Brachial plexus anatomy: roots to branches, and the nerve lesions that follow

From the neck to the axilla: how the plexus is assembled

The brachial plexus is the ventral rami of C5, C6, C7, C8 and T1, which emerge between scalenus anterior and scalenus medius beneath the prevertebral fascia and reorganise four times on the way to the arm. The rule at each level is fixed. C5 and C6 unite as the upper trunk, C7 continues alone as the middle trunk and C8 and T1 unite as the lower trunk; the trunks cross the posterior triangle above the clavicle, the lower one lying behind the third part of the subclavian artery on the first rib. Behind the middle third of the clavicle each trunk splits into an anterior and a posterior division. The three posterior divisions form the posterior cord, the anterior divisions of the upper and middle trunks form the lateral cord and the anterior division of the lower trunk continues as the medial cord. The cords take their names from their position around the second part of the axillary artery, behind pectoralis minor, before giving the terminal branches. The functional logic is simpler than the diagram: anterior divisions carry fibres for the flexor compartments and posterior divisions for the extensors, so every muscle that extends the shoulder, elbow, wrist or fingers is supplied by the posterior cord through the axillary and radial nerves, and every flexor by the lateral or medial cord through the musculocutaneous, median and ulnar nerves. About half of people depart from the textbook: a prefixed plexus takes a large contribution from C4, a postfixed plexus from T2.

Relations that explain the clinical picture

• Roots lie between the scalenes with the phrenic nerve on the front of scalenus anterior, so an interscalene block almost always paralyses the ipsilateral hemidiaphragm and may give a transient Horner's syndrome.
• Trunks cross the first rib, where the lower trunk lies behind the subclavian artery. A cervical rib or fibrous band lifts both, so thoracic outlet syndrome presents with T1 symptoms (wasting of the small hand muscles, numbness of the medial forearm) or a cold, pulseless hand.
• Divisions sit behind the clavicle, protected by subclavius; they are rarely injured by a clavicular fracture, but a supraclavicular block here risks a pneumothorax.
• Cords surround the second part of the axillary artery behind pectoralis minor, the medial cord between artery and vein. In an axillary node clearance the long thoracic nerve on the medial wall, the thoracodorsal bundle on the posterior wall and the intercostobrachial nerve crossing the fat are the structures at risk.
• Branches each have a relation that explains their classic injury: the axillary nerve passes through the quadrangular space round the surgical neck of the humerus; the radial nerve lies in the spiral groove with the profunda brachii artery; the ulnar nerve lies behind the medial epicondyle; the median nerve runs medial to the brachial artery in the cubital fossa and then through the carpal tunnel.

Root and trunk lesions: Erb's and Klumpke's

Traction pulls the plexus in one of two directions. Forcing the head away from a depressed shoulder, in a difficult vertex delivery or a motorcyclist landing on the shoulder, tears the upper trunk at Erb's point, where C5 and C6 meet. The abductors and external rotators of the shoulder (deltoid, supraspinatus, infraspinatus), the elbow flexors (biceps, brachialis) and the supinators lose their supply, so the arm hangs adducted and internally rotated with the elbow extended and the forearm pronated: the waiter's tip. Sensation is lost over the lateral arm and forearm and the biceps and brachioradialis reflexes disappear. Pulling the abducted arm upward, when a falling person grabs a branch or a breech infant is delivered by the arm, stretches the lower roots and gives Klumpke's palsy: paralysis of the intrinsic muscles and long finger flexors produces a claw hand with wasting between the metacarpals, and sensation is lost over the medial forearm and ulnar hand. If T1 is torn from the cord proximal to the exit of its sympathetic fibres the patient also has a Horner's syndrome. A Horner's syndrome, a paralysed hemidiaphragm, a winged scapula or a preserved sensory nerve action potential in anaesthetic skin each means the injury is at root level, proximal to the dorsal root ganglion, where the roots cannot be repaired by grafting.

The peripheral nerve lesions Part A loves

Root values, myotomes and reflexes