Median nerve lesions

Definition:

The median nerve is a peripheral nerve originating at the brachial plexus from the lateral and medial cords of the ventral roots of C6, C7, C8 and T1.
The median nerve enters the arm through the axilla at the inferior border of the teres major muscle. Its descent continues running alongside the brachial artery, towards the ventral portion of the elbow joint.
The median nerve continues its descent caudally passing through the carpal tunnel.

It innervates

• Wrist flexors
• 1st & 2nd lumbircals
• The flexors of the wrist
• Pollicis brevis
• Flexor pollicis
• Abductor pollicis
• The palmar skin of the thumb, index, middle & half of the ring finger
• Nail bed of the above digits.

Causes:

The median nerve can be lesioned at different sites along its course:

Above the elbow: the median nerve may be damaged as it emerges from the brachial plexus as a result of traction or compression, foraminal encroachment and space occupying lesions. The nerve may also be damaged as a result of humeral fractures. An anatomical anomaly: the ligament of Struthers may also compress the median nerve. Other causes include crutch compression, sleep palsy, tourniquets and anterior dislocation of the humerus.

At the elbow: here the median nerve may be compressed due to joint effusion, pronator teres syndrome, ventral dislocation of the radial head, fracture at the distal humerus and other bony constituents of the elbow articulation. Other causes include supracondylar spurs,

At the forearm: anterior interosseous syndrome, deep lacerations.

At the wrist: the most common lesion to the median nerve is carpal tunnel syndrome,

Causes include:
• Genetic
• Aneurysms
• Wrist injury
• Gout
• Diabetes
• Thyroid disorders
• Pregnancy

Examination:

The presentation of a lesion to the median nerve will vary depending on the site of the lesion.

Above the elbow: Injury to the median nerve at this level results in the paralysis or weakness of all the muscles supplied, with “Ape hand deformity” in advanced lesions: hyperextended and adducted thumb, flattened thenar eminence, and loss of sensation or reduced sensation in the skin supplied by the median nerve. Objective tests should include: muscle strength testing, testing of the skin sensation, and observing the hand for any abnormal appearance.

At the wrist: clinical features include: pain and/or numbness and/or paraesthesia in the area of the hand innervated by the median nerve, which commonly intensify at night and upon wakening often relieved by the patient shaking their hand. Grip strength is often reduced, which may affect dexterity. As the condition progresses, wasting of the thenar eminence may be appreciated. The physical examination ought to include Phalen’s and reverse Phalen’s; which involves the patient placing the dorsal portion of their wrist together and applying pressure, thus reducing the amount of space available within the carpal tunnel. An aggravation in symptoms is noted as positive. Percussing the flexor retinaculum and carpal tunnel may also aggravate symptoms, this is known as Tinel’s test.