The Brachial Plexus: Anatomy and Ultrasound-Guided Nerve Blocks

Anatomical Overview and Clinical Correlation^1,2

Overview

• The brachial plexus is formed from the anterior rami of C5–T1 spinal nerves.
• It is organized into Roots → Trunks → Divisions → Cords → Terminal Branches.
• The brachial plexus is located between the anterior and middle scalene muscles, passing under the clavicle and into the axilla.

Key Landmarks by Region¹

• Roots: emerge between the anterior and middle scalene muscles in the anterolateral neck
• Trunks: Converge into superior (C5-C6), Middle (C7), and Inferior (C8-T1) trunks, which course adjacent to the subclavian artery, superior and posterior to the clavicle.
• Divisions: At the level posterior to the clavicle, each trunk divides into an anterior and a posterior division.
• Cords: In the axilla, divisions regroup into lateral, posterior, and medial cords, which are named relative to the axillary artery.
• Branches: Five major terminal nerves arise in the axilla and continue into the distal arm.

As demonstrated by the dermatomal distribution figure above, the sensory component of the medial aspect of the upper arm is innervated by the intercostobrachial nerve, which is derived from T2. Therefore, nerve blocks that target the brachial plexus will not cover this region.

Roots and Trunks^1,2

• Roots (C5–T1) emerge from the spinal column and travel between the anterior and middle scalene muscles. These roots converge to form the superior trunk (C5-C6), middle trunk (C7), and inferior trunk (C8-T1).
• At this level, two popular peripheral nerve blocks can be performed to target shoulder and upper arm surgery: the interscalene block and the superior trunk block.
• It is important to recognize that two nerves come off the roots of the brachial plexus: the dorsal scapular nerve, which innervates muscles contributing to scapular elevation and stabilization, and the long thoracic nerve, which is important for scapular abduction.
• Additionally, closely adjacent to the brachial plexus, at this level, descending anterior to the anterior scalene muscle and superior to the brachial plexus roots is the phrenic nerve, which innervates the diaphragm.

Interscalene Block

• Objective: Localization of C5-C7 nerve roots to target the lateral shoulder and upper arm and classically spares the ulnar nerve (C8–T1) territory.

• Common surgeries: Shoulder arthroscopy, rotator cuff repair, lateral clavicle procedures (when supplemented), proximal humerus surgery
• Probe placement: Transverse orientation at the level of C6 (cricoid cartilage), lateral neck.
• Anatomy visualized:
  • “Stoplight” appearance: C5, C6, and C7 roots between the anterior and middle scalene muscles.
  • The carotid artery is medial, and the sternocleidomastoid muscle is superficial.

• Complications: hemidiaphragmatic paresis (100% risk), Horner syndrome, vertebral artery injection, pneumothorax

Superior Trunk Block

• Objective: Selective anesthesia of the superior trunk (C5–C6) of the brachial plexus, often used as a diaphragm-sparing alternative to the interscalene block for shoulder procedures.^3,4
• Common surgeries: Shoulder arthroscopy, rotator cuff repair, lateral clavicle, proximal humerus surgery
• Probe placement: Transverse or oblique orientation at the lateral neck, just above the clavicle, scanning caudally from an interscalene view toward the supraclavicular fossa

• Anatomy visualized:
  • Superior trunk typically appears posterior/lateral to the subclavian artery, deep to the sternocleidomastoid muscle.
  • Between the anterior and middle scalene muscles, C5 and C6 often appear as separate hypoechoic nodules; however, they coalesce into the superior trunk as you scan caudally.

• Advantage of lower risk of phrenic nerve blockade (about 5–16.7% vs. 71-100% with interscalene)^3,4
• Complications: Horner’s syndrome, vascular injection

Divisions and Cords^1,2

Divisions

• After the formation of the three trunks, each trunk splits into an anterior and a posterior division, resulting in a total of six divisions:
  • 3 anterior divisions (one from each trunk)
  • 3 posterior divisions (one from each trunk)
• These divisions occur posterior to the clavicle and are not typically targets for regional anesthesia due to their depth and relative inaccessibility under ultrasound.
• Functional separation:
  • Anterior divisions: generally supply the flexor compartments of the upper extremity.
  • Posterior divisions: generally supply the extensor compartments.
• No terminal branches arise directly from the divisions, but they serve as a transition point toward the cords.
• At the distal end of the trunks and level of the divisions is the location of the supraclavicular nerve block.

Supraclavicular Block

• Objective: Localization of the trunks and divisions of the brachial plexus to provide dense anesthesia of the upper extremity distal to the shoulder; often referred to as the “spinal of the arm” due to its rapid onset and high efficacy
• Common surgeries: Upper extremity procedures involving the arm, elbow, forearm, and hand
• Probe placement: Transverse orientation in the supraclavicular fossa, superior to the clavicle and lateral to the clavicular head of the sternocleidomastoid

• Ultrasound anatomy:
  • The subclavian artery serves as the key landmark.
  • The brachial plexus appears as a compact, hypoechoic cluster (“bundle of grapes”) posterolateral to the artery.
  • The first rib and pleura form a deep boundary.

• Advantages: Rapid onset and dense block with relatively low volume; consistent coverage of terminal branches for surgery below the shoulder
• Complications: High risk of phrenic nerve palsy (50–67%), possible pneumothorax, vascular puncture due to proximity to subclavian vessels, and pleura

Cords

• The cords are named based on their position relative to the axillary artery and are formed by the recombination of divisions:
  • Lateral cord: formed by the anterior divisions of the superior and middle trunks (C5–C7)
  • Posterior cord: formed by the union of all three posterior divisions (C5–T1)
  • Medial cord: continuation of the anterior division of the inferior trunk (C8–T1)
• The cords are located in the infraclavicular fossa, deep to the pectoralis minor muscle.
• Several clinically relevant nerve branches originate here:
  • Lateral cord: gives rise to the musculocutaneous nerve and part of the median nerve
  • Posterior cord: gives rise to the axillary and radial nerves
  • Medial cord: gives rise to the ulnar nerve, medial cutaneous nerves, and part of the median nerve
• At this level, the most popular peripheral nerve block targeting the cords is the infraclavicular block.

Infraclavicular Block

• Objective: Localization of the lateral, posterior, and medial cords of the brachial plexus to anesthetize the distal upper extremity, including the elbow, forearm, and hand; typically spares proximal shoulder innervation
• Common surgeries: Distal upper extremity surgery (below the shoulder), including elbow, forearm, and hand procedures
• Probe placement: Transverse (more like an oblique longitudinal) orientation just below the clavicle in the deltopectoral groove

• Ultrasound anatomy:
  • Cords appear as hyperechoic oval or triangular structures immediately lateral to the axillary artery. The lateral cord is the most superficial, the medial cord is deepest, and the posterior cord is the most lateral.

• Advantages: Lower risk of phrenic nerve blockade (0-11%**) compared to interscalene and supraclavicular blocks
• Complications: Pneumothorax (rare but possible with high needle insertion or poor visualization)

Terminal Branches^1,2

Terminal Branch Blocks

Axillary Block

• Objective: Localization of the terminal branches of the brachial plexus at the level of the axilla. The musculocutaneous nerve typically branches proximally and may require separate blockade. The axillary nerve cannot be blocked with this approach due to its departure towards the posterior shoulder proximal to the axilla.
• Common surgeries: Forearm, wrist, and hand procedures; suitable for the distal upper extremity that does not include the shoulder or upper arm.
• Probe placement: Transverse orientation in the axillar, perpendicular to the arm with the arm abducted to 90 degrees.

• Ultrasound anatomy:
  • The axillary artery serves as the primary landmark and appears as a pulsatile anechoic circle. The median, ulnar, and radial nerves are visualized surrounding the artery, while the musculocutaneous nerve lies within the coracobrachialis muscle, superficial and lateral to the artery, and often requires a separate injection.

• Advantages: Safe and superficial approach; minimal risk of pneumothorax or phrenic nerve blockade
• Complications: Incomplete block if the musculocutaneous nerve is missed

Selective Nerve Blocks

• Objective: Selective blockade of individual terminal branches of the brachial plexus to provide targeted anesthesia or analgesia to specific regions of the forearm, wrist, or hand; useful for distal procedures and motor-sparing techniques. They can also be used as “rescue blocks” for incomplete brachial plexus blocks.
• Common surgeries: Hand and wrist procedures, digit surgery, carpal tunnel release, trigger finger release, and selective nerve injury management
• Median nerve: Appears as an oval or honeycomb-shaped structure medial to the brachial artery in the antecubital fossa
  • Probe placement: Transverse orientation over the anterior (volar) forearm at the mid-forearm or just proximal to the wrist.

• Ulnar nerve: Medial to the ulnar artery at the distal forearm; superficial and medial to the flexor carpi ulnaris tendon at the wrist. Tracks posteriorly from the brachial artery
  • Probe placement: Transverse orientation over the medial forearm at the mid-forearm or just proximal to the wrist near the ulnar artery.

• Radial nerve: Typically leaves the spiral groove of the humerus and then runs between the brachialis and brachioradialis.
  • Probe placement: Transverse orientation over the posterior lateral distal arm or forearm, typically at the spiral groove or just proximal to the lateral epicondyle.