Lower level of formation of lateral cord, variable formation of median nerve and communications between median and musculocutaneous nerve

Brachial plexus are the bunch of nerves which are formed in the neck and axilla for supplying the upper limb. During routine dissection of the axilla for academic purpose we have noticed the variation in the formation of lateral cord, median nerve and its communications with the musculocutaneous nerve on both the upper limbs. The variations of brachial plexus even though they are common, have got very important clinical and surgical importance, knowledge of which is very important for surgeons, orthopedicians and anesthetists. An attempt has made to add the information of case to the existing knowledge in the literature.


Left side
Lateral cord is terminated by dividing into the thin lateral root of median nerve and thick musculocutaneous nerve at the level of second part of axillary artery. Median nerve is formed by the combination of medial root from medial cord and lateral root fr om lateral cord normally at the level of second part of the axillary artery. In the following course the thick musculocutaneous nerve and the median nerve again joined together at the lower border of teres major to form a single nerve and continued into the arm, measuring 6.2cms. In the arm the single trunk was medial to the coracobrachialis muscle and lateral to the brachial artery, finally terminated by dividing into muscul ocutaneous nerve and median nerve. The musculocutaneous nerve later followed normal in course and distribution.

Discussion
Gupta et al 12 have reported that the formation of lateral cord was distal than usual, in relation to the second part of axillary artery behind the pectoralis minor muscle by the union of anterior division of upper trunk and anterior division of middle trunk of brachial plexus. The similar variation we have observed in our case on right side.
Valeria et al 13 have found that the median nerve formed by 2 lateral roots and 1 medial root (from the medial cord) in 28 (52%) cases, being 25 from male and 3 from female, 9 from white and 19 from black cadavers. In 4 cases, one of the two lateral roots came from the anterior division of the middle trunk and 1 from the lateral cord. On the other 24 cases, the two lateral roots came from the lateral cord of the plexus. Darji et al 14 have studied 100 limbs and in which one limb (1%) showed the formation of median nerve by two lateral roots and one medial root. Dahiphale et al 15 have observed the formation of median nerve by three roots in 5% of cases.
Sargon et al 16 have described the formation of the median nerve by three roots on a male cadaver. In this case, the authors reported the presence of two roots coming from the lateral cord, one of them with a very close course over the axillary artery. one must be aware that this kind IJBAR (2014) 05 (09) www.ssjournals.com of variation is more prone to injury in surgical operations of the axilla and that, the very close course of the second lateral root of the median nerve to the axillary artery may lessen the blood supply of the upper extremity by compressing the vessel. This is to very similar to our case on right side. Le Minor et al 17 have studied the variations of median and musculocutaneous nerves and classified the findings into five types. The formation of median nerve by two roots in our case fits into type III variety and small communicating branches between median nerve and musculocutaneous nerve in the right limb fits into type II variety. The musculocutaneous nerve joining with the median nerve trunk and in the arm branched out again leaving the median nerve to supply the flexor of arm which we have observed in the left limb fits into type IV variety. It was found sometimes that if lateral root was small and the musculocutaneous nerve was connected with median nerve in the arm 18 .
In a recent study by Choi et al 8 the communications between musculocutaneous nerve and median nerve have been broadly classified into three types.4 In type I: the musculocutaneous nerve and median nerve were fused; in type II: there was one connecting branch between the musculocutaneous nerve and median nerve and in type III: two connecting branches were present between musculocutaneous nerve and median nerve. Our observation on left side fits into type I variety and on right side into type III variety.

Embryological Basis
The upper limb muscles develop from mesenchyme of paraxial mesoderm in the fifth week of intrauterine life. The motor axons arrive at the base of limb bud; they mix to form brachial plexus in upper limb. The growth cones of axons continue in the limb bud 21 .As the guidance of the developing axons is regulated by the expression of chemo-attractants and chemo-repulsants in a highly coordinated site specific fashion any alterations in signalling between mesenchymal cells and neuronal growth cones can lead to significant variations 22 .

Conclusions
The anatomical variation in the median nerve has practical implication, since any injury caused to this nerve in the axilla o r arm could cause unexpected paralysis of the flexor musculature of the elbow and hypoesthesia of the lateral surface of the forearm. Knowledge on the variations of the brachial plexus is also important during nerve blocks which are formed on infraclavicular part of brachial plexus. Normal functioning of the limbs may not be altered by these variations, but it is important to remember these variations during performance of surgical and anaesthesiological procedures 23 .The existence of communicating branches may be of importance in the evaluation of unexplained sensory loss after trauma or surgical intervention in a particular area 24 .
The variations of brachial plexus in the axilla and arm are very important for general surgeons, oncosurgeons, vascular surge ons, radiologists, orthopedicians and anaesthesiologists. Being anatomists feel very happy to add the information by presenting this case to the existing knowledge in the literature.