The axillary approach to the brachial plexus targets the four terminal braches of
the brachial plexus namely median, ulnar, radial and Musculocutaneous nerves.
One of the widely practiced blocks done by anaesthetists, emergency physicians and surgeons.
Apart from blood vessels and nerves there are not many vital structures in this region (e.g. cervical spine, viscera etc). Provided intravascular injection is avoided this block is very safe to perform.
It does however, have a significant failure rate with non-ultrasound methods.
This is due to the fact that:
1. The nerves are not close together like in the supraclavicular fossa where the trunks are tightly packed together and a single injection can block all the trunks.
The musculocutaneous nerve leaves the plexus within the axilla and is separated from the other three nerves in the axilla.
The other three nerves even though they are closer to the artery, their position around the artery is quite variable as will be discussed later.
2. The fascial sheath that surrounds the axillary neurovascular bundle is not continuous.
There are definite septae and attachments within this sheath which restrict free flow of the local anaesthetic deposited in the sheath.
The above picture is from Gray's anatomy.
The axilla is a pyramidal shaped region situated between the chest wall and the upper arm.
The chest wall forms the medial boundary and the humerus the lateral boundary.
Superiorly lies the pectoral fold.
Latissimus dorsi forms the inferior boundary.
The neurovascular bundle running through axilla consists of the Axillary artery, vein/s, and terminal branches of brachial plexus.
The Musculocutaneous nerve in most cases lies superior to the artery in the coracobrachialis muscle.
The other three nerves lie around the artery.
The position of the other three nerves in relation to the Axillary artery is highly variable.
This is shown in the diagram below.
Imagine that the person being scanned is supine. The arm is abducted at the shoulder, elbow flexed and forearm supinated.The probe is placed over the axilla such that a cross sectional view of the axilla is obtained. Following is a diagrammatic representation of the ultrasound image. In this diagram the space around the Axillary artery (Red circle in the centre) is divided into 8 pie chart sectors.
The median nerve is found most of the time superior and anterior (nearer the skin) to the Axillary artery in region 6,7and 8.
The ulnar nerve can be seen in region 1, 2 and 3 (Inferior and anterior to artery).
The radial nerve in the region 2, 3, 4 and 5 (posterior to artery and inferior).
The regions indicate where the nerve can be found mostly.
It is possible for the nerves to lie anywhere else as well.
1. Surgery on the elbow, forearm and hand.
1. Patient refusal
2. Coagulopathy, although if this is the only anaesthetic you can give to a patient due to his medical problems then this block can be done with minor coagulopathy as blood vessels can be easily compressed to stop bleeding.
3. Infection at the site of the block.
1. There are four nerves to be blocked and they are not close to each other. Therefore a single nerve injection is unlikely to block all nerves.
2. The musculocutaneous nerve always needs to be blocked separately as it is not enclosed in the so called axillary sheath. It needs to be blocked to prevent tourniquet pain.
3. The arm is not a perfect cylinder and nerves do not travel in a straight line in it. In order to get the best picture of the nerve the probe will have to be angled cephalad and caudal and rotated clockwise and anticlockwise.
4. Look for the structure which can be easily identified first i.e pulsating blood vessels. Then look for the nerves as detailed in locating the nerves in relation to them.
1. Ultrasound machine with high frequency probe, probe cover and ultrasound gel
2. Insulated stimulating needle (I use 50 mm stimulating needle).
3. Local anaesthetic: I use 20 ml of 0.5% levobupivacaine if block is done for anaesthesia. For postoperative analgesia 0.25% levobupivacaine is also sufficient
4. 2% chlorhexidine
Locating the nerves:
The patient is supine. The arm is abducted at the shoulder, elbow flexed and forearm supinated.
The ultrasound machine and the operator are positioned as shown in the picture:
A linear high frequency ultrasound probe is used. The probe is placed as high in the axilla under the pectoral fold.
The probe is placed such that cross sectional view of the artery is obtained.
To start with do not make any attempt to identify the nerves.
Find the structure that you can easily identify.
That will be the axillary artery which will be seen as a pulsating structure which cannot be compressed.
Once the axillary artery is identified ease the pressure off the probe and you will see the axillary vein filling up. After the vascular structures are identified then look for the nerves.
As discussed before the position of the nerves in relation to the artery is variable.
Keep the artery in the centre of picture and scan from the axilla towards the elbow. You will see a nerve which stays with the artery all the time.
It starts superiorly (usually) and around midarm level crosses over the artery to lie on its medial side.
This is the median nerve.
Once you have found the median nerve, go back to the axilla and focus your attention on finding the ulnar nerve.
Start in axilla and follow the artery down towards the arm.
This time keep the artery in upper part of the screen. Just above the midforearm level you will see a structure breaking away from the inferior side of the artery going away.
It lies above the muscles and just underneath the subcutaneous plane.
As you follow it down towards the elbow it seems to be heading towards the medial epicondyle and then at the elbow seems to go behind the medial epicondyle ( so called ‘Sun set’ sign). This is the ulnar nerve.
The radial nerve although with the axillary artery in the axilla, it sharply breaks away from it to go behind the humerus in the spiral groove to emerge four finger breadths above the lateral epicondyle and then it travels in the groove between biceps and brachioradialis muscle.
To start looking for the radial nerve start in the axilla and then scan towards elbow.
You will see a band or a ribbon shape or a railtrack like structure breaking away from the axillary artery going towards the humerus.
If you follow it back towards the axilla you will see it resting between the axillary artery and triceps muscle.
At the level of the axilla, the musculocutaneous nerve is meant to lie in the substance of coracobrachialis muscle.
It has a very shiny appearance It is oval in shape. It normally seems to dive between the muscle planes like a fish.
There is considerable variation in anatomy of musculocutaneous nerve.Francis Remerand et.al. (Anesth Analg 2010;110:1729–34) have described the variation very nicely. They studied all the patients undergoing ultrasound guided axillary block between Dec 2006 and Dec 2008. They found that the musculocutaneous nerve was outside the coracobrachialis muscle in 22% of cases. It was near the axillary artery in 6% of patients. The musculocutaneous and median nerves appeared as a common neural structure in 16% of individuals studied.
Tracing back the radial nerve:
Following video also shows how to find nerves for axillary block
Performing the nerve block:
This block needs to be performed in-plane.
The needle tip has to be visualized at all times. The patient lies supine with arm abducted at the shoulder, elbow flexed and forearm supinated.
The anaesthetist sits/stands at the head end of the patient with ultrasound machine positioned opposite him as shown in the picture.
This is the most ergonomic position for doing the block. We use 50 mm insultaed needle.
We combine ultrasound with nerve stimulation. Median, ulnar and musculocutaneous nerves are quite easily visible . The radial nerve can sometimes be tricky.
Try to surround the nerves with local anaesthetic, aim to block all four nerves (If you can’t find all four nerves block at least three out of four nerves; always block the musculocutaneous nerve, other two neves depending on the site of surgery).
You can always block median, ulnar and radial nerves more distally. Locate and block the nerves as discussed before.
Please check this video from RA-UK website. You will see Dr Nigel Bedforth expertly doing an axillary block on Dr James French. Both are Consultant Anaesthetist at Nottingham, UK. Video courtesy ‘Nerve Imaging Group’. It works best on a PC.
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