Leonard V. Bunting, M.D.; and Beatrice Hoffmann, M.D., Ph.D., RDMS
Axillary Plexus Block - Axillary Approach to the Brachial Plexus
The axillary approach to the brachial plexus block (termed axillary block from here forward) provides anesthesia for many painful upper extremity conditions presenting to the ED. Its straight forward anatomy and few dangerous complications make it an excellent choice for novices. A block at this level provides dense anesthesia to the hand and forearm.
Illustration 1: Distribution of anesthesia with axillary plexus block – anterior and posterior view.
The target nerves are the terminal branches of the brachial plexus. In the axilla, the median, radial and ulnar nerves travel in a neurovascular bundle with the axillary artery, medial to the humerus. The musculocutaneous nerve, which breaks off proximally from the others, is located in the flexor compartment, either between the biceps and coracobrachialis or within the body of the coracbrachialis. For complete anesthesia of the forearm, the musculocutaneous nerve should be included in the block. Although dense anesthesia at the elbow can occur, anesthesia at the elbow is better achieved using a more proximal block (see infraclavicular or supraclavicular).
Illustration 2: Course of the brachial plexus.
Illustration 3: Overview of brachial plexus.
The patient should lie supine on the stretcher with their arm abducted to 90 degrees and externally rotated. Injured extremities should be supported during positioning. Pillows under the arm may improve patient comfort. (POPUP- Room setup) A high-frequency (10-18 MHz), linear array probe is used. The ultrasound probe is placed transversely on the proximal, medial upper arm. The probe indicator is towards the biceps on the right arm and towards the triceps on the left.
Figure 1: Probe placement in the axilla (red – axillary artery).
Figure 2: Ultrasound image of the axilla (m – median nerve, u – ulnar nerve,
r – radial nerve, mc – musculocutaneous nerve, A – axillary artery).
The musculocutaneous nerve is located by searching within the hyperechoic borders of the flexor compartment. Moving the probe distally and proximally may help highlight the nerve as it changes conformation from a layer of hyperechoic circles to a traditional nerve structure.
Generally the area of needle insertion is over the biceps, using an in-plane approach. Block needles should be at least 5cm long and 22 gauge or larger. After the appropriate equipment is setup, the skin is anesthetized and the block needle is inserted 1cm into the skin. The needle tip is located and followed throughout the procedure (POPUP – visualizing the tip).
Given the variation in nerve position, no standard approach to this block is possible. In most patients it should be possible to block all 4 nerves from one entry point at the biceps with an adequate length needle. If all four nerves are visualized, the block can proceed in any order.
Ideally each nerve should be surrounded by LA. Adequate anesthesia is traditionally achieved with 20-40cc’s of anesthestic, but some suggest lower volumes are possible when using ultrasound guidance.
Figure 3: Needle placement in the axilla (red – axillary artery).
If all nerves cannot be seen initially, a regional approach to the ultrasound guided axillary block has been described. For this approach, the most common locations for the nerves are targeted. The region of the median nerve (location 1) is blocked first, attempting to separate the median nerve from the artery. Then the needle is passed between the median nerve and artery to anesthetize the medial region of the artery, where the ulnar nerve generally sits (location 2). Next the needle is partially withdrawn and redirected towards the inferior border of the artery to block the radial nerve, ensuring to dissect it away from the artery (location 3). Finally, the needle is withdrawn to the skin and redirect to block the musculocutaneous nerve in the flexor compartment (location 4). Once completed, the area is once again scanned to identify the individual nerves. With the local anesthetic in place, the nerves should be easily visualized and areas of poor LA distribution can be addressed with further injection.
Figure 4: Regional approach to the axillary nerve block
(red – median, yellow – ulnar, green – radial, blue – musculocutaneous).
Video 1: Axillary nerve block.
Pearls and Pitfalls
The three main nerves targeted in this block closely surround the axillary artery. If nerve visualization is poor, initial injections are aimed at surrounding the artery with local anesthetic. Once achieved, the nerves will become prominent and areas of poor anesthetic distribution can be addressed.
The axillary vein is easily compressed during this procedure, leading to the possibility of inadvertent intravascular injection. Good needle control and frequent aspiration should avoid this complication.
This area has potential for anatomical variations, the most frequent found were double axillary artery, numerous axillary veins, variant location of the musculocutaneous nerve and posterior location of the brachial plexus in relation to the axillary artery.
1. Koscielniak-Nielsen, Z.
Axillary Brachial Plexus Block. In: Hadzic A .(ed): Textbook of Regional Anesthesia. McGraw-Hill, 2007, pp 449.
2. Koscielniak-Nielsen Z.
Axillary Brachial Plexus Block. In Hadzic A (ed): Textbook of Regional Anesthesia. McGraw-Hill, 2007, pp 446.
3. Gray, A.
Ultrasound Guidance for Regional Anesthesia. In Miller R (ed): Miller: Miller’s Anesthesia, 7th edition. Churchill Livingstone, 2009; Chapter 53: 21.
4. O’Donnell BD, Iohom G.
An estimation of the minimum effective anesthetic volume of 2% lidocaine in ultrasound-guided axillary brachial plexus block. Anesthesiology, 2009; 111(1): 25-9.
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Anatomical basis for ultrasound-guided regional anaesthesia at the junction of the axilla and the upper arm. Surg Radiol Anat. 2010 Mar;32(3):299-304.