Sural Nerve Block

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.[1] 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.[2, 3, 4] In one study, regional anesthesia of the foot and ankle, when performed by surgeons, was completely successful 95% of the time.[5] 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.[6]

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).

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 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.

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[7]  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.[8]

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.[9]

Reports of significant hemorrhage during sural nerve blockade are rare since this block is superficial and in a compressible area.[10] 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%.[8, 11] Reactions range from delayed hypersensitivity (type IV) to anaphylactic (type I).[12] 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.[8]

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.[11, 13] 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. 

Patient Education & Consent

Explain the procedure, benefits, risks, and complications to the patient and/or patient’s representative, and inform the patient of the possibility of paresthesia during the procedure.

Obtain informed consent in accordance with hospital protocol.

Equipment

Equipment for sural nerve block includes the following:

• Needle, 4 cm, 25 G

• Needle, 18 G

• Syringe, 10 mL

• Marking pen

• Sterile gloves

• Antiseptic solution (eg, povidone iodine [Betadine] or chlorhexidine gluconate [Hibiclens]) with skin swabs

• Alcohol swabs

• Sterile drape

• Lidocaine 1%, 10 mL

• Facial mask with eye shield

• Sterile gauze

Equipment is shown in the image below.

Patient Preparation

Anesthesia

The 2 main classes of local anesthetics currently in use are amino esters and amino amides. Both inhibit ionic fluxes required for the initiation and conduction of nerve impulses.[14] Lidocaine, the most commonly used anesthetic, has a fast onset of action and a duration of action of 30-120 minutes, which is increased to 60-400 minutes with the addition of epinephrine. The total cumulative dose of lidocaine to be infiltrated is 5 mg/kg (not to exceed 300 mg) if lidocaine without epinephrine is used, and 7 mg/kg (not to exceed 500 mg) if lidocaine with epinephrine is used.

Anesthetic preparations that contain epinephrine are commonly used in the emergency department. Epinephrine induces vasoconstriction, which decreases the amount of local bleeding at the site of injection. In addition, it increases the duration of action of the anesthetic with which it is combined. Despite these advantages, the vasoconstrictive properties of epinephrine may contribute to tissue hypoxia, and its use should be avoided in areas of poor perfusion (ie, fingers, toes, penis, ears, nose).

Topical anesthetics may be needed in children or uncooperative adults. Consider the use of a lidocaine, epinephrine, and tetracaine topical preparation or lidocaine/prilocaine emulsion (EMLA) cream.

Pediatric or elderly patients may require additional sedation for compliance.

Consider a hematoma block to attain more effective analgesia when a fracture exists or when more extensive manipulation of the foot is expected.[15, 16]

Adding a buffering solution, like sodium bicarbonate, can decrease the pain of the injection when performing a nerve block while hastening the onset of blockade.[10, 8] Add 1 mL of sodium bicarbonate (44 meQ/50 mL) to 9 mL of lidocaine.

Warming the anesthetic solution to body temperature can significantly decrease the pain of the injection.[8]

When unassisted, tape a bottle of lidocaine upside down to the wall before starting the procedure. If more anesthetic is needed during the procedure, it can be obtained from this bottle without compromising the sterility of gloves and equipment.

Positioning

Position the patient prone, with the ankle elevated by a pillow or rolled sheet to optimize comfort.

Alternatively, the patient may sit or lay supine with the affected leg internally rotated and the ankle elevated on pillow or rolled sheet.

Equipment preparation and proper patient positioning may make the difference between success and failure.

Approach Considerations

Perform and document neurovascular and musculoskeletal examinations prior to the procedure. Testing the sural nerve prior to block includes sensation of posterolateral calf and sensation of the lateral fifth digit (see the images below).

Approach for Sural Nerve Block

Using nonsterile gloves, expose the area of injection and identify the landmarks. Locate the posterior border of the lateral malleolus and the Achilles tendon (see the image below).

Mark the site just lateral to the Achilles tendon, between the 2 landmarks, as shown in the images below.

Be sure to not inject within the Achilles tendon.

Wipe the area with an alcohol pad and clean the site thoroughly with antiseptic solution, moving outward in a circular fashion (see the image below).

Open a sterile drape and place the syringe, needle, and gauze on the tray, maintaining sterility.

Put on sterile gloves. Attach the 18-G needle to the 10-mL syringe and draw up the lidocaine. Then, change to the 25-G needle.

Place a skin wheal at the site marked. Advance the needle through the skin wheal, angling toward the lateral malleolus. Inject 5-7 mL of lidocaine in a transverse line until the lateral malleolus is reached. See the image below.

Calor and rubor of the foot from loss of sympathetic tone may be noted initially.

Successful anesthesia of the areas noted heralds a successful sural nerve block.

Ultrasound of Sural Nerve

The use of ultrasound allows for direct visualization of the sural nerve thus lowering the volume of local anesthesia needed for successful nerve block while minimizing the chance of a neural or vascular injection.

Ultrasound Technique

To perform an ultrasound-guided sural nerve block place the patient in the supine position with the lateral surface of the foot readily exposed and prepped with chlorhexidine. Use a linear ultrasound transducer set to a freqeuncy of 10–18 MHz to best expose this superficial nerve. Place the ultrasound probe posterior to the lateral malleolus. The key landmarks to identify on the ultrasound image are the small saphenous vein and sural nerve, which are adjacent to one another. Inject 3–10 cc of local anesthesia around the nerve. An in-plane block is recommended to allow for better needle visualization.