CONCURRENT VARIATIONS IN THE FORMATION OF LATERAL CORD AND MEDIAN NERVE OF BRACHIAL PLEXUS

Lateral cord and median nerve are associated with variations. We observed concurrent variations in the formation of lateral cord and median nerve in thirty two formalin fixed cadavers. In 29.6% upper limbs Lateral cord was formed by anterior division of upper trunk only. In 21.8% of these cases the anterior division of middle trunk formed two additional roots for the median nerve and in 7.8% cases anterior division of middle trunk joined with medial root of median nerve. In 14% upper limbs the upper trunk was not formed and the lateral cord was formed by union of anterior division of C5, C6 and C7 roots. In 6.2% of such cases where upper trunk was not formed, the median nerve received an additional root from lateral cord. We believe that prior knowledge of such anatomical variations is of interest to the anatomist and clinician alike. Surgeons who perform procedures involving neoplasms or repairing trauma need to be aware of these variations.


INTRODUCTION
The brachial plexus (BP) is a major and complicated plexus at the root of neck.It is formed by the union of the ventral rami of inferior four cervical (C 5 -C 8 ) and first thoracic (T 1 ) nerves.Upon exit from the intervertebral foramina, the ventral rami of C 5 and C 6 cervical nerves unite to form upper trunk.Ventral ramus of C 7 nerve continues as the middle trunk and the ventral rami of C 8 and T 1 nerves unite to form the lower trunk.Each trunk bifurcates into anterior and posterior divisions.The anterior division of upper and middle trunk forms the lateral cord (LC).

Lateral cord and median nerve
Rev Arg de Anat Clin;2013, 5 (3) The anterior division of lower trunk continues as the medial cord.The posterior divisions of all three trunks unite to form posterior cord (Snell, 2004;Standring et al, 2005).Normally the LC after giving the lateral pectoral nerve divides into musculocutaneous nerve and lateral root of the median nerve (MN).The lateral root then joins the medial root from the medial cord to form the median nerve (Venieratos and Anagnostopoulou, 1998).Understanding of the anatomical variations of the BP important for diagnosing unexplained clinical cases during surgical procedures to the neck and axillary region.It also facilitates the anesthesia in that region (Orebaugh and Williams, 2009;Sassoli Fazan et al, 2003;Loukas et al, 2008).

MATERIAL AND METHOD
Thirty two formalin fixed cadavers (sixty four upper limbs) obtained from Anatomy department of L.N. Medical College constitute the material for study.During routine dissection of root of neck and axilla, various muscles were reflected to note the variations in the formation of lateral cord and median nerve of brachial plexus.

RESULTS
We observed formation and branching pattern of LC in thirty two formalin fixed cadavers (sixty four upper limbs).In thirty six (56.2%) upper limbs LC showed usual formation by union of anterior division of upper and middle trunk.In nineteen (29.6%) upper limbs LC was formed by anterior division of upper trunk only (Fig. 1, Fig. 2).In fourteen (21.8%) of these cases the anterior division of middle trunk formed two additional roots for the MN (Fig. 1) and in five (7.8%) cases anterior division of middle trunk joined with medial root of median nerve (Fig. 2).In nine (14%) upper limbs the upper trunk was not formed.In these cases the LC was formed by union of anterior division of C 5 , C 6 and C 7 roots (Fig. 3).In four (6.2%) of such cases where upper trunk was not formed the MN received an additional root from lateral cord (Fig. 3).In five (7.8%) upper limbs MN formation was normal by lateral and medial root even in absence of upper trunk.

DISCUSSION
The knowledge of variations in the formation, course and branching of the lateral cord of brachial plexus assumes importance while performing shoulder arthroscopy and shoulder reconstructive surgeries (Chitra, 2007).Uzun and Bilgic (1999) in their study on one hundred and thirty brachial plexus observed that in four brachial plexus, LC was formed by continuation of anterior division of upper trunk alone.In the present study we also observed formation of the LC by continuation of anterior division of upper trunk alone in 29.6% cases (Fig. 1).Matejcik (2003) described in three cases and Villamere et al (2009) and Singla et al, (2011) in their case reports did not find formation of upper trunk.In these cases C 5 and C 6 root united with that of C 7 root to form LC. In our study upper trunk was not formed in 14% cases (Fig. 3).The LC was formed by union of anterior division of C 5 , C 6 and C 7 roots.The brachial plexus without upper trunk may increase the chance of nerve root avulsion due to downward traction injury (Stevens, 1988).These variations can be correlated embryologically.Position and width of a limb bud determines its innervation.Limb bud is supplied Lateral cord and median nerve Rev Arg de Anat Clin; 2013, 5 (3): 208-212 __________________________________________________________________________________________ by nerves of region where it is implanted.Segregation of the developing structure within the limb directs growing nerve fibers (axons) and determines their grouping into bundles leading to formation of roots and trunks (Keibel et al, 1912).As the expression of chemoattractants and chemorepulsants regulates the growing nerve fibers (axons) in a highly coordinated site specific fashion, any alterations in signaling between the mesenchymal cells and neuronal growth cones can lead to significant variations (Sannes et al, 2000).In the present study non formation of upper trunk appears to be a result of over expression of chemo attractants/repulsants, leading to separation of the C 5 and the C 6 roots.Miller (1932) in different vertebrates observed the pattern of roots, trunks, cords and branches of brachial plexus.He reported no trunk formation in amphibians, reptiles and dogs.In our study 14% cases showed absence of the upper trunk (Fig. 3).These cases partially fit into this category of amphibians, reptiles and dogs indicating ontogeny repeats phylogeny.Median nerve is associated with several variations which include communication with musculocutaneous nerve (Chauhan and Roy, 2002), splitting of the median nerve (Sundaram et al, 2008) and unusual innervations of the flexor muscles of arm (Nayak, 2007).Variations in the MN formation with an additional root that arose from LC were also reported earlier (Pais et al, 2010;Sontakke et al, 2011).In the present study we observed variations in the formation of MN in concurrence with unusual formation of lateral cord.In our study in 21.8% cases anterior division of middle trunk provides additional roots to the MN (Fig. 1).In 7.8% cases anterior division of middle trunk joins with medial root of median nerve to provide C 7 fibers to MN (Fig. 2).We believe that prior knowledge of such anatomical variations of lateral cord and median nerve is of interest to the anatomist and clinician.Variations assume significance during surgical exploration of the axilla and can even lead to failure of nerve block of the infraclavicular part of brachial plexus.Surgeons who perform procedures involving neoplasm or reparing trauma needs to be aware of these variations.