General overview of the brachial plexus

The brachial plexus runs within the interscalene triangle (formed by the anterior scalene anteriorly, the middle scalene posteriorly, and the superior border of the first rib inferiorly). The brachial plexus is also located within the posterior triangle of the neck (formed by the sternocleidomastoid (SCM) medially, the trapezius laterally, and the clavicle inferiorly).

The brachial plexus is the network of nerves that provides sensation and function to the upper extremity. It is formed from the ventral primary rami of the lowest four cervical nerve roots (C5-

* Corresponding author. E-mail address: [email protected] (A.Y. Shin).

C8) and that of the first thoracic nerve (T1; Fig. 1). Although a frequent percentage of brachial plexuses have contributions from C4 (prefixed) [1-3] or T2 (postfixed) [2,4,5], these contributions have little clinical significance.

The true form of the brachial plexus has been best described by Kerr [2], who performed detailed anatomic dissections on 175 specimens (see Fig. 1). In the ''true form,'' the components of the brachial plexus include the following: roots, trunks, divisions, cords, and terminal branches. Five roots form three trunks, which form six divisions. These divisions form three cords, which ultimately form five terminal branches. Roots and trunks are found supraclavicularly; divisions are located retroclavicularly; and cords and terminal branches comprise the infraclavicular portion.

C5 and C6, and C8 and T1 roots merge to form the upper and lower trunks. C7 becomes the middle trunk. The point at which C5 and C6 merge is known as Erb's point. The upper trunk trifurcates; the suprascapular nerve emerges from the upper trunk and the two divisions are formed. Each trunk divides into an anterior and posterior division, and passes beneath the clavicle. The posterior divisions from the trunks merge to become the posterior cord, and the anterior divisions of the upper and middle trunk merge to form the lateral cord. The anterior division from the lower trunk forms the medial cord. The lateral cord splits into two terminal branches: the musculocutaneous nerve and the lateral cord contribution to the median nerve (the so-called ''sensory'' part). The posterior cord forms the axillary nerve and the radial nerve, and the medial cord gives off the medial cord contribution to the

Mabc Nerve

Fig. 1. The five portions of the brachial plexus are drawn out and separated into roots, trunks, division, cords, and terminal branches. LSS, lower subscapular nerve; MABC, medial antebrachial cutaneous nerve; MBC, medial brachial cutaneous nerve; TD, thoracodorsal nerve; USS, upper subscapular nerve. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

Fig. 1. The five portions of the brachial plexus are drawn out and separated into roots, trunks, division, cords, and terminal branches. LSS, lower subscapular nerve; MABC, medial antebrachial cutaneous nerve; MBC, medial brachial cutaneous nerve; TD, thoracodorsal nerve; USS, upper subscapular nerve. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

median nerve (the so-called "motor" part) and the ulnar nerve.

There are a few terminal branches that come off the roots, trunks, and cords. The branches from the C5 root include the dorsal scapular nerve (rhomboid muscles), a branch to the phrenic nerve (with C3 and C4), and a branch to the long thoracic nerve (serratus anterior muscle). Branches from the C6 and C7 nerve also contribute to the long thoracic nerve. The branches off the upper trunk include the nerve to the subcla-vius muscle (clinically unimportant) and the suprascapular nerve. The lateral cord gives off the lateral pectoral nerve, whereas the posterior and medial cords each have three branches. The posterior cord typically gives off branches (proximal to distal) that include the upper subscapular nerve, the thoracodorsal nerve, and the lower subscapular nerve. The medial cord gives off the medial pectoral nerve, the medial brachial cutaneous nerve, and the medial antebrachial cutaneous nerve.

Common variations of the brachial plexus

Overall variations to the brachial plexus have been reported in more than 50% of cases [6]. The most common variations of the brachial plexus are related to the contributions of C4 and T2—the prefixed and postfixed brachial plexus. It has been estimated that C4 will be contributory in 28% to 62% of patients based on the dissection of brachial plexuses in cadavers (Fig. 2) [1-3]. The incidence of postfixed brachial plexuses ranges from 16% to 73% (Fig. 3) [2,4,5]. These branches range from very small to significant size.

Variations in the trunk level are relatively uncommon. Approximately 90% of upper trunks are formed by the confluence of C5 and C6, whereas in 8%, the upper trunk does not ex-ist—C5 and C6 immediately split into divisions [2]. In the remaining 2% of upper trunks, C7 joined C5 and C6, and then divided into two parts. The middle trunk, which is the continuation of C7, was the normal finding in 93.7% of

Prefixed Plexus
Fig. 2. The most common variation in the brachial plexus is variability of contributions of C4 to the brachial plexus. This contribution of C4 nerve fibers to the brachial plexus is also known as a prefixed plexus. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

specimens, whereas 3% of specimens had the middle trunk divide into two anterior divisions and one posterior division. The lower trunk was formed by the confluence of C8 and T1 in 95.4% of specimens [2].

A common variation of the lateral cord is for it to contribute to the ulnar nerve, and this variation has been reported to occur as frequently as 42.9% [2]. Another common variation is the size of the lateral cord contribution to the median nerve: when this is small, there is often a communication of the musculocutaneous nerve to the median nerve in the arm. The anatomy of the medial cord is relatively constant; it has been found to receive contributions from C8 and T1 in 94.6% of specimens and has few reported variations. The posterior cord has been reported to be absent in 20.8% of cadavers. In these specimens, the radial and axillary nerves arose independently from the brachial plexus.

Variations in the terminal branches are common. A discussion of every reported variation is beyond the scope of this article; only the most commonly encountered variations in the terminal branches are discussed. The suprascapular nerve has been found to come from the upper trunk or its anterior or posterior divisions in more than 82% of specimens [2]. Occasionally, C4 may contribute directly to the suprascapular nerve. It also may be a terminal branch off of C5 with a small contribution to C6 [7]. The musculocutaneous nerve is commonly associated with variations. Kaplan and Spinner [8] noted that this nerve may seem absent because of a ''double musculocutaneous nerve'' or a combined median/musculocutaneous nerve. In 24% of specimens, there were C7 fibers present in the musculocutaneous nerve, and these fibers passed through a communication from the mus-culocutaneous nerve to the median nerve [8]. A low take-off of the musculocutaneous nerve from the lateral cord may confuse the surgeon, especially if he or she is using a small infraclavicular incision and does not identify this nerve branch within the surgical field. The posterior cord, with its terminal branches, the axillary and radial nerve, is also frequently variant. The radial nerve comes off the posterior cord in its classic position in 79% of specimens [2]. The variations include the radial nerve coming off the posterior division of the upper and middle trunk. The axillary nerve arises

Prefixed Postfixed Brachial Plexus
Fig. 3. The postfixed brachial plexus has a variable contribution of T2 to the brachial plexus. (Courtesy of the Mayo Foundation, Rochester, MN; with permission.)

in its classic position in 79.9% of specimens but can also arise from the upper and middle trunk divisions directly [2].

The axillary artery and its relationship to the brachial plexus elements may also be variant. Typically, at the level of the coracoid, the three cords are named for their anatomic relationship to the axillary artery: lateral, medial, and posterior cords (Fig. 4). Occasionally, a large superficial branch of the axillary artery will emerge between the medial and lateral cord contribution to the median nerve to course into the arm as a superficial radial, ulnar, brachial, or median artery. In addition to this variation, Miller [9] reported several vascular anomalous relationships (occurring in 8 of 480 specimens) where the median nerve below the convergence of the medial and lateral contributions was penetrated and divided by a branch of the axillary artery. Overall, the author reported 8% arterial and 4% venous variations associated with the brachial plexus.

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