【Median Nerve】Functional Anatomy and Summary of Entrapment Neuropathy

The median nerve is one of the major peripheral nerves involved in the motor and sensory functions of the upper limb, and various symptoms arise from compressive neuropathy.

The median nerve controls many muscles responsible for finger flexion, wrist palmar flexion, forearm pronation, and thumb flexion, abduction, and opposition. It also conveys sensation from the palmar side and the dorsal side of the fingertips from the thumb to the radial half of the ring finger, making it a crucial nerve for hand function.

Below is a detailed summary of the functional anatomy of the median nerve and the main compressive neuropathies.

Median Nerve

The median nerve is an important nerve responsible for both motor and sensory functions in the upper limb.

The median nerve is formed by fibers from the lateral cord of the brachial plexus (C5, C6, C7 nerve roots) and the medial cord (C8, T1 nerve roots). It originates from the spinal nerve roots C5 to Th1.

Motor Nerve

As a motor nerve, it controls many muscles in the forearm and hand, playing a crucial role in precise hand and finger movements and gripping, such as finger flexion, wrist palmar flexion, forearm pronation, and thumb flexion, abduction, and opposition.

The muscles primarily controlled by the median nerve are as follows:

The thenar muscles are involved in thumb abduction, opposition, and flexion.

The lumbricals assist in flexing the proximal phalanges and extending the middle and distal phalanges of the index and middle fingers, enabling fine finger movements.

The Anterior Interosseous Nerve (AIN) innervates the deep flexor muscles of the forearm, such as the flexor pollicis longus, flexor digitorum profundus (especially the 2nd and 3rd fingers), and pronator quadratus.

The function of the AIN can be tested by evaluating the ability to make an "OK sign" with the thumb and index finger. If the thumb's DIP joint hyperextends and a complete circle cannot be formed, resulting in a shape like a "tear drop," it suggests AIN paralysis. This is referred to as a "Tear Drop Sign" positive.

The recurrent motor branch innervates most of the thenar muscles (abductor pollicis brevis, opponens pollicis, superficial head of flexor pollicis brevis). This enables thumb abduction and opposition (bringing the thumb and other fingers together). Damage to the recurrent motor branch results in loss of thumb motor function and atrophy of the thenar muscles, sometimes referred to as "ape hand."

Sensory Nerve

The median nerve transmits sensations (touch, pain, temperature, etc.) from specific areas of the hand and fingers to the brain.

In low-level median nerve paralysis, such as carpal tunnel syndrome, sensory symptoms like pain, tingling, numbness, and burning sensations typically appear in these sensory areas (thumb, index, middle, and radial half of the ring finger). Symptoms are often reported to worsen at night.

The anterior interosseous nerve does not have sensory branches, so sensory disturbances are usually not observed in anterior interosseous nerve paralysis.

The sensory distribution can be divided into the palmar and dorsal sides as follows:

Course

The median nerve travels from the axilla through the arm, heading anteriorly to the elbow joint at the distal arm.
In the arm, it is located lateral to the brachial artery but crosses medially at the level of the coracobrachialis.

Around the elbow joint, it is positioned medial to the biceps tendon and brachial artery, passing beneath the bicipital aponeurosis and anterior to the brachialis muscle.

As it enters the forearm, it passes between the humeral and ulnar heads of the pronator teres, branching into the anterior interosseous nerve at the distal part of the pronator teres. It then travels between the flexor digitorum superficialis and flexor digitorum profundus towards the wrist.

In the forearm, it sends branches to the pronator teres, flexor carpi radialis, and flexor digitorum superficialis.

About 5 cm before the wrist, it emerges from the deep layer of the flexor digitorum superficialis to the superficial layer, passing through the carpal tunnel to reach the palm.

The terminal branches are the muscular branches to the thenar muscles and the sensory branches to the palmar side of the radial three and a half fingers.

The carpal tunnel is formed by the carpal bones as the floor and sides, and the transverse carpal ligament (flexor retinaculum) as the roof, protecting not only the median nerve but also the tendons of the flexor digitorum profundus, flexor digitorum superficialis, and flexor pollicis longus from compression.

Compressive Disorders of the Median Nerve

The median nerve can be compressed as it passes through various narrow passages from its origin in the brachial plexus to its terminal branches. These include the bicipital aponeurosis (Lacertus Fibrosus), pronator teres, the tendon arch of the flexor digitorum superficialis (Sublimis Bridge), and the carpal tunnel.

Compression at these sites can lead to various neuropathies.

Neuropathies are thought to arise from adhesions of the surrounding tissues and traction on the nerve.

Normally, nerves glide and stretch during body movements, but if the surrounding tissues are adhered and tissue gliding is reduced, excessive traction on the nerve can lead to symptoms.

The following is a summary of the most common compressive disorder of the median nerve due to the carpal tunnel, known as carpal tunnel syndrome.

Carpal Tunnel Syndrome

Carpal Tunnel Syndrome (CTS) is the most common compressive neuropathy caused by compression of the median nerve within the carpal tunnel.

The carpal tunnel is a tunnel formed by the bony groove of the wrist's carpal bones and the strong transverse carpal ligament (flexor retinaculum) covering it. The median nerve and the flexor tendons of the fingers pass through this tunnel.

Causes

The main cause of CTS is an increase in pressure within the carpal tunnel beyond what is necessary. This pressure increase occurs when the contents of the carpal tunnel increase or the size of the tunnel itself decreases.

Specifically, the following various factors are cited as the main causes of carpal tunnel syndrome:

Idiopathic

This is the most frequently observed case where no clear cause can be identified.

In many cases, it is thought to be related to an increase in connective tissue, such as fibrosis of the tendon sheaths within the carpal tunnel.

Occupational Factors

Repetitive hand and wrist flexion or twisting tasks, use of vibrating tools, work with the wrist in prolonged flexion or extension, tasks requiring high hand force, and highly repetitive tasks can be significant risk factors for work-related CTS.

Underlying Conditions

・Diabetes
・Thyroid Disorders
・Rheumatic Diseases

These conditions are thought to affect the nerves and surrounding tissues, making it easier for pressure to rise within the carpal tunnel.

Trauma

Wrist fractures, for example, can reduce the volume of the carpal tunnel and compress the nerve.

Others

・Hemorrhagic Disorders
・Vascular Disorders
・Abnormal Anatomical Structures (though anatomical variations are sometimes considered rare causes)
・Rare Hereditary Disorders (HNPP, familial CTS) may be related.

Symptoms

Common symptoms of carpal tunnel syndrome include pain, tingling (prickling sensation), burning sensation, numbness, or a combination of these.

These symptoms usually appear on the palmar side of the fingers, especially the thumb, index, middle, and radial half of the ring finger.

Sometimes, numbness or tingling may occur in the entire hand, especially the radial palmar area. However, since the palmar branch does not pass through the carpal tunnel but passes anterior to the flexor retinaculum, the sensation of the radial palmar area is preserved in carpal tunnel syndrome.

Symptoms usually occur in episodes and often worsen at night, causing awakening during sleep. This is often thought to be related to the arm's position during sleep.

Symptoms may also occur upon waking.
Symptoms can appear during the day, especially when the hand is in the same position for a long time (driving, reading, holding a phone), making them more likely to be triggered.

In more advanced stages, symptoms like numbness and tingling may become persistent.

Pain may occur as an early symptom, and it can radiate beyond the hand to the forearm and shoulder.

As the condition progresses, motor symptoms appear.
Patients may report stiffness, clumsiness, and difficulty handling objects. Atrophy of the thenar muscles may appear later.

Differential Evaluation

To aid in diagnosis, the following two provocation tests are primarily used:

Phalen Test (positive if symptoms are reproduced after fully flexing the wrist for 60 seconds)
Tinel Sign (positive if symptoms are reproduced when repeatedly tapping over the carpal tunnel)

Sensory disturbances are mainly seen at the fingertips, but sensory testing may be normal in the early stages of the condition.

Additionally, patients may perform hand or arm shaking movements to relieve symptoms, known as the "Flick Sign."

Also, consider the condition known as "Double Crush Syndrome."

This refers to a state where the nerve is compressed not only at a distal site like the carpal tunnel but also at proximal sites such as cervical disc herniation, foraminal stenosis, brachial plexus injury, thoracic outlet syndrome, or pronator syndrome.

These factors combine, leading to compression of the median nerve within the carpal tunnel, resulting in symptoms of carpal tunnel syndrome.

In this case, focusing only on carpal tunnel syndrome may not lead to improvement.
It is necessary to evaluate and prioritize whether other conditions are the main issue or if carpal tunnel syndrome is the primary concern.

Median Nerve Paralysis

In high-level median nerve paralysis (above the elbow), a wide range of muscles such as the flexor digitorum superficialis, flexor digitorum profundus (index and middle fingers), flexor pollicis longus, thenar muscles, and lumbricals are affected, resulting in extensive impairment of finger flexion function.

In low-level median nerve paralysis, sensory symptoms such as pain, tingling, numbness, and burning sensations typically appear in these sensory areas (thumb, index, middle, and radial half of the ring finger). Symptoms are often reported to worsen at night. This is the aforementioned carpal tunnel syndrome.

Flexor Digitorum Superficialis (FDS)

Controlled by the main trunk of the median nerve, it flexes the proximal interphalangeal joints (PIP joints) of all four fingers from the index to the little finger. In high-level median nerve paralysis, the function of this muscle is impaired, making it difficult to flex at the proximal interphalangeal joints of these fingers.

Flexor Digitorum Profundus (FDP)

The anterior interosseous nerve, a branch of the median nerve, controls the flexor digitorum profundus of the index and middle fingers.

These muscles flex the distal interphalangeal joints (DIP joints) of the fingers. In many cases of high-level median nerve paralysis, the AIN is also involved, making it impossible to flex the distal interphalangeal joints of the index and middle fingers.

Flexor Pollicis Longus (FPL)

This muscle is also controlled by the AIN and flexes the distal phalanx (IP joint) of the thumb. In high-level median nerve paralysis, this function is also lost.

Flexor Pollicis Brevis (FPB)

The superficial head is controlled by the recurrent motor branch of the median nerve. In high-level paralysis, the function of this muscle is also lost, impairing flexion of the proximal phalanx (MP joint) of the thumb.

Lumbricals 1 & 2

Controlled by branches of the median nerve, these muscles assist in flexing the proximal phalanges and extending the middle and distal phalanges of the fingers. In high-level paralysis, these functions are also affected.

Particularly, impairment of thumb distal phalanx flexion (flexor pollicis longus) and index and middle finger distal interphalangeal joint flexion (flexor digitorum profundus) results in an abnormal "OK sign" (Tear Drop Sign).

The typical hand shape when a patient attempts to bend their fingers, with the index and middle fingers (sometimes the ring finger) remaining extended, is sometimes referred to as "Hand of benediction" or "Preacher's hand." However, due to anatomical variations in the proximal interphalangeal joint flexion of the middle finger by the ulnar nerve, it may clinically appear as a "Pointer Finger" with only the index finger extended.
(In my clinical experience, I have never seen a patient in this state, so I honestly do not know what it looks like.)

In low-level median nerve paralysis, paralysis of the thenar muscles (abductor pollicis brevis, opponens pollicis, superficial head of flexor pollicis brevis) impairs thumb abduction and opposition with other fingers, resulting in an appearance known as "ape hand."

References

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