Musculoskeletal System
Erb's palsy and the C5 and C6 roots
Erb's palsy is damage to the C5 and C6 roots (the parts of the spinal nerves just outside the spinal cord) of the brachial plexus, the arm's nerve network.
Remember
Erb's palsy is typically a stretch injury of the top two roots, C5 and C6, which in severe cases leaves the arm hanging by the side in the waiter's tip position; damage to the bottom two roots, C8 and T1, is Klumpke's palsy, which affects the hand.
The brachial plexus is the network of nerves running from the neck into the arm. It is built from the C5, C6, C7, C8 and T1 roots (the parts of the spinal nerves just outside the spinal cord). The roots combine into trunks: C5 and C6 form the upper trunk, C7 continues alone as the middle trunk, and C8 with T1 form the lower trunk.
An upper brachial plexus injury, also called Erb's palsy, typically damages the C5 and C6 roots, though some references describe it as extending to C7. It typically follows a stretch that forces the neck and shoulder apart, such as a heavy fall onto the shoulder, or shoulder dystocia (the baby's shoulder gets stuck in the birth canal) at birth.
C5 and C6 supply the muscles that raise the arm sideways, turn it outwards and bend the elbow, so damage to these roots weakens these muscles. Pectoralis major (the large chest muscle) is largely spared, and with little pulling against it, it turns the arm inwards. The biceps is weak too, so the forearm is pronated (palm facing backwards); the wrist is also held flexed. In a severe injury, the arm therefore hangs by the side, turned inwards, in the waiter's tip position.
Sensation is reduced over the C5 and C6 dermatomes (the areas of skin each root supplies): the lateral arm, the lateral forearm and the thumb and index finger. The small muscles of the hand are spared. Damage to the lower roots, C8 and T1, causes a clawed hand instead, in Klumpke's palsy.
Memory aid
A waiter discreetly holding out a hand for a tip
Picture the arm hanging by the side, turned inwards, like a waiter discreetly holding out a hand for a tip.
Key facts
- Roots
- C5 and C6 form the upper trunk, and damage to them is Erb's palsy.
- Usual cause
- Stretching that forces the neck and shoulder apart, as in shoulder dystocia at birth or a heavy fall onto the shoulder.
- Weak movements
- Shoulder abduction (lifting the arm out to the side), lateral rotation (turning it outwards) and elbow flexion.
- Posture
- Waiter's tip in a severe injury: arm hangs by the side, turned inwards, elbow extended, forearm pronated and wrist flexed.
- Skin
- Numbness along the lateral arm, lateral forearm, thumb and index finger.
- Contrast
- Damage to the lower roots, C8 and T1, causes Klumpke's palsy with a claw hand.
Exam traps
- C8 and T1 are not the roots of Erb's palsy, because they form the lower trunk and their injury weakens the small muscles of the hand, giving a clawed hand rather than a shoulder problem.
- Paralysis of the whole arm and hand does not fit Erb's palsy, because that requires damage to all the roots, while an upper injury of C5 and C6 spares the small muscles of the hand.
- C6 and C7 is not the pair for Erb's palsy, because it leaves out C5, and C7 forms the middle trunk, while the upper trunk is C5 and C6.
- Damage to the whole plexus, C5 to T1, would affect the limb throughout, including the hand, which is not the selective picture of Erb's palsy.
- An isolated musculocutaneous nerve injury (the nerve to the biceps and brachialis) explains weak elbow flexion and lateral forearm numbness, but not the inward-rotated shoulder or the lateral arm numbness.
Read the full notes
Tested in 12 SimpleMed+ questions
Practise it in Musculoskeletal System, with the answer and a full explanation after every question.
Practise these questions