Variation of the brachial artery emerging through an opening in the median nerve: a case report

The median nerve maintains a close relationship with the brachial artery in the upper limb. Variability in the formation and distribution of upper extremity vessels is frequently encountered during donor body dissections or surgical procedures. This case presents a rare anatomical variation of the brachial artery emerging through an opening in the median nerve. The median nerve formed a small opening without the contribution from additional nerve roots and the remaining vasculature of the upper limb was otherwise normal. An abnormal arrangement of a neurovascular bundle in the arm can cause sensory and motor issues, as well as vascular implications. This variation has clinical significance in surgical interventions and potentially daily functioning. To our knowledge, this is the first report of a small median nerve opening pierced by the brachial artery.


Introduction
The median nerve (MN) is a mixed peripheral nerve of the upper limb containing fibers from spinal nerves C5 to T1.It is formed by the merging of the medial and lateral roots, which are branches of the medial and lateral cords of the brachial plexus, respectively.The MN descends lateral to the brachial artery (BA) before crossing anteriorly approximately at the midpoint of the humerus.Below the midpoint, the MN travels medial to the BA and enters the cubital fossa deep to the bicipital aponeurosis.The MN passes through the arm without providing muscular or sensory branches.It innervates musculature of the anterior forearm and hand, and supplies sensation to areas of the hand.The MN controls muscles involved in pronation of the forearm, flexion and abduction of the wrist, and flexion, abduction, and opposition of the thumb, as well as flexion of the digits.Sensory distribution of the MN includes the lateral aspect of the palm, in addition to the nailbeds and palmar surface of the lateral 3.5 digits [15].
The primary source of blood supply in the upper limb is the BA, originating from the axillary artery at the inferior border of the fused tendons of the teres major and latissimus dorsi muscles.The BA travels along the anterior surface of the arm and remains closely associated with the MN.A major branch of the BA is the profunda brachii artery which courses along the radial groove and supplies the posterior compartment of the arm.In addition, the BA has several branches associated with anastomotic networks around the shoulder and elbow.Distal to the cubital fossa, the BA bifurcates into the radial and ulnar arteries to supply the forearm and hand [15].
Anatomical variability of the MN and BA is widely documented in the literature.Common variations of the MN include additional nerve roots, formation below the axillary region, crossing posterior to the BA, and communicating branches with the musculocutaneous and ulnar nerves [4,7,8,16].Variations of the BA involve a high bifurcation, a unique branching pattern, or an abnormal relationship with the MN.The superficial brachial artery, a variant of the BA, courses anterior to the MN in the arm [6].In rare instances, the MN forms a nerve opening and surrounds upper extremity blood vessels.Nerve openings arise either from abnormal connections between nerves or by an artery puncturing the nerve.The MN has reportedly been pierced by a muscular branch of the BA, persistent median artery, and superficial brachial artery [9,10,13,14].However, there are no recorded cases of BA penetrating the MN.Thus, we present an atypical variation of the MN in the arm of a donor body.To our knowledge, this is the first report that describes a narrow opening of the MN traversed by the BA.

Case report
A rare unilateral variation of the right upper extremity was identified during a routine dissection for medical students.The variation was discovered in a 101-year-old female donor body that was embalmed using Maryland State Anatomical Solution Concentrate.After the BA gave off the profunda brachii artery, it descended posterior to the MN.The BA then emerged from posterior to anterior through a small fusiform opening in the MN.The approximate diameter of the MN opening was 1.2 cm (vertical) by 0.8 cm (horizontal).This opening was located 6.0 cm from the initial formation of the MN in the axilla.The diameter of the MN proximal and distal to the opening was 0.5 cm.The BA had an estimated diameter of 0.6 cm as it emerged through the opening.Distal to the opening, the MN was located posterior and lateral to the BA.Around the midpoint of the humerus, the MN crossed to the posterior and medial aspect of the BA and descended in this position until reaching the cubital fossa (Figs. 1 and 2).At the level of the elbow, normal relationships between the MN and BA were observed.The remaining neurovasculature in the cadaver body lacked notable variations and the cause of death was unrelated to the observed variation.

Discussion
Development of the upper limb begins during the 3rd week of gestation with the formation of the limb buds.Expression of sonic hedgehog (Shh) genes from the notochord initiates the growth of limb buds from surrounding germ layers.Limb buds contain a core of mesoderm covered by a layer of ectoderm.The mesodermal core consists of somatic and lateral plate mesoderm.Muscles, nerves, and vasculature arise from the somatic mesoderm, whereas connective tissue develops from the lateral plate mesoderm [1].Blood vessels of the limb bud originate by day 26 as capillary networks branch from the dorsal aorta.The selective differentiation and regression of capillaries establish the normal pattern of arteries in the limb.Nerve trunks enter the limb bud on day 33 and divide into anterior and posterior divisions.By day 37, the anterior division is pierced by the subclavian artery as it extends into the tip of the limb bud.At this stage in development, variations of the BA have been detected [12].
Alterations in the pathway of neuro vasculature during limb bud development provide an embryological basis for the observed variation in this case report.
Variations in the upper limb, such as abnormalities in the origin, location, and branching of the MN, appear frequently in donor body dissections and intraoperative studies.Approximately 21-27% of donor bodies receive three roots from the medial and lateral cords of the brachial plexus [4,7].The presence of additional roots forms a nerve opening that encompasses arteries in the upper extremity.Paraskevas et al. described two cases of a superficial brachial artery enclosed by an MN loop in a cadaver body [9,10].An MN loop was also reported surrounding the axillary artery by Bhat et al. [2].In these cases, the MN openings were formed by a connection with an additional lateral root.Blood vessels and muscles can also pierce the MN, effectively forming a nerve loop.There are several cases of a persistent median Fig. 1 Image of the anatomical variation of the brachial artery and median nerve in the arm.An opening in the median nerve is observed where the brachial artery emerges from posterior to anterior before descending in the arm.Dissection was completed by medical students and the authors of this case report artery piercing the MN [11,14].A muscular branch of the BA penetrating the MN was noted by Roy [13].In addition, Chou et al. reported the MN enclosing a variant of the palmaris longus muscle [3].The variation described in this report is unique because the MN is penetrated by the BA.These variations, as well as that described in this case report, may lead to several clinical implications.
While the side effects that the donor experienced from this abnormality are unknown, the variation may have impacted the function of the MN causing a wide range of motor and sensory symptoms.Impairment of the MN results in weakness or loss of flexion at the proximal interphalangeal joints of the 1st-3rd digits and possible weakness in the 4th and 5th digits.Additional symptoms include the loss of flexion at the distal interphalangeal joints of the 2nd and 3rd digits.The ability to flex the metacarpophalangeal joints of the 2nd and 3rd digits may also be affected because digital branches of the MN supply the 1st and 2nd lumbricals.This would result in the inability of the 2nd and 3rd digits to completely flex when making a fist ("hand of benediction").Weakness or loss of adduction, abduction, flexion, and opposition of the thumb may also occur [15].
In addition, the intimate relationship between the MN and BA may present complications with vascular pathology.Increased adherence of nerves and blood vessels, such as the variation described in this report, requires separation during surgery to prevent hemorrhage [5].An unexpected connection between the MN and BA could result in severe blood loss.Therefore, surgeons and anesthesiologists should carefully consider the variations of the MN before performing procedures on the arm.Additionally, external forces on the variation site may result in inaccurate vital signs since the BA is the main artery used for blood pressure monitoring.Furthermore, pulsations or an aneurysm of the BA could impinge the MN and cause motor or sensory issues described above.

Conclusion
Nerve loops of the MN have been previously described.However, this case report details a small opening in the MN, through which the BA emerges anteriorly.It is assumed that this variation may result in neural or vascular complications during normal function and/or clinical intervention.Knowledge of MN and BA variations assists clinicians with the management of peripheral nerve and vascular disease in the upper limb.