Brachial Plexus Blocks – Anatomy
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Direct answer
Where you place the needle on the brachial plexus decides which roots, trunks, divisions, cords or branches you block. An interscalene injection between the anterior and middle scalene muscles at C6 catches predominantly C5-C7 — ideal for shoulder surgery, and famous for 100% ipsilateral phrenic nerve paresis. A supraclavicular block flattens the divisions as they cross the first rib — the "spinal of the arm", covering most of the upper limb at the price of pneumothorax risk from the lung apex behind. An infraclavicular block targets the cords around the axillary artery (classically lateral, posterior and medial at the 9, 6 and 3 o'clock positions on ultrasound), while an axillary block surrounds the terminal branches — and characteristically misses the musculocutaneous nerve, which has already left for coracobrachialis. Ultrasound anatomy has replaced paraesthesia-seeking, but the landmark anatomy is exactly the dissection you already know from the cadaver.
What you must remember
- Interscalene: between anterior and middle scalene at the level of C6 (Chassaignac's tubercle, cricothyroid plane); blocks C5-C7 reliably; misses C8-T1, so it fails the ulnar side; phrenic paresis virtually universal, Horner syndrome and recurrent laryngeal block recognised side effects — avoid in severe respiratory disease.
- Supraclavicular: the plexus lies postero-lateral to the subclavian artery on the first rib; the dome of the pleura lies behind — pneumothorax is the classical complication; the "spinal of the arm" for mid-forearm-to-hand surgery.
- Infraclavicular: cords around the axillary artery below the clavicle at the coracoid; a single injection posterior to the artery spreads to all three cords — good for elbow, forearm and hand surgery with a catheter-friendly site.
- Axillary: terminal branches within the axillary sheath — around the artery, the mnemonic "MARMU" (median anterior, radial posterior, musculocutaneous lateral outside in coracobrachialis, ulnar medial); the musculocutaneous nerve is the classic miss unless separately blocked.
- Divisions save the day: at the supraclavicular level the block is at divisions, so the coverage is near-complete limb; at interscalene, roots (shoulder); at infraclavicular, cords (forearm-hand emphasis).
- Complication ladder: local anaesthetic systemic toxicity (perioral tingling, tinnitus, seizures, arrhythmia) from intravascular injection — treat with 20% lipid emulsion; vertebral artery or intrathecal injection at interscalene level causing immediate CNS signs; Horner syndrome from stellate spread.
- Nerve of Latarget: the thoracodorsal or the long thoracic? — quote the anatomy honestly: the long thoracic nerve (bell's palsy of serratus, winged scapula) arises from C5-C7 roots and can be caught by interscalene spread.
Choosing the level for one operation
Take three operations in one morning list. First, a shoulder arthroscopy in a 30-year-old: interscalene — the shoulder is C5-C6 territory, an interscalene catheter gives perioperative analgesia, and you counsel the patient about a hoarse voice and a droopy eyelid (Horner's) as expected events, while excluding severe COPD because one hemidiaphragm will be paralysed. Second, a distal radius fracture fixation in an 80-year-old on rivaroxaban: an axillary or infraclavicular block — away from the pleura and the neuraxis, compressible if punctured — with ultrasound showing the median, ulnar and radial nerves around the artery; you deliberately infiltrate coracobrachialis for the musculocutaneous nerve since lateral forearm sensation lives there. Third, a hand tendon repair where the surgeon wants stillness for an hour: supraclavicular, one injection, fast and dense, with the probe angled to keep the first rib and pleura in view throughout. Same plexus, three levels, three risk profiles — the decision is pure topographic anatomy plus patient comorbidity, and the viva marks exactly that reasoning.
Where the viva probes
The exam favourite is the missed nerve question: "which nerve is classically spared by an axillary block and why?" — the musculocutaneous, because it leaves the sheath to pierce coracobrachialis an average of a few centimetres distal to the coracoid; the fix is a separate injection into the muscle. The second probe maps complications to levels: phrenic and Horner at interscalene (both sympathetic chain and phrenic lie on anterior scalene), pneumothorax at supraclavicular, intravascular risk with the axillary artery at every level, and vertebral artery injection at interscalene producing instant seizure — the reason for aspiration and incremental injection. Indian anaesthesia vivas also ask for cord positions at the infraclavicular level; answer with the ultrasound clock-face (lateral, posterior, medial around the artery) and add the caveat that positions vary, which is precisely why ultrasound guidance replaced blind coracoid techniques.
Frequently asked questions
Which block is best for shoulder surgery?
The interscalene block at C5-C7, with the caveat of near-universal ipsilateral phrenic paresis and Horner syndrome as accepted side effects.
Why does an axillary block often miss the musculocutaneous nerve?
Because that nerve leaves the axillary sheath early to pierce coracobrachialis, so it must be blocked separately within the muscle.
Which brachial plexus block carries pneumothorax risk?
The supraclavicular block, since the plexus lies on the first rib with the pleural dome immediately posterior.
What are the cord positions for an infraclavicular block?
Around the axillary artery — classically lateral, posterior and medial cords at about the 9, 6 and 3 o'clock positions, though ultrasound shows frequent variation.
How is local anaesthetic systemic toxicity treated?
Stop injection, call for help, suppress seizures, and give 20% lipid emulsion bolus followed by infusion — the specific antidote to bupivacaine cardiotoxicity.