INTRODUCTION

The lateral cord of the brachial plexus usually bifurcates at the infraclavicular level, giving origin to the musculocutaneous nerve (MCN) and the lateral root of the median nerve (MN) (^{Ballesteros et al., 2015}). The fibers for the MCN originates from C5, C6 and C7 nerve roots. The MCN gives off a branch for the coracobrachialis muscle (CbM) before piercing it, and then innervates the biceps brachii muscle and brachialis muscle (^{Standring, 2016}). In its trajectory, the MCN goes through the deep fascia, lateral to the biceps tendon and then continues as a sensitive nerve, the lateral antebrachial cutaneous nerve (^{Osborne et al., 2000}; Standring).

The anatomic variations of the MCN are present in one of every three arms (^{Choi et al., 2002}). The most frequent anatomic variations of the MCN could be divided in two main groups: communicating branches with the MN and variations in relation to the origin, which in turn can be subdivided into absence of the MCN and non-perforation of the CbM. These variations have been reported in ranges between 0.8-8.3 % and 1.3-11.1 % respectively (^{Venieratos & Anagnostopoulou, 1998}; Choi et al.; ^{Loukas & Aqueelah, 2005}; ^{Guerri-Guttenberg et al., 2009}).

Injuries to the MCN are associated with weakness of arm flexion and sensory loss along the lateral aspect of the forearm. Unusual clinical symptoms and/or unusual physical examination in patients with motor disorders, could be explained by anatomic variations of the MCN (^{Beheiry, 2004}). This study was therefore conducted to determine the variations of the MCN in relation to its origin, in a cadaveric sample of Colombian mestizo population, constituting valuable information for clinical diagnosis, electromyographic procedures, nerve blocks and surgical approaches to the arm.

MATERIAL AND METHOD

This descriptive cross-sectional study was designed to determine the variations of the MCN in relation to its origin in 106 fresh upper extremities of 53 individuals who were undergoing necropsy at the National Institute of Forensic Medicine in Bucaramanga, Colombia. The sample met the following inclusion criteria: male individuals over 18 years of age. Furthermore, the following exclusion criteria were established: subjects with evidence of direct trauma or scars in upper extremities. A longitudinal incision along the midaxillary line was made from the coracoid process to the upper third of the forearm, which involved skin, subcutaneous tissue and brachial fascia. Subsequently, the muscular structures of the anterior compartment of the arm and the nerves derived from the lateral and medial cords of the brachial plexus were dissected. The presence or absence of the MCN was evaluated and whether the MCN pierced the coracobrachialis muscle or not. Using a digital micrometer (Mitutoyo, Japan), the lengths of the motor branches and the distances from its origins to the coracoid process were measured. Continuous quantitative variables were described with their averages and standard deviations. A student's t test was carried out accepting an alpha error of up to 5 %. The statistical analysis was performed using IBM® SPSS®.

RESULTS

In 96 arms (90.5 %) the usual origin pattern of the MCN was observed from the lateral cord of the brachial plexus. In 10 arms (9.5 %) anatomic variations in relation to the origin of the MCN were observed, corresponding to six left and four right arms. No statistically significant differences were found between the sides and the presence of a variant origin pattern (p > 0.05).

In six arms (5.7 %) the MCN did not pierce the CbM. In this group, the MCN originated directly from the lateral cord of the brachial plexus in five arms and in the remaining the MCN originated from the median nerve (Fig. 1). In three arms of those with an origin of the MCN from the lateral cord of the brachial plexus, the MCN arose at the level of the latissimus dorsi tendon, accompanied by a lateral root of the median nerve with mean length of 45.6 mm (Fig. 2). In the other two arms, the origin of the MCN was inferior to the latissimus dorsi tendon, accompanied by a lateral root of the median nerve with mean length of 12.0 mm (Fig. 3).

Fig. 1 Anterior compartment in a left arm. Origin of the musculocutaneous nerve from the median nerve. MCN: Musculocutaneous nerve. RN: Radial nerve. MN: Median nerve; UN: Ulnar nerve; BBM: Biceps brachii muscle. (*): Medial root of the median nerve; (**): Lateral root of the medial nerve. 

Fig. 2 Anterior compartment in a left arm. Origin of the musculocutaneous nerve at the level of the upper edge of latissimus dorsi tendon and non-perforation of the coracobrachialis muscle. MCN: Musculocutaneous nerve. RN: Radial nerve. MN: Median nerve; UN: Ulnar nerve; CbM: Coracobrachialis muscle; BBM: Biceps brachii muscle; (s): Latissimus dorsi tendon. 

Fig. 3 Anterior compartment in a right arm. Origin of the musculocutaneous nerve inferior to the latissimus dorsi tendon and non-perforation of the coracobrachialis muscle. MCN: Musculocutaneous nerve. RN: Radial nerve. MN: Median nerve; UN: Ulnar nerve; CbM: Coracobrachialis muscle; BBM: Biceps brachii muscle; (*): Lateral cord of the brachial plexus; (**): Medical cord of the brachial plexus; (s): Latissimus dorsi tendon. 

In four arms (3.8 %) the MCN was absent and the branches for the muscles of the anterior compartment of the arm and the lateral antebrachial cutaneous nerve originated from the MN (Fig. 4). We found bilateral case of absence of the MCN and 1 bilateral case of nonperforation of the CbM.

Fig. 4 Anterior compartment in a left arm. Absence of the musculocutaneous nerve. MN: Median nerve; UN: Ulnar nerve; CbM: Coracobrachialis muscle; BBM: Biceps brachii muscle; (*): Branch for the Coracobrachialis muscle; (**): Branch for the Biceps brachii muscle; (***): Branch for the Brachialis muscle. 

In the variant arms, the branches for the biceps brachii muscle and brachialis muscle originated at distance from the coracoid process at a mean of 106.6±23.2 mm and 202.7±20.2 mm respectively. The length of the branch for the brachialis muscle was at a mean of 37.7±15.4 mm and for the short and long heads of the biceps 27.6±11.7 mm and 33.2±10.1 mm respectively. No statistically significant differences were found between the sides and the morphometric variables (p > 0.05).

DISCUSSION

The MCN can travel with the lateral root of the MN and subsequently arise from the MN (^{Rea, 2016}). In this pattern, the MCN gives off a branch to the CbM but without piercing it (Choi et al.). This corresponds to ^{Le Minor (1990}) classification type IV. Furthermore, the MCN originated directly from the lateral cord of the brachial plexus may have a trajectory in the arm without perforating the CbM. This pattern has been included in previous classifications of MCN and MN communications: Venieratos & Anagnostopoulou type III and Loukas & Aqueelah, type III. Some authors classified Le Minor types IV and V in a same group: Choi et al. type 1, ^{Maeda et al. (2009}) type E and ^{Hayashi et al. (2017}) type 0. Conceptually Le Minor types IV and V correspond to a fusion between MCN and MN. In the first one the fusion dissolves and in the second it continues indefinitely.

During the embryologic development, the MCN nerve is derived relatively late. Its absence may correspond to an incomplete differentiation of the brachial plexus (^{Bulla et al., 2012}). In the absence of the MCN, its fibers are fused with the MN forming a common trunk. In this case, the branches for the muscles of the anterior compartment of the arm and the lateral antebrachial cutaneous nerve arises from the median nerve (^{Maeda et al., 2009}; Bulla et al.). This corresponds to Le Minor classification type V. Studies by Beheiry reported that in MCN absence the branch for the CbM may arise from the lateral cord of the brachial plexus and not from the median nerve.

The frequencies of the anatomic variations in relation to the origin of the MCN varies in different population groups (Table I). The frequency of the non-perforation of the CbM has been reported in a range between 1.3 % and 11.1 %. We found a prevalence of 5.7 % which is in an intermediate range with respect to previous findings. On the other side, the prevalence of the absence of the MCN has been reported in a range between 0.8 % and 8.3 %. Our findings are consistent with the literature concerning to the frequency of this variation. Studies by ^{Prasada Rao & Chaudhary (2001}) and Bulla et al. reported a high prevalence, however, the sample is smaller than other studies with lower frequencies. We consider that factors such as the sample size and the biological characteristics of the populations may explain the variability of the results.

Table I Frequencies of the anatomic variations in relation to the origin of the MCN in different population groups. 

Regarding to the morphometric, in our series the distances from the coracoid process to the emergence of the branches for the biceps brachii are slightly shorter compared to the findings of previous studies in non-variant arms (^{Yang et al., 1995}; ^{Apaydin et al., 2009}). However, the distance from the coracoid process to the origin of the branch for the brachialis muscle is slightly longer compared to prior studies (Yang et al.; Apaydin et al.). Our findings are consistent with the literature concerning to the length of the branches (Yang et al.; Apaydin et al.).

The MCN has a protected location and course, deep within the arm (Tubbs, 2015). When the MCN do not pierce the CbM, is exposed for a traumatic or iatrogenic injuries. On the other side, when the MCN is absent, an injury to the median nerve will lead to unusual clinical symptoms and/or unusual physical examination.

An adequate knowledge of anatomical variations that compromise nerve structures at the arm level is relevant for clinical practice. It allows assessment and appropriate management in those patients with motor disorders who present unusual symptomatology. In addition, this knowledge is also important during electromyography procedures, nerve blocks and surgical approaches to the arm by decreasing the risk of iatrogenia.