Keywords brachial plexus - trunks
Introduction
Most of the upper limb nerves arise from by the brachial plexus (BP), which has a
complex anatomical structure that begins in the root of the neck and extends to the
axilla (armpit region). The BP is formed by the anterior rami of the lower four cervical
nerves (C5-C8) and first thoracic nerve (T1), which form the roots of the brachial
plexus.[1 ]
[2 ]
[3 ] Generally, the roots of BP nerves pass through the space between the anterior and
middle scalene muscles along with the subclavian artery and join with each other by
anastomosis leading to the formation of the upper trunk (C5 and C6), medium trunk
(C7), and lower trunk (C8 and T1), accompanying the axillary artery under cover of
the pectoralis major muscle. These three trunks pass through the cervicoaxillary canal,
posterior or anterior the clavicle or behind; each trunk splits into anterior and
posterior divisions: the anterior divisions from the upper and middle trunks form
the lateral cord, the anterior divisions from the lower trunk form the medial cord,
and the three posterior divisions from the posterior cord.[2 ]
[3 ]
[4 ] The supraclavicular branches (descending branches) of BP arise from the trunk or
from cervical nerves, parallel to the trunk, and are as follows: dorsal scapular nerve,
suprascapular nerve, long thoracic nerve, and subclavian nerve. The short infraclavicular
branches, namely axillary nerve, subscapular nerves, thoracodorsal nerve, and medial
and lateral pectoral nerve, arise from the cords and innervate the shoulder muscles.
The long infraclavicular branches, which extend longitudinally to the upper limb,
are: the radial nerve, the ulnar nerve, and the median nerve.[4 ]
[5 ]
Variations in the formation of the BP are common and are usually related to the contribution
of anterior rami of C4 and T2 nerve roots. When the superior root of the plexus is
C4 and the inferior root is C8, it is a prefixed Brachia Plexus . When the superior root is C6 and the inferior root is T2, it is a postfixed BP. Variations may also occur in the formation of trunks, divisions and cords, in the
origin and/or combination of branches, and in the relationship with the axillary artery
and scalene muscles.[2 ]
[3 ]
[5 ]
The BP is of great importance in medical and surgical procedures and in making a diagnosis
of orthopedic injuries, such as neurovascular compression of the subclavian artery
and BP injuries.[6 ] Brachial plexus blocks, such as the interscalene and inter-sternocleidomastoid blocks,
are commonly employed anesthesia techniques for proximal upper extremity surgery.[7 ] The knowledge of the anatomical variations of the BP is very important to anatomists,
radiologists, anesthesiologists, and surgeons, mainly for making accurate diagnosis
of nerve injuries and providing the most effective and appropriate treatment for injuries
in the axillary region and upper limb. Moreover, from a clinical point of view, in
terms of anesthesia and surgical interventions, it is necessary to understand BP variations,
morphometry, and possible anastomosis.[8 ]
[9 ]
Due to their importance in surgical procedures involving the upper limb, variations
of the brachial plexus have been addressed in the scientific literature in terms of
their origin, formation, and distribution. Therefore, studies aimed at contributing
to the morphological knowledge of BP and its variations are of paramount importance
as a source of information and guidance for safe surgical procedures in brachial plexus
injuries. Accordingly, the present study addresses a bilateral anatomical variation
in the formation of trunks of the brachial plexus.
Case Report
During a routine dissection of a 60 year-old Caucasian male cadaver fixed in 10% formalin,
in the Human Anatomy Laboratory of the University of Santa Cruz do Sul, RS, Brazil,
we found a bilateral variation in the formation of the trunks of the brachial plexus;
there were four trunks instead of the usual three.
The right brachial plexus/RBP ([Fig. 1 ]) was composed of six roots (C4-T1), which formed four trunks: first trunk (C4-C5);
second trunk (C6); third trunk (C7) and fourth trunk (C8-T1). The four trunks formed
anterior divisions, and only the three superior trunks formed posterior divisions.
Fig. 1 Right brachial plexus: C4 (Roots of the C4), C5 (Roots of the C5), C6 (Roots of the
C6), C7 (Roots of the C7), C8 (Roots of the C8), T1 (Roots of the T1). A1 (First trunk),
A2 (Second trunk), A3 (Third trunk) and A4 (Fourth trunk). RC (Rear Cord), MC (Medial
cord) and LC (Lateral Cord). N1 (Axillary Nerve), N2 (Musculocutaneous Nerve), N3
(Radial Nerve), N4 (Median Nerve) and N5 (Ulnar Nerve). FD (Former Divisions) and
PD (Posterior Divisions).
The posterior cord was formed by posterior divisions of the three superior trunks
(C4-C7); the medial cord was formed by anterior divisions of the two lower trunks
(C7-T1); and the lateral cord was formed by anterior divisions of the first three
trunks (C4-C7).
The median and ulnar nerves were formed by anterior divisions of the four trunks,
where one branch originated from the lateral cord, called lateral root (C4-C7), and
the other branch originated from the medial cord, called medial root (C8-T1). The
axillary nerve was formed by the union of the two posterior divisions of the first
(C4-C5) and second trunk (C6). The radial nerve originated from the posterior cord
(C4-C7) and had no communication with the C8 and T1 roots; the musculocutaneous nerve
was formed by the three anterior divisions of the three superior trunks (C4-C7).
The left brachial plexus/LBP ([Fig. 2 ]) was composed of five roots (C5-T1), which formed four trunks: first trunk (C5);
second trunk (C6); third trunk (C7), and fourth trunk (C8 and T1). All four trunks
formed anterior and posterior divisions.
Fig. 2 Left brachial plexus: C5 (Roots of the C5), C6 (Roots of the C6), C7 (Roots of the
C7), C8 (Roots of the C8), T1 (Roots of the T1), A1 (First trunk), A2 (Second trunk),
A3 (Third trunk) and A4 (Fourth trunk). RC (Rear Cord), MC (Medial cord) and LC (Lateral
Cord). N1 (Axillary Nerve), N2 (Musculocutaneous Nerve), N3 (Radial Nerve), N4 (Median
Nerve) and N5 (Ulnar Nerve). FD (Former Divisions) and PD (Posterior Divisions).
The posterior cord was formed by posterior divisions of the four trunks (C5-T1); the
medial cord was formed by anterior divisions of the third (C7) and fourth trunk (C8
and T1); the lateral cord was formed by anterior divisions of the first three trunks
(C4-C7).
The median nerve was formed by anterior divisions of the four trunks (C5-T1), where
one branch originated from the lateral cord, called lateral root (C5-C7), and the
other branch was originated from the medial cord, called medial root (C7-T1). The
ulnar nerve was formed by anterior divisions of the third (C7) and fourth trunk (C8
and T1). The axillary nerve was formed by posterior divisions of the first (C5) and
second trunk (C6). The radial nerve was formed by the posterior divisions of the four
trunks (C5-T1); the musculocutaneous nerve was formed by anterior divisions of the
two superior trunks (C5 and C6).
The Tables below show a summary of the contributions of the roots to the formation
of the trunks of the brachial plexus: [Table 1 ], typical pattern described in the literature according to Standring[3 ]; [Table 2 ], right brachial plexus; and [Table 3 ], left brachial plexus.
Table 1
typical pattern described in the literature according to Standring[3 ]
Cords
Nerve
Roots
Lateral
Musculocutaneous
C5, C6, C7
Roots lateral of the nerve median
(C5), C6, C7
Medial
Ulnar
C7, C8, T1
Roots medial of the nerve median
C8, T1
Posterior
Axillary
C5, C6
Radial
C5, C6, C7, C8, (T1)
Table 2
Right brachial plexus
Cords
Nerve
Roots
Note
Lateral
Musculocutaneous
C4, C5, C6, C7
+ C4
Roots lateral of the nerve median
C4, C5, C6, C7
+ C4
Roots lateral of the nerve ulnar
C4, C5, C6, C7
+ C4
Medial
Roots medial of the nerve ulnar
C8, T1
- C7
Roots medial of the nerve median
C8, T1
Posterior
Axillary
C4, C5, C6
+ C4
Radial
C4, C5, C6, C7
+ C4 / - C8, - T1
(+): inclusion in the formation; (-): absence in the formation.
Table 3
Left brachial plexus
Cords
Nerve
Roots
Note
Lateral
Musculocutaneous
C5, C6
- C7
Roots lateral of the nerve median
C5, C6, C7
Medial
Ulnar
C7, C8, T1
- C7
Roots medial of the nerve median
C7, C8, T1
+ C7
Posterior
Axillary
C5, C6
Radial
C5, C6, C7, C8, T1
(+): inclusion in the formation; (-): absence in the formation.
Discussion
The present study demonstrates a bilateral variation in the formation of the BP trunks,
in which the RBP was composed of six roots (C4-T1) and the LBP of five roots (C5-T1).
The nerve fibers that contribute to the formation of the upper limb nerves showed
variations due to the presence of four trunks. There are very few reports in the scientific
literature addressing cases of BP with four trunks. Investigating 60 BPs which belonged
to 30 cadavers, Chaudhary et al[10 ] found four trunked brachial plexuses on the left side in three cases. Of the three
cases, one BP (prefixed and postfixed) was composed of roots C4-T2 which formed four
trunks: first trunk (C4-C6), second trunk (C7), third trunk (C8), and fourth trunk
(T1 and T2). In the other two cases, the BPs (postfixed) were composed of C5-T2 roots,
which formed four trunks: first trunk (C5 and C6), second trunk (C7), third trunk
(C8), and fourth trunk (T1 and T2). Although the same variation of four trunked plexus
was found in the present study, these authors found different variations in terms
of the fibers from which they originate and trunk endings.
During routine dissection of a female cadaver, Singla et al.[11 ] found two trunks instead of the usual three in the RBP: upper trunk (C5 and C6)
and the lower trunk (C7, C8, and T1) with an absence of the middle trunk. Yildiz et
al[12 ] found the same variation in a male cadaver in the LBP.
In another study, Shetty et al[13 ] found unilateral variations in the formation of the trunks in five (11.3%) of 44
cadavers investigated. In one cadaver (2.27%), the middle trunk was formed by the
union of C7 and C8 roots, and the lower trunk was formed by the T1 root. In another
cadaver (2.27%), the upper and middle trunks were fused with each other. In the other
three cadavers (6.81%), the C5 root pierced the anterior scalene muscle before joining
C6 to form the upper trunk.
The prevalence of anatomical variants of the brachial plexus is addressed in different
specimens in the literature. One of the first common variants described is the prefixed
or postfixed types of BP.[14 ] In the present study, a prefixed RBP was found, in which the C4 roots contribute
to its formation, but the T1 root is not smaller or absent and is similar in caliber
to the other roots. Ongoïba et al[15 ] and Bowden[16 ] argue that the prevalence rates for the prefixed type range from 12 to 30%, and
that the postfixed type occurs in 0% to 11% of the cases. In a study conducted in
Brazil, it was found a prevalence of the prefixed type in 20% to 24% of cases, and
the postfixed type in 0% to 6% of the cases.[17 ]
Fazan et al.,[1 ] evaluated 54 BPs in 27 cadavers and found normally formed BPs in 39 cadavers (72%)
and prefixed BPs in 13 cadavers (24%); two males and eleven females (54% RBPs and
46% LBPs). Only three BPs were postfixed and they were found in male cadavers, two
in the right antimere and one in the left antimere. According to the authors, intercordal
neural communications between the lateral and medial cords are relatively common,
especially between the lateral cord and the ulnar nerve or between the median nerve
and the musculocutaneous nerve. They found communications between the lateral cord
and the ulnar nerve in 30% of cases. The same variation was found by Harris[14 ] in 26 (86.66%) of the 30 upper limbs studied by this author.
Investigating 32 upper limbs in 16 formalin-fixed Brazilian cadavers of both sexes,
Olave et al[18 ] found a communicating branch between the musculocutaneous nerve and the median nerve
in ten cases (31.3%), nine males, four in the right antimere and six in the left antimere,
and two with bilateral variation. Chiarapattanakom et al[19 ] found the same communication branch in 21.8% of 57.3% cases investigated. In 75
(90.5%) of the cases studied by Tountas and Bergman,[20 ] the musculocutaneous nerve originated from the BP, 2% originated from the median
nerve, and 1.4% from the posterior cord. In the present study, it was found a communicating
branch between the musculocutaneous and median nerves in the BP in both antimeres.
In the RBP, the radial nerve was formed by the posterior cord (C4-C7) showing no communication
with the C8 and T1 roots, and the axillary nerve was formed from the fusion of the
two posterior divisions of the first (C4-C5) and second trunk (C6).
According to Albertoni et al.,[17 ] the prevalence rates of anatomical variants of the BP varies depending on the population
studied and can be up to 48%. In some individuals, the trunk divisions or the cord
formations may be absent in different parts of the BP; However, the makeup of the
terminal branches is invariable. Moreover, the lateral or medial cords may receive
fibers from anterior divisions directly inferior or superior to the usual levels respectively,
resulting in the fusion of different calibers the terminal branches.[1 ]
[2 ] argue that variations in the BP are not influenced by gender or race and are more
common in a certain antimere.
Gonzáles et al[21 ] stated that severe BP injuries can cause shoulder paralysis due to disrupted innervation
of the muscles. An epidemiological study on traumatic BP injuries in adults performed
by Flores[22 ] showed that the BP is very susceptible to injuries due to its anatomical position
and relationship with other anatomical structures; therefore knowledge of anatomy
is essential for treatment and injury prevention.
According to Olave et al,[18 ] there may be nerve fibers that run for some distance in another nerve, to reach
their innervation territory and return to the nerve they belong to be distributed,
which are sometimes denominated anastomotic communications and allow the understanding
of the variations in the innervation territories of nearby nerves. Moreover, the presence
of anatomical variations of the peripheral nervous system is often used to explain
unexpected clinical signs and symptoms. Therefore, the knowledge of the distribution,
route, and patterns of the rami of nerves, cords, trunks, and roots that make up the
BP are fundamental for clinical investigations involving peripheral nerve injuries.
Considering that every individual may have some type of anatomical variation, the
present study aims to deepen the knowledge of anatomical variations of the BP, which
can be an important source of information to help healthcare professionals to acknowledge
and interpret BP variations make accurate diagnosis, improve surgical safety, and
provide effective treatment facilitating recovery.
