Median Nerve Injury

Excerpt

The median nerve, also called the 'eye of the hand,' is a mixed nerve with a role of primary importance in the functionality of the hand. It innervates the group of flexor-pronator muscles in the forearm and most of the musculature present in the radial portion of the hand, controlling abduction of the thumb, flexion of the hand at the wrist, and flexion of the digital phalanx of the fingers. The nerve allows the sensory innervation to the palmer face of the thumb, index, middle and radial side of the ring finger, and the entire palmar region of the radial half of the hand. It also provides sensitivity to the dorsal skin of the last two phalanges of the index and middle fingers.

The nerve forms in the cervical area of the spinal cord from the medial and lateral cord of the brachial plexus. These cords form from the ventral primary rami of cervical nerve roots five to eight, as well as the first thoracic spinal segment. The median nerve descends medially to the brachial artery at the level of the humerus and enters the forearm between the two heads of pronator teres. The nerve is very superficial in the cubital fossa and lies deep to bicipital aponeurosis. In the forearm, the median nerve lies deep to the flexor digitorum superficialis and superficial to flexor digitorum profundus. It then enters the palm under the flexor retinaculum lateral to the tendon of flexor digitorum superficialis and posterior to the tendon of palmaris longus. Pathology and injury to the median nerve can occur anywhere along the length of the median nerve.

Of note, in the arm, there are no muscles innervated by the median nerve. Although a branch to pronator teres is innervated proximal to the elbow joint, there are a few vascular branches of the median nerve that supply to the brachial artery, and articular branches of the median nerve innervate the elbow joint. In the forearm, the median nerve innervates the flexor digitorum superficialis, pronator teres, the medial half of the pronator quadratus, the palmaris longus, flexor carpi ulnaris, and flexor carpi radialis. Furthermore, in the hand, the flexor pollicis longus and flexor digitorum profundus are innervated by the anterior interossei branch of the median nerve. Articular branches of the median nerve feed the carpal joints, distal radioulnar, and radiocarpal joint. Multiple communicating branches of the median nerve connect to the ulnar nerve. The median nerve innervates the muscles of the thenar compartments of the palm, flexor pollicis longus, abductor pollicis brevis, opponens pollicis, and adductor pollicis. Also, the palmar cutaneous branch of the median nerve innervates the skin over the thenar eminences and lateral two and a half fingers on the palmar aspect of the hand and the skin over the two and a half fingers over the dorsum of the hand.

The median nerve can be affected by acute traumatic, chronic micro traumatic, and compressive lesions. The nerve can also become damaged during multiple-cause degenerative processes and neuropathies. The different types of lesions can affect the median nerve at various levels along its long path from the brachial plexus and axilla to the hand. Neuropathies mainly concern the distal territory. Peripherally, the median nerve can become compressed under the fascial sheath of the flexor retinaculum, which often causes burning pain, numbness, and tingling (neuropathic pain). This condition is known as entrapment syndrome or carpal tunnel syndrome. The carpal tunnel syndrome pain is explainable as a needle and pin sensation along the distribution of the median nerve. The condition is idiopathic and is also associated with hypothyroidism, pregnancy, and diabetes. Decreased sensation over a patient's thenar eminence is an indication of a medial nerve injury that is proximal to the carpal tunnel. The sensation of the thenar eminence receives its nerve supply by a branch of the median nerve, which is proximal to the carpal tunnel, the palmar cutaneous branch of the median nerve. Clinically, symptoms can be intermittent with flares and remissions.

Although a strong history can be clinically suggestive of median nerve pathology, there are several modalities that can aid in diagnosis. Plain film images, including a carpal tunnel view X-ray, can assist in diagnosis. Ultrasound is another imaging modality that is finding increasing use in diagnosing nerve pathology. Median nerve mononeuropathy is most common at the carpal tunnel. However, the prevalence of entrapment along other sites is estimated to be 7 to 10 percent. Other sites include the ligament of Struthers, lacertus fibrosis, between the heads of the pronator teres, and the flexor digitorum superficialis. Electromyography (EMG) also plays an important role in confirming the diagnosis and localizing the nerve and location. Treatment options vary depending on location. Non-invasive therapy is attempted first, including options such as braces to relieve pressure at sites of compression, physical therapy, and lifestyle modifications to avoid repetitive stress. If those measures fail, surgical evaluation can be considered.