Anatomical variation in formation of brachial plexus and its branching.

Variant brachial plexus formation with two trunks and two cords is uncommon and has clinical implications as it may result in failure of regional brachial or axillary block. During routine anatomical dissection, unilateral variation in the formation of brachial plexus accompanied by unusual positional relationship with axillary artery was discovered in the left upper extremity of a 52-year-old Indian male cadaver. Brachial plexus showed two trunks formed by ventral rami of C5, C6 and C7, C8, T1 spinal nerves, respectively, which first split and then reunited in an unusual manner to form two cords: posterior and lateral instead of three. Medial cord was absent. The branching pattern of the brachial plexus also showed important variations. Second part of axillary artery was found lying inferomedial to brachial plexus instead of passing between medial and lateral cords. Transverse cervical artery was found to be coursing between two trunks instead of passing superficial to brachial plexus. Median nerve was observed to be formed from a single root, instead of usual two roots. Embryologically, this rare variation may be due to the development of axillary artery from ninth segmental artery instead of usual seventh cervical intersegmental artery. Such rare variation is clinically important as this knowledge may help the anesthesiologists and the surgeons to avoid any inadvertent damage to nerves and axillary artery during blocks and surgical interventions.