Upper Extremity Main Nerves and Nerve Lesions

Introduction and Importance:
- It is a vital nerve of the upper limb because it provides the motor supply to the deltoid muscle, which is the primary abductor of the arm.
Origin:
- It is the smaller terminal branch of the posterior cord of the brachial plexus.
- Its root values are $C5-C6$ .
Course:
- It arises from the lower part of the axilla.
- It enters and passes through the quadrangular space.
Termination:
- It terminates by dividing into two terminal branches: the anterior branch and the posterior branch.
Clinical Relevance:
- It is commonly involved in clinical cases involving dislocations of the shoulder or fractures occurring at the surgical neck of the humerus.

Introduction:
- This is the primary nerve serving the front (anterior compartment) of the arm.
Origin:
- It is a branch derived from the lateral cord of the brachial plexus.
- Its nerve root values are $C5-C7$ .
- It arises in the lower part of the axilla, where it accompanies the third part of the axillary artery.
Course:
- It enters the anterior compartment of the arm by piercing through the coracobrachialis muscle.
- It then runs downwards and laterally, situated between the biceps brachii and the brachialis muscles.
Termination:
- It terminates approximately $2\,cm$ above the bend of the elbow.
- At this point, it continues into the forearm as the lateral cutaneous nerve of the forearm.

Course and Relational Anatomy:
- The median nerve maintains a close relationship with the brachial artery throughout its entire course in the arm.
- Upper part of the arm: The nerve lies lateral to the brachial artery.
- Middle of the arm: The nerve crosses the artery from the lateral side to the medial side.
- Lower part of the arm: The nerve remains on the medial side of the artery until it reaches the elbow.
Course in the Hand:
- It enters the palm by running deep to the flexor retinaculum within the carpal tunnel.
Termination:
- It terminates by dividing into two sets of branches:
  - Muscular branches: Supplies the thenar muscles.
  - Cutaneous branches: Supplies the lateral $3 + 1/2$ digits (including the thumb).

Introduction:
- Known as the main nerve of the hand.
Course:
- It runs on the medial side of the brachial artery until it reaches the level of the insertion of the coracobrachialis muscle.
- At this level, it pierces the medial intermuscular septum to enter the posterior compartment of the arm.
Course in the Hand:
- Unlike the median nerve, it lies superficial to the flexor retinaculum.
Termination:
- It terminates by dividing into:
  - Superficial branch: Provides cutaneous supply to the medial $1 + 1/2$ fingers (specifically the little finger and the medial side of the ring finger).
  - Deep branch: Supplies the hypothenar eminence.

Introduction:
- It is the largest branch of the posterior cord of the brachial plexus.
- Its root values are $C5-C8$ .
Course:
- At the origin of the brachial artery, the radial nerve is positioned posterior to the artery.
- It exits the vicinity of the artery by entering the radial (spiral) groove located on the posterior aspect of the arm.
- Inside the spiral groove, it is accompanied by the profunda brachii artery.
- It runs behind the third part of the axillary artery and descends down toward the lateral epicondyle, ultimately entering the cubital fossa.
Termination:
- Its deep terminal branch is known as the posterior interosseous nerve.
- This branch enters the posterior compartment of the forearm by passing through the supinator muscle.

Functional Loss:
- Loss of arm abduction from the beginning of the movement ( $0^{\circ}$ ) up to $90^{\circ}$ .
Sensory Loss:
- Sensory loss occurs over the lower half of the deltoid muscle.
- This specific area of clinical presentation is referred to as the regimental sign or badge sign.

Root Involvement: $C5$ , $C6$ , and $C7$ .
Sites of Deficit: Front of the arm.
Motor Effects:
- Paralysis of the biceps brachii and brachialis muscles.
Sensory Effects:
- Sensory loss on the lateral side of the forearm (due to the terminal lateral cutaneous nerve of the forearm).

Root Involvement: $C5$ , $C6$ , $C7$ , $C8$ , and $T1$ .
Functional and Motor Deficits:
- Injury in the Axilla: Leads to no extension of the elbow (paralysis of the triceps).
- Wrist Drop: This is the hallmark sign, caused by paralysis of the extensor muscles. It can occur due to injury in the axilla, the radial sulcus, the anterolateral side of the lower arm, or the deep branch in the cubital fossa.
Sensory Loss:
- Loss of sensation on the dorsum of the hand.

Root Involvement: $C5$ , $C6$ , $C7$ , $C8$ , and $T1$ .
General Injury Manifestations:
1. Weak flexion of the wrist.
2. Loss of forearm pronation.
3. Loss of flexion at the proximal interphalangeal and distal interphalangeal joints of the index and middle fingers.
4. Loss of flexion at the interphalangeal joint of the thumb.
5. Loss of the thenar eminence (atrophy).
6. Sensory, trophic, and vasomotor changes.
Carpal Tunnel Syndrome:
- Caused by the compression of the median nerve under the flexor retinaculum.
- Clinical Presentation:
  - Paralysis of thenar eminence muscles.
  - Resulting deformity is called 'ape-like' or 'monkey-like' hand.
  - Sensory loss in the lateral $3\,1/2$ digits including the nail beds.
  - Clawing of the index and middle fingers.

Root Involvement: $C7$ , $C8$ , and $T1$ .
Injury Manifestations:
1. Flattening of the medial border of the forearm.
2. Loss of flexion at the distal interphalangeal joints of the $4th$ and $5th$ digits.
3. Loss of the hypothenar eminence.
4. Loss of adduction of the thumb (due to adductor pollicis paralysis).
5. Loss of abduction of all fingers EXCEPT the little finger.
6. Loss of adduction of all fingers (interossei paralysis).
7. Slight clawing of the $2nd$ and $3rd$ digits (index and middle).
8. Marked clawing of the $4th$ and $5th$ digits (ring and little fingers).
9. Sensory, trophic, and vasomotor changes.
Cubital Tunnel Syndrome:
- Mechanism: Entrapment of the ulnar nerve between the two heads of the flexor carpi ulnaris muscle.
- Paralysis involves:
  - Medial half of the flexor digitorum profundus.
  - Muscles of the hypothenar eminence.
  - All interossei muscles.
  - Adductor pollicis.
  - $3rd$ and $4th$ lumbricals.
- Result: Clawing of the medial two digits and loss of the hypothenar eminence.

Ulnar Nerve: Medial side of the hand (anterior and posterior surfaces), including the little finger and half of the ring finger.
Radial Nerve: Primarily the lateral portion of the dorsum (posterior surface) of the hand and the thumb base.
Median Nerve: Lateral aspect of the palm (anterior surface), lateral $3\,1/2$ digits, and the posterior tips (nail beds) of those same digits.