Median Nerve Lesion

Question 1

The Path

Answer 1

Originates from the medial and lateral cords of the brachial plexus (C5-T1)

It travels down the radial aspect of the arm from the axilla to the cubital fossa

It leaves the cubital fossa and runs between the heads of the pronator teres muscle

It then penetrates deeper, and then in the forearm, it is well protected from a traumatic injury, although susceptible to compression

As it approaches the wrist, the nerve becomes more superficial and before it reaches the carpal tunnel, it gives off the palmar cutaneous branch which travels superior to the carpal tunnel, turns radially and supplies sensory innervation the to the skin of the thenar area

The nerve becomes increasingly flat as it enters the carpal tunnel along with the tendons of the flexor pollicis longus, flexor digitorum superficialis and flexor digitorum profundus muscles

Upon leaving the tunnel, the medial nerve passes under the palmar fascia and branches into the hand

Question 2

Muscles Innervated by the Median Nerve

Answer 2

Pronator teres
Flexor carpi radialis
Flexor digitorum profundus (lateral half)
Palmaris longus
Pronator quadratus
Abductor pollicis brevis
Flexor pollicis longus
Flexor pollicis brevis
Opponens pollicis
1st and 2nd lumbricals

Question 3

Causes of Lesions of the Median Nerve

Answer 3

• Fractures:

At the elbow, wrist and carpal bones. This nerve is rarely injured at the upper arm

• Dislocations:

Of the elbow, wrist or carpal bones, primarily the lunate and scaphoid

• Compression:

From fibrosis and hypertonicity in overused muscles or the carpal tunnel which can lead to entrapment of the nerve and for example, pronator teres syndrome and carpal tunnel syndrome.

• Trauma:

Such as a traction injury, contusion or laceration, especially at the wrist or hand

Question 4

Symptom Picture

Answer 4

Presentation of a complete median nerve lesion is known as a ape hand or oath hand
Ape hand refers to the thumb lying in the same plane as the rest of the hand since there is a loss of opposition

Oath hand presentation is observed as the person attempts to make a fist. The person cannot this action because only the 3rh and 4th digits can be flexed

There is a loss of thenar flexors and opponens pollicis as well as most of the flexors of the index and middle finger

There is flaccidity in the opponens pollicis muscle with a complete lesion

If innervation is lost, thumb opposition is severely compromised, especially medial rotation of the thumb

There is difficulty holding a pen or firmly grasping an object between the thumb and finger

The median nerve is rich in autonomic fibres, therefore, a lesion results in vasomotor and trophic changes. There is edema in the hand, fingers and thumb, nail changes (ridges) and skin changes (skin is thin, glossy and lacking lines)

Depending on the location of the lesion, atrophy of the forearm flexors and thenar muscles is observed

Question 5

Sensory Dysfunction as a Result of a Lesion

Answer 5

Altered sensation is experienced on the thumb, index, middle and ½ of the 4th digit on the flexor surface, including the distal ⅔ of the palm. This extends from the middle to the distal phalanx on the extensor surface

Anesthesia (area of isolated supply) occurs at the distal interphalangeal joint of the index and middle fingers