Bilaterally Symmetrical Dual Origin of Muculocutaneous Nerve.

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Oct 10, 2013 (3 years and 6 months ago)

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Bilaterally Symmetrical Dual Origin of Muculocutaneous Nerve.
Avinash Abhaya, Bhardwaj R. * and Prakash R. *
King George Medical University, Lucknow & *UCMS & G.T.B. Hospital, Delhi
Abstract: Variations of the arrangement and distribution of the lateral cord and its branches in the infraclavicular
part of the brachial plexus are common and are of significance to the neurologists, surgeons, anesthetists and the
anatomists. The present report describes a rare variation of the musculocutaneous nerve having a bilaterally
symmetrical dual origin, observed during routine dissection of a 33-year-old Indian male cadaver. The
musculocutaneous nerve and its twigs should be identified and protected, keeping in mind the variations in
anatomy while planning any exploration in the region of axilla and upper arm. The clinical importance of the
variation is discussed.
Key words: Musculocutaneous nerve, Median nerve, Anatomical variations.
Introduction:
Variations of the brachial plexus regarding its origin,
level of junction or separation of cords, composition of
fiber bundle, pre-fixation, post-fixation, relations with
subclavian and axillary artery and absence or
communication between its branches are common and
are being reported by several authors (Kerr, 1918; Linell,
1921; Miller, 1934; Iwamotto et al, 1990; Nakatani et
al, 1997; Sud and Sharma, 2000; Chauhan and Roy,
2002; Choi et al, 2002; Abhaya et al, 2003).
Brachial plexus is a complex of nerves originating
in the neck and axilla and is formed by the union of the
ventral rami of fifth, sixth, seventh, eighth cervical, and
the first thoracic spinal nerves which then unite, divide
and unite again to form three trunks (upper, middle,
and lower), three cords (medial, lateral, and posterior)
and the nerves of the upper extremities. The lateral
cord contains the fibers from C
5
, C
6
and C
7
and also
from C
4
if these join the plexus, while the medial cord
from C
8
and T
1
. The lateral and medial cord represents
the anterior divisions of the brachial plexus and their
branches supply the anterior muscles of the limb. The
posterior cord is formed by the union of all the posterior
divisions and so receives fibers from all the nerves
entering in the plexus (Hollinshed, 1979).
Normally, the lateral cord gives its first branch, the
lateral pectoral nerve to supply the pectoralis major
muscle and then it divide into musculocutaneous and
the lateral root of the median nerve. The
musculocutaneous nerve (C 5, 6, 7) pierces the
coracobrachialis muscle and than passes obliquely
down to the lateral side of the arm between the biceps
brachii and brachialis muscle, pierces the deep fascia
lateral to the tendon of the biceps brachii near elbow
and is continued as the lateral cutaneous nerve of the
forearm. In its course through the arm it supplies the
coracobrachialis, biceps brachii and the greater part
of the brachi al i s muscl e. The branch to the
coracobrachialis is given off from the musculocutaneous
nerve close to its origin, and in some instances as a
separate filament from the lateral cord of brachial
plexus. The branches to the biceps brachii and
brachialis are given off after the musculocutaneous has
pierced the coracobrachialis; that supplying the
brachialis gives a filament to the elbow joint. The nerve
also sends a small branch to the bone, which enters
the nutrient foramen with the accompanying artery.
The median nerve (C5-T1) is formed anterior or
anterolateral to the third part of the axillary artery by
the union of its medial root from the medial cord and
lateral root from the lateral cord of the brachial plexus.
The median nerve passes in the arm at first lateral to
brachial artery and near the insertion of coracobrachialis
it crosses in front of (rarely behind) the artery,
descending medial to it in the cubital fossa, where it
passes posterior to the bicipital aponeurosis and
anterior to brachialis muscle. (Williams et al, 1995).
Case Report
During routine dissection of the axilla and arm
region of a 33 yr old Indian male cadaver in the
department of Anatomy, University College of Medical
Sciences, Delhi it is observed in the infraclavicular part
of brachial plexus that the musculocutaneous nerve is
having a dual origin and the variation of its origin, course
and distribution is symmetrical bilaterally. The higher
origin is reduced to a thin nerve, arising normally from
the lateral cord of brachial plexus and supplies only
the coracobrachialis muscle (Fig 1 and 2), while the
lower origin is of normal usual thickness is separated
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J.Anat.Soc. India 55 (2) 56-59 2006
from the lateral side of the median nerve, supplies
the biceps brachii and the brachialis muscles and then
become continuous as the lateral cutaneous nerve of
the forearm after piercing the deep fascia lateral to the
tendon of biceps brachii (Fig 1 and 2). The first branch
of the lateral cord, the lateral pectoral nerve arises
normally bilaterally just below the outer border of the
first rib to supply the pectoralis major muscle. Bilaterally
the relations of all the three cords of the brachial plexus
with the second part of the axillary artery and the further
course, branchi ng and termi nati on of the
musculocutaneous and median nerve in the forearm
and hand follow the normal usual pattern. No
communication is observed between both the origins
of the musculocutaneous nerve and in between the
musculocutaneous and the median nerve. No other
vascular or muscular variation is observed in both the
upper limbs.
Keeping the upper limb at 90 degree abducted
position the distance from the tip of the coracoid
process is recorded on both the sides. The higher origin
of the musculocutaneous nerve arise from the lateral
side of the lateral cord at 42 mm (Rt) while 44 mm (Lt)
and enter into the coracobrachialis muscle from its
medial side. The lateral and medial root of the median
nerve unite to form the median nerve at 64 mm (Rt) and
65 mm (Lt) lying anterolateral to the axillary artery.
The lower origin of the musculocutaneous nerve is
separated from the lateral side of median nerve at 73
mm (Rt) and 75 mm (Lt). The nerve to biceps brachii
originate from the lower origin of musculocutaneous
nerve at 101 mm (Rt) and 106 mm (Lt) and enter into
the biceps brachii at 128 mm (Rt) and 153 mm (Lt)
from the undersurface of the muscle from its medial
side. The branch to the brachialis muscle splits from
the lower origin of musculocutaneous nerve at 211 mm
(Rt) and 205 mm (Lt) and enter into muscle at 230 mm
(Rt) and at 240 mm (Lt). The lower musculocutaneous
continue to run along the lateral border of the forearm
and pierces the deep fascia lateral to the tendon of the
biceps brachii at 324 mm (Rt) and 325 mm (Lt) to
become the lateral cutaneous nerve of the forearm.
Discussion
The musculocutaneous nerve ordinarily enters
coracobrachialis muscle from its medial aspect
approximately 5 cm. distal to the tip of coracoid process
but is shown to have frequent variations. It may run
behind the coracobrachialis muscle or adhere for some
distance to the median nerve and pass behind the
biceps or may be accompanied by fibers from the
median nerve as it transits coracobrachialis (Kaus and
Wotowicz, 1995; Williams et al, 1995; Venieratos and
Anagnostopoulou, 1998; Sevki, 2001); less frequently
the reverse occurs, the median nerve sending a branch
to the musculocutaneous nerve. Occasionally it
supplies the pronator teres and may replace the radial
branch to the dorsal surface of the thumb (Williams et
al, 1995). Rarely the lateral cord pierces the
Fig.1 Photograph of the dissected left axilla and upper arm
showing the dual origin of the musculocutaneous
nerve. The higher origin (MC-1, Black arrow head) is
reduced to a thin nerve supplying only the
coracobrachialis muscle (CB), while the lower origin
(MC-II, Black arrow head) split from the lateral side of
the median nerve (MN) and give the nerve (NBB) to
biceps brachii muscle (BB) and continue and give
branches to the brachialis (BR) as shown as NBR in
the figure and than later become continuous as lateral
cutaneous nerve of the forearm (LCF) after piercing
the deep fascia near elbow and lateral to the tendon
of the biceps muscle. (LR), lateral root and (MR),
medial root of the median nerve join in front of the
axillary artery (AA) to form the median nerve. (UN)
Ulnar nerve.
Fig.2 Photograph of the dissected right axilla and upper
arm showing the symmetrical variation of the dual
ori gi n, course and di stri buti on of the
musculocutaneous nerve.
Bilaterally Symmetrical Dual Origin ....... Avinash Abhaya, Bhardwaj R. * and Prakash R. *
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J.Anat.Soc. India 55 (2) 56-59 2006
coracobrachialis muscle and then divides into
musculocutaneous and the lateral root of median nerve
(Le Minor, 1990, Abhaya et al, 2003).
During shoulder reconstruction surgery it is
important to identify or palpate the musculocutaneous
nerve, as it is vulnerable to injury from the retractors
placed under the coracoid muscle. The operative
management by coracoid graft transfers in the recurrent
dislocations of the shoulder and frequent shoulder
arthroscopies could be source of lesions to the
structures piercing the muscle. (Flatow et al, 1989,
Laburthe-Tolra, 1994-95). The lesion of the
musculocutaneous nerve produces weakness of elbow
flexion and supination and loss of sensation on the
lateral aspect of forearm.
The brachial plexus lesions may occur following
trauma, traction, compression of nerves, shoulder
dislocation, intraoperative nerve damages, traumatic
delivery in infants and malposition of the patient during
general anesthesia (Cooper-Daniel et al. 1988;
Schwartzman, 1991; Miller 1993; Mcllveen 1994),
resulting in complete or partial palsy of the musculature
innervated. By surgery of the lesions in continuity and
nerve grafting in cases with complete interruption,
recovery can be achieved. If only one suitable root is
present than the supraclavicular nerve and lateral cord
(musculocutaneous nerve, lateral pectoral nerve and
lateral root of median nerve) are given priority (Alnot et
al. 1981). The priority of the restoration of function is
also an important consideration. Elbow flexion should
be given the first priority, followed by wrist extension,
finger flexion and shoulder abduction in the order.
(Sedal-Laurent 1988).
The knowledge of the variations of the course and
distribution of the lateral cord of brachial plexus is
important while performing neurotization of brachial
plexus lesions, shoulder arthroscopy by anterior
glenohumeral portal and shoulder reconstructive
surgery so that these structures can be identified and
protected.
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