2 General Overview
The brachial plexus is formed by the anterior rami of the C5–C8 and T1 spinal nerves. The brachial plexus also contains contributions from C4 in over 60% of people and from T2 in over 30% (Fig. 2.1).
The roots of the spinal nerves exit from the spinal canal behind the vertebral artery and cross the transverse process of the corresponding vertebra. They then join to form three trunks and run together toward the first rib.
The upper trunk arises from the union of the roots of C5/6, where the suprascapular nerve arises immediately as a lateral branch from the upper trunk.
The middle trunk is formed by the root of C7.
The lower trunk is formed by the roots of C8/T1.
The trunks, which here lie on top of one another, pass through the interscalene groove (posterior interscalene groove) between the scalenus anterior and scalenus medius muscles; the subclavian artery is positioned in front of the lower trunk in the caudal area of the space and thus also passes through the space.
Just above the clavicles, each of the trunks splits into an anterior and a posterior division. The three posterior divisions join to form the posterior cord, the anterior divisions of the upper and middle trunks form the lateral cord, and the medial cord is the continuation of the anterior division of the lower trunk.
In the interscalene region, we thus have the trunks and in the immediate supraclavicular and infraclavicular regions initially still the trunks, then their branches, and then the cords.
The cords lie very close together in the infraclavicular region:
The lateral cord is the most superficial (lateral to and in front of the subclavian artery).
The posterior cord is a little deeper and in the immediate infraclavicular region slightly lateral to the lateral fascicle!
The medial cord lies deep (behind the subclavian artery).
Here, the cords rotate by about 90° around the axillary artery, with the medial cord passing under the artery. It is now positioned medial to the artery and then gives off a medial root that unites with the lateral root of the lateral cord to form the median nerve. The median nerve is usually located lateral to the axillary artery. When the chords enter the axillary region, they are actually located medially, laterally, and posteriorly in accordance with their names.
The ulnar nerve, the medial cutaneous nerve of the arm and the medial nerve of the forearm, and the medial root of the median nerve arise from the medial cord. After the musculocutaneous nerve has arisen from the lateral cord, it combines with parts of the medial cord to form the median nerve (Fig. 2.6, Fig. 2.7, Fig. 2.8). The posterior cord divides into the axillary nerve and radial nerve (see Fig. 2.1 and Fig. 2.5).
From its passage through the (posterior) interscalene groove as far as the axillary region, the entire brachial plexus is surrounded by a firm sheath of connective tissue (Fig. 2.9). This sheath, which encloses the anterior neck muscles, is called the prevertebral fascia. It continues in lateral and caudal direction and covers the trunks like a cloth. The space below it continues in caudal direction to the infraclavicular region, but in medial and cranial direction to the intervertebral foramen and in the broader sense to the epidural space.
As well as the nerves, this sheath also contains the blood vessels (axillary artery and vein). The subclavian artery passes with the brachial plexus through the (posterior) interscalene groove, while the subclavian vein does not join them until after they pass through the anterior interscalene groove (between the sternocleidomastoid located in front and the scalenus anterior positioned adjacent and to the rear). Variations in the branches of the subclavian artery in the supraclavicular fossa may increase the incidence of vascular puncture or intravascular injection of local anesthetic while performing the block (Kohli et al 2014).
There are connective-tissue septa inside this neurovascular sheath. However, in the majority of people, these do not appear to impede the steady spread of local anesthetic, so that a complete block of the brachial plexus is possible with a single injection, particularly in the supraclavicular, infraclavicular, and also axillary regions.
2.2 Important Topographical Anatomical Relations in the Region of the Brachial Plexus
The phrenic nerve runs on the belly of scalenus anterior enclosed in the prevertebral fascia described above (see Fig. 2.2, Fig. 2.3, and Fig. 2.10). If a response to stimulation of the phrenic nerve is produced during interscalene block (contraction of the diaphragm), the position of the needle tip must be corrected laterally and posteriorly. Phrenic nerve paresis can be produced by the local anesthetic effect.
Recurrent laryngeal nerve.
Recurrent laryngeal nerve block with hoarseness occurs occasionally (Fig. 2.10), which is a sign of the block spreading medial to the scalenus anterior.
Cervical and thoracocervical sympathetic ganglia.
These ganglia are in the immediate vicinity of the brachial plexus (Fig. 2.10 and Fig. 2.11), but always medial to the interscalene groove and in the same region as the recurrent laryngeal nerve. Horner syndrome (miosis, ptosis, enophthalmos) can be triggered by the local anesthetic effect. It is argued that bronchospasm can be triggered in asthmatic patients by the sympatholytic effect, but this is not undisputed.
Dome of the pleura.
The dome of the pleura extends clearly above the body of the first rib, but never above the neck of the first rib and is in the immediate vicinity of the structures described here (Fig. 2.11). The risk of pneumothorax must therefore be borne in mind with the corresponding techniques in the supraclavicular and infraclavicular space.
The vertebral artery lies anterior to the exit of the spinal nerves through the intervertebral foramina (Fig. 2.11). After arising from the subclavian artery medial to the scalenus anterior, it runs in cranial direction and disappears in the transverse foramen of the transverse process of the sixth cervical vertebra and continues with the transverse part (V2 segment) in cranial direction.
If the needle is inserted in the wrong direction during interscalene block using the Winnie approach or in deep cervical plexus block, intravascular injection of the local anesthetic can occur. Only a few milliliters are sufficient to cause a seizure as the local anesthetic reaches the brain directly through the artery.