Radial Nerve Block

Anatomy and Physiology

The radial nerve, along with the axillary artery, originates as a branch of the brachial plexus, specifically from the posterior cord of the plexus. The nerve is comprised of fibers from C5-T1 spinal roots. The radial nerve is posterior and inferior to the axillary artery. Once it exits the axilla, the nerve descends into the arm by passing anterior to the latissimus dorsi muscle insertion. Then, it passes between the medial and long heads of the triceps muscle to eventually lie on the posterior surface of the humerus. The radial nerve has multiple branches, including the sensory posterior cutaneous branch of the forearm, the superficial radial nerve, and the motor deep radial nerve. The posterior cutaneous branch divides from the radial nerve proximal to the humeral lateral epicondyle, providing sensation to the elbow and the posterior forearm. The radial nerve continues distally over the humeral spiral groove and emerges close to the lateral epicondyle. It subsequently pierces the lateral intermuscular septum to arrive at the anterior compartment of the arm, lying in between the brachialis and brachioradialis muscles. Here, at the level of the antecubital fossa, it divides into its 2 terminal branches, the superficial and deep branches. The superficial branch is close to the radial artery and is responsible for afferent innervation. This is a sensory branch, innervating the geographical area of the wrist dorsally and part of the thumb and index and middle fingers proximal to the DIP dorsally. The deep branch is responsible for motor innervation to the extensors of the forearm.[2][3]

It is important to understand the anatomical path of the radial nerve as it has significant implications when attempting to achieve an anesthetic block along its course. The ideal locations of the anesthetic block described in the literature include locations above, at, and below the elbow. Above the elbow (above the bifurcation of the radial nerve into its terminal branches), the block can be done 4 inches above the humeral lateral epicondyle between the brachialis and brachioradialis muscles. The nerve can also be blocked at the elbow level in the antecubital fossa before the division into its terminal branches. Lastly, the superficial radial nerve can be anesthetized below the elbow at the level of the proximal third of the forearm.[2]

Indications

Indications for radial nerve block include:

• Multiple lacerations along the distribution of the radial nerve requiring repair
• Forearm and hand surgical procedures, including excision of skin lesions, incision and drainage, foreign body removal, finger amputation
• Standalone or adjunctive analgesia for acute painful injuries and burns
• Metacarpal and phalangeal fractures and dislocation
• Rescue analgesia for failed/inadequate regional blocks
• Supplemental to the median nerve block
• Radial tunnel syndrome
• Recalcitrant lateral epicondylalgia
• Post-operative pain following arthrolysis and tenolysis [4]

Contraindications

Contraindications to the radial nerve block include:

• Lack of informed consent/patient refusal
• Infection/abscess/cellulitis over the site of injection
• Allergy to the local anesthetic class
• Potential for local anesthetic systemic toxicity in regards to administering near-toxic threshold doses. Refer to Table 1 for a detailed review of anesthetic pharmacokinetics.[4]

Equipment

For an ultrasound-guided radial nerve block, the practitioner needs the following:

• Ultrasound machine with a high-frequency linear array transducer (10 to 15 MHz).
• Ultrasound probe cover (sterile) or appropriate Tegaderm dressing.
• Ultrasound gel (sterile).
• Skin preparation (sterile) (2% chlorhexidine or appropriate disinfectant).
• A 22-G to 25-G, 38-mm to 50-mm, short-bevel block needle.
• Appropriate sterile gloves.
• Local anesthetic: A 3 to 5 ml volume suffices for each nerve. The goal is a circumferential spread of anesthetic around the nerve sheath. To choose the most suitable anesthetic for the block, the use of anesthetic concentration versus duration must be made before starting the procedure. 
• Ergonomics: The recommendation is for the individual doing the anesthetic block to stand on the ipsilateral side as the block with the ultrasound on the contralateral side.[4][5]

Personnel

Properly trained personnel with ultrasound-guided regional anesthesia experience should perform the procedure with adequate support staff.[4]

Preparation

After obtaining informed consent, position the patient depending on which approach to the radial nerve block is being attempted, as outlined below in the technique section. Subsequently, povidone-iodine or chlorhexidine 2% can prepare the skin over the anesthetic injection site. Next, applying a sterile probe cover to the ultrasound probe is recommended, followed by applying sterile gel to the intended injection site. Lastly, a local anesthetic is drawn up into a syringe. Ensure appropriate monitoring and access to 20% lipid emulsion in local anesthetic systemic toxicity cases.[4]

Technique or Treatment

There are 3 techniques to attempt an anesthetic block to the radial nerve.

Proximal Third of the Forearm

An anesthetic block can be performed in this location before the superficial radial nerve joins the lateral aspect of the radial artery proximal to its division into smaller branches.

1. The patient can be supine or sitting upright. Have the patient pronate their arm to facilitate placement of the needle, with the operator standing on the lateral (cephalad) side of the armboard.
2. Here, the superficial radial nerve is covered by the brachioradialis muscle. Place the linear ultrasound transducer, preferably in an in-plane approach from the lateral side of the forearm. A 25-gauge needle, preferably 38 mm long, is used for this procedure.
3. Using ultrasound, identify the radial nerve at an initial depth of 25 mm under the brachioradialis muscle. Upon scanning, the radial nerve is seen as a hyperechoic structure underneath the brachioradialis muscle.
4. Place the needle tip in an in-plane view from lateral to medial through the brachioradialis muscle adjacent to the superficial radial nerve.
5. Confirm that there is no inadvertent entry into the vasculature via negative aspiration.
6. Inject the local anesthetic of choice and observe for layering of local anesthetic under the brachioradialis muscle and around the superficial radial nerve.[6]

At the Elbow in the Antecubital Fossa

An anesthetic block can be performed here at the antecubital fossa before the nerve divides into superficial (sensory) and deep (motor) branches.

1. The patient can be supine or sitting upright. To facilitate placement of the needle, have the patient supinate their arm with the operator standing on the lateral (cephalad) side of the armboard.
2. Place the ultrasound using the linear transducer, preferably in an in-plane approach from the lateral side of the elbow. For this procedure, use a 25-gauge needle, preferably 50 mm in length.
3. Identify the radial nerve between the brachioradialis and the biceps muscle in this location. Our preferred approach is to start proximally above the elbow and identify the radial nerve as a triangular hyperechoic structure coming off the distal humerus. We suggest following this distally to the antecubital fossa, where the nerve branches into its superficial and deep branches.
4. After identification, choose a point of injection before the bifurcation of the radial nerve.
5. Place the needle tip in an in-plane view from lateral to medial until you enter the fascial plane adjacent to the radial nerve.
6. Confirm that the tip is not in a vascular space by negative aspiration
7. Inject the local anesthetic of choice and observe for layering of a local anesthetic around the radial nerve.[6]

Above the Elbow Just Distal to the Humeral Spiral Groove

An anesthetic block can be performed here at the level of the spiral groove on the posterolateral aspect of the humerus to block the deep and superficial branches of the radial nerve.

1. The patient can be supine with the arm placed in shoulder adduction and internal rotation with the elbow flexed and rested on the patient’s chest. The operator should be standing on the arm-board's lateral (cephalad) side.
2. Place the ultrasound, using the linear transducer, preferably in an in-plane approach, on the spiral groove of the posterolateral humerus distally. For this procedure, use a 25-gauge needle, preferably 50 mm in length.
3. Identify the radial nerve as a triangular hyperechoic structure coming off the humerus at this location.
4. Place the needle tip in an in-plane view from lateral to medial until it enters the fascial plane adjacent to the radial nerve.
5. Confirm that the tip is not in a vascular space by negative aspiration
6. Inject the local anesthetic of choice and observe for layering of a local anesthetic around the radial nerve.[6]

Complications

Complications include:

• Nerve injury, including neuropraxia or neurolysis
• Hematoma formation
• Myalgias
• Intravascular injections
• Systemic local anesthesia toxicity
• Infection
• Bleeding at the site of puncture
• Allergic reaction to a local anesthetic
• Myotoxicity and myonecrosis with frequent or prolonged injections
• Reflex sympathetic dystrophy [4]

Clinical Significance

The use of local peripheral nerve blocks for surgical anesthesia, post-operative pain, acute and chronic pain syndromes, and many other indications has significantly increased with the advent of ultrasound-guided techniques. Furthermore, using ultrasound has improved the overall success and efficiency of the peripheral nerve block while minimizing local anesthetic toxicity risk.[7]

Enhancing Healthcare Team Outcomes

The utilization of peripheral ultrasound-guided nerve blocks helps provide a viable alternative to procedural sedation, which has potential risks and is labor-intensive and time-intensive for both patients and physicians. The American Society of Regional Anesthesia and Pain Medicine has classified the use of ultrasound as Level 1b evidence for faster block onset and higher rates of block success. An ultrasound-guided radial nerve block, combined with other treatments, is a therapeutic resource for treating multiple types of pain and clinical entities.