Regional Anesthesia - Regional Anesthesia of the Upper Extremity
- The shoulder, axilla, and upper extremity are innervated by the brachial plexus except for the medial aspect of the upper arm that is innervated by the intercostobrachial nerve formed by the second thoracic nerve root.
- The brachial plexus is formed from the anterior roots of the spinal nerves from C5–C8 and T1, with frequent contributions from C4 and T2.
- Each root exits posterior to the vertebral artery and travels laterally in the trough of its cervical transverse process, where it is directed toward the first rib and fuses with the other four roots to form the three trunks (upper, middle, and lower) of the plexus. The roots are sandwiched between the fascial sheaths of the anterior and middle scalene muscles.
- The trunks pass over the first rib through the space between the anterior and middle scalene muscles, in association with the subclavian artery, which shares the same fascial sheath. The roots and trunks have several branches, innervating the neck, shoulder girdle, and chest wall.
- As the trunks pass over the first rib and under the clavicle, they split into divisions, which then reorganize to form the three cords (lateral, medial, and posterior) of the plexus. The cords descend into the axilla, where each has one major branch in addition to several minor branches, before becoming a major terminal nerve of the upper extremity. Branches of the lateral and medial cords form the median nerve. The lateral cord gives off a branch that forms the musculocutaneous nerve, whereas the posterior cord becomes the axillary and radial nerves. The medial cord also forms the ulnar, medial antebrachial, and brachial cutaneous nerves. In the axilla, the median nerve classically lies lateral to the axillary artery, the radial nerve posterior, and the ulnar nerve medial, but variation in these relative positions may occur. The axillary and musculocutaneous nerves exit the sheath high up in the axilla, the musculocutaneous nerve traveling through the substance of the coracobrachialis muscle before becoming subcutaneous below the elbow, and the axillary nerve traveling through the quadrilateral space (bordered by the humeral shaft, long head of the triceps, and teres major and minor muscles) before dividing into its terminal branches. The median cutaneous nerves of the arm and forearm are minor branches of the medial cord (Fig. 18.4). The cutaneous peripheral nerve supply of the upper extremity is summarized in Figure 18.5.
- The dermatome and sclerotome distribution of the nerves of the body is summarized in Figure 18.6. Cutaneous innervation does not necessarily correlate with deep structures; therefore, knowledge of the sclerotomes can be very useful in predicting the ultimate success of any regional technique.
- The major motor functions of the five nerves are as follows:
- Axillary (circumflex nerve): shoulder abduction (deltoid contraction)
- Musculocutaneous: elbow flexion (biceps contraction)
- Radial: elbow (triceps contraction), wrist, and finger extension (extensor carpi radialis longus)
- Median: wrist and finger flexion (flexor carpi radialis).
- Ulnar: wrist and finger flexion (flexor carpi ulnaris)
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