Musculocutaneous Nerve absence ; a research study in sixty upper limb specimens

Absence of musculocutaneous nerve was observed during study of 60 dissected upper limb specimens. The area of innervation was supplied by median nerve. Knowledge of this anatomical variation is of clinical significance to neurologists, traumatologists and surgeons as injury to median nerve in this case would lead to unexpected paralysis of flexor muscles of arm and hypoaesthesia of lateral surface of forearm.


Introduction
Musculocutaneous Nerve (MCN) is a branch from lateral cord of brachial plexus arising opposite lower border of teres minor. It pierces the coracobrachialis and descends laterally between biceps and brachialis to lateral side of arm. Just above the elbow, it pierces the deep fascia lateral to tendon of biceps, continuing as lateral cutaneous nerve of forearm. It supplies the coracobrachialis, both heads of biceps and brachialis 1 .
Median Nerve (MN) has two roots from lateral and medial cords, which embrace the third part of axillary artery, uniting either anterior or lateral to it. MN enters the arm at first lateral to brachial artery, near insertion of coracobrachialis crosses the artery, descends medial to it in the cubital fossa. Here it lies deep to bicipital aponeurosis and anterior to brachialis. It usually enters the forearm between heads of pronator teres 1 .

Materials and methods
The materials used for this study consisted of 60 upper limbs of 30 human cadavers of either sex and age group between 30-60 years. The study was done over a period of 2 years from August 2010 to July 2012 in the department of Anatomy at Vijayanagar Institute of Medical Sciences(VIMS), Bellary. The study was done by dissection method and observations were made.

Observation and Results
The study included the origin and course of MCN, MN and their relation with the surrounding structures.
A variation of brachial plexus, characterised by the absence of MCN on right arm, was found during the dissection of 55-60 year old male cadaver. MN supplied the muscles of front of arm from its lateral aspect . Branches each to coracobrachialis, biceps and brachialis were given 5.2cms,14.7cms and 17.6cms from the outer border of first rib respectively. Lateral cutaneous nerve of forearm was given from lateral side of MN just above the elbow. The course and distribution of MN in the forearm did not show any variation. The course and distribution of MCN and MN in the left upper limb of same cadaver was normal.

Discussion
Anatomical variation of peripheral nerves constitute a potentially important clinical and surgical issue. There is little data in literature regarding absence of MCN.
In man, forelimb muscles develop from the mesenchyme of paraxial mesoderm during fifth week of intrauterine life 2 . Axons of spinal nerves grow distally to reach the mesenchyme. Significant variations in the nerve patterns may be a result of altered signalling between mesenchymal cells and neuronal growth cones or circulatory factors at the time of fusion of brachial plexus 3 .
Rao and Chaudhary 4 observed MCN was absent in two cases (8%) of 24 dissected upper limbs. MN took over the area of supply of MCN by giving both muscular and sensory branches. Hollinshed 5 (1982) has stated that very occasionally the MCN is not found and its various branches arise from MN. The absence of MCN was also noted by Le Minor 6 (1990), Nakatani et al 7 (1997), Sud and Sharma 8 (2000) and Gumusburn 9 (2000). Beheiry 10 (2004) observed absence of MCN in one limb (1.7%) out of 60 arms dissected in the age group between 30-67 years. The present study coincides with the study done by Beheiry.
Precise knowledge of variation of MCN and MN may be of great value in traumatology of arm, as well as in plastic and reconstructive surgeries.