Key points
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Blockade of peripheral nerves of the upper extremity is often accomplished by brachial plexus approaches. However, conditions such as infections to brachial plexus sites, coagulopathy, single nerve distribution, minor procedures (not requiring a tourniquet), and rescue supplementations of brachial plexus block may require individual nerve blockade.
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Distal peripheral nerve blocks are associated with a slightly higher likelihood of nerve injury, possibly because of the anatomical location of these sites where the nerve is contained within bony and ligamentous surroundings.
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As most distal forearm and hand surgery procedures are performed using a tourniquet, the patients may require deeper sedation to tolerate the high tourniquet inflation pressures.
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A high-frequency linear array transducer is preferred for these peripheral nerves; also it is easier to locate a nerve in short axis at a determined landmark and then follow in a cranial to caudal direction.
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Continuous nerve catheters are not recommended, perhaps because these nerves are confined in a tight space and carry the risk of compartment syndrome. Nevertheless, if prolonged analgesia is needed, axillary continuous catheters may be considered.
Sonoanatomy
The three major peripheral nerves of the upper extremity, namely median, radial, and ulnar, may be blocked at various levels as follows: arm, elbow, forearm, and wrist.
The median nerve is composed of both motor and sensory components derived from the medial and lateral cords of the brachial plexus. It is bounded in a neurovascular bundle in the upper arm where it accompanies the brachial artery. At the elbow, the median nerve courses medial to the brachial artery, and lies between the humeral and ulnar heads of the pronator teres muscle ( Fig. 10.1 ). At mid-forearm, the median nerve separates from ulnar artery and is sandwiched between the muscle of the flexor digitorium superficialis (FDS) and flexor digitorum profundus (FDP) before entering through the carpal tunnel at the wrist. Motor branches (anterior interosseous nerve) supply the deep volar muscles in the forearm and thenar eminence of the hand, whereas sensory distribution is limited to the radial aspect of the hand. Notably, the median nerve does not provide any sensory distribution to the forearm; however, it innervates all muscles of the forearm except flexor carpi ulnaris and the ulnar aspect of flexor carpi radialis.
The ulnar nerve originates from the C8 and T1 nerve roots as the terminal branch of the medial cord of the brachial plexus and has mixed motor-sensory components. At the upper level of the arm, the ulnar nerve is medial to the axillary artery, and posterior to the brachial artery and median nerve. It does not provide any motor or sensory innervation at this level. However, in midarm, it descends along the posteromedial aspect and passes between the olecranon process and medial epicondyle to enter the forearm, where it lies superficial to the FDP and medial to the ulnar artery. At the forearm, the ulnar nerve can be located medial to the ulnar artery in close proximity, and hence facilitates sonographic determination. At the level of wrist, the nerve runs lateral to flexor carpi ulnaris and enters the hand superficial to the flexor retinaculum.
The radial nerve is a mixed motor-sensory nerve originating from the posterior cord of the brachial plexus via C5 to T1 nerve roots. It travels from medial to lateral within the spiral groove of the humerus and further descends along the medial and lateral heads of the triceps muscle to lie anterior to the lateral epicondyle in the elbow (see Fig. 10.1 ). At this level, the nerve divides into superficial and deep branches.
Technique
Median nerve
Landmark technique. At the level of elbow—antecubital crease, the median nerve is located 1 cm medial to pulsation of the brachial artery approximately 1–2 cm deep ( Fig. 10.2 ). The needle is inserted at 45 degrees cephalad, and a resistance click of the bicipital aponeurosis may be felt 1–2 cm deep to the skin. At this point, paresthesia may be achieved, and after confirming, 5–10 mL of local anesthetic (LA) can be injected.
Ultrasound technique. Distal to the elbow, at the level of the mid-forearm, the median nerve is found as a hyperechoic structure embedded in hypoechoic FDS and FDP ( Fig. 10.3 ). The nerve should be confirmed by fanning the probe along its course. Using an in-plane view, with a high frequency linear probe, the needle tip is advanced towards the base of the median nerve and 2–3 mL of LA is injected. The needle is then readjusted to the superior border of the nerve in order to effectively surround the nerve completely with LA. The total volume should be limited to 5–7 mL.
