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Sural Nerve Block

Overview

Background

Practitioners in the emergency department frequently encounter patients who have sustained trauma to the lower leg or foot and require anesthesia for repair. The regional sural nerve block allows for rapid anesthetization of the posterolateral calf and laterodorsal foot, including part of the dorsal fifth digit. Regional blocks have several advantages compared to local infiltration, such as fewer injections required to attain adequate anesthesia, smaller volume of anesthetic required, and less distortion of the wound site.
Because of the lower number of injections, regional block is better tolerated by the patient and limits the chance of a needle stick injury to the provider. Regional anesthesia allows for selective analgesia and can eliminate the need to provide the patient with sedation or opioids for pain control while providing longer-lasting results.

This procedure is safe, is relatively easy to perform, and can provide excellent anesthesia to the foot and lower leg.
In one study, regional anesthesia of the foot and ankle, when performed by surgeons, was completely successful 95% of the time.
Because the sural nerve is relatively superficial, it is easily blocked at multiple levels at or above the ankle. In fact, because the nerve is so accessible, sural nerve biopsy specimens have been used to study inflammatory demyelinating peripheral neuropathies.

Indications

Indications for sural nerve block include the following:

Wound repair or exploration of the lateral posterior calf or dorsolateral fifth digit

As part of an ankle block required to manipulate a fractured or dislocated ankle

Incision and drainage of an abscess in the lateral posterior calf or laterodorsal fifth digit

Removal of a foreign body in the lateral posterior calf or dorsolateral fifth digit

A combination of posterior tibial, saphenous, superficial peroneal, deep peroneal, and sural nerve blocks results in complete block of sensory perception beneath the ankle (see the image below).

Areas of anesthetization to complete an ankle bloc

Areas of anesthetization to complete an ankle block. This block requires anesthetization of 5 nerves for complete sensory block below the ankle. The areas to anesthetize include a line along the anterior ankle for the superficial peroneal nerve (blue line), the deep peroneal nerve (red star), the saphenous nerve (pink star), the sural nerve (green arrow), and the posterior tibial nerve (orange arrow).

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Contraindications

Contraindications to sural nerve block include the following:

Allergy to anesthetic solution or additives (eg, ester, amide)

Injection through infected tissue

Severe bleeding disorder or coagulopathy

Preexisting neurological damage

Patient uncooperativeness (pediatric or elderly patients may need sedation)

Technical Considerations

Procedure Planning

Understanding the arborization of the sural nerve is crucial to a regional block of this nerve. The sural nerve has a contribution from both the tibial nerve and the common peroneal nerve, each of which originates from the sciatic nerve. The contribution from the tibial nerve is the medial sural cutaneous nerve; the common peroneal nerve’s contribution is the sural communicating branch. These two contributions come together to form the sural nerve, which arises in the popliteal fossa and courses superficially after piercing the deep fascia in the posterior calf (see the image below).

The sural nerve as it travels down the posterior c

The sural nerve as it travels down the posterior calf.

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The sural nerve continues down the posterior calf and supplies the skin of the posterolateral lower third of the lower leg. Entering the foot posterior to the lateral malleolus, this nerve supplies the lateral aspect of the foot, including the lateral fifth digit, via the lateral dorsal cutaneus nerve. It supplies the lateral heel via the lateral calcaneal branches. See the images below.

Sural nerve dermatome at the level of the posterio

Sural nerve dermatome at the level of the posterior calf.

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Sural nerve dermatome at the level of the sole of

Sural nerve dermatome at the level of the sole of the foot.

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Complication Prevention

Infection occurs when the puncture site is not clean. Avoid needle insertion through infected skin or a skin lesion. Be sure to use a sterile technique during the procedure, as the risk of infection is insignificant when sterility is properly maintained.

Intra-arterial injection
 may result in vasospasm and lead to ischemia of the limb tissue. Intravenous injection can lead to systemic toxicity in high doses. Tissue texture changes revealing pallor, bogginess, and cool temperature may indicate that either intravascular injection or vascular compression has occurred. Always aspirate prior to injection and every 3-5 cc to ensure that intravascular placement has not taken place. Alpha-adrenergic antagonists (eg, phentolamine 0.5-5 mg diluted 1:1 with NaCl 0.9%) can be administered by local infiltration to relieve arterial vasospasm secondary to intra-arterial injection.

Patients may develop paresthesia, sensory deficits, or motor deficits secondary to inflammation of the nerve. Most often, this type of neuritis is transient and resolves completely. During the procedure, pull back gently after inducing a paresthesia to avoid an intraneural injection. Make sure to document a complete neurovascular examination both before and after the procedure.

Reports of significant hemorrhage during sural nerve blockade are rare since this block is superficial and in a compressible area.
A hematoma may develop with intravascular puncture. If prolonged bleeding occurs, apply direct pressure and elevate the limb.

Allergic reactions to local anesthetics occur at a rate of 1%.
Reactions range from delayed hypersensitivity (type IV) to anaphylactic (type I).
Although rare, the most common cause of such allergic reaction is the preservative in the local anesthetic solution. Preservative-free lidocaine (found in cardiac preparations) is an alternative because it does not contain the preservative. Alternatively, a 1-2% diphenhydramine solution can be used as a local anesthetic.

The dose of 1% lidocaine without epinephrine should not exceed 5 mg/kg. If lidocaine with epinephrine is used, total dose should not exceed 7 mg/kg. Systemic toxicity manifests in the central nervous and cardiovascular systems.
Signs such as tremors, convulsions, tachycardia, or respiratory compromise should alert the physician to stop the procedure and reassess the patient. One should be familiar with the LAST (Local Anesthetic Systemic Toxicity) protocol and have intralipid readily available. 

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