Median Nerve Injuries
Sohaib K. Malik
Alan R. Koester
INTRODUCTION
Pathoanatomy
Median nerve derived from medial and lateral cords of brachial plexus contains fibers from all five nerve roots (C5-T1).
Median nerve runs down the medial aspect of the arm in the anterior compartment, medial to the biceps and brachialis (initially lateral to brachial artery), then crosses over brachial artery, and becomes medial to it just before the antecubital fossa.
Enters forearm between pronator teres and under the bicipital aponeurosis (lacertus fibrosus).
Median nerve gives motor branches to pronator teres, flexor carpi radialis (FCR), palmaris longus, and flexor digitorum superficialis (FDS).
Median nerve then travels between flexor digitorum profundus (FDP) and FDS and emerges between FDP and flexor pollicis longus (FPL).
Anterior interosseous nerve branches proximally, runs along the interosseous membrane between FPL and FDP, gives sensory innervation to volar wrist capsule, and motor to FDP to 2nd/3rd digits, FPL, and pronator quadratus.
The palmar cutaneous branch divides 5 cm proximal to wrist and runs between the FCR and palmaris longus, supplying sensation to lateral hand and palm.
The nerve then enters the hand via the carpal tunnel, along with the tendons of FDS, FDP, and FPL.
Motor recurrent branch divides after (50%), under (30%), or through (20%) the transverse carpal ligament and innervates abductor pollicis brevis, opponens pollicis, and flexor pollicis brevis (superficial head only).
Palmar digital branches and proper palmar digital branch supplies sensory to the volar aspects of the thumb, index, middle and radial aspect of the ring finger, as well as the dorsal/distal aspects of these fingers.
The radial two lumbricals are also supplied by the median nerve.
Mechanism of injury
Etiologies include penetrating trauma, crush, traction, ischemia, and less common mechanisms such as thermal and electric shock, radiation, percussion, and vibration.1
Lacerations
Usually because of penetrating trauma, fractures, gunshot wound (GSW)
Stretching/compression/crush
Blunt trauma, fracture malunion, iatrogenic with prolonged tourniquet use or retraction
8% elongation will diminish nerve’s microcirculation, 15% elongation will disrupt axons.
Epidemiology
Estimated 2% to 3% of patients admitted to trauma centers have peripheral nerve injuries, and this percentage is higher if plexus and root injuries are included.2
Traumatic median nerve injury is the second most common isolated nerve injury behind ulnar nerve injury.3,4
2700 admissions in 2006 for median nerve injuries costing in the range of 28 000 dollars3
EVALUATION
History
Paresthesia and numbness of palm, volar thumb, index, long, and radial ring finger, as well as dorsal/distal tips of these digits
History of blunt or penetrating trauma, fractures, traction injuries to shoulder
Physical examination
Inspection of any open wounds if acute injuries. If long-term nerve injury, thenar atrophy as well as hand deformities apparent
Injury to median nerve at different levels causes different syndromes.
Above the elbow (eg, a supracondylar humerus fracture vs GSW/stab wound)
Loss of pronation of forearm, weakness in flexion of the hand at the wrist, loss of flexion of radial half of digits and thumb, loss of abduction and opposition of thumb5
Presence of ape hand deformity when hand is at rest because of hyperextension of index finger and thumb, and an adducted thumb
▲ See Figure 38.1
Presence of benediction sign when attempting to form a fist because of loss of flexion of radial half of digits5
▲ See Figure 38.2
Sensory deficit: loss of volar sensation in thumb, index, long, and radial ring finger, as well as distal/dorsal aspect of these digits, and loss of sensation to the palm5
In proximal forearm (eg, tight cast/splint or forearm fracture)
Anterior interosseous nerve syndrome
Loss of pronation of forearm, loss of flexion of radial half of digits and thumb
At the wrist (ie, wrist laceration)
Loss of abduction and opposition of thumb
Presence of ape hand deformity when hand is at rest may be likely secondary to denervation of the thenar muscles
Sensory examination can reveal impaired sensation of the thumb, index, long, and ring finger using two-point discrimination or Semmes-Weinstein monofilament testing.
Patients with a nerve injury will often develop Tinel’s sign over the site of nerve injury. This is performed by gently tapping along the nerve to discern the level of injury, which causes the patient to experience an electrical sensation that propagates distally along the nerve distribution.5Related posts:
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