. cyst or tumor) or a systemic nerve disorder may be involved.

When Should the Nerve Be Checked?

Important Note

Sudden weakness or paralysis should be taken seriously. In rare cases, serious conditions such as a slipped disc or even a stroke may be the cause. If numbness is unclear, symptoms worsen, or the same problem keeps coming back, medical evaluation should not be delayed.

The Role of High-Resolution Nerve Ultrasound

One of the tools that can support diagnosis is high-resolution nerve ultrasound. This method provides a detailed and painless view of the nerve directly beneath the skin – without radiation.

Illustration of a high-resolution ultrasound scanner for nerves to perform nerve ultrasound. — Peripheral nerves can be visualized in detail using nerve ultrasound.

The radial nerve can often be visualized very clearly at the spiral groove of the upper arm. The following changes can be identified:

Illustration of a nerve ultrasound showing nerve fascicles in high detail. — High-resolution nerve ultrasound can visualize internal nerve structures – including the fascicles, depending on location and image quality.

Local swelling or narrowing of the nerve
Altered internal structure (fascicles)
Signs of irritation or entrapment

Even subtle changes in nerve size or echotexture may offer important diagnostic clues when examined by an experienced clinician.

What Does the Ultrasound Show?

Normally, the entire course of the radial nerve at the upper arm can be followed and compared to the healthy side. In the area of compression, the nerve often appears thickened and less sharply defined.

Compared to standard nerve conduction studies (NCS), ultrasound can sometimes reveal early structural changes – even if the electrical signal conduction is still intact or borderline.

Electrophysiology: Functional Testing of the Nerve

In some cases, electrophysiological tests can help assess how well the radial nerve is functioning.

llustration of the electrodes used for nerve conduction studies (neurography / NCS / ENG). — Nerve conduction studies (neurography) are performed using a stimulation electrode and adhesive skin electrodes.

These include:

Nerve conduction studies, which check how quickly signals travel through the nerve
Electromyography (EMG), which looks at how the muscles respond to those signals

These tests are especially useful when:

The symptoms persist over time
The exact location or severity of the injury is unclear
Recovery needs to be monitored

Because certain changes may only appear after two to three weeks, these tests are often performed with a delay or repeated later.

What Is the Outlook?

In many cases, the symptoms gradually resolve on their own – especially if the compression was brief. Depending on the severity, full recovery may take several weeks to months.

Follow-up ultrasound exams can help monitor progress and assess whether the nerve is regenerating – or whether further diagnostic steps are needed.

Summary

Park bench palsy (also known as Saturday night palsy) is a common type of radial nerve injury caused by compression at the upper arm. It typically leads to wrist drop and numbness on the back of the hand.

High-resolution nerve ultrasound offers a valuable, non-invasive tool to visualize the nerve, detect changes, and guide further assessment. In unclear cases, a targeted examination can help to clarify the cause and monitor recovery.

Disclaimer: The information provided on this page is for general informational purposes only and does not constitute medical advice. It is not a substitute for personal consultation, examination, or treatment by a physician or other qualified healthcare professional. No guarantees of healing or cure are made. While we strive for accuracy and completeness, we cannot guarantee that all content is up to date or free from errors, and we accept no liability for any inaccuracies.