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Ultrasound Guided Nerve Blocks

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Leonard V. Bunting, M.D.

Sural Nerve Block – Ultrasound-guided Ankle Blocks

Introduction

The sural nerve is a small and superficial nerve innervating the foot that can be difficult to visualize with ultrasound. Despite this, the use of ultrasound does improve the success rate of this nerve block at the ankle.

There are five nerves that provide sensory innervation to the foot at or below the level of the ankle.  These are four distal branches of the sciatic nerve (posterior tibial, sural, deep and superficial peroneal nerves) and one branch from the femoral nerve (saphenous nerve).  Local anesthetic injection can be extremely painful or even impossible in foot injuries with extensive and diffuse cutaneous and subcutaneous trauma or foreign body impaction.  Like the wrist block, the straightforward anatomy and low risk for complications make ultrasound-guided ankle blocks an excellent choice for novices.  A comprehensive ankle block involving several nerves requires position changes and may not be ideal for uncooperative patients or patients with injuries that make repositioning uncomfortable.  However, not all nerves need to be blocked for every patient, as injuries to the foot often fall under a single nerve's territory.  Documentation of the neurovascular exam findings before application of the block is essential, as this will help prevent missing neurovascular injuries prior to anesthesia.

Innervation areas of the 5 nerve blocks are shown in illustration 1. 
The three illustrations below may also help you choose which nerves to block based on the location of the injury.  As a simplified rule, two nerves innervate the volar aspect of the foot, three nerves innervate the dorsal aspect of the foot.


Volar Foot:
- Posterior tibial nerve
- Sural nerve

Dorsal Foot:
- Saphenous nerve
- Anterior tibial nerve (also called the deep peroneal nerve)
- Superficial peroneal nerve (also called the dorsal cutaneous or musculocutaneous nerve)

 

             
  SuralNerve   SuralNerve2   SuralNerve3  
             
  Illustration 1: Sensory innervation of the foot, dorsal view.   Illustration 2: Sensory innervation of the foot, plantar view.   Illustration 3: Sensory innervation of the lateral view.  
             

 

Anatomy

The sural nerve is a formed of branches of the common peroneal nerve and the tibial nerve.  The nerve initially courses posterior between the heads of the gastrocnemius muscle. It emerges antero-lateral to the Achilles tendon and passes around the posterior lateral malleolus.

The sural nerve is a sensory nerve. It provides cutaneous innervation to the posterior calf, lateral ankle, lateral heel, and foot (Illustration 1-3).

Scanning Technique

A high-frequency (10 – 18 MHz) linear array probe is used with appropriate depth adjustment.  Maintaining a transverse plane, the probe is moved posterior to the lateral malleolus until the small saphenous vein is identified. The vein will collapse easily in most patients.  A tourniquet placed around the calf and light probe pressure can aid in detection (Figure 1-3). Posterior to the vein should be the small, oval sural nerve.  If the nerve is not well visualized, use the vein as the target landmark.

Figure 1:  Ultrasound image of anatomical area. Vein (blue) and sural nerve (yellow)
are shown next to the lateral malleolus (light blue).

Nerve Block

Given the superficial lie of the nerve and shallow needle angle, a smaller gauge needle (25 gauge, 2.5cm long) is generally used for this block.  The needle is inserted in an in-plane orientation from the side of the probe (Figure 2).  Once the tip is identified, the needle is advanced to the deep border of the nerve, or the small saphenous vein, if the nerve is not apparent.  Once movement of the needle causes movement of the nerve, a small amount of local anesthetic, usually 3-5 ml, is injected. The anesthetic should spread evenly around the nerve and separate it from the vein.  If anesthetic spread is inadequate, the needle may need to be directed towards the anterior surface of the nerve.
If the nerve remains obscured, surround the vein with local anesthetic instead.  In this case, inject the local anesthetic in a band line distribution between the lateral malleolus and Achillis tendon, about one 1 cm superior to the lateral maleolus.

Figure 2:  In-plane approach for sural nerve block, patient and needle position
(blue – saphenous vein, yellow –sural nerve, light blue – lateral malleolus).



Figure 3:  Out-of-plane approach for sural nerve block; patient and needle position (blue – saphenous vein, light blue – lateral malleolus).


Video 1:  Sural nerve block in-plane approach.


Video 2:  Sural nerve block in-plane approach.

 

Pearls and Pitfalls:

Place the patient supine with the lateral malleolus exposed.  Assist the patient by supporting an injured extremity during positioning.  Minimize ambient light to improve image quality.  Placing a spotlight on the block site and darkening the rest of the room can have a dramatic effect on the quality of the ultrasound image on the screen.  Given their superficial lie, some nerves may be very difficult to visualize using standard emergency medicine bedside ultrasound equipment.  Be sure to optimize the depth setting.  An acoustic stand-off pad (such as small bag of saline) can be used between the skin and probe to enhance superficial resolution.  Remember, nerves that are not initially visualized may become obvious after injection of local anesthetic.

References

1.  Redborg KE.
Ultrasound improves the success rate of a sural nerve block at the ankle. Reg Anesth Pain Med, 2009; 34(1):24-8.

 

 

 

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