The Sural Nerve: A Target To Miss!


by Diana L. Neuhardt, RVT

As a common alternative to traditional surgery, endovenous thermal ablation (EVTA) is a minimally invasive treatment method to eliminate truncal reflux. Reflux is detected more commonly in the Great Saphenous Vein (GSV) when compared to the Small Saphenous Vein (SSV),(1) though the prevalence of SSV reflux may be underestimated due to investigative technique.(2)

Reported adverse events following thermal ablation of the SSV suggest a higher rate of nerve injuries,(3) although arguably there is less data regarding SSV treatment compared to the GSV. Based on its’ anatomical close proximity to the SSV, the sural nerve (SN) is at greatest risk for injury during intervention in the distal calf. Paresthesia and dysesthesia, though usually transient, are the manifestations of damage to the SN. The SN is a sensory nerve and supplies the skin of the posterolateral aspect of the distal calf, lateral malleolus, lateral foot and toe. Motor fibers and unmyelinated autonomic fibers have been discovered within the SN, up to 4.5%.(4) Based on this evidence, what techniques can we use to avoid SN complications?

In most instances, EVTA is performed in an ambulatory setting under local regional anesthesia with the aid of ultrasound guidance. Visualization with ultrasound enables percutaneous vein access, placement of sheath and catheter, and anesthetic delivery.

Ultrasound is also an excellent resource to visualize structures other than veins. Ultrasound images are not selective. In other words, imaging from the skin line demonstrates subcutaneous fat, muscle, tendons, arteries, veins, nerves, joint space, periarticular bursae, and bone.Identification of the sural nerve by ultrasound may allow one to safely gain vein access for EVTA treatment.

Though variation exists, the SN is best visualized in the mid and lower calf. A high frequency linear transducer is used to identify the SSV in transverse orientation. Immediately adjacent to the SSV is the SN – a round structure with mixed internal hypo-echogenic characteristics. Movement of the transducer along the skin line will demonstrate the variable proximity of the SN to the SSV. (Figures 1 and 2) Identification of the SN in the mid to lower calf may assist in determining the most advantageous point for percutaneous venous access. With Ultrasound detection, the area in which the SN is closest to the SSV would be ideal to avoid.

Dr. Stefano Ricci of Rome, Italy, was among the first to describe the ultrasound anatomic features of the nerve structures in the popliteal fossa, including the sciatic, peroneal, and tibial nerve.(5) Dr. Ricci’s profound description of the sciatic nerve is “always visible, never seen” and serves as a reminder to phlebologists to expand their use of ultrasound to include the identification of nerves.

Awareness of the visualization techniques to identify nerves, their location and proximity to the vein to be treated, may reduce the risk of nerve injury. This information may be considered in planning treatment for patients who undergo venous procedures, including EVTA, ultrasound-guided sclerotherapy, stripping, and AP.

Diana Neuhardt, RVT is President of CompuDiagnostics, Inc. in Scottsdale, Arizona. She is a member of the Board of Directors and the Ultrasound Section Chair of the American College of Phlebology.

References
1. Engelhorn CA, Engelhorn AL, Cassou MF, Salles-Cunha SX. Patterns of saphenous reflux in women with primary varicose veins. J Vasc Surg. 2005;41:645-651
2. Neuhardt DL, Salles-Cunha SX, Morrison N. Prevalence and Patterns of Small Saphenous Vein Reflux. J Vasc Ultrasound 2009;33:19-22
3. Merchant RF, Pichot O, for the Closure Study Group. Longterm outcomes of endovenous radiofrequency obliteration of saphenous reflux as a treatment of superficial venous insufficiency. J Vasc Surg 2005; 42:402-509
4. Sankar D, Bhanu P, et al. Variant Formation of Sural Nerve and its Distribution at the Dorsum of the Foot. International Journal Anatomical Variations 2009; 1:33-34
5. Ricci S. Ultrasound Observation of the Sciatic Nerve and Its Branches at the Popliteal Fossa: Always Visible, Never Seen. Eur J Vasc Endovasc Surg 2005;30:659-663


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