Applied Anatomy of Median Nerve

The median nerve is called the “labourer’s nerve” because it controls coarse, powerful movements of the hand (it supplies most of the long flexor muscles of the forearm). It is also called the “eye of the hand” because it is sensory to most of the palmar aspect of the hand and is critical for fine tactile discrimination, especially of the thumb, index, and middle fingers.

Median nerve injury is more disabling than ulnar nerve injury because it compromises the pincer grip (thumb–index opposition) essential for precision activities.

1. Injury at the Elbow (High Median Nerve Injury)

Common cause: Supracondylar fracture of the humerus.

Features:

• Flexor pollicis longus and the lateral half of flexor digitorum profundus are paralysed → patient cannot flex the terminal phalanx of the thumb or index finger when the proximal phalanx is held fixed.
• Pointing index finger: while making a fist, the index (and to a lesser extent the middle) finger remains extended, due to paralysis of the long flexors of these digits.
• The forearm is kept in a supine position due to paralysis of the pronators (pronator teres and pronator quadratus).
• The hand is adducted at the wrist due to paralysis of flexor carpi radialis; flexion at the wrist is weak.
• Ape thumb (simian hand) deformity: the thumb lies in the same plane as the fingers due to paralysis of the thenar muscles, with loss of opposition.
• Sensory loss: corresponds to the median nerve’s cutaneous distribution — palmar aspect of the lateral 3½ digits and their nail beds, and the dorsal aspect of the distal phalanges of these digits.
• Vasomotor and trophic changes: skin over the affected area is warm, dry, and scaly (loss of sympathetic supply); nails crack easily.

2. Injury at the Mid-Forearm (Anterior Interosseous Nerve Syndrome)

• Isolated injury to the anterior interosseous nerve (a pure motor branch).
• Results in weakness of flexor pollicis longus, lateral half of flexor digitorum profundus, and pronator quadratus.
• Produces the same pointing index finger sign on attempted fist-making (inability to flex the distal interphalangeal joints of the thumb and index finger).
• No sensory loss, since the anterior interosseous nerve is purely motor.

3. Injury at the Wrist / Distal Forearm

• Median nerve and its palmar cutaneous branch may be injured by lacerations at the wrist.
• Results in paralysis of the thenar muscles and the lateral two lumbricals, with sensory loss over the palmar lateral 3½ digits including the thenar eminence (since the palmar cutaneous branch is also cut here, unlike in carpal tunnel syndrome).
• Partial claw hand / “Hand of Benediction” type appearance: on attempting to make a fist, the index and middle fingers lag behind in flexion at the metacarpophalangeal joints due to paralysis of the first and second lumbricals (loss of MCP flexion with IP extension in these digits).

4. Carpal Tunnel Syndrome (CTS)

The most common entrapment neuropathy, caused by compression of the median nerve within the carpal tunnel, deep to the flexor retinaculum.

Clinical features:

• Thenar wasting producing an “ape-like hand” appearance.
• Hypoaesthesia (reduced sensation) to light touch over the palmar aspect of the lateral 3½ digits.
• The skin over the thenar eminence is spared, because the palmar cutaneous branch arises proximal to the flexor retinaculum and does not pass through the carpal tunnel.
• Phalen’s test: acute flexion of the wrist for about 60 seconds reproduces or worsens the pain/paraesthesia in the median nerve distribution.
• Tinel’s sign: tapping over the flexor retinaculum produces tingling (paraesthesia) in the median nerve distribution.
• Nerve conduction studies show reduced conduction velocity across the wrist segment of the median nerve.

Key Differentiating Points

Feature	Elbow (High) Lesion	AIN Syndrome	Carpal Tunnel Syndrome
Sensory loss	Present (lateral 3½ digits + thenar)	Absent	Present (lateral 3½ digits, thenar spared)
Pointing index finger	Present	Present	Absent
Forearm pronation	Lost	Lost	Normal
Thenar wasting	Present	Absent	Present
Special tests	—	—	Phalen’s, Tinel’s