Keywords
communication - lateral cord - macroscopic human anatomy - median nerve - musculocutaneous
nerve
Introduction
The brachial plexus is formed by the ventral rami of the lower four cervical nerves
and by the first thoracic nerve, and it supplies the upper limbs. C5, C6, C7, C8 and
T1 roots unite to form trunks (upper, middle, and lower) that divide into anterior
and posterior divisions. The anterior divisions of the upper and middle trunks unite
to form the lateral cord, which lies laterally to the axillary artery. The musculocutaneous
nerve is a continuation of the lateral cord and leaves the axilla by piercing the
coracobrachialis muscle. It supplies all of the muscles of the anterior compartment
of the arm and continues as a lateral cutaneous nerve of the forearm. The median nerve
is formed by two roots; the medial root of the median nerve from the medial cord,
and the lateral root of the median nerve from the lateral cord. Both roots join to
form the median nerve in front of the third part of the axillary artery. The median
nerve crosses the brachial artery from the lateral to the medial side in the middle
of the arm and does not originate any branches in the arm. In the literature, various
anatomical variations were described by many authors. The knowledge regarding these
variations can serve as a useful guide for surgeons operating in the axillas and in
the arms.[1]
Materials and Methods
The present study was conducted on 60 upper limbs dissected in the anatomy department
of the Kasturba Medical College, Manipal, India, over a period of 2 years. The cadavers
were embalmed and preserved in a weak formalin solution. The infraclavicular part
of the brachial plexus was dissected according to the guidelines of the Cunningham's
manual of Practical Anatomy.[2] During the dissection, the normal pattern, as well as variations from the normal
pattern, were noted and photographed. The number of the variations was noted and the
result was tabulated using a regular statistics method. The study was started after
obtaining the Institutional Ethical Clearance. Any variations from normal was noted
and tabulated ([Table 1]).
Discussion
Variations of the lateral cord are not rare and have been reported by many authors
in the past, such as Venieratos et al,[3] Beheiry,[4] Budhiraja et al,[5] and Chitra.[6] The comparison with similar studies has been shown in [Table 2]. The causes for these anatomical variations in these peripheral nerves are not well
understood. It is mentioned that, in humans, the muscles of the upper limbs are derived
from the paraxial mesoderm during the 5th week of development.[7] The axons of the spinal nerves grow toward the mesenchyme. If there is any altered
signaling between them, it results in significant variations in the nerve pattern.[8]
Venieratos et al[3] described three different types of communications between the musculocutaneous and
median nerves in relation to the coracobrachialis muscle. In 16 out of 79 cadavers,
22 communications were found between the musculocutaneous and median nerves. In six
subjects, they were present bilaterally. There were three types based on the sites
of communications.
-
Type I: The communication was proximal to the entrance of the musculocutaneous nerve
into the coracobrachialis muscle;
-
Type II: The communication was distal to the coracobrachialis muscle;
-
Type III: The nerve, as well as the communicating branch, did not pierce the coracobrachialis
muscle. Bilateral communications were not necessarily of the same type.
In the present study, the communication between the median nerve and the musculocutaneous
nerve was distal to the coracobrachialis muscle.
In the present study, the musculocutaneous nerve was absent in 3 cases out of 60 (5%).
Among these three cases, one case of bilateral absence of musculocutaneous nerve was
seen. The coracobrachialis muscle was innervated by a thin branch directly from the
lateral cord. The rest of the muscles of the anterior compartment were supplied by
the median nerve and, finally, one branch from the median nerve continued as a lateral
cutaneous nerve of the forearm. This case was similar to one reported by Beheiry.[4] There was no communication between the median nerve and the musculocutaneous nerve
observed in this case. The third case of absence of musculocutaneous nerve in the
present study was seen in the right upper limb of an adult male cadaver. In the absence
of the musculocutaneous nerve, all of the muscles of the anterior compartment of the
arm were supplied by the median nerve, which later originated a branch that continued
as a lateral cutaneous nerve of the forearm. Budhiraja et al[5] reported the absence of the musculocutaneous nerve in 13 cases out of 116 (11%).
In the present study, the musculocutaneous nerve was seen piercing the coracobrachialis
muscle in all of the cases in which it was present.
Communication between the median and the musculocutaneous nerves was observed in 8
cases out of 60 (13.33%). Among these noted communications, five were seen on the
left side, and three on the right side. In 1 case out of 60 communications between
the median and the musculocutaneous nerves was seen on both arms of an adult male
cadaver. According to the studies of Beheiry,[4] Budhiraja et al,[5] Dahiphale et al.,[11] Chitra,[6] and Choi et al.,[12] communications between the median and the musculocutaneous nerves was seen in 5%
of the cases, in 20.7% of the cases, in 25% of the cases, in 26% of the cases, and
in 46.4% of the cases, respectively. Joshi et al[13] reported absence of the musculocutaneous nerve in 5.5% of the cases, and communication
between the median and musculocutaneous nerves was noted in 14% of the cases. The
result of the present study is related very closely to this study. In the present
study, the communication between the median and the musculocutaneous nerves was more
common on the left side, as seen in the study by Choi et al.[12]
In the present study, formation of the median nerve by three roots was noted in 8
out of 60 cases (13.33%). In all of these cases, two roots were coming from the lateral
cord, and one root from the medial cord (four on the left side and four on the right
side). In one case, there was also communication between the median and the musculocutaneous
nerves. A similar case of formation of the median nerve by three roots was described
by Sargon et al.,[14] Saeed et al.,[15] and by Das et al.[16]; with two roots from the lateral cord and one from the medial cord. Balachandra
et al (2015) observed 3 roots of the median nerve in 5% of the cases with absence
of the musculocutaneous nerve.[17]
The variations reported in the present study are of importance to surgeons during
arthroscopic shoulder reconstructive surgery, nerve block, and in any surgery performed
for pathologies involving the coracobrachialis muscles. Knowledge regarding these
nerve variations may also be of use in case of treatment for fractures of the midshaft
of the humerus. Chances of damage to these nerves can be due to trauma, to tractions,
and to compression. It is observed that variant nerves are more prone to compression
neuropathy. The knowledge of these variations also helps in correlating particular
clinical manifestations with the involved nerve damage. In patients of breast carcinoma
undergoing mastectomy, the coracobrachialis muscle is used as a flap to cover the
defect after the mastectomy. Therefore, it becomes important to have knowledge regarding
the normal anatomy as well as any observed variations in the muscle.[18]
Fig. 1 Three roots of the median nerve with communication between the median nerve (MN)and
the musculocutaneous nerve(MCN).
Fig. 2 Absence of musculocutaneous nerve and muscles of the anterior compartment of the
arm supplied by the median nerve. Abbreviations: LCNF, lateral cutaneous nerve of
the forearm; MN, median nerve.
Fig. 3 Communication between the median nerve (MN) and the musculocutaneous nerve(MCN).
Absence of musculocutaneous nerve. Median nerve supplying all of the muscles of the
anterior compartment of the arm.
