A working knowledge of ultrasound-guided peripheral nerve blocks of the upper extremities covering the median and ulnar nerves, in combination with the radial nerve block discussed last month, will enable emergency physicians to block the hand and wrist. This is useful in dealing with complex lacerations, fractures, and other procedures.
The median nerve supplies sensation to the palmar aspect of the hand over the thenar eminence and lateral palm as well as the volar surface of the thumb, the index and middle fingers, and a portion of the ring finger. It also supplies sensation to the nail beds of the index and middle fingers, as well as a portion of the ring finger. The median nerve can be found easily with ultrasound in the mid-forearm, where it lies between the flexor digitorum superficialis and flexor digitorum profundus muscles. (Image 1.)
Using a high-frequency transducer in transverse orientation, the median nerve is seen as a triangular, hyperechoic (bright white) bundle lying between these two muscles bellies. (Image 2.) The median nerve can also be blocked at the wrist. Identifying the nerve can be challenging at this level because tendons can have a similar appearance in the transverse orientation. Following the suspected nerve proximally will help to discriminate between the structures because tendons will be seen to terminate in a muscle belly while the nerve will continue proximally toward the antecubital fossa.
The ulnar nerve supplies sensation to the medial aspect of the hand, specifically the skin over the small finger and a portion of the ring finger and to the corresponding proximal area of the hand and wrist. The nerve innervates the volar and dorsal aspect of the hand and wrist. The ulnar nerve can be easily identified at multiple locations by first identifying the ulnar artery. A high--frequency transducer in transverse orientation can be applied at the wrist, elbow, or mid--forearm on the ulnar aspect. (Image 3.) The ulnar artery can be seen as a rounded, hypoechoic, pulsatile structure. The nerve will be seen as a hyperechoic, slightly triangular structure just medial to the artery. A block can be performed at the wrist, forearm, or elbow, but many practitioners find that the mid-forearm offers the best approach because the nerve is easily identified and is slightly more separated from the artery.
As with the radial nerve, the block can be done either in-plane (introducing the needle from the long end of the transducer) or out-of-plane (introducing the needle from the middle of the transducer). Either way, ultrasound will enable the EP to place approximately 3-5 cc of anesthetic directly around the nerve.
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