◊ BACKGROUND:
The aim of this study was to describe topographic variations in the arrangement of the four main brachial plexus nerves at the junction of the axilla and the upper part of the arm.
◊ METHODS:
In 153 patients undergoing upper arm surgery using axillary block, we studied nerve arrangements with a three-step approach, combining: (A) cross-sectional ultrasound imaging using a 12 MHz linear ultrasound probe; (B) distal shift of the ultrasound scanhead from the axilla to the elbow joint following the paths of individual nerves; and (C) identifying the distal motor response to electrical nerve stimulation of each nerve. These results were then converted into a 12-section pie chart with the axillary artery (AA) as the axis.
◊ RESULTS:
The order of the nerves around the AA was median, ulnar, radial, and musculocutaneous in all cases. The most frequent arrangement was observed in 65% of the patients. Five less frequent variations were observed in 4–20% of the patients, with four other variations seen in <2% of the patients. In 78% of the cases, the four nerves were seen separately using static ultrasound imaging. The musculocutaneous nerve was close to the artery in 18% of the patients.
◊ CONCLUSIONS:
Topographic variations of the four main nerves at the axilla were found to be numerous, the most frequent arrangement being seen in less than two-thirds of the patients. Four separate nerves were seen on static ultrasound imaging at this sectional level of the axilla in only 78% of the cases.
◊ Reviewed by Z. J. Koscielniak-Nielsen MD, PhD, FRCA.
Ass. Professor,
Orthopaedic Anaesthesia Research, Rigshospital, Blegdamsvej, Copenhagen, Denmark.
This study investigated in detail anatomical variations of the four main terminal nerves of the brachial plexus in the axilla: the median, ulnar, radial, and musculocutaneous nerve. The authors have to be congratulated for their effort because axillary block is widely used for hand surgery and many anaesthesiologists still perform this block without visual techniques.
The methodology was correct and clear. Nerves were traced distally and their identity was confirmed by electrical stimulation. Excessive transducer pressure, which could have distorted the anatomy was avoided. Results are presented as comprehensible pie diagrams.
The two main conclusions of this study are:
- In about two thirds of patients the location of the four nerves in the four quadrants around the axillary artery is predictable: The median nerve is superficial and cephalad, the ulnar nerve is superficial and caudad, the musculocutaneous nerve is deep and cephalad, and the radial nerve is deep and caudad. With the exception of the musculocutaneous nerve, which is usually located at an average distance of 1 cm from the artery, all other nerves remain close to the arterial wall.
- In 22% of the patients not all four nerves could be identified by ultrasound alone.
Other important findings of the study are the superficial location of all nerves at less than 3 cm under the skin and the close proximity of the musculocutaneous nerve to the axillary artery in 18% of the patients. As correctly discussed, main results are in accordance with the so called “normal” anatomy previously described in anatomical [1,2] and ultrasonographic studies [3,4].
Minor drawbacks are the lacking specification of difficulties in visualizing nerves, of image quality, and of the time needed to perform the block including tracing the nerves to the elbow and back. I often have problems to visualize the radial nerve, which is also regarded as most challenging by others [5,6].In a recently published study, Wong et al. [7] identified the radial nerve with ultrasound and nerve stimulation in 46 out of 51 patients (90%). Out of these 90%, one third was difficult to trace downwards. However, their study confirmed that the radial nerve in most patients is located deep and caudad to the axillary artery. For an anaesthesiologist working in a busy operating theatre the results of both studies have profound clinical relevance. Using ultrasound as primary method, LA injections should be adjacent to musculocutaneous, median, and ulnar nerves. If the radial nerve is visible, the fourth injection should be close to it. If it is not, the LA should be injected in the quadrant deep and caudad to the axillary artery, perhaps using a greater volume. Using nerve stimulator as primary method, the LA should be injected at three sites, after obtaining elbow flexion, wrist and fingers’ flexion and wrist and fingers’ extension [8]. For anaesthesiologists using other methods, the LA should ideally be injected in the four quadrants around the artery, as proximal as possible.
◊ REFERENCES
[1] Partridge BL, Katz J, Benirschke K. Functional anatomy of the brachial plexus sheath: implications for anesthesia. Anesthesiology 1987; 66: 743-47.
[2] Choi D, Rodriguez-Niedenfuhr M, Vazquez T et al. Patterns of connections between the musculocutaneous and median nerves in the axilla and arm. Clin Anat 2002; 15: 11-17.
[3] Retzl G, Kapral S, Greher M et al. Ultrasonographic findings of the axillary part of the brachial plexus. Anesth Analg 2001; 92: 1271-75.
[4] Van Geffen GJ, Moayeri N, Bruhn J et al. Correlation between ultrasound imaging, cross-sectional anatomy and histology of the brachial plexus. Reg Anesth Pain Med 2009; 34: 490-97.
[5] Marhofer P, Greher M, Kapral S. Ultrasound guidance in regional anaesthesia. Br J Anaetsh 2005; 94: 7-17.
[6] Chan VW, Perlas A, McCartney CJ et al, Ultrasound guidance improves success rate of axillary brachial plexus block. Can J Anaesth 2007; 54: 176-82.
[8] Wong DM, Gledhil S, Thomas R et al. Sonographic location of the radial nerve confirmed by nerve stimulation during axillary brachial plexus blockade. Reg Anesth Pain Med 2009; 34: 503-07.
[8] Koscielniak-Nielsen ZJ. Multiple injections in axillary block: where and how many? Reg Anesth Pain Med 2006; 31: 192-95.
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