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Ultrasound Guided Nerve Block

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Illustration 1: Distribution of anesthesia, anterior view.

Illustration 2: Distribution of anesthesia, posterior view.

Figure 1:  Hand placement for median nerve blocks at the elbow.

Figure 2:  Image of the median nerve at the elbow.

Figure 3:  Hand placement for median nerve blocks in the mid-forearm.

Figure 4:  Image of the median nerve in the mid-forearm.

Video 1:  Median nerve located at forearm.

Figure 5:  Hand placement for median nerve blocks at the wrist.

Figure 6:  Image of the median nerve at the wrist.

Video 2:  Anisotropy at wrist – labeled.

Video 3:  Median nerve at wrist.

 

Leonard V. Bunting, M.D.; and Beatrice Hoffmann, M.D., Ph.D., RDMS

Median Nerve Block


Anatomy

The median nerve is a mixed sensory and motor nerve that is formed from elements of the medial and lateral cords of the brachial plexus. It travels in a neurovascular bundle in the upper arm and remains tightly associated with the brachial artery as it passes through the cubital fossa in the elbow. In the forearm, the median nerve is found between the muscle bodies of the flexor digitorum superficialis and flexor digitorum profundus before passing through the carpal tunnel in the wrist. Its motor branches supply the deep volar muscles in the forearm and thenar eminence of the hand. Its sensory distribution is limited to the radial aspect of the palm.

 



Illustration 1:  Distribution of anesthesia, anterior view.



Illustration 2:  Distribution of anesthesia, posterior view.



The median nerve can be blocked in three locations below the axilla:
the elbow, the forearm and the wrist. These areas will be covered in separate sections below.

Median Nerve – Elbow
Scanning Technique
Distal to the elbow, the median nerve gives off the anterior innerosseous nerve, which supplies the deep volar muscles of the forearm. Blocking the median nerve at the elbow leads to greater paralysis without greater distribution of anesthesia. In the antecubital fossa, the median nerve lies medial to the brachial artery. With the arm extended and externally rotated, a high-frequency (8-18 MHz) probe is placed tranversely over the brachial artery pulse. Once the brachial artery is identified, depth is adjusted to visualize just beyond the artery.  The brachial vein location varies and a tourniquet will aid in identifying it. Medial to the artery, the median nerve is exhibits its classic honeycomb appearance.

 



Figure 1:  Hand placement for median nerve blocks at the elbow.


Figure 2:  Image of the median nerve at the elbow.


Nerve Block
For an in-plane approach, the needle is held in the dominant hand and carefully inserted medial to the probe. After identifying the needle tip, the needle is advanced slowly towards the base of the median nerve. Local anesthetic is injected to surround the nerve. Readjustment of the needle to the superficial border of the nerve is often necessary to fully surround the nerve and dissect it away from the artery. An out-of-plane approach may aid in separating the nerve from the artery. Typical block volumes are 5-7 cc’s.(1,2)
Median Nerve – Forearm

Scanning Technique
At the level of the mid-forearm, the median nerve is found embedded in the muscles of the volar compartment. Its classic honeycomb appearance should stand out against the surrounding hypoechoic muscle. If the nerve is difficult to identify, it may be traced from the carpal tunnel.(1)  The median nerve will persist when followed, unlike the tendons on the carpal tunnel, which will transition into muscle bodies.


Figure 3:  Hand placement for median nerve blocks in the mid-forearm.


Figure 4:  Image of the median nerve in the mid-forearm.

Nerve Block
A high-frequency (8-18 MHz) probe is used for the block. Although this text demonstrates an in-plane approach, an out-of-plane approach is possible. The needle is held in the dominant hand and carefully inserted in the forearm. After identifying the needle tip, the needle is advanced slowly towards the base of the median nerve. Local anesthetic is injected to surround the nerve. Readjustment of the needle to the superficial border of the nerve is often necessary to fully surround the nerve. Typical block volumes are 5-7 cc’s. (1,2)



Video 1:  Median nerve located at forearm.


Median Nerve – Wrist

As with the landmark based technique, the median nerve can be blocked in the carpal tunnel. This site has the same sensory distribution, but spares paralyzing the forearm muscles. 

Scanning Technique
This block is performed using a high-frequency (8-18 MHz) linear array probe. Scanning starts on the volar wrist, transversely at the first carpal crease. Depth is set to 2-3 cm. Several round, fibrinous structures are identified superficial to the carpal bones representing the tendons and median nerve in the carpal tunnel. Fanning or rocking the probe and will change the appearance of these structures from hypo-echoic to hyper-echoic. This effect is term anisotropy and is more apparent in tendons than in nerves. The median nerve is identified by its less dramatic anisotropy and lack of movement with flexion and extension of the digits.



Figure 5:  Hand placement for median nerve blocks at the wrist.



Figure 6:  Image of the median nerve at the wrist.

Video 2:  Anisotropy at wrist – labeled.

This video shows the dynamic appearance of structures in the carpal tunnel with rocking of the probe distally and proximally on its face.  This is termed Anisotropy and is more dramatic in tendons than nerves.
Alternatively, the structures can be traced into the forearm, where the median nerve persists and the tendons transition into muscle.


Nerve Block

Given the very superficial lie of the median nerve in the carpal tunnel, an out of plane approach is generally preferred. The needle is inserted perpendicular to the ultrasound plane. Once the needle tip is identified, it is slowly directed to one side of the median nerve. Anesthetic is injected until the entire nerve is surrounded. Redirecting to the opposite side of the nerve may be necessary. Local anesthetic volumes are usually between 5-7 cc’s. (1,2)


Video 3: Median nerve block at wrist.

Pearls and Pitfalls
Exercise caution when considering these techniques in patients at risk of a compartment syndrome, as regional anesthesia can mask the signs and symptoms of this condition. Use the minimum volume necessary when injecting into compartments in the forearm to minimize the risk of increasing compartment pressures.

If uncertain whether a structure is the median nerve or a tendon, having the patient flex their fingers and wrist will show motion in tendons and help distinguish them from nerves. Also, tendons will transition into muscle if followed proximally.


References
1.  Buckenmaier C, Bleckner L.
In: Buckenmaier C and Bleckner L (eds.) Military Advanced Regional Anesthesia and Analgesia Handbook, 2008. The Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc; Chapter 11: page 44.

2.  Sites BD, Spence BC.
Sites BD and Spence BC: Ultrasound-Guided Rescue Blocks: A Description Of A Technique For The Median And Ulnar Nerves. The Internet Journal of Anesthesiology. 2005, Volume 10 Number 1.

3. Liebmann O, Price D, Mills C, Gardner R, Wang R, Wilson S, Gray A.
Feasibility of Forearm Ultrasound-Guided Nerve Blocks of the Radial, Ulnar and Median Nerves for Hand Procedures in the Emergency Department. Ann Emerg Med, 2006; 48: 558-561.

 


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