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NEW Written to support the Anatomy & Regional MCQ deck — the plexus, its branches, the four blocks and the injury patterns.

The brachial plexus

Roots to branches, the four block sites and what each one misses, and the classic injury patterns.

The architecture

Real Texans Drink Cold Beer — Roots, Trunks, Divisions, Cords, Branches. Formed from the anterior rami of C5–T1.

ROOTS TRUNKS DIVISIONS CORDS BRANCHES C5 C6 C7 C8 T1 UPPER MIDDLE LOWER anterior posterior ×3 anterior LATERAL POSTERIOR MEDIAL Musculocut. Median (lat root) Axillary Radial Median (med root) Ulnar All three posterior divisions unite to form the posterior cord — the only cord with a single source. The median nerve is the only terminal branch with two roots, one from the lateral and one from the medial cord.

Where each branch comes off

LevelBranchSupplies / clinical point
RootsDorsal scapular (C5)Rhomboids, levator scapulae
RootsLong thoracic (C5,6,7)Serratus anterior — "C5,6,7 keeps the wing from heaven"; injury gives winged scapula
Upper trunkSuprascapularSupra/infraspinatus, and ~70% of the shoulder capsule — the phrenic-sparing shoulder block
Upper trunkNerve to subclaviusThe only other trunk-level branch
Lateral cordLateral pectoral · musculocutaneous · lateral root of medianMusculocutaneous is lateral cord — the classic exam trap
Posterior cordUpper & lower subscapular · thoracodorsal · axillary · radialULTRA — the posterior cord mnemonic
Medial cordMedial pectoral · medial cutaneous nerves of arm & forearm · ulnar · medial root of medianC8–T1 territory — the fibres an interscalene block misses

The four blocks — and what each one misses

BlockLevel & landmarkCovers / misses · main risk
InterscaleneRoots / upper trunk. Between scalenus anterior and medius, at about C6Shoulder. Misses C8–T1 (ulnar sparing). Phrenic palsy approaching 100% — never bilateral, avoid in severe respiratory disease. Horner's, hoarseness, vertebral artery injection
SupraclavicularDivisions. Lateral to the subclavian artery, on the first ribThe whole arm below the shoulder — the "spinal of the arm". Most compact point of the plexus. Pneumothorax; phrenic palsy ~30–50%
InfraclavicularCords. Deep to pectoralis minor, around the axillary arteryElbow and below. Deep but stable for a catheter. Vascular puncture, pneumothorax
AxillaryTerminal branches. Around the axillary artery in the axillaForearm and hand. Misses musculocutaneous (already in coracobrachialis) and intercostobrachial (T2, tourniquet pain) — block both separately. Safest for pleura

The axillary clock face

  • Around the axillary artery: median superficial and lateral, ulnar medial, radial posterior.
  • The musculocutaneous has already left the sheath and lies in the body of coracobrachialis — a separate injection.

Injury patterns

Erb's palsy — C5, C6 (upper trunk)

  • "Waiter's tip": arm adducted, internally rotated, elbow extended, forearm pronated.
  • Loss of deltoid, biceps, brachialis, supraspinatus, infraspinatus, brachioradialis.
  • Cause: downward traction on the shoulder — shoulder dystocia, a fall onto the shoulder, or excessive arm abduction under anaesthesia.

Klumpke's palsy — C8, T1 (lower trunk)

  • Claw hand from loss of the intrinsic muscles, with sensory loss on the medial forearm and hand.
  • Horner's syndrome may accompany it, because T1 carries the sympathetic supply to the head.
  • Cause: upward traction on the abducted arm — a breech delivery, or grabbing a support during a fall.

Perioperative plexus injury — the point that matters in the exam

  • Most perioperative nerve injury is positional, surgical or tourniquet-related — not the block. Keep abduction under 90°, avoid extension and external rotation, pad the arm boards.
  • Sternal retraction in cardiac surgery injures the lower plexus, sometimes with first rib fracture.
  • Document the pre-block neurological state, the technique, and the timing of onset. A deficit present on waking is rarely the block.
  • Seddon: neurapraxia (conduction block, full recovery) → axonotmesis (axon disrupted, endoneurium intact, regrows ~1 mm/day) → neurotmesis (complete transection). Deficit beyond 6 weeks warrants nerve conduction studies.

Quick recall

C5–T1 anterior rami
Roots between scalenus anterior & medius
Divisions behind the clavicle — no block there
Cords named for the axillary artery
ULTRA posterior cord branches
C5,6,7 keeps the wing from heaven
C3,4,5 keeps the diaphragm alive
Musculocutaneous = LATERAL cord