The brachial plexus arises from the anterior rami of spinal nerves C5 to C8 and T1. Damage to these nerve roots tends to give rise to one of two distinct clinical presentations.
Upper brachial plexus injuries (Erb palsy) involve the upper plexus nerve roots C5 and C6. Injury usually arises from forced separation of the head and shoulder as may occur during delivery or during a motorcycle accident. Most often, the axillary, musculocutaneous and suprascapular nerves are affected (less so the upper and lower subscapular nerves):
- Paralysis of deltoid (axillary nerve), and supraspinatus (surascapular nerve) leads to absence of shoulder abduction.
- Biceps and brachialis paralysis (musculocutaneous nerve) cause absent elbow flexion and forearm supination.
- The intrinsic muscles of the hand are unaffected.
- Sensory deficit occurs over the posterior and lateral arm, radial aspect or the forearm and over the thumb and first finger (C5, C6 dermatome)
- The patients presents with the affected arm adducted and internally rotated, the forearm pronated and wrist flexed (Waiters tip position).
Lower Brachial plexus injuries (Klumpke’s Paralysis) arises from damage to the C8,T1 nerve roots. It is much less common than upper brachial plexus injury and caused by forced traction of the fully abducted arm. Typically, this occurs during a traumatic delivery, where the abducted arm is used to pull the baby from the birth canal, or when a falling person grasps at a branch above them. Lower brachial plexus injuries are characterised by paralysis of the muscles of the forearm and hand:
- The intrinsic muscles of the hand (interossie and muscles of the thenar and hypothenar eminences) are paralysed, as are the flexors of the wrist and fingers (flexor carpi ulnaris and flexor digitorum profundus)
- sensory deficit: anaesthesia over the ulnar aspect of the forearm, hand and ulnar 1 and ½ digits
- The patient presents with a claw hand deformity, where the forearm is supinated and the wrist and fingers flexed.
- T1 involvement may also be associated with a Horner’s syndrome (ipsilateral miosis and ptosis)
Compete brachial plexus injuries, involving all roots of the brachial plexus ( C5 – T1) can occur but are rare and particularly devastating with complete loss of movement and sensation in the affected arm.