Relations of the Brachial Plexus
The brachial plexus has a supraclavicular part (roots and trunks) in the neck and an infraclavicular part (cords and branches) in the axilla. Its relations differ accordingly.
Supraclavicular Part — Relations in the Neck
Roots
- Located in the posterior triangle of the neck, deep to the scalenus anterior muscle
- Roots emerge between the scalenus anterior (anteriorly) and scalenus medius (posteriorly)
Trunks
- The three trunks occupy the cleft between scalenus medius (behind) and scalenus anterior (in front)
- Pass over the 1st rib to enter the axilla through the cervico-axillary canal
Relations of the Supraclavicular Part
| Direction | Relation |
|---|---|
| Anteriorly | Skin, platysma, investing fascia, sternocleidomastoid muscle |
| Posteriorly | Scalenus medius; serratus posterior superior; longissimus cervicis |
| Medially | Scalenus anterior (separating the roots from the subclavian artery in the scalene triangle) |
| Superiorly | Subclavian artery (runs in the scalene triangle between the scalenus anterior and medius, medial to the roots) |
The subclavian artery lies between the brachial plexus and the subclavian vein at the root of the neck — the vein is anterior to scalenus anterior.
Cervical Rib — Clinical Relevance
- A cervical rib (arising from C7 vertebra) may compress the lower trunk (C8, T1) of the brachial plexus as it passes over the rib
- Produces features of lower trunk compression (Klumpke-type symptoms) + vascular compression of the subclavian artery → thoracic outlet syndrome
Infraclavicular Part — Relations in the Axilla
Entry into the Axilla
The brachial plexus (as cords) enters the axilla through the apex (cervico-axillary canal) together with the axillary artery, enclosed in the axillary sheath (derived from prevertebral fascia).
Relations of Cords to the Axillary Artery
The cords are named by their position relative to the second part of the axillary artery (posterior to pectoralis minor):
| Cord | Position |
|---|---|
| Lateral cord | Lateral side of axillary artery |
| Medial cord | Medial side of axillary artery |
| Posterior cord | Posterior to axillary artery |
Relations by Part of the Axillary Artery
| Part of Axillary Artery | Relation to Brachial Plexus |
|---|---|
| 1st part (medial to pectoralis minor; above it) | All three cords lie posteriorly |
| 2nd part (posterior to pectoralis minor) | Cords in their named positions: lateral, medial, posterior |
| 3rd part (lateral/inferior to pectoralis minor) | Cords give off their terminal branches which surround the artery |
Cunningham: “The cords first lie posterior to the first part of the axillary artery, but lower down posterior to the pectoralis minor, they surround the second part of the artery in positions which correspond to their names.”
Axillary Sheath
- The brachial plexus and axillary artery are enclosed together in the axillary sheath — a prolongation of the prevertebral fascia from the neck
- This forms a continuous fascial compartment from the neck into the axilla
- Anaesthetic injected into this sheath (brachial plexus block) can spread up to the scalene muscles
Floor of the Axilla (Brachial Plexus in the Axilla)
The infraclavicular brachial plexus is related to all walls of the axilla:
| Wall / Structure | Nerve Relations |
|---|---|
| Anterior wall (pectoralis minor) | Lateral and medial pectoral nerves pierce / pass around pectoralis minor |
| Posterior wall (subscapularis, teres major, latissimus dorsi) | Subscapular, thoracodorsal, axillary nerves pass to the posterior wall |
| Medial wall (serratus anterior) | Long thoracic nerve descends on the medial wall |
| Lateral wall (humerus + biceps) | Musculocutaneous nerve pierces coracobrachialis on the lateral wall |
Long Thoracic Nerve — Special Relation
- Arises from the posterior aspect of the roots (C5, C6, C7) in the neck
- Descends behind the brachial plexus and the axillary artery
- Runs on the lateral surface of serratus anterior (which it supplies)
- Its long course in the axilla makes it vulnerable to surgical injury during axillary dissection for breast cancer
Intercostobrachial Nerve
- The lateral cutaneous branch of the 2nd intercostal nerve — not a brachial plexus branch
- Pierces the lateral thoracic wall and crosses the axilla to supply the skin of the medial side of the arm and the axilla
- Communicates with the medial cutaneous nerve of the arm (from medial cord)
- At risk during axillary dissection — its division causes anaesthesia/dysaesthesia of the medial upper arm
Structures at Risk During Axillary Surgery
During axillary lymph node dissection (for breast cancer), the following nerves in the axilla are at risk:
| Nerve | Risk | Consequence |
|---|---|---|
| Intercostobrachial nerve | Division | Anaesthesia/pain along medial arm |
| Long thoracic nerve | Division | Winging of scapula |
| Thoracodorsal nerve | Division | Weakness of latissimus dorsi — impaired adduction + medial rotation of arm |
| Thoracodorsal artery | Damage | Bleeding |
Viva Pearls
- Roots of brachial plexus lie between scalenus anterior (anterior) and scalenus medius (posterior)
- Cords named by relation to 2nd part of axillary artery
- All three cords lie posterior to the 1st part of axillary artery
- Axillary sheath = prevertebral fascia prolongation; used for brachial plexus block
- Long thoracic nerve descends behind the plexus → on lateral surface of serratus anterior
- Cervical rib compresses lower trunk (C8, T1) → Klumpke-type + vascular = thoracic outlet syndrome
- Intercostobrachial nerve = lateral cutaneous of 2nd intercostal nerve; medial arm skin; at risk in axillary dissection
- Structures at risk in axillary dissection: intercostobrachial nerve, long thoracic nerve, thoracodorsal nerve