Brachial plexus

Overall plan

The brachial plexus is formed by the ventral rami of C5, C6, C7, C8 and T1. It is reorganised in five stages: roots, trunks, divisions, cords and terminal branches. The roots and trunks lie in the neck (supraclavicular part); the divisions lie behind the clavicle; the cords and branches lie in the axilla (infraclavicular part).

• The cords are the part of the plexus found in the axilla, and they are named by their position relative to the axillary artery, not the clavicle or the ribs.
• Anterior divisions supply the flexor (anterior) compartments; posterior divisions supply the extensor (posterior) compartments. This single rule explains why the posterior cord carries the radial and axillary nerves.

Branches above the clavicle

These come from the roots and the upper trunk, so their involvement tells you a lesion is high.

• Dorsal scapular nerve (C5 root): rhomboid major and minor, levator scapulae.
• Long thoracic nerve (C5, C6, C7 roots): serratus anterior. It runs down the medial wall of the axilla on the outer surface of the muscle, so it is exposed in axillary surgery and to carrying heavy loads on the shoulder. Loss gives winging of the scapula when pushing against a wall.
• C5 contribution to the phrenic nerve: a high C5 avulsion may paralyse part of the hemidiaphragm.
• Suprascapular nerve (upper trunk, C5–6): supraspinatus and infraspinatus, and sensory twigs to the shoulder joint; passes through the suprascapular notch below the superior transverse scapular ligament.
• Nerve to subclavius (upper trunk, C5–6).
• The middle and lower trunks give no named branches.

Cords and their branches

• The median nerve is formed by a lateral root (lateral cord) and a medial root (medial cord) that join in front of the third part of the axillary artery. So the medial root of the brachial plexus contributes to the median nerve.
• The cord formed from two anterior divisions is the lateral cord; the cord formed from all three posterior divisions is the posterior cord.
• Pectoralis minor is supplied by the medial pectoral nerve (medial cord), which pierces it; pectoralis major receives both pectoral nerves. Confusingly, the medial pectoral nerve lies lateral to the lateral pectoral nerve in the chest wall: the names come from the cords, not from their final position.
• Latissimus dorsi paralysis points to the thoracodorsal nerve, a branch of the posterior cord; it is at risk in axillary clearance.

Erb–Duchenne palsy: upper trunk, C5–C6

Cause: anything that forcibly widens the angle between the neck and the shoulder: shoulder dystocia with traction on the head, a fall onto the shoulder, motorcycle accidents. The site of injury is Erb's point, where the C5 and C6 roots unite to form the upper trunk.

• Posture ("waiter's tip"): arm adducted (abductors lost, pectoralis major unopposed), medially rotated (lateral rotators lost, subscapularis unopposed), elbow extended and forearm pronated (flexors and supinators lost).
• Sensory loss over the lateral arm and lateral forearm (C5–6). Biceps and supinator jerks are lost; in a newborn the Moro reflex is absent on that side.
• A C5/C6 injury (for example traction in a whiplash-type injury) therefore removes shoulder abduction and lateral rotation, elbow flexion and supination. Forward flexion of the shoulder (anterior deltoid, clavicular head of pectoralis major, coracobrachialis, biceps) is also C5–6, so loss of forward movement of the arm points to the upper trunk.

Klumpke palsy: lower trunk, C8–T1

Cause: the arm is forcibly pulled upwards (grabbing a branch while falling, breech delivery with the arm above the head). Slow compression of the lower trunk occurs with a cervical rib and an apical lung (Pancoast) tumour.

• All small muscles of the hand are lost (ulnar and median supply), with some weakness of the long finger flexors (C8).
• Claw hand of all fingers: metacarpophalangeal joints extended (lumbricals and interossei lost, long extensors unopposed) and interphalangeal joints flexed (long flexors pull unopposed).
• Sensory loss along the medial forearm, medial hand, ring and little fingers.
• Horner syndrome (ptosis, miosis, anhidrosis, apparent enophthalmos) if the T1 root is torn close to the spinal cord, because the preganglionic sympathetic fibres to the head leave the cord in T1 and pass up to the stellate ganglion.

Remember Erb is the "waiter's tip" of an upper-trunk stretch; Klumpke is the claw hand of a lower-trunk stretch, often with Horner syndrome.

Localising the lesion

Myotomes, dermatomes, reflexes and disc prolapse

• Cervical nerve roots exit above the vertebra of the same number (C8 exits between C7 and T1). A posterolateral disc prolapse compresses the root that exits at that level, so a C5/C6 disc compresses the C6 root: weak elbow flexion and wrist extension, reduced supinator and biceps jerks, numb thumb.
• A C6/C7 disc compresses C7: weak triceps, lost triceps jerk, numb middle finger.

Exam trap A C5/C6 disc herniation affects the C6 root, not C5: count the root that exits at that level.

Brachial plexus blocks

• The axillary sheath is a sleeve of prevertebral fascia around the artery and the plexus. The musculocutaneous nerve leaves the sheath high up, entering coracobrachialis, so an axillary block often spares it: the hand is numb but the patient can still flex the elbow (biceps, brachialis) and feel pinprick on the lateral forearm (lateral cutaneous nerve of the forearm).

Thoracic outlet and cervical rib

The lower trunk and subclavian artery cross the first rib behind scalenus anterior (the vein passes in front of it). A cervical rib or fibrous band from C7 lifts them: wasting of small hand muscles (especially thenar), paraesthesia along the medial forearm, and sometimes post-stenotic dilatation of the subclavian artery with distal emboli. Symptoms worsen with the arm raised.

Exam trap Weakness on raising the arm after surgery in the posterior triangle is usually the accessory nerve (trapezius), not the brachial plexus.

How FCPS asks about this

• Cords are named by relation to: the second part of the axillary artery; cords are the part of the plexus in the axilla.
• Cord formed by two anterior divisions: lateral cord (C5–7); posterior cord takes all three posterior divisions.
• Nerve from the upper trunk: suprascapular nerve (supraspinatus, infraspinatus); long thoracic and dorsal scapular come from roots.
• Erb palsy muscles and posture: deltoid, supraspinatus, infraspinatus, biceps, brachialis, brachioradialis, supinator; arm adducted, medially rotated, pronated.
• Klumpke palsy: claw hand, numb medial forearm, Horner syndrome if T1 avulsed; Pancoast tumour and cervical rib compress the lower trunk.
• Latissimus dorsi or pectoralis minor paralysed: thoracodorsal nerve (posterior cord); medial pectoral nerve (medial cord).
• Axillary block spares elbow flexion and lateral forearm: musculocutaneous nerve leaves the sheath high in coracobrachialis.