The median nerve is one of the major nerves of the upper limb, originating from the brachial plexus in the neck and shoulder. Its path extends down the arm, passing through the elbow and forearm before entering the hand. When this nerve is compressed or impinged along its route, it can cause significant functional issues. Understanding the anatomical locations and mechanisms of compression is the first step toward effective diagnosis and relief.
Anatomy and Function of the Median Nerve
The median nerve travels from the neck and shoulder down the arm, lying close to the brachial artery, and enters the forearm at the elbow. In the forearm, it runs between muscle layers, providing motor control to muscles responsible for wrist and finger flexion. It is a mixed nerve, carrying both sensory and motor signals.
The nerve’s motor function controls specific muscles in the forearm and hand, necessary for fine motor skills like gripping, pinching, and rotating the forearm. Its sensory function provides feeling to the palm side of the hand, including the thumb, index finger, middle finger, and the thumb-side half of the ring finger. Compression can impair either or both functions, leading to different combinations of symptoms.
Primary Sites and Mechanisms of Compression
The most frequent site of median nerve compression is at the wrist, known as carpal tunnel syndrome. The carpal tunnel is a narrow, rigid passageway formed by the wrist bones and the tough, fibrous transverse carpal ligament. This tunnel accommodates the median nerve along with nine flexor tendons that control finger movement.
Compression occurs when the internal volume of the tunnel decreases or the volume of the structures within it increases, raising the pressure. This rise in pressure physically squeezes the nerve tissue and compromises its blood supply. Inflammation of the protective sheaths surrounding the flexor tendons, called tenosynovitis, is a common mechanism, as the swollen tissue takes up extra space. Repetitive motion, prolonged wrist posture, fluid retention (such as during pregnancy), or local trauma can contribute to this inflammatory response and pressure increase. Less common compression sites exist higher up the arm, such as the elbow, where the nerve can be pinched between the two heads of the pronator teres muscle, a condition known as pronator teres syndrome.
Clinical Signs and Symptoms of Impingement
Compression of the median nerve leads to a predictable set of subjective experiences, primarily affecting the nerve’s sensory distribution. Patients commonly report paresthesia, described as a pins-and-needles or tingling sensation, in the thumb, index, middle, and half of the ring finger. This can progress to hypoesthesia, or numbness, in the same distribution, making it difficult to differentiate textures or temperatures.
Pain is also a frequent symptom, often radiating from the wrist up the forearm toward the shoulder. A distinct characteristic of compression at the wrist is the worsening of symptoms at night, often waking the person from sleep. This is thought to be related to sleeping with the wrist flexed. As the condition advances, motor symptoms develop, including weakness in the muscles at the base of the thumb. This weakness makes activities requiring a strong pinch or grip challenging, sometimes leading to objects being dropped.
Medical Provocation Tests for Diagnosis
Clinicians use specific maneuvers, called provocation tests, to intentionally compress the median nerve to confirm a suspected diagnosis. These tests reproduce the patient’s symptoms, providing objective evidence of nerve irritation. One such maneuver is Phalen’s Test, where the patient presses the backs of their hands together with the wrists fully flexed for up to one minute.
This forced flexion reduces the space within the carpal tunnel, directly increasing pressure on the median nerve. A positive result occurs if the characteristic tingling or numbness is reproduced in the median nerve distribution within that minute. Another common diagnostic tool is Tinel’s Sign, which involves the clinician lightly tapping the skin directly over the median nerve at the wrist. The test is considered positive if this percussion causes a sudden, electric shock-like tingling sensation to shoot into the fingers, indicating an irritated nerve sensitive to mechanical stimulation.
Treatment Options for Decompression
Treatment for median nerve compression aims to reduce the pressure surrounding the nerve, starting with conservative methods for mild to moderate cases. Rest and activity modification are initial steps, particularly avoiding activities that require repetitive gripping or prolonged wrist posture. Wrist splinting is often prescribed to keep the wrist in a neutral position, especially at night, which minimizes pressure within the carpal tunnel.
Non-steroidal anti-inflammatory drugs (NSAIDs) may be used to manage pain and inflammation. A corticosteroid injection can also be delivered directly into the carpal tunnel for temporary relief of symptoms. If symptoms persist or if nerve damage is severe, surgical intervention known as carpal tunnel release surgery is considered. The goal of this procedure is to physically cut the transverse carpal ligament, which immediately increases the space within the tunnel and removes the primary source of compression.