The brachial plexus is a group of nerves located from the back of the neck to the axillary fossa, called the armpit, and innervating the upper limb.

Position. Two in number, the brachial plexuses are arranged symmetrically with respect to the midline. The brachial plexus begins at the level of the cervical spinal nerves and continues into the axillary region, corresponding to the armpit region.

Structure. From the neck down to the armpit, the brachial plexus is made up of roots, trunks, divisions and bundles.

The roots of the brachial plexus originate from (1):

• Position. Two in number, the brachial plexuses are arranged symmetrically with respect to the midline. The brachial plexus begins at the level of the cervical spinal nerves and continues into the axillary region, corresponding to the armpit region.
• Structure. From the neck down to the armpit, the brachial plexus is made up of roots, trunks, divisions and bundles.

These roots extend and pass between the scalene muscles. They then group together to divide into three trunks, also called primary trunks:

• the upper trunk, from roots C5 and C6;
• the middle trunk, formed by the root C7;
• the lower trunk, coming from roots C8 and T1.

Each trunk extends behind the collarbone and splits into two divisions, an anterior and a posterior. These divisions come together to form the three bundles, also called the secondary trunks:

• the lateral bundle, coming from the anterior divisions of the upper and middle trunks;
• the medial bundle, originating from the anterior division of the lower trunk;
• the posterior bundle, coming from the posterior divisions of the three trunks.

Continuing on from these bundles are the terminal branches constituting some of the main nerves of the upper limb such as the radial nerve, axillary nerve or musculocutaneous nerve (2). Other main nerves of the upper limb also emerge from different parts of the plexus, forming lateral branches.

The brachial plexus can be damaged resulting in partial or total paralysis of the upper limb (3). The cause of these paralyzes is most often accidental pulling or stretching. These lesions can in particular be an avulsion, that is to say a tearing of the roots of the brachial plexus, or a rupture of the nerves.

Orthopedic treatment. Immobilization of the upper limb may be necessary with, for example, the wearing of an orthopedic vest.

Surgical treatment. Depending on the type of lesions, surgery may be performed under brachial plexus anesthesia. An anesthetic solution is injected into the brachial plexus in order to stop the nerve impulses and to numb the upper limb.

Physical examination. First, a clinical examination is performed in order to observe and assess the symptoms perceived by the patient.

Medical imaging examination, such as an MRI or an x-ray may be performed to complete or confirm a diagnosis.

Electromyogram. This technique measures the activity of a nerve or muscle.

In some people, the brachial plexus may have a different makeup. The roots of the brachial plexus can vary. The ventral branch of C4 may be the first root and C8 the last to form a prefixed brachial plexus. Conversely, the first root can be formed by C6 and the last by T2 to form a postfixed brachial plexus (1).