Peripheral nerves of the lower extremities are often encoun-tered during preoperative venous ultrasound examination or venous procedures. Nerves are not a region of interest in routine practice for most vascular specialists. However, basic knowledge of their course and sonographic appearance is helpful in evaluating venous pathology and, more importantly, to avoid nerve damage during venous interventions. Ultrasound (US) examination is increasingly being used for imaging peripheral nerves. A previous study showed greater sen-sitivity in the detection of peripheral nerve pathology with US than with magnetic resonance imaging (93% vs. 67%) (1). Furthermore, US is noninvasive, well-tolerated by patients, easily repeatable, can guide venous interventions and assess the outcome immediately, and can be used for follow-up. An appropriate scanning technique is essential for accurate anatomical confirmation of nerves using US. In this review, we present a brief description of the anatomy of peripheral nerves, their ultrasonographic appearance, and clinical significance in venous procedures.

Commonly encountered peripheral nerves of the lower extremity in routine clinical practice are the femoral, sciatic, tibial, common peroneal, sural, and saphenous nerves. Nonsaphenous reflux is often associated with sciatic or tibial nerve vein reflux. The common peroneal, sural, and saphenous nerves can be damaged by venous interventions. The femoral and sciatic nerves can be easily located during scanning, while blocks of these nerves are performed with US guidance. Injury to the lower extremity nerves can lead to several complications, such as paresthesia (hypoesthesia- numbness, hyperesthesia – pain), and reduced or even abolished motor function.

1) Instrumentation and appearance at US

The peripheral nerves of the lower extremity can be scanned using a high-frequency linear array transducer. Superficial nerves may be scanned with a 12∼17 MHz linear-array transducer, while deeper nerves may be scanned with a 5∼12 MHz transducer. A convex transducer may be used when scanning the posterior thigh in patients with obesity.

Nerves are composed of bundled fascicles, and each fiber is surrounded by the endoneurium (Fig. 1). Each fascicle is held together and surrounded by the perineurium. The epineurium is a dense sheath of connective tissue that covers the outside of the nerve. Nerves often travel along the blood vessels.

Fig. 1. Illustration of the anatomy of a typical peripheral nerve.

The structure of a nerve has a characteristic “honeycomb” appearance on US in the short axis, representing an uninterrupted fascicular pattern (Fig. 2). The epineurium, made of dense collagen, appears bright on US, whereas the perineurium, made of fat cells, appears dark. As nerves are not compressible, higher pressure can be applied with the transducer to improve imaging when necessary. Tendons have a fibrillar pattern of parallel hyperechoic lines, such as nerves. However, the nerves are very close to the vessels and slide away during compression.

Fig. 2. Ultrasound appearance of the nerve. (A) Honeycomb appearance in the short axis. (B) Hypoechoic structure that runs longitudinally with the hyperechoic cover (epineurium) in the long axis.

2) Femoral nerve

The femoral nerve is most easily seen within the femoral triangle lateral to the common or superficial femoral artery (Fig. 3A). The femoral nerve receives nerve fibers from L2 to L4 and runs between the iliacus and psoas major muscles. The femoral nerve exits the pelvis into the anterior thigh compartment. Approximately 2∼4 cm below the inguinal ligament, the femoral nerve then divides into the anterior and posterior divisions. It terminates as the saphenous nerve. Femoral nerve injury is most commonly caused by traction injury, direct surgical trauma, or compression injury. Ischemic injury is less common because it receives redundant blood supply from the iliac branch of the iliolumbar artery, deep circumflex iliac artery, and lateral circumflex femoral artery in the femoral triangle (2). Femoral nerve injuries manifest as an isolated motor or sensory deficit (3). Physical signs may include paresis or flaccid paralysis of the quadriceps muscle, diminished or absent extension of the knee, and reduced or absent patellar reflex. Sensory distur-bances, including numbness, paresthesia, or hyperesthesia, are present along the course of the saphenous nerve, over the anteromedial aspect of the thigh, and the inner aspect of the calf down to the medial malleolus.

Fig. 3. Anatomy (A) and ultrasound image (B) of the femoral nerve (FN). The FN is found lateral to the common femoral artery (CFA) (B-left). The nerve is smaller than the CFA and the artery is smaller than the adjacent common femoral vein (CFV). Compression with the transducer occludes the CFV leaving the FN and CFA (B-right). Femoral nerve block is administered with ultrasound guidance, which is easy to perform and has a low risk of complications.

A US examination of the femoral nerve was performed in the supine position, and the femoral vessels were used as important anatomic landmarks for locating the femoral nerve (Fig. 3B). The femoral nerve is often visualized during the femoral nerve block. The femoral nerve block is usually performed for surgery on the anterior aspect of the thigh and superficial surgery on the medial aspect of the leg below the knee, such as saphenous vein stripping or harvest, femoral endarterectomy, and groin lymph node excision.

3) Lateral femoral cutaneous nerve (lateral cutaneous nerve of the thigh)

The lateral femoral cutaneous nerve arises from the dorsal division of L2 and L3. It emerges from the lateral border of the psoas major approximately at its middle and crosses the iliacus muscle obliquely toward the anterior superior iliac spine. It then passes under the inguinal ligament, through the lacuna musculorum, and then over the sartorius muscle into the thigh, where it divides into an anterior and a posterior branch (Fig. 3A). It can be injured during ligation, stripping, or avulsion. It supplies sensation to the skin of the thigh and causes meralgia paresthetica, characterized by tingling, numbness, and burning pain in the outer thigh when injured.

4) Sciatic nerve

The sciatic nerve is the longest and thickest peripheral nerve in the body. It can reach a diameter of up to 2 cm. It is formed by the L4∼S3 nerve roots and exits the lesser pelvis via the greater sciatic foramen, emerging from beneath the piriformis muscle in most cases with some anatomical variants. At this level, it is difficult to locate the nerve with US (4). Below the piriformis muscle, the sciatic nerve courses posteriorly around the posterior acetabulum and beneath the gluteus maximus muscle. The sciatic nerve descends posteriorly in the fascial plane deep to the biceps femoris and superficial to the adductor magnus muscle, and it is surrounded by a large amount of connective and adipose tissue (5). In the mid-thigh, it is usually visible through US in the groove between the semimembranosus and biceps femoris muscles (Fig. 4A, B). At the apex of the popliteal fossa, the sciatic nerve is usually divided into the peroneal and tibial nerves. When it is difficult to identify, the tibial nerve can be located first and then tracked proximally along its course. However, the bifurcation level is variable (4).

Fig. 4. Anatomy (A) and ultrasound image (B) of the tibial nerve (arrow).

Trauma, fracture, or hip surgery are the most common causes of sciatic nerve injuries. Sciatic nerve injury commonly occurs in patients with lower weight on hard tables during surgery. Stretch, compression, and ischemia are the primary mechanisms. Injury to the nerve can occur in the frog leg position in vertebral surgeries and in prolonged surgeries in the sitting position. Sciatic nerve injuries occur less commonly in the midthighs. Symptoms and signs of sciatic nerve injury are weakness of the affected muscles, including the hamstrings. Sensory loss involves the entire peroneal, tibial, and sural territories.

The sciatic nerve can be observed when a nonsaphenous vein reflux is suspected, which is defined as the reflux in the superficial veins that are not part of the great or small saphenous systems (Fig. 4C) (6). The sciatic nerve vein insufficiency was found in 10% of the cases of nonsaphenous superficial vein reflux (7). In these patients, the most common presenting symptom was pain along the sciatic nerve that was worse while sitting, usually improved with walking and standing, and was reproducible with point pressure in the area of tenderness where varices disappear (8,9). Similar to the saphenous reflux test, the sciatic nerve vein is examined in a standing position through distal augmentation. Sciatic nerve vein reflux can be treated with phlebectomy or ultrasound-guided sclerotherapy with excellent results (8,10). A more extensive surgical approach should be avoided because of the high risk of nerve injury (9).

5) Tibial nerve

The tibial nerve is the thicker terminal branch of the sciatic nerve and predominantly contains fibers from the L5∼S3 roots. The tibial nerve travels through the middle of the popliteal fossa, along the dorsal surface of the popliteus muscle, between the popliteal vessels surrounded by abundant connective and adipose tissue (Fig. 5A). It then passes between the two heads of the gastrocnemius muscle. The tibial nerve is sometimes referred to as the posterior tibial nerve at a level below the characteristic fibrous arch of the soleus muscle. It is accompanied by the posterior tibial artery and vein, initially on the anterior side, but more distally medial. Thereafter, the term tibial nerve is used throughout the course of the nerve. The tibial nerve gives off an anastomotic branch to form the sural nerve.

Fig. 5. Anatomy (A) and ultrasound image (B) of the sciatic nerve. The sciatic nerve vein reflux can be documented as a dilated vein with significant reflux along the sciatic nerve, when the patients have varicosities behind the thigh or knee (C).

Tibial nerve injury may occur due to compression below the flexor retinaculum of the ankle (tarsal tunnel syndrome), trauma, posterior dislocation of the knee, and fracture. Injury to the tibial nerve can cause motor and sensory deficits in the calf and foot, depending on the site of involvement.

Because of its large diameter, linear course, and accompanying neurovascular bundle, the tibial nerve is an easy target for US assessment. On US, the tibial nerve accompanies the popliteal vein through the popliteal fossa and then continues distally within the posterior tibial neurovascular bundle (Fig. 5B). Using the tibial vessel as a landmark, the tibial nerve below the knee can be identified slightly medial to the long axis of the fibula in the transverse view.

The tibial nerve can be observed if a nonsaphenous vein reflux is suspected at the posteromedial calf. The prevalence of tibial nerve vein reflux is reported to be lower than that of sciatic nerve vein reflux (8). Reflux in the tibial nerve veins has been observed together with that of the sciatic nerve (Fig. 6), and symptoms from compression of the nerve have been reported alone or in combination with the popliteal vein compression (11).

Fig. 6. Female patient with 2 preg-nancies presented with tingling sensation and posterior calf varicosities. She had sciatic (A) and tibial nerve varicose veins (B). The yellow line indicates the width of the sciatic nerve. The tibial nerve is in contact with the popliteal vein seen below it.

6) Common peroneal nerve (CPN)

The CPN is the smaller terminal branch of the sciatic nerve and originates from the L4∼S2 nerve roots. In the popliteal fossa, it runs deep to the biceps femoris and then descends obliquely around the fibular head (Fig. 7A). It courses in the subcutaneous fat and enters the fibular tunnel, formed between the origin of the peroneus longus muscle and the fibular head. The CPN wraps around the proximal fibula and then trifurcates into the deep peroneal nerve, superficial peroneal nerve (SPN), and recurrent articular branch (Fig. 7B). The trifurcation most commonly occurs at or distal to the fibular neck. The deep peroneal nerve continues distally, accompanied by the anterior tibial artery on the interosseous membrane. The SPN descends in the lateral compartment, between the peroneus longus and brevis muscles in approximately 70% of cases or in the anterior compartment in the remaining cases (12).

Fig. 7. Anatomy at the popliteal fossa (A) and upper calf (B), and ultrasound image (C) of the common peroneal nerve (CPN) and the tibial nerve (TN). The CPN and TN are close to the vein of popliteal fossa (VPOPF) at its union with the popliteal vein (POPV). The nerves are seen just below their branching off from the sciatic nerve. Further down, the tibial nerve lies over the POPV. The CPN will run lateral and split into superficial and deep peroneal nerves. In this patient the VPOPF was very dilated and had reflux. Because of its proximity to the nerves, ligation of this vein could lead to potential nerve damage.

Common peroneal neuropathy is the most common mononeuropathy in the lower extremity (13), which may occur due to nerve injury following the surgical removal of the small saphenous vein (SSV) in cases of high location of the saphenopopliteal junction (SPJ) above the popliteal skin crease or having a more lateral course (Fig. 7C) (14). Injury of CPN in patients with reflux in the vein of the popliteal fossa can occur during ligation as this vein unites with the popliteal vein at a higher level and lateral to the SPJ (15). Furthermore, because the CPN courses around the neck of the fibula distally, injury could occur after phlebectomy in that area. Injuries to the CPN can cause numbness, tingling, pain, extensor weakness, and a gait problem called foot drop. Entrapment of the SPN is not uncommon and it can cause pain in the sensory distribution of the dorsum of the foot with or without paresthesia (16). The SPN block is useful when edema or infection contraindicate a more distal ankle block.

7) Sural nerve

The sural nerve is a pure sensory nerve that is formed by the union of the medial sural cutaneous nerve (which originates from the tibial nerve at the popliteal fossa) and the lateral sural cutaneous nerve (which originates from the CPN). It provides sensory innervation to the posterolateral aspects of the distal third of the lower leg and the lateral aspect of the ankle and foot. The sural nerve usually descends in the posterior midline between the two heads of the gastrocnemius muscle and penetrates the deep fascia in the proximal calf, with a high rate of anatomical variation (17). It runs alongside the SSV in the subcutaneous fat of the posterior calf (Fig. 8).

Fig. 8. Ultrasound image of a female patient with small saphenous vein (SSV) reflux. The patient had SSV reflux from the saphenopopliteal junction (SPJ) to the lateral malleolus (red color in the drawing) with long duration as seen in the upper calf (A). The sural nerve (yellow line in the drawing) is not close to the SSV in the mid-calf (B) but is in contact with the SSV at the lower calf (yellow arrow) (C). The perimeter of the nerve has been marked with a yellow dashed line.

The sural nerve is well visualized on US examination because of its superficial course. Using probe compression of the SSV, the sural nerve can be visualized in the subcutaneous fat lateral to the SSV.

Because of its proximity to the SSV, it is at risk during harvest or ablation of the SSV, as well as dissection of the SPJ. Ablation of the SSV may cause sural nerve injury in up to 4% of patients (18). When the sural nerve is injured, burning pain and sensory alterations, including hyperesthesia or paresthesia over the posterolateral aspect of the distal third of the leg and lateral aspect of the foot, heel, and ankle. Sural nerve injury from ablation procedures can be reduced by using a tumescent solution or nonthermal technique (19,20).

8) Saphenous nerve

The saphenous nerve is a terminal cutaneous branch of the posterior division of the femoral nerve, originating from the L2∼L4 nerve roots. It provides pure sensory innervation to the anterior and medial aspect of the distal thigh, as well as the anteromedial knee and medial lower leg. The saphenous nerve descends in the subfascial plane of the femoral canal and accompanies the superficial femoral artery. It then courses from the femoral vessels penetrating the superficial fascia toward the medial subcutaneous tissue and joins the greater saphenous vein approximately 10 cm proximal to the knee. Below the level of the knee, the saphenous nerve descends along the medial border of the tibia, accompanying the greater saphenous vein.

A US examination of the saphenous nerve is performed in the supine position with slight flexion and external rotation of the knee in the same position as when scanning the lower extremity veins. At the joining point 10 cm proximal to the knee, the saphenous nerve can be identified by compressing the greater saphenous vein as a landmark (Fig. 9).

Fig. 9. Ultrasound image of the saphenous nerve at the lower third of the calf in contact with the great saphenous vein. (A) Cross sectional view, (B) longitudinal view.

The saphenous nerve can be injured during thermal or surgical saphenous vein procedures or saphenous vein harvesting involving the medial knee, because of its proximity to the greater saphenous vein (15,21). When injured, patients present with saphenous neuropathy with paresthesia or numbness of the medial calf or medial foot depending on the level of injury (15,22).

Most nerves run together with vessels having a standard location with only a few important variations and are therefore easy to identify. Vascular specialists who are accustomed to the ultrasonographic appearance of vessels need to become familiar with the nerves. This will help in treatment planning and reducing the complication rates.