Ultrasonographic findings of the axillary part of the brachial plexus

In this prospective study we sought to determine anatomic variations of the main brachial plexus nerves in the axilla and upper arm via high-resolution ultrasonography (US) examination. Positions of nerves were studied via US in three sectional levels of the upper arm in 69 healthy volunteers (31 men and 38 women, median age 28 yr). Analysis was done by subdividing the US picture into eight pie-chart sectors and matching sectors for the position of the ulnar, radial, and median nerves. Shortly after the nerves pass the pectoralis minor muscle, they begin to diverge. At the middle level 9%-13%, and at the distal level, 30%-81% of the nerves are not seen together with the artery in the US picture. At the usual level of axillary block approach, we found the ulnar nerve in the posterior medial position in 59% of the volunteers. The other two nerves had two peaks in distribution: the radial nerve in posterior lateral (38%) and anterior lateral (20%) position, and the median nerve in anterior medial (30%) and posterior medial (26%) position. Applying light pressure distally can displace nerves to the side, especially when they are positioned anterior to the axillary artery. We conclude that an axillary block should be attempted as proximal as possible to the axilla.