Brachial Plexus
On this page
Direct answer
Erb's palsy is an upper trunk (C5–C6) injury producing the waiter's tip posture — arm adducted and medially rotated, elbow extended, forearm pronated, wrist flexed — classically from downward traction of the shoulder at birth. Klumpke's palsy is a lower trunk (C8–T1) injury producing claw hand, adding Horner syndrome when T1 preganglionic fibres are involved. Between these poles, FMGE questions turn on branches by root value — long thoracic, axillary, musculocutaneous — and their one-sign signatures such as winged scapula.
What you must remember
- Five roots C5–T1 form upper, middle and lower trunks; each trunk splits into divisions that reunite as lateral, posterior and medial cords, named for their position around the second part of the axillary artery.
- Upper trunk injury (Erb–Duchenne): biceps jerk lost, abductors and lateral rotators paralysed, giving the waiter's tip or policeman's tip posture.
- Lower trunk injury (Klumpke): claw hand with ulnar sensory loss, plus ptosis, miosis and anhidrosis when T1 sympathetic outflow is damaged — think violent upward traction or a Pancoast tumour.
- Long thoracic nerve (C5–C7) to serratus anterior: winged scapula, inability to push off a wall, loss of the last 30 degrees of overhead abduction.
- Musculocutaneous nerve (C5–C7): coracobrachialis, biceps, brachialis, continuing as the lateral cutaneous nerve of the forearm.
- Axillary nerve (C5–C6, posterior cord) passes through the quadrangular space with the posterior circumflex humeral artery to deltoid and teres minor.
- Medial cord gives the medial pectoral nerve, medial cutaneous nerves of arm and forearm, the ulnar nerve and the medial root of the median nerve; the lateral cord gives the lateral pectoral, musculocutaneous and the lateral root of the median; the posterior cord runs upper subscapular, thoracodorsal, lower subscapular, then axillary and radial nerves in order.
- A cervical rib compresses the lower trunk (thoracic outlet syndrome): C8–T1 sensory loss, thenar wasting, positive Adson test.
How to work through a newborn arm posture
A baby after a difficult vertex delivery holds the right arm close to the trunk, medially rotated, forearm pronated, hand flexed; the grasp reflex is preserved. Reason movement by movement. Medial rotation and adduction mean the lateral rotators and abductors — all C5–C6 muscles — are silent; the pronated forearm adds loss of biceps as a supinator. Preserved finger flexion and grasp show C8–T1 is intact, excluding Klumpke's; the absent biceps jerk with a normal grasp completes an upper trunk lesion: Erb's palsy. If the same baby instead had a claw hand held upward with a small pupil on that side, the same logic descends to the lower trunk and the sympathetic chain. The stem always hides the level in one or two spared movements, so identify what still works before naming what does not.
Where the exam sets traps
Two landmarks share one name. Erb's point in plexus anatomy is the junction of the C5 and C6 roots where an upper trunk injury occurs; Erb's point on the neck is the surface spot at the posterior border of sternocleidomastoid where the upper trunk emerges and becomes palpable — a block or injection question means the surface landmark. The second trap is cord naming: cords are named for their relation to the second part of the axillary artery behind pectoralis minor, not to the humerus. Candidates also forget that the phrenic nerve (C3–C5) arises directly from the roots and is not a plexus branch — the reason it is spared in most upper trunk injuries.
Frequently asked questions
Which injury produces the waiter's tip posture?
An upper trunk lesion of the brachial plexus (C5–C6), Erb's palsy, from forceful downward traction of the head and shoulder.
What causes winged scapula?
Long thoracic nerve palsy (C5–C7) paralysing serratus anterior, seen after axillary node surgery or a blow to the chest wall.
Which plexus lesion is associated with Horner syndrome?
A lower trunk (C8–T1) lesion involving preganglionic T1 sympathetic fibres, as in Klumpke's palsy or a Pancoast tumour.
Around which part of the axillary artery are the cords named?
The second part, behind pectoralis minor — lateral, posterior and medial describe their position relative to that segment.
Which nerve supplies biceps and continues as a cutaneous nerve?
The musculocutaneous nerve (C5–C7), ending as the lateral cutaneous nerve of the forearm.
What is tested by thenar wasting with medial arm anaesthesia in a cervical rib?
Compression of the lower trunk and T1 root — thoracic outlet syndrome with Klumpke-type hand signs.