Median nerve

Nervus medianus

Also known as: anterior interosseous nerve, recurrent branch of median nerve, carpal tunnel nerve

The main nerve of the anterior forearm and of the thumb (thenar) muscles (C5/6–T1). It forms from the lateral and medial cords, passes through the carpal tunnel, and supplies the skin of the palmar lateral three and a half digits. Compression causes carpal tunnel syndrome, the commonest entrapment neuropathy.

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Overview

RootsC(5)6–T1

Branches in the armNone

Commonest entrapmentCarpal tunnel syndrome

Hand muscles suppliedLOAF (5 muscles)

The median nerve forms in front of the third part of the axillary artery from a lateral root (lateral cord, C5–C7) and a medial root (medial cord, C8–T1). It descends lateral to the brachial artery and crosses in front of it at mid-arm to lie medial to it in the cubital fossa. It gives no branches in the arm. It enters the forearm between the two heads of pronator teres and runs deep to flexor digitorum superficialis, adherent to its deep surface. From it arises the anterior interosseous nerve (AIN), which runs on the interosseous membrane.

About 5 cm above the wrist it gives the palmar cutaneous branch, which passes superficial to the flexor retinaculum. The main nerve then passes through the carpal tunnel deep to the retinaculum, with the nine flexor tendons. In the palm it gives the recurrent (thenar, motor) branch to the thenar muscles. This is called the "million-dollar nerve" because cutting it is so disabling. Its common palmar digital nerves supply lumbricals 1 and 2 and the skin of the palmar lateral 3½ digits, including their dorsal tips and nail beds.

Location: Arm (with the brachial artery) → cubital fossa → anterior forearm (between FDS and FDP) → carpal tunnel → lateral palm.

Parts & subdivisions

Anterior interosseous nerve
A purely motor branch to FPL, the lateral FDP and pronator quadratus. AIN syndrome (after a supracondylar fracture, or neuralgic amyotrophy) makes the patient unable to make an "OK" sign: the tip pinch of thumb and index collapses into a pulp pinch. There is no sensory loss.
Recurrent (thenar) branch
It curves back into the thenar muscles just beyond the retinaculum. Superficial lacerations of the thenar eminence can sever it, causing a loss of opposition with no sensory loss.

Anatomy

Innervation: All anterior forearm muscles except flexor carpi ulnaris and the medial half of FDP; the LOAF muscles of the hand; and the skin of the lateral palm and the palmar lateral 3½ digits.
Nerve roots: C5 (variable), C6, C7, C8, T1 (lateral and medial cords).
Course: Union of the lateral and medial roots anterior to the axillary artery → lateral to the brachial artery, crossing anteriorly at mid-arm to lie medial → cubital fossa (medial to the brachial artery, deep to the bicipital aponeurosis) → between the heads of pronator teres → under the FDS arch → between FDS and FDP → carpal tunnel (deep to the flexor retinaculum) → recurrent branch and digital nerves in the palm.
Branches: - Forearm muscular branches: pronator teres, flexor carpi radialis, palmaris longus and FDS.
• Anterior interosseous nerve: flexor pollicis longus, the lateral half of FDP (index and middle fingers) and pronator quadratus.
• Palmar cutaneous branch: skin over the thenar eminence and lateral palm. It passes over the retinaculum, so it is spared in carpal tunnel syndrome.
• Recurrent branch: abductor pollicis brevis, flexor pollicis brevis (superficial head) and opponens pollicis.
• Digital branches: lumbricals 1–2 and the skin of the palmar lateral 3½ digits.

Function

Pronation of the forearm (pronator teres, pronator quadratus)
Flexion of the wrist (FCR) and fingers (FDS; FDP to the index and middle fingers)
Flexion of the thumb IP joint (FPL)
Thumb opposition and abduction (thenar muscles), essential for precision grip
Sensation of the palmar lateral 3½ digits, the most functionally important skin of the hand

Clinical significance

Carpal tunnel syndrome

Compression in the carpal tunnel causes paraesthesia and pain in the lateral 3½ digits, worse at night, relieved by shaking the hand. Thenar wasting and weak abduction and opposition of the thumb follow later. Sensation over the thenar eminence is preserved because the palmar cutaneous branch is spared. Tinel (tapping) and Phalen (wrist flexion) signs are positive. Associations include pregnancy, hypothyroidism, diabetes, rheumatoid arthritis, acromegaly, obesity and dialysis amyloid. Diagnosis is by nerve conduction studies. Treatment is a splint, steroid injection or surgical division of the flexor retinaculum.

Proximal lesion: hand of benediction and ape hand

A lesion at the elbow, for example a supracondylar fracture, causes loss of forearm pronation, weak wrist flexion with ulnar deviation, and the "hand of benediction" when the patient tries to make a fist: the index and middle fingers stay extended while the ring and little fingers flex through the ulnar-supplied FDP. Long-standing median palsy produces an "ape hand": thenar wasting with the thumb adducted and lying in the plane of the palm, unable to oppose.

Mnemonics

Median-supplied hand muscles

"LOAF": Lateral two lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis.

Median vs ulnar in the forearm

The median nerve supplies all flexors except 1½. Those 1½ are flexor carpi ulnaris and the medial half of FDP, which are supplied by the ulnar nerve.

Test yourself

In carpal tunnel syndrome, sensation over the thenar eminence is usually preserved. Why?

The thenar skin is supplied by the radial nerve only
The palmar cutaneous branch of the median nerve arises proximal to the tunnel and passes superficial to the flexor retinaculum
The ulnar nerve takes over sensation in the palm
Sensory fibres are more resistant to compression than motor fibres

Answer: The palmar cutaneous branch of the median nerve arises proximal to the tunnel and passes superficial to the flexor retinaculum

The palmar cutaneous branch leaves the median nerve in the distal forearm and runs over the flexor retinaculum, so it escapes compression. Paraesthesia in the lateral 3½ digits, with thenar (LOAF) weakness and wasting but a normal palm, therefore points to the carpal tunnel.