Brachial plexus anatomy

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Why this matters to the anaesthetist

The brachial plexus is the single most frequently blocked nerve structure in regional anaesthesia. Every upper-limb block — shoulder, arm, forearm, hand — is an exercise in knowing which stage of the five-stage sequence sits at which anatomical location, which nerves will and will not be covered, and which vital structures sit next door (phrenic nerve, pleura, subclavian artery, vertebral artery). The viva rewards exact relations and block-choice logic, not generic "block the plexus" answers [1][2].

The five-stage organisation (Read That Damn Cadaver)

Prefixed plexus = C4 contribution; postfixed = T2 contribution. Both alter dermatome maps slightly and are best recognised under ultrasound [7].

Exact relations along the course

Terminal branches — origin, motor, sensory

The four classic ultrasound-guided blocks

Ultrasound sonoanatomy — exam points

• Interscalene: linear probe, short-axis, mid-neck. Scalenus anterior and medius form a "stoplight" of 2–3 hypoechoic roots. Identify carotid artery medially; keep needle tip lateral to vessels. Greater auricular nerve may be seen superficial over scalenus medius [8].
• Supraclavicular: probe in supraclavicular fossa parallel to clavicle. Subclavian artery is the landmark; plexus sits superolateral ("grapes"); first rib and pleural line are deep — never inject below the rib line.
• Axillary: probe high in axilla short-axis to artery. Median (superficial), ulnar (medial), radial (deep/posterior) around artery; musculocutaneous as hyperechoic oval in coracobrachialis — find it deliberately or the lateral forearm wakes up [7].

Injuries examiners love

• Erb's palsy (upper trunk C5–C6): waiter's tip — arm adducted, internally rotated, elbow extended, forearm pronated, wrist flexed. Birth traction or motorcycle fall.
• Klumpke's palsy (lower trunk C8–T1): claw hand ± ipsilateral Horner syndrome if T1 sympathetics involved [6].
• Inadvertent intraneural / arterial injection and local anaesthetic systemic toxicity are procedure hazards, not plexus anatomy — but the vertebral artery proximity at interscalene makes seizure risk real.

Variation that changes the block

Musculocutaneous nerve may pierce coracobrachialis late, run with the median nerve, or send a communicating branch — the commonest reason an axillary block misses the lateral forearm [7]. Prefixed/postfixed plexuses and anomalous cord formation are recognised under ultrasound before injection.

SAQ answer scaffold

1. Draw/list the five stages (roots → trunks → divisions → cords → branches) with root contributions.
2. Name the five terminal branches with cord of origin, motor and sensory.
3. For a given operation (e.g. shoulder arthroscopy vs wrist ORIF), choose the block and justify coverage and risks.
4. Explain why interscalene causes hemidiaphragmatic paresis and how to mitigate.
5. State how you would complete an axillary block if the lateral forearm remains intact. [1]

Viva stem bank and model phrases

• "Describe the course of the brachial plexus from foramina to axilla." → "C5–T1 roots unite in the interscalene groove between scalenus anterior and medius; trunks cross the posterior triangle over the first rib; divisions form behind the clavicle; cords surround the axillary artery; terminal branches arise in the axilla."
• "Why prefer costoclavicular over classic infraclavicular?" → "Cords are more tightly clustered between clavicle and second rib, improving single-injection spread and catheter stability [4]."
• "Patient with severe COPD needs shoulder surgery — what do you avoid?" → "Classical interscalene because of near-universal phrenic block; discuss alternative (superior trunk low-volume, general alone, or carefully counselled low-volume techniques)."

Common traps

• Thinking interscalene covers the whole arm including ulnar — it often spares C8–T1.
• Forgetting to block musculocutaneous separately at the axilla.
• Ignoring phrenic risk in COPD / obesity / contralateral phrenic palsy.
• Confusing Erb (upper trunk, waiter's tip) with Klumpke (lower trunk, claw ± Horner).
• Injecting deep to the first rib at the supraclavicular level (pleura). [1]

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Red flags

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References

1. [1]Soor B, et al. Ultrasound-guided Interscalene Block as a Safer Alternative to General Anesthesia in a Patient with Ankylosing Spondylitis Undergoing Proximal Humerus Fracture Fixation Ann Afr Med, 2026.PMID 42318963
2. [2]Verbeke AL, et al. The diaphragm-sparing effect of interscalene block with a low-volume of ropivacaine 0.1% vs. 0.5%: A double-blind, controlled, randomised trial Eur J Anaesthesiol Intensive Care, 2026.PMID 42244823
3. [3]Rani N, et al. Ultrasound-Guided Selective Trunk Block Combined With Supraclavicular Nerve Block for Awake Shoulder Arthroscopy: A Case Series Cureus, 2026.PMID 42220761
4. [4]Yazar V, et al. Comparison of Costoclavicular Block and Infraclavicular Block Effects on Tissue Oxygen Saturation in Upper Extremity Surgery: A Randomized, Assessor-Blinded Controlled Trial Diagnostics (Basel), 2026.PMID 42279582
5. [5]Solomos D, et al. Effect of Ketamine Supplementation in Axillary Plexus Blockade: A Comparative Study Cureus, 2025.PMID 41583169
6. [6]Cimilli E, et al. Cognitive and Executive Function Profiles in School‑Age Children with Obstetric Brachial Plexus Injury Phys Occup Ther Pediatr, 2026.PMID 42322142
7. [7]Lv Y, et al. An anatomical variation of the musculocutaneous nerve featuring early bifurcation and a transient common trunk with the median nerve: a case report Folia Morphol (Warsz), 2026.PMID 42345218
8. [8]Kabra N, et al. Asymmetrical Brachial Plexus Involvement with Greater Auricular Nerve Involvement in Borderline Tuberculoid Leprosy: An Electrophysiological and Radiological Correlation Indian J Radiol Imaging, 2026.PMID 42344197