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5. Upper Extremity Neuroanatomy

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5. Upper Extremity Neuroanatomy 5. UPPER EXTREMITY compartments of the NEUROANATOMY arm). The brachial plexus divisions INTRODUCTION pass posterior to the mid-point of Regional anesthesia of the upper extremity in- the clavicle through volves two major nerve plexuses, the cervical plexus the cervico-axillary and the brachial plexus. A detailed understanding of canal. the anatomy of these nerve plexuses and surrounding • Three cords. The structures is essential for the safe and successful prac- divisions coalesce tice of regional anesthesia in this area of the body. to form three cords. The anterior CERVICAL PLEXUS divisions of the superior and middle The cervical plexus is formed from a series of nerve trunk form the loops between adjacent anterior rami of cervical nerve lateral cord. The roots C1 through C4. The cervical plexus is deep to the anterior division of sternocleidomastoid muscle and medial to the scalene the inferior trunk muscles. The deep branches of the plexus are motor becomes the medial nerves. They include the phrenic nerve (diaphragm cord. The posterior muscle) and the ansa cervicalis nerve (omohyoid, divisions of all sternothyroid, and sternohyoid muscles). The named three trunks unite nerves of the superficial cervical plexus are branches to form the posterior from the loops and emerge from the middle of the ster- Figure 5-1. Dissection of the superficial cervical plexus in the posterior triangle cord. The cords are nocleidomastoid muscle (Figure 5-1): BRACHIAL PLEXUS named based on their relationship to the axillary artery (as this • Lesser occipital nerve (C2): innervates the skin The brachial plexus is formed from the five roots neurovascular bundle passes in its sheath into posterior to the ear. (anterior rami) of C5–T1. Occasionally contributions to the axilla). • Great auricular nerve (C2–C3): innervates the ear the brachial plexus come from C4 (prefixed plexus) or • Five terminal branches. The cords give rise to and angle of the mandible to the mastoid process. from T2 (postfixed plexus). There are seven described five terminal branches. The musculocutaneous • Transverse cervical nerve (C2–C3): innervates the variations of brachial plexus anatomy, with the most nerve (C5–C7) arises from the lateral cord and anterior neck. common variant (Figure 5-2) occurring 57% of the time. innervates the coracobrachialis, biceps brachii • Supraclavicular nerve (C3–C4): innervates the area Asymmetry between the left and right brachial plexus and brachialis muscles, and the skin to the lateral over the clavicle and shoulder. in the same individual occurs 61% of the time. Brachial forearm. The median nerve is a compilation of the plexus anatomy includes the following parts: lateral cord (C6–C7) and the medial cord (C8, The spinal accessory nerve (CN XI) emerges at the T1). It innervates muscles of the anterior forearm posterior border of the sternocleidomastoid muscle, • Three trunks. The five roots unite to form the three and the thenar half of the muscles and skin of the passing superficial to the levator scapulae muscle to in- trunks of the brachial plexus; superior (C5 and C6), palm. The ulnar nerve is a branch of the medial nervate the trapezius muscle. Stimulation of this nerve middle (C7), and inferior (C8 and T1). The trunks cord (C7–T1) and innervates the forearm and during interscalene block, which causes the shoulder pass between the anterior and middle scalene hand medial to the midpoint of digit four. The to shrug, is occasionally mistaken as stimulation of the muscles. axillary nerve (C5–C6) is a branch of the posterior brachial plexus. Injection of local anesthetic based on • Six divisions. Each trunk divides into an anterior cord and innervates the shoulder joint and lateral this stimulation pattern will result in a failed intersca- division (anterior flexor compartments of the arm) skin over the deltoid muscle. The radial nerve lene block. and a posterior division (posterior extensor (C5–T1), which is also a branch of the posterior 21 5 UPPER EXTREMITY NEUROANATOMY Figure 5-2 Figure 5-3. Sheath prior to injection with saline cord, innervates all of the muscles of the posterior 5-4, the omohyoid muscle has been compartments of the arm and forearm and most retracted, and the sheath has been of the posterior skin of the upper extremity. filled with normal saline. The nerves Although there are numerous other named of the brachial plexus can now be seen branches of the brachial plexus, familiarization through the “window” created by the with the plexus as outlined above is adequate fluid-filled sheath. for most upper extremity regional anesthesia The existence of nerve sheaths procedures. is not unique to the brachial plexus and can be demonstrated on Considerable controversy has arisen about the neurovascular structures throughout existence of a nerve “sheath” surrounding the the human body. The practice of brachial plexus and including the artery, vein, and regional anesthesia depends on the investing connective tissue. Anatomical dissection anatomical fact of the sheath. The of the brachial plexus consistently reveals a distin- sheath improves the success of single guishable sheath of fibrous tissue surrounding the injection blocks and continuous brachial plexus, vasculature, and loose investing peripheral nerve catheters by connective tissue. In Figure 5-3, the platysma mus- containing the local anesthetic near cle has been reflected, exposing the brachial plexus nervous tissue targets and allowing sheath just posterior to the omohyoid muscle and the anesthetic to surround and bathe Figure 5-4. Sheath injected with normal saline. Note the nerve tissue lateral to the sternocleidomastoid muscle. In Figure the nerves. visible within the sheath. 22.
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