Forearm nerve blocks should be performed using the high-frequency linear (usually 10-5 MHz) transducer held in a transverse orientation. A soft-tissue or nerve preset if available is ideal to locate the nerves in the forearm. For consistency, we recommend pointing the probe marker toward the right side of the patient with the directional indicator on the left of the screen (as per standard emergency ultrasound convention).

Specifics for Locating Each Nerve

Systematically scan the forearm starting distally and moving proximally to find each nerve. Remember that the goal is to locate each nerve at a point where there is sufficient distance from vascular structures to avoid inadvertent vascular puncture. This must be balanced against selecting a site where nerve depth makes the procedure more difficult.

Median nerve. The median nerve does not have an associated vascular structure except in rare cases. To locate the median nerve, start at the wrist with the mid-point of the transducer over the middle of the wrist crease. Move the probe proximally and look for the nervous structure that exists at the junction of several fascial planes. The median nerve lies in the fascial plane between the flexor digitorum superficialis and profundus. As the probe moves more proximal, tendon structures will disappear, while the classic “honeycomb” bundle of the median nerve will persist (photo 1).

Radial nerve. The radial nerve is located radially to the radial artery. Starting at the wrist, locate the pulsatile radial artery. Immediately radial to the radial artery, a collection of nerve bundles is seen.

Often, the radial nerve is difficult to identify as a separate structure at the wrist because of its close proximity to the artery. Trace the radial artery proximally, and by approximately mid-forearm the radial nerve should be located at a safe distance from the radial artery (photo 2).

Ulnar nerve. The ulnar nerve is located at the ulnar aspect of the ulnar artery. Starting at the wrist, locate the pulsatile ulnar artery. Immediately ulnar to the ulnar artery, a small nerve bundle can be visualized. Like the radial nerve, the ulnar nerve lies close to the ulnar artery. As the provider moves the probe in a more proximal manner, the ulnar nerve will separate from the artery, allowing an ideal location for injection while reducing risk of inadvertent arterial puncture (photo 3).