A Prefixed Four-Trunk Brachial Plexus with Anomalous Anatomy of All Subsequent Divisions, Cords, and Terminal Branches: A Cadaveric Case Report

Introduction

The brachial plexus is a complex structure of nerves that provides motor and sensory innervations to the upper limb. The roots and trunks of the brachial plexus are typically found posterior to the anterior scalene muscle and are formed by the anterior rami of spinal nerves C5-T1 [1]. There are traditionally 3 trunks: an upper trunk (C5–C6), middle trunk (C7), and lower trunk (C8–T1), each bifurcating into anterior and posterior divisions. The anterior divisions of the upper and middle trunks combine to form the lateral cord, while the anterior division of the lower trunk forms the medial cord. The posterior divisions of all 3 trunks fuse to form the posterior cord. Each cord subsequently divides into peripheral nerves that supply the upper limb [2].

Although the previously described arrangement of the brachial plexus is most common, numerous anatomical variations of the brachial plexus have been described in the literature [3–5]. Variations may consist of differences in the positioning of the plexus relative to the scalene muscles, as well as variations among the roots, trunks, divisions, and cords of the plexus. As opposed to passing posteriorly to the anterior scalene muscle, the roots and trunks may pierce through the muscle belly of the anterior scalene muscle or pass anterior to the anterior scalene muscle [6–9]. Also, variations in the brachial plexus’ nerve origins have been reported. A prefixed brachial plexus consists of a contribution from the fourth cervical root (C4) to the origin, while postfixed consists of a contribution from the second thoracic spinal root (T2) [10]. Of the 2, prefixed brachial plexuses are more common [3,4].

The composition and number of trunks may also vary [11,12]. The middle trunk has been found to arise from C7 and C8, with the inferior trunk receiving contribution from only T1 [13]. The upper trunk may be formed by the anterior rami of C5–C7 rather than C5–C6. Regarding the number of trunks, there have been reports of brachial plexuses with both 2 and 4 trunks [11,12]. Given these findings, it is evident that variations within the structure of the brachial plexus are diverse.

Awareness of brachial plexus variations is important for surgical planning, performing brachial plexus nerve blocks, imaging, and other procedures involving the neck and shoulder. The present case report describes a prefixed brachial plexus with 4 trunks, leading to further variations in the anatomy of the divisions, cords, and branches, which has not been previously described in the medical literature.

Case Report

ANOMALIES IN THE ROOTS:

Two anomalies were identified at the level of the roots. First, the brachial plexus had a contribution from the C4 nerve root, making it a prefixed plexus. Secondly, the C4 and C5 nerve roots emerged anterior to the anterior scalene muscle rather than between the anterior and middle scalene muscles (Figure 2).

ANOMALIES IN THE CONTRIBUTIONS TO THE TRUNKS AND DIVISIONS:

The anomalous plexus had 4 trunks: the anterior rami of nerve roots C4 and C5 united to form the superior trunk, nerve roots C8 and T1 joined to formed the inferior trunk, and nerve roots C6 and C7 each formed independent trunks. The trunk composed of nerve root C6 was called the superior middle trunk (SMT), and the trunk composed of nerve root C7 was called the inferior middle trunk (IMT) (Figures 1–3). Each of the 4 trunks divided into an anterior and posterior division, leading to the development of 8 total divisions (Figures 1, 2).

ANTERIOR DIVISIONS:

The anterior division of the superior trunk (C4/C5) joined the anterior division superior middle trunk (C6) to form the lateral cord. The lateral cord gave off the lateral pectoral nerve and then divided distally into the musculocutaneous nerve and lateral root of median nerve.

The anterior division of the inferior middle trunk (C7) alone formed a novel cord with novel branching pattern, referred to here as the middle cord. The middle cord gave rise to an anomalous medial pectoral nerve with innervation arising from nerve root C7 (Figure 1). The middle cord then divided into a medial and lateral branch. The lateral branch of the middle cord joined the lateral root of median nerve, and the medial branch of the middle cord joined the medial root of median nerve (Figures 1, 3).

The anterior division of the inferior trunk formed the medial cord. The medial cord gave rise to the medial antebrachial cutaneous nerve and then divided distally into the ulnar nerve and medial root of median nerve. We could not find a medial brachial cutaneous nerve, but assumed it arose from the medial cord, as in normal anatomy. This nerve was likely inadvertently removed during dissection due to its small size and superficial nature.

The overall structure of the anomalous brachial plexus created a unique geometric appearance due to the complex interconnectivity of the distal cords and branches. The novel middle cord bifurcated into a medial and lateral branch. The joining of these medial and lateral branches with the medial and lateral cords created 2 M-shaped structures with mirror image symmetry: an inverted M composed of the lateral cord, me-dial and lateral branches of the middle cord, and medial cord (Figure 3, green dotted line), and an upright M composed of the musculocutaneous nerve, medial and lateral roots of the median nerve, and the ulnar nerve (Figure 3, blue dotted line). When the 2 Ms faced each other, they created a novel diamond-shape pattern by the union of their central limbs.

POSTERIOR DIVISIONS: The brachial plexus formed 2 posterior cords, with the posterior divisions of superior trunk and superior middle trunks forming the second, anomalous posterior cord, referred to here as the accessory posterior cord (Figure 4). The first branch coming from the accessory posterior cord was a root contributing to the posterior cord. The second branch of the accessory posterior cord was the upper subscapular nerve, and the third branch of accessory posterior cord was the lower subscapular nerve. After giving off the lower subscapular nerve, the accessory posterior cord continued as the axillary nerve through the quadrangular space (Figure 4).

The posterior cord received contributions from 3 sources: the posterior divisions of the inferior and inferior middle trunks, and the accessory posterior cord (see above). The posterior cord proper gave off the thoracodorsal nerve and then continued as the radial nerve with the deep brachial artery into the triangular interval (Figure 4).

Discussion

PREFIXED PLEXUS AND POSITION RELATIVE TO THE ANTERIOR SCALENE MUSCLE:

The occurrence of a prefixed brachial plexus is a common finding and is well documented in the literature [1,3,4,16]. A brachial plexus is considered prefixed if there is contribution from the C4 nerve root [4,10]. Some definitions state that a prefixed brachial plexus consists of a large contribution from C4 with or without contribution from T1 or a plexus with C4–C8 nerve root contributions [4,16]. Our findings of a prefixed brachial plexus are consistent with the literature and characteristics of a prefixed plexus.

Variations among the positioning of the roots relative to the scalene muscles has also been documented [6,9,17]. In a typical brachial plexus, the roots pass posterior to the anterior scalene muscle. However, the roots and trunks have been found to perforate the anterior scalene muscle, with variants in which specific roots pierce the muscle [6,7]. Other positional differences include passage of the C5–C6 roots anterior to the anterior scalene muscle [8].

While our findings of a prefixed plexus and positional variation relative to the anterior scalene muscle are not novel findings in themselves, there is clinical significance to these findings. Intermittent compression in the region of the anterior scalene muscle, such as from a backpack, can lead to neurogenic compression due to temporary pressure [8,18]. Interscalene blocks may require additional anesthetic injection to the areas with variant nerve roots [15]. There may also be anomalous sensory or motor innervation to the upper limb with prefixed and postfixed brachial plexuses [16]. Based on the more superficial positioning of the nerve roots, we postulate a brachial plexus with nerve roots passing anterior to the anterior scalene muscle might be more prone to injury with neck and/ or shoulder trauma.

ANTERIOR DIVISIONS AND TERMINAL BRANCHES:

Anomalies of the anterior division of the brachial plexus have been previously described [3,19]. The anterior division of the middle cord may split with the superior portion ultimately supplying the lateral cord and the inferior portion supplying the medial cord. The anterior divisions of all 3 trunks may combine to form a single anterior cord [3] Wozniak et al (2013) described the extent of anterior divisions, finding that variation among the middle anterior division of the middle trunk was highest. One notable finding relevant to our case study is the splitting anterior division of the middle cord to give contributions to the lateral and medial roots of the median nerve [19]. This is similar to findings in the present case study with the same root contribution (C7) to the division; however, the contributions to the median nerve come from a novel cord rather than directly from the trunk. Additionally, the lateral branch of the middle cord contributes to the median nerve in conjunction with the lateral cord. This, in combination with the prefixed nature of the brachial plexus we present, is a unique abnormality.

Abnormalities of the anterior division can also involve the terminal branches. The anterior division of the middle trunk may give independent contributions to the musculocutaneous and median nerves [13]. A common variation of the medial pectoral nerve is to receive contributions from the medial and lateral cords [20]. There have also been many anomalies involving the median nerve, including several receiving contributions from the lateral cord, independent contributions from the anterior division of the middle trunk, and communicating branches arising from the median nerve [3,13,20,21].

The “M” branching may be normal or abnormal. The “M” arrangement refers to the arrangement the musculocutaneous, median, and ulnar nerves create in a normal plexus. Thus, variations involving these nerves will impact the “M” structure. About 40% of donors have been shown to demonstrate the classic M arrangement [20]. While anomalies of the M pattern have been reported, our literature search did not show any similar findings of an inverted “M” with mirror symmetry to the typical “M” pattern, caused by a novel middle cord arising from the anterior division of the inferior middle trunk that is composed of lateral and medial cords.

POSTERIOR DIVISIONS, POSTERIOR CORD PROPER, ACCESSORY POSTERIOR CORD, AND TERMINAL BRANCHES:

Previously described variations within the posterior cord include differences in the nerve composition of the posterior cord, such as only receiving contributions from the middle and inferior trunks with no superior trunk, or being formed by the superior and middle trunks alone with no inferior trunk [22,23]. An accessory branch of the posterior cord was noted in a case report by Banik et al (2022), in which an additional branch arose from the posterior cord and supplied the triceps muscle [24]. Another notable anomaly is the split posterior cord, in which the posterior cord was formed normally by the superior, middle, and inferior trunks, but splits to give off 2 separate branches, one giving off the axillary nerve and another gave off the radial nerve [3]. These reported variations in the posterior cord somewhat resemble the current case report; however, our posterior cord proper received contributions from the accessory posterior cord (composed of the superior and superior middle trunks), the middle trunk, and the inferior trunk. Our posterior cord proper gave off a branch forming the thoracodorsal nerve, which continued further as the radial nerve. This contrasts with the split posterior cord noted by Benes et al (2022), which has each branch forming the axillary and radial nerves [3]. In this way, not only is the composition of our posterior cord proper different, but there was an entirely separate accessory posterior cord, which alone formed the axillary and subscapular nerves.

While posterior cord anomalies are common, awareness of these anomalies is important to avoid injury to variant posterior cords and terminal branches during neck and axillary dissection [24]. The spread of anesthesia is also impacted with cord variations, which may cause undesirable nerve blockades and the inability to predict the dissemination of local anesthetics [22,25].

Conclusions

The current case report presents a series of brachial plexus anomalies not previously described in the literature. We present a prefixed (C4-T1) brachial plexus that is positioned anterior to the anterior scalene muscle with anomalies of the trunks, divisions, cords, and terminal branches. The anterior division of the middle trunk forms a new middle cord, and there was an anomalous second M shape to the presented brachial plexus, due to the presence of the novel middle cord and its me-dial and lateral branches, creating a unique double M pattern.

Regarding the posterior division, there was both an accessory posterior cord and the posterior cord proper, each giving independent different terminal branches. Our findings are clinically relevant regarding neurogenic compression, interscalene blocks, and trauma to the upper limb. Knowledge of these anomalies may better equip both anatomists and clinicians to understand the pathology and surgical intervention of the upper limb.