Anatomic Variations in Relation to the Origin of the Musculocutaneous Nerve: Absence and Non-Perforation of the Coracobrachialis Muscle. Anatomical Study and Clinical Significance

Abstract

SUMMARY:The most frequent anatomic variations of the musculocutaneous nerve could be divided in two main groups: communicating branches with the median nerve and variations in relation to the origin, which in turn can be subdivided into absence of the nerve and non-perforation of the coracobrachialis muscle. Unusual clinical symptoms and/or unusual physical examination in patients with motor disorders, could be explained by anatomic variations of the musculocutaneous nerve. A total of 106 arms were evaluated, corresponding to 53 fresh male cadavers who were undergoing necropsy. The presence or absence of the musculocutaneous nerve was evaluated and whether it pierced the coracobrachialis muscle or not. The lengths of the motor branches and the distances from its origins to the coracoid process were measured. In 10 cases (9.5 %) an unusual origin pattern was observed, of which six (5.7 %) correspond to non-perforation of the coracobrachialis muscle and four (3.8 %) correspond to absence of the nerve. The length of the branch for the brachialis muscle was 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. The study showed that our population has similar prevalence of absence of the musculocutaneous nerve and non-perforation of the coracobrachialis muscle compared to previous reports in different populations. Moreover, there was no statistical correlation between the sides and the evaluated variables.

A Review of the Typical Course of the Musculocutaneous Nerve into the Coracobrachialis Muscle: Its Variability and Possible Clinical Implications

“…Α total of 126 studies were identified, 28 of which were assessed as full-text articles (Figure 1). Out of these, 23 manuscripts (1,2,15,(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38) were used for estimation of the MCN's typical course, 10 (6, 12, 15, 17, 27, 34, 36, 39-41) for calculation of the distance between the CP and the entrance point of the MCN into the CB, and 7 (2,6,12,15,17,36,40), for computation of the distance between the CP and the point of origin of the proximal branches.…”

“…Developmentally, the MCN derives late during arm development. Disruption in BP differentiation may lead to the MCN's absence (24,42,43). In these circumstances, the MCN fibers are fused with the median nerve (MN) in a common trunk, responsible for the anterior arm compartment's motor innervation (21).…”

“…In these circumstances, the MCN fibers are fused with the median nerve (MN) in a common trunk, responsible for the anterior arm compartment's motor innervation (21). In embryology "the nerve follows the muscle" and as a result a developmental problem in muscle differentiation also leads to abnormal innervation (24,42,43). In the current review, in cases of MCN absence, the CB is usually innervated by the MN's branches, and less frequently by a branch arising directly from the BP's lateral cord.…”

Coracobrachialis muscle morphology and coexisted neural variants. A cadaveric case series

“…The muscles' formation is completed before nerves' formation. Thus, "the nerve follows the muscle", and as a result, a developmental problem in muscle differentiation leads to the abnormal innervation [4,15,28]. After the formation of the muscles and nerves supplying the entire limb, begin to form a plexus, in the form of loops, between nerve bers, and in this way the MCN originates from the MN [3], late during arm development.…”

“…After the formation of the muscles and nerves supplying the entire limb, begin to form a plexus, in the form of loops, between nerve bers, and in this way the MCN originates from the MN [3], late during arm development. Disruption in the brachial plexus differentiation may lead to the MCN absence [4,15,28].…”