The sural nerve forms from branches of the common peroneal (lateral) and tibial (medial) components of the sciatic nerve. Although the sural usually receives both contributions, anatomic variation in its composition is common.¹

The sural nerve lies adjacent to the small saphenous vein within the subcutaneous tissue of the lateral leg. The sural nerve provides sensory innervation to the lateral foot.² Because it is a sensory nerve, the sural nerve is sometimes used for biopsy or harvest. Although subcutaneous infiltration is an effective means of blocking the distal sural nerve, more proximal block may be indicated in patients with infection or edema of the foot.

Suggested Technique

Because of its small size, the sural nerve can be difficult to image. The sural nerve can be blocked proximal to the lateral malleolus by applying a calf tourniquet to help identify the small saphenous vein. In this location the sural nerve lies adjacent to the vein within the subcutaneous tissue of the leg. An in-plane approach can be used to distribute local anesthetic around the sural nerve.

A sural contribution from the tibial nerve can often be imaged between the medial and lateral heads of the gastrocnemius muscle. This contribution emerges between these muscles to pierce the fascia lata and join the lesser saphenous vein and the common peroneal nerve contribution within the subcutaneous tissue of the lateral aspect of the lower leg. The sural nerve can be blocked with an in-plane approach from the lateral aspect of the leg with the patient in supine position and the leg elevated. Although prone position is optimal for sural nerve imaging, the former approach is more practical and useful in most patients.

Key Points

Sural Nerve Block	The Essentials
Anatomy	The SuN emerges between the medial and lateral heads of the GC.
	The SuN lies adjacent to the SSV within SQ tissue of the lateral leg.
	The SuN is about 2.5 mm in diameter.
Positioning	Supine with leg elevated
Operator	Standing at side of patient
Display	Across the table
Transducer	High-frequency linear, 23- to 38-mm footprint
Initial depth setting	20 mm
Needle	25 gauge, 38 mm in length
Anatomic location	Apply a calf tourniquet to help identify the SSV.
	Begin by scanning the lateral leg proximal to the lateral malleolus.
Approach	Perform the block proximal to the lateral malleolus.
	SAX view of SuN, in-plane from anterior to posterior
Sonographic assessment	The injection should track along the SuN and SSV.
Anatomic variation	The SuN may divide proximally.

GC, Gastrocnemius muscle; SAX, short axis; SSV, small saphenous vein; SuN, sural nerve.

Clinical Pearls

• The sural nerve emerges between the medial and lateral heads of the gastrocnemius to enter the subcutaneous tissue of the lower calf.

• In this location the sural nerve lies lateral to the small saphenous vein. When the sural nerve consists of separate branches, they typically flank either side of the small saphenous vein.¹

• The sural nerve and small saphenous vein pass posterior to the lateral malleolus. Because the sural nerve lies in subcutaneous tissue in the lower calf, it is not mistaken for a nearby tendon.

FIGURE 49-1 Course of the sural nerve.

(From Drake RL, Vogl W, Mitchell AWM. Gray’s anatomy for students. Philadelphia: Churchill Livingstone; 2004.)

FIGURE 49-2 External photograph showing the approach to sural nerve block in the distal leg (A). A calf tourniquet has been applied to improve the visibility of the small saphenous vein behind the lateral malleolus. The corresponding sonogram is shown (B). The sural nerve is adjacent to the small saphenous vein within the subcutaneous tissue of the posterolateral leg. The nerve and vein are viewed in short axis.

FIGURE 49-3 Short-axis view of the sural nerve in the distal leg. In some patients the sural nerve and adjacent small saphenous vein are divided.

FIGURE 49-4 Image sequence showing sural nerve block in the distal leg. Before injection the sural nerve is identified adjacent to the small saphenous vein (A). An in-plane approach is demonstrated where the needle tip is placed adjacent to the sural nerve within the subcutaneous tissue of the leg (B). Local anesthetic surrounds the sural nerve and compresses the small saphenous vein.

FIGURE 49-5 Proximal sural nerve block. The sural nerve also can be blocked in the posterior calf where it emerges between the medial and lateral heads of the gastrocnemius muscle into the subcutaneous tissue of the leg (A and B).