Median Nerve Injury

The median nerve, known as the "eye of the hand," plays a crucial role in hand function. This nerve supplies the flexor-pronator muscles of the forearm and most muscles in the radial portion of the hand, controlling thumb abduction, wrist flexion, and flexion of the finger phalanges. Sensory innervation covers the palmar surface of the thumb, index, middle, and radial half of the ring finger, and the entire palmar region of the radial hand. Additionally, the median nerve supplies sensation to the dorsal skin of the distal phalanges of the index and middle fingers. Originating in the cervical spinal cord, the median nerve forms from the medial and lateral cords of the brachial plexus, which derive from the ventral primary rami of cervical roots C5 to C8 and the first thoracic segment. The nerve descends medial to the brachial artery at the humerus level, passing between the 2 heads of the pronator teres muscle in the forearm (see . Nerve Distribution in the Arm and Hand). The nerve lies superficially in the cubital fossa, beneath the bicipital aponeurosis, then courses deep to the flexor digitorum superficialis and superficial to the flexor digitorum profundus. Entering the palm beneath the flexor retinaculum, the nerve passes lateral to the flexor digitorum superficialis tendon and posterior to the palmaris longus tendon. Injury or pathology may occur at any point along this course. Of note, the median nerve does not supply any muscles in the arm. A branch to the pronator teres arises proximal to the elbow joint. Several vascular branches of the median nerve supply the brachial artery, and articular branches provide innervation to the elbow joint. In the forearm, the median nerve innervates the pronator teres, flexor digitorum superficialis, palmaris longus, and flexor carpi radialis. The nerve also supplies the lateral half of the flexor digitorum profundus and the pronator quadratus via its anterior interosseous branch (see . Course of the Median Nerve in the Forearm). Additionally, the anterior interosseous nerve supplies the flexor pollicis longus. Articular branches from the median nerve supply the carpal joints and the distal radioulnar and radiocarpal joints. Multiple communicating branches connect the median nerve with the ulnar nerve in the forearm and hand. In the hand, the median nerve innervates the thenar muscles, including the abductor pollicis brevis, opponens pollicis, and the superficial head of the flexor pollicis brevis. The palmar cutaneous branch of the median nerve provides sensation to the skin over the thenar eminence. The median nerve supplies sensation to the palmar surface of the lateral two and a half digits—the thumb, index finger, middle finger, and the radial half of the ring finger—and the dorsal distal phalanges of these same digits. The median nerve may be affected by acute traumatic, chronic microtraumatic, and compressive lesions. Damage may also result from multiple-cause degenerative processes and neuropathies. These different lesion types can involve various levels along the nerve’s long course from the brachial plexus and axilla to the hand. Neuropathies primarily affect the distal territory. Compression frequently occurs beneath the fascial sheath of the flexor retinaculum, leading to burning pain, numbness, and tingling—symptoms characteristic of neuropathic pain. This condition, entrapment or carpal tunnel syndrome, produces a needle-and-pin sensation along the median nerve distribution. Carpal tunnel syndrome often arises idiopathically but has associations with hypothyroidism, pregnancy, and diabetes. Decreased sensation over the thenar eminence indicates a median nerve injury proximal to the carpal tunnel. The palmar cutaneous branch, which supplies this area, originates proximal to the carpal tunnel. Clinically, symptoms may present intermittently, with periods of flares and remissions. A strong clinical history often suggests median nerve pathology, but several diagnostic modalities can aid confirmation. Plain radiographs, including dedicated carpal tunnel views, assist in identifying underlying causes. Ultrasound use is increasingly becoming a valuable tool in diagnosing nerve abnormalities. Median nerve mononeuropathy most commonly occurs at the carpal tunnel. However, entrapment at other sites accounts for approximately 7% to 10% of cases. These sites include the ligament of Struthers, lacertus fibrosus, between the heads of the pronator teres, and near the flexor digitorum superficialis. Electromyography is key in confirming the diagnosis and precisely localizing the lesion. Treatment varies according to the entrapment site. Initial management typically involves noninvasive measures such as braces to relieve compression, physical therapy, and lifestyle modifications to reduce repetitive stress. Surgical intervention may be considered if conservative therapies prove ineffective.