The Brachial Plexus
The brachial plexus is a network of nerves that supplies innervation to the skin and musculature of the upper limb. It is subdivided into roots, trunks, divisions, cords and branches and the order in which these division occur can be remembered using the mnemonic ‘Rugby Teams Drink Cold Beers’:
There are typically 5 roots, 3 trunks, 6 divisions, 3 cords and 5 terminal branches as can be seen in the diagram below:
The roots, trunks, divisions, cords and terminal branches of the brachial plexus (from Gray’s Anatomy)
The five roots of the brachial plexus are the five anterior rami of the spinal nerves C5, C6, C7, C8 and T1. These roots pass anterior to scalenus medius and posterior to scalenus anterior before entering the base of the neck.
The roots converge in the base of the neck to form three trunks, which are named according to their anatomical position:
- The C5 and C6 roots combine to form the superior trunk
- The C7 root continues to form the middle trunk
- The C8 and T1 roots combine to form the inferior trunk
The trunks pass laterally across the posterior triangle of the neck, where they are palpable and pass over the 1st rib posterior to the 3rd part of the subclavian artery before descending behind the clavicle.
Behind the middle third of the clavicle each trunk divides into two branches to form six divisions. One division passes anteriorly and one posteriorly and are therefore known as the anterior and posterior divisions:
- Anterior divisions of the upper, middle and lower trunks
- Posterior divisions of the upper, middle and lower trunks
The divisions then leave the posterior triangle of the neck and pass into the axilla.
After entering the axilla the six divisions regroup to become the three cords. The cords lie in the axilla related to the second part of the axillary artery and are named according to their position respective to it:
- The three posterior divisions of the trunks (C5-C8, T1) reform into the posterior cord
- The anterior divisions of the upper and middle trunks (C5-C7) reform into the lateral cord
- The anterior division of the lower trunk (C8-T1) continues on as the medial cord
The three cords give rise to five major branches in the axilla and the proximal part of the upper limb:
- Axillary nerve (C5 and C6)
- Musculocutaneous nerve (C5-C7)
- Radial nerve (C5-C8 and T1)
- Median nerve (C6-C8 and T1)
- Ulnar nerve (C8 and T1)
In addition to the five major branches there are several smaller branches that arise from all various parts of the brachial plexus:
A simple stylized version of the brachial plexus that can be easily drawn is shown below:
Stylized version of the brachial plexus. © Medical Exam Prep
Upper brachial plexus lesion – Erb’s palsy
Erb’s palsy is named after Wilhelm Heinrich Erb (1840-1921), the German neurologist who described a case in 1874. The French neurologist Guillaume Duchenne (1806-1875) also described a case two years earlier in 1872 and for this reason it is sometimes referred to as the Erb-Duchenne palsy.
Erb’s palsy is a paralysis of the arm that is caused by an injury to the upper roots of the brachial plexus. The most commonly involved root is C5 but in some cases C6 is affected also.
The most common cause of Erb’s palsy is traction on the arm during difficult childbirth but it can also occur in adults secondary to shoulder trauma. Approximately 50% of cases are associated with shoulder dystocia.
Clinically the arm will hang at the side with the elbow extended and the forearm pronated (waiter’s tip sign).
On examination there will be loss of:
- Shoulder abduction (deltoid, supraspinatus)
- Shoulder external rotation (infraspinatus)
- Elbow flexion (biceps, brachialis)
Erb’s palsy can be contrasted with Klumpke’s palsy, which affects the lower roots of the brachial plexus (C8 and T1) and presents with ‘claw hand’ (paralysis of the intrinsic hand muscles) and sensory loss along the lateral side of the forearm and hand. If T1 is affected a Horner’s syndrome may also be present.
Lower brachial plexus lesion – Klumpke’s palsy
A Klumpke’s palsy, also referred to as Dejerine-Klumpke palsy, is a paralysis of the arm that is caused by an injury to the lower roots of the brachial plexus. The most commonly involved root is C8 but T1 can also be affected. The most common cause of Klumpke’s palsy is traction on the arm in an abducted position during difficult childbirth. It can also occur in adults due to apical lung carcinoma (Pancoast’s syndrome).
Clinically it presents with ‘claw hand’ deformity (paralysis of the intrinsic hand muscles) and sensory loss along the medial side of the forearm and hand. If T1 is affected a Horner’s syndrome may also be present.
Kumpke’s palsy can be contrasted with Erb’s palsy, which affects the upper roots of the brachial plexus (C5 and sometimes C6) and presents with the arm hanging at the side with the elbow extended and the forearm pronated (waiter’s tip sign). In Erb’s palsy there is loss of shoulder abduction, external rotation and elbow flexion.
Summary of Erb’s and Klumpke’s palsies: