Leonard V. Bunting, M.D.; and Beatrice Hoffmann, M.D., Ph.D., RDM
Radial Nerve Block
The radial nerve is a mixed sensory and motor nerve that arises from the posterior cord of the brachial plexus. It travels in the spiral groove of the humerus in the upper arm and descends anterior to the lateral epicondyle in the elbow. At the elbow it divides into the superficial branch, which provides cutaneous innervation to part of the hand, and the deep branch, which innervates the dorsal compartment of the forearm. The superficial branch travels with the radial artery in the mid-forearm, but divides away from the artery in the distal forearm as it separates into smaller cutaneous branches.
Illustration 1: Distribution of anesthesia, anterior.
Illustration 2: Distribution of anesthesia, posterior.
The radial nerve is found sonographically by tracing the radial artery from distal to proximal. At the level of the mid-forearm, the cutaneous branches will reform the radial nerve lateral to the artery. The nerve may appear triangular or oval at this level.
Proximal to the elbow, the nerve can be visualized and blocked at the level of the upper third of the humerus. The probe is placed over the triceps muscle in transverse orientation localizing the brachial artery. Lateral to it, the radial nerve can be found crossing over the humeral shaft in mid-upper arm.
Radial Nerve – Forearm
A high-frequency (8-18 Mhz) probe is used for the block. Although this text describes an in-plane approach, an out-of-plane approach is possible and may aid in separating the nerve from the artery. The needle is held in the dominant hand and carefully inserted from the radial side of the forearm. After identifying the needle tip, the needle is advanced slowly towards the base of the radial nerve. Local anesthetic is injected to surround the nerve. Readjustment of the needle to the superficial border of the nerve is often necessary to fully surround the nerve. Typical block volumes are 3-5 cc’s and should achieve anesthesia within 15 minutes.
Figure 1: Placement of Hands for block at the level of the forearm.
Figure 2: Ultrasound Image of the radial nerve at the level of the forearm.
Video 1: Radial nerve block, in-plane.
Video 2: Radial nerve block, out-of-plane.
Radial Nerve – Upper Arm
The probe is placed over the triceps muscle in transverse orientation approximately between upper and mid third of the humerus. The brachial artery is localized deep and close to the humerus shaft, color Doppler can assist in identification of the vessel. Lateral to it, the nerve can be found crossing over the humeral shaft in mid-upper arm (Figure 3 and 4). Often, the nerve can be seen branching into superficial and deep branches when followed closer to the elbow (Video 3).
Figure 3: Probe position for a radial nerve block in the upper arm. (placeholder)
Figure 4: Radial nerve in upper arm in close proximity to the brachial artery.
Video 3: Radial nerve shown in upper arm. The brachial artery is seen pulsating,
the nerve is seen passing immediately inferior to it when the probe is moved.
Pearls and Pitfalls
Color Doppler can assist identifying and distinguishing brachial artery and radial nerve in the proximal third of the upper arm as well as forearm.
Blocking the nerve more proximal (level of upper or mid-humerus) assures a larger area of anesthesia.
1. McCartney CJ, Xu D, Constantinescu C, Abbas S, Chan VW.
Ultrasound examination of peripheral nerves in the forearm. Reg Anesth Pain Med. 2007;32(5):434-9. Erratum in: Reg Anesth Pain Med. 2008;33(2):188.
2. Loewy J.
Sonoanatomy of the median, ulnar and radial nerves. Can Assoc Radiol J. 2002;53(1):33-8. Review.