Brachial Plexus-The Axillary Sheath and a Brachial Plexus Nerve Block

Brachial Plexus

The nerves entering the upper limb provide the following important functions: sensory innervation to the skin and deep structures, such as the joints; motor innervation to the muscles; influence over the diameters of the blood vessels by the sympathetic vasomotor nerves; and sympathetic secretomotor supply to the sweat glands. At the root of the neck, the nerves form a complicated plexus called the brachial plexus. This allows the nerve fibers derived from different segments of the spinal cord to be arranged and distributed efficiently in different nerve trunks to the various parts of the upper limb. The brachial plexus is formed in the posterior triangle of the neck by the union of the anterior rami of the 5th, 6th, 7th, and 8th cervical and the 1st thoracic spinal nerves.

The plexus can be divided into roots, trunks, divisions, and cords. The roots of C5 and 6 unite to form the upper trunk, the root of C7 continues as the middle trunk, and the roots of C8 and T1 unite to form the lower trunk. Each trunk then divides into anterior and posterior divisions. The anterior divisions of the upper and middle trunks unite to form the lateral cord, the anterior division of the lower trunk continues as the medial cord, and the posterior divisions of all three trunks join to form the posterior cord.

The roots, trunks, and divisions of the brachial plexus reside in the lower part of the posterior triangle of the neck and are fully described on page XXX. The cords become arranged around the axillary artery in the axilla. Here, the brachial plexus and the axillary artery and vein are enclosed in the axillary sheath.

Cords of the Brachial Plexus All three cords of the brachial plexus lie above and lateral to the first part of the axillary artery. The medial cord crosses behind the artery to reach the medial side of the second part of the artery . The posterior cord lies behind the second part of the artery, and the lateral cord lies on the lateral side of the second part of the artery . Thus, the cords of the plexus have the relationship to the second part of the axillary artery that is indicated by their names.

Most branches of the cords that form the main nerve trunks of the upper limb continue this relationship to the artery in its third part .

The branches of the different parts of the brachial plexus are as follows:

■■ Roots

Dorsal scapular nerve (C5)

Long thoracic nerve (C5, 6, and 7)

■■ Upper trunk

Nerve to subclavius (C5 and 6)

Suprascapular nerve (supplies the supraspinatus and

infraspinatus muscles)

■■ Lateral cord

Lateral pectoral nerve

Musculocutaneous nerve

Lateral root of median nerve

■■ Medial cord

Medial pectoral nerve

Medial cutaneous nerve of arm and medial cutaneous

nerve of forearm

Ulnar nerve

Medial root of median nerve

■■ Posterior cord

Upper and lower subscapular nerves

Thoracodorsal nerve

Axillary nerve

Radial nerve

The Axillary Sheath and a Brachial Plexus Nerve Block

Because the axillary sheath encloses the axillary vessels and the brachial plexus, a brachial plexus nerve block can easily be obtained. The distal part of the sheath is closed with finger pressure, and a syringe needle is inserted into the proximal part of the sheath. The anesthetic solution is then injected into the sheath, and the solution is massaged along the sheath to produce the nerve block. The position of the sheath can be verified by feeling the pulsations of the third part of the axillary artery.

Hematoma of the Rectus Sheath-Abdominal Muscles, Abdominothoracic Rhythm, and Visceroptosis

Hematoma of the Rectus Sheath

Hematoma of the rectus sheath is uncommon but important, since it is often overlooked. It occurs most often on the right side below the level of the umbilicus. The source of the bleeding is the inferior epigastric vein or, more rarely, the inferior epigastric artery. These vessels may be stretched during a severe bout of coughing or in the later months of pregnancy, which may predispose to the condition. The cause is usually blunt trauma to the abdominal wall, such as a fall or a kick. The symptoms that follow the trauma include midline abdominal pain. An acutely tender mass confined to one rectus sheath is diagnostic

Abdominal Muscles, Abdominothoracic Rhythm, and Visceroptosis

The abdominal muscles contract and relax with respiration, and the abdominal wall conforms to the volume of the abdominal viscera. There is an abdominothoracic rhythm. Normally, during inspiration, when the sternum moves forward and the chest expands, the anterior abdominal wall also moves forward. If, when the chest expands, the anterior abdominal wall remains stationary or contracts inward, it is highly probable that the parietal peritoneum is inflamed and has caused a reflex contraction of the abdominal muscles. The shape of the anterior abdominal wall depends on the tone of its muscles. A middle-aged woman with poor abdominal muscles who has had multiple pregnancies is often incapable of supporting her abdominal viscera. The lower part of the anterior abdominal wall protrudes forward, a condition known as visceroptosis. This should not be confused with an abdominal tumor such as an ovarian cyst or with the excessive accumulation of fat in the fatty layer of the superficial fascia.