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Regional Anesthesia Anatomy — Deep Dive

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Regional III · 9 min read

Brachial plexus roots/trunks/divisions/cords/branches, lumbosacral plexus, fascial planes. Knowing the sonoanatomy = predicting block success.

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Map nerve roots to dermatomes + myotomes.
Identify ultrasound landmarks for major blocks.
Recall block-specific complications.
Plan needle approach by anatomic plane.

Brachial plexus — roots to branches

C5-T1 rootsupper/middle/lower trunks (above clavicle)anterior/posterior divisionslateral/posterior/medial cords (below clavicle)terminal branches (musculocutaneous, axillary, radial, median, ulnar).

Mnemonic: 'Robert Taylor Drinks Cold Beer' (Roots, Trunks, Divisions, Cords, Branches).

Interscalene targets roots/trunks covers shoulder + upper arm, misses ulnar (C8-T1).

Supraclavicular targets trunks/divisions 'spinal of the arm', entire UE.

Infraclavicular targets cords elbow/wrist.

Axillary targets terminal branches elbow/wrist (skip musculocutaneous — separate injection).

Brachial plexus anatomy: roots (C5-T1) -> trunks (upper/middle/lower) -> divisions (anterior/posterior) -> cords (lateral/posterior/medial) -> branches; mnemonic: Read Through Drinks Cold Beer; clinically relevant for block selection.

Lumbar plexus anatomy

KeyL1-L4 roots in psoas major.

Branches

iliohypogastric
ilioinguinal
genitofemoral (L1-L2)
lateral femoral cutaneous (L2-L3)
femoral (L2-L4, largest)
obturator (L2-L4)

Femoral n. innervates anterior thigh + medial leg via saphenous.

Saphenous (terminal branch of femoral) is purely sensory below knee — adductor canal block isolates this.

Obturator innervates medial thigh adductors.

Posterior approach lumbar plexus block ('psoas compartment') covers all 3 major branches.

Sciatic nerve anterior approach: needle traverses quadriceps and adductors to reach sciatic nerve medial to femur; alternative to posterior approach when patient cannot be positioned prone or laterally.

Sacral plexus

KeyL4-S4 roots converging to sciatic nerve.

Sciatic n. = tibial + common peroneal in single sheath until popliteal fossa where it bifurcates.

Tibial plantar foot.

Common peroneal dorsum + lateral.

Popliteal sciatic block: target above bifurcation (visible split = better spread).

Pudendal n.

(S2-S4) for perineal blocks.

Posterior femoral cutaneous innervates posterior thigh — common reason for partial anesthesia in 'just sciatic' blocks.

Sacral plexus (L4-S4) anatomy: superior/inferior gluteal, sciatic, posterior femoral cutaneous and pudendal nerves; foundation for sciatic, pudendal, and posterior lumbar approaches.

Fascial plane anatomy — the new wave

TAP (transversus abdominis plane): between IO and TA — classical lateral approach covers T10-L1 abdominal wall; subcostal TAP variant extends coverage to T6-L1 for upper abdominal incisions.

ESP (erector spinae plane): deep to ESP muscle on transverse process — covers thoracic + abdominal segments by paravertebral spread.

QL (quadratus lumborum): various approaches (anterior, lateral, posterior) — analgesia for lower abdomen + hip.

Pec1 + Pec2: between pec major/minor + serratus — breast + chest wall.

Serratus anterior plane: between serratus + lat dorsi — T2-T9 chest wall.

These spread LA via fascial planes; analgesia not surgical-grade anesthesia.

Erector spinae plane (ESP) block: ultrasound-guided fascial plane block at transverse process for paravertebral spread; analgesia for thoracic, breast, abdominal, hip surgery. Local anesthetic 20-30 mL ropivacaine 0.25-0.5%.

Sonoanatomy landmarks

KeyInterscalene: ASM + MSM with brachial plexus 'stoplight' (3 hypoechoic dots) between.

Supraclav: 'cluster of grapes' lateral to subclavian artery.

Infraclav: cords surrounding axillary artery in cluster.

Femoral: lateral to femoral artery, deep to fascia iliaca.

Popliteal: sciatic 5-7 cm above popliteal crease — bifurcation visible.

ESP: transverse process tip + trapezius/rhomboid/ESP muscles overhead.

TAP: triple-line 'sandwich' EO/IO/TA at mid-axillary line.

Hydrodissection with 1-2 mL saline confirms correct plane before LA injection.

Dermatomes for surgery-block mapping

Key landmarks

C4 clavicle
T4 nipples
T6 xiphoid
T10 umbilicus
L1 inguinal
S2-S4 perineum

For each surgery: shoulderC5-C6 (interscalene + cervical plexus for skin), handC6-T1 (axillary or supraclav), kneeL2-L4 + obturator + sciatic posterior, footL4-S2 (popliteal sciatic ± saphenous), inguinal herniaT12-L1 (ilioinguinal/iliohypogastric or TAP), thoracotomyT2-T10 (paravertebral or epidural).

Dermatomes key landmarks: C4 shoulder, T4 nipple, T6 xiphoid, T10 umbilicus, L1 inguinal, S2-S4 perineum; reference for spinal/epidural block level assessment.

⚠ Common pitfalls

Injecting without confirming spread on ultrasound — high failure + LAST risk.
Forgetting that anatomic variants are common — what you 'know' is right is often slightly off.
Reaching for stimulation in a deep block when ultrasound is available — modern technique.
Skipping aspiration before injection — vascular puncture detection.

💎 Clinical pearls

Brachial plexus: roots → trunks → divisions → cords → branches ('Real Texans Drink Cold Beer').
Interscalene = shoulder; supraclavicular = arm + hand; axillary = below-elbow.
Femoral nerve under fascia iliaca, lateral to artery — short-axis ultrasound view.
ESP block: erector spinae plane covers ventral + dorsal rami; T5 for thoracotomy.

Recap

Brachial plexus: roots → trunks → divisions → cords → branches ('Real Texans Drink Cold Beer').
Interscalene = shoulder; supraclavicular = arm + hand; axillary = below-elbow.
Femoral nerve under fascia iliaca, lateral to artery — short-axis ultrasound view.
ESP block: erector spinae plane covers ventral + dorsal rami; T5 for thoracotomy.

Mark each section done to complete the module.