Forearm nerve anatomy (ultrasound)

Case Discussion

Knowledge of the normal sonographic anatomy and anatomical relationships of the radial, ulnar, and median nerves in the mid-forearm is important when considering regional anaesthesia for pain or anticipated procedures involving the hand. 

Unlike the radial or ulnar nerve, the median nerve traverses the mid-forearm without an associated artery. It may be difficult to distinguish distally at the level of the wrist from similarly anisotropic tendons as they converge at the carpal tunnel, but may be easily identified as the lone echogenic, honeycomb-like structure running in the fascial plane between the flexor digitorum superficialis (FDS) and profundus (FDP) ¹.

The ulnar nerve is found ulnar (medial) to the associated ulnar artery, which it travels in close apposition with in the distal forearm. As the transducer follows the structures proximally the nerve commonly diverges from the artery, providing a more favourable anatomical relationship for blockade ².

The radial nerve is often the least conspicuous of these nerves on ultrasound. At the level of the mid to distal forearm the structure imaged is the superficial radial nerve which accounts for the relevant sensory innervation of the hand; bifurcation occurs more proximally in the radial tunnel with the posterior interosseous nerve (PION) thereafter travelling between the heads of the supinator. The (superficial) radial nerve is typically located in the mid-forearm by noting the position of the radial artery and scanning just radial (lateral).

The nerve may be more easily identified above the elbow as it exits the spiral groove of the humerus and courses within the fascial plane defined by the brachialis and brachioradialis, however performing a radial nerve block at this level will result in motor blockade and a radial nerve palsy (i.e. wrist drop) ³.