Axilla, Brachial Plexus, Arm & Elbow – Comprehensive Study Notes

Axilla – Macro-Landmarks, Shape & Walls

• Pyramidal transition space linking neck ↔ upper limb (“arm-pit”)
• Fat-filled; traversed by neurovascular bundles; pathway for lymphatics & breast tail
• Bony/muscular boundaries
• Apex (cervico-axillary canal)
• Bound by clavicle (anterior), 1st rib (medial), superior scapula (posterior)
• Base
• Skin + fascia of armpit; concave when arm abducted
• Four walls
• Anterior wall – pectoralis major & minor + clavipectoral fascia → anterior axillary fold
• Posterior wall – subscapularis (superior), teres major & latissimus dorsi → posterior axillary fold
• Medial wall – thoracic cage (ribs 1–4) + serratus anterior
• Lateral wall – intertubercular sulcus of humerus (long head biceps tendon runs here)
• Surface lines
• Anterior, posterior & mid-axillary lines used in clinical exam/ECG lead placement

Axillary Contents (inside the axillary sheath)

• Axillary artery & vein (continuations of subclavian → brachial respectively)
• Cords & branches of brachial plexus (lateral, posterior, medial named to artery)
• Axillary lymph-node groups (≈ $30{-}50$ nodes)
• Long thoracic nerve, intercostobrachial cutaneous n.
• Portions of biceps (short head, coracobrachialis), fat

Arterial Highway – Subclavian → Axillary → Brachial

• Subclavian a.
• Becomes axillary after crossing outer border of rib 1; pulse felt posterior to clavicle
• Axillary a. (rib 1 → inferior teres major)
• Divided by pectoralis minor into 3 parts; rich collateral anastomoses round scapula
• Gives circumflex humeral aa. (with axillary n.) ≈ $5\,\text{cm}$ inferior to acromion
• Brachial a. (inf. teres major → cubital fossa)
• Palpable medial to biceps tendon; catheterised here
• Main branch profunda brachii a. + radial n. in spiral groove (injured by mid-shaft fractures)
• Surgical relevance
• Segments of axillary a. may be clamped individually owing to scapular collaterals

Lymphatics & Nodes

• Functional roles
• Immune surveillance, pathogen clearance, debris removal, tumour defence
• Five axillary groups

Apical 2. Humeral (lateral) 3. Central 4. Subscapular (posterior) 5. Pectoral (anterior)
• Drainage territories
• Upper limb, ant/post thoracic wall, lateral breast (axillary tail)
• Clinical points
• Breast Ca → early metastasis to axillary nodes → sentinel-node biopsy, axillary clearance
• Post-operative/radiotherapy scarring → lymphoedema (non-pitting fluid accumulation)
• Surgery risk to long thoracic n. (winged scapula) & thoracodorsal n. (lat. dorsi)

Brachial Plexus – Blueprint

• Formed by anterior rami $C5–T1$ (may receive C4/T2 contributions)
• Sequence mnemonic: Roots → Trunks → Divisions → Cords → Branches (“Rugby Teams Drink Cold Beer”)
• Roots: pass between anterior & middle scalene, posterior to subclavian a.
• Trunks: Superior (C5–6), Middle (C7), Inferior (C8–T1) lying on 1st rib
• Divisions: each trunk → anterior & posterior (behind clavicle)
• Cords (around axillary a.)
• Lateral = ant divisions of sup & middle trunks (C5–7)
• Medial = ant division of inferior trunk (C8–T1)
• Posterior = all three post divisions (C5–T1)
• Terminal branches in axilla
• Musculocutaneous (lat cord) • Median (lat + med cords) • Ulnar (med cord)
• Axillary (post cord) • Radial (post cord)
• Key pre-terminal branches & clinical pearls
• Long thoracic n. (C5–7) → serratus anterior → winging of scapula if cut
• Dorsal scapular (C5) → rhomboids/lev scap → retraction weakness
• Suprascapular (upper trunk) → supraspinatus/infraspinatus → ↓ abduction start & ER
• Lateral pectoral (lat cord) → clavicular head pect maj; Medial pectoral (med cord) → sternal head & pect min

Major Peripheral Nerves – Origins, Supply, Lesions

• Musculocutaneous $C5–7$
• Motor: anterior arm (BBC – biceps, brachialis, coracobrachialis)
• Sensory: lateral forearm (lat. cutaneous n. of forearm)
• Lesion rare – weak elbow flex/supination
• Median $C5–T1$
• Forearm anterior (except FCU & medial FDP); thenar (recurrent br.); 1st–2nd lumbricals
• Sensory: palmar lat $3.5$ digits via digital br.
• Carpal tunnel syndrome: nocturnal paraesthesia, thenar wasting; palmar cutaneous br. spared
• Ulnar $C8–T1$
• Motor: FCU, medial FDP; hand intrinsics (hypothenar, interossei, 3rd–4th lumbrical, adductor pollicis)
• Sensory: medial $1.5$ digits (palmar & dorsal) + ulnar palm
• Lesion @ medial epicondyle/wrist → claw hand; “ulnar paradox” (higher lesion → less clawing)
• Radial $C5–T1$
• Motor: triceps (via axilla branches), posterior arm & forearm extensors; divides at lat epicondyle → superficial sensory & posterior interosseous (motor only)
• Sensory: posterior arm/forearm, dorsal lat $3.5$ digits (not nail beds)
• Spiral groove fracture → wrist-drop, preserved elbow extension
• Axillary $C5–6$
• Deltoid + teres minor; sensory regimental badge
• Damaged in surgical-neck fracture or GH dislocation → weak abduction $15–90^\circ$

Dermatomes & Myotomes

• Upper-limb dermatomes spiral laterally — clinically test over autonomous zones
• C5 lat arm; C6 thumb; C7 middle finger; C8 little finger; T1 med forearm
• Myotomes (key movements/reflexes)
• $C5$ shoulder abduction (deltoid) + biceps reflex with $C6$
• $C6$ wrist extension
• $C7$ elbow extension (triceps reflex) & wrist flexion
• $C8$ finger flexion
• $T1$ finger ab-/adduction (interossei)

Plexus Injury Patterns

• Upper trunk (C5–6) = Erb–Duchenne palsy
• Mechanism: neck–shoulder stretch (birth, fall)
• Waiter’s tip: adducted, IR arm; elbow extended; wrist flexed
• Lower trunk (C8–T1) = Klumpke palsy
• Upward pull on arm, cervical rib, Pancoast tumour
• Claw hand, Horner syndrome (sympathetic chain)
• Thoracic outlet syndrome – neurovascular bundle compressed by anomalous structures → paraesthesia, vessel obstruction

Arm Compartments & Transition Zones

• Anterior compartment
• Muscles: biceps brachii, brachialis, coracobrachialis
• Nerve: musculocutaneous • Artery: brachial
• Bicipital aponeurosis forms roof of cubital fossa, protects median n. & brachial a.
• Popeye sign: rupture LH biceps at supraglenoid tubercle
• Posterior compartment
• Triceps (long, lat, med heads) + anconeus
• Nerve: radial (gives branches before spiral groove)
• Artery: profunda brachii
• Important spaces
• Quadrangular: axillary n. + post circumflex humeral a.
• Triangular interval: radial n. + profunda brachii a.

Osteology & Vascular Relationships

• Surgical neck humerus # → axillary n.
• Spiral groove mid-shaft # → radial n. + profunda brachii a.
• Medial epicondyle trauma → ulnar n.
• Posterior elbow dislocation, supracondylar # (kids) → brachial a., median n. & risk of Volkmann ischaemic contracture

Elbow Joint Complex

• Compound synovial sharing one capsule
• Humeroulnar + humeroradial (hinge flex/ext)
• Proximal radioulnar (pivot pron/sup)
• Stabilising ligaments
• Medial (ulnar) & lateral (radial) collateral, annular (encircles radial head)
• Pulled-elbow (nursemaid) = sublux radial head under annular lig (children)
• Bursae
• Olecranon (subcut) → “student/miner elbow”; subtendinous triceps bursa
• Movements & innervation
• Flexion – brachialis (MC n.), biceps (MC n.), brachioradialis (radial n.)
• Extension – triceps (radial n.)
• Pronation – pronator teres/quadratus (median n.)
• Supination – biceps (MC n.), supinator (radial n.)

Cubital Fossa

• Triangular anterior elbow window
• Borders: line between epicondyles (sup), pronator teres (med), brachioradialis (lat)
• Roof: skin, fascia, bicipital aponeurosis (+ median cubital vein)
• Floor: brachialis (prox), supinator (distal)
• Contents (med → lat): Median n. | Brachial a. (→ radial & ulnar) | Biceps tendon | Radial n. (+poses mnemonic “My Blood Turns Red”)
• Clinical: venepuncture in median cubital v.; BP measurement over brachial a.

Clinical Corner – Common Lesions

• Supracondylar humerus # – may injure brachial a. & median n.; check radial pulse & OK sign
• Epicondylitis – overuse tendinopathy
• Lateral = tennis elbow (CET extensor origin)
• Medial = golfer elbow (CFT flexor origin)
• Clavicle # mid-third – possible subclavian vessels & lower plexus trunks damage; shoulder droop
• Pancoast tumour – C8/T1 root pain, hand wasting, Horner triad
• Carpal tunnel vs Ulnar tunnel (Guyon) – differentiate by palmar cutaneous sparing, claw severity
• Thoracic outlet – cervical rib/hyper-abduction/backpack → vascular & neurological symptoms

Upper-Limb Veins (brief)

• Superficial: cephalic (lat), basilic (med) joined by median cubital at elbow
• Deep: venae comitantes of arteries → axillary v.

Numerical & Reflex Reminders

• Axillary lymph nodes ≈ $30\text{–}50$
• Biceps tendon reflex $C5{+}C6$ ; Triceps reflex $C7{+}C8$
• Radial pulse assessed lateral to FCR tendon at wrist; brachial pulse ≈ midway cubital fossa

Ethical / Practical Points

• Knowledge of transition zones essential for IV lines, nerve blocks, trauma management
• During breast surgery, deliberate preservation of long thoracic & thoracodorsal nerves prevents functional morbidity
• Early recognition of compartment syndrome or vascular compromise in supracondylar # avoids limb-threatening ischaemia

Integrative Links

• Parallels with lower-limb lumbosacral plexus crossing fibres → partial loss not total paralysis in single-root lesions
• Immunology relevance: axillary nodes prime adaptive response to upper-limb infections & vaccinations (COVID-19 deltoid dose → temporary node enlargement)