Ultrasound imaging of lower limbs nerves: implications to venous interventions

Key words

lower limb nerves - damage - ultrasound imaging - varicose veins - venous intervention

Schlüsselwörter

Nerven der unteren Extremitäten - Schädigung - Ultraschallbildgebung - Varikose - venöse Intervention

Varicose veins (VVs) are one of the most common vascular pathologies among adults in Western countries. It affects up to 30 % of population [1] [2]. Many of CVD patients are candidates for invasive venous procedures. For decades, open surgery, which combines high ligation, stripping and phlebectomy was the method of choice for varicose veins patients.

Being effective in elimination of varicose veins open surgery often leads to complications related to the invasiveness of procedure and its technique. One of the most disappointing and disturbing complication is a nerve damage that is registered in up to 40 % of patients [3] [4] [5] [6]. Nerve injury is mainly related to stripping of great or small saphenous veins. Frequency of this adverse event may be reduced by shortening the segment of saphenous vein to be stripped. So-called short stripping allows to decrease risk of nerve damage significantly. Nevertheless, even after less invasive stripping around 7 % of patients experience symptoms of neuropathy, related to sensitive branches damage [6].

High ligation is another possible source of nerve injury if open surgery is performed. While this is usually not the case for GSV ligation, common peroneal nerve is damaged in 4.7 % of patients underwent short saphenous vein interruption in popliteal fossa [7]. The most severe complication that may happen in this case is a motor weakness with dropping foot.

Minimally invasive venous interventions, i. e. thermal ablative techniques have been widely substituting open surgery for varicose veins in the last two decades in many countries. Thermal ablation is considered to be safer for patients as both high ligation and stripping are avoided. But, paresthesia is not that rare after laser or radiofrequency. Nerve damage occurs in 3.8–5.2 % of patients [8] [9] [10]. Those rates are much less than that of registered after open surgery. But, it has to be taken into account that the wider thermal ablation is spread the more patients underwent venous interventions. From 1996 to 2014 in USA number of venous ablation procedures per year increased by 4529 % [11]. It means that much more patients are at risk of nerve injury than it was before, in the era of high ligation and stripping.

One of the ways to avoid nerve damage during venous intervention is to visualize “risk points” where nerves lie in close proximity to veins that are going to be ablated. This can be done by ultrasound imaging. The aim of this article is to present possibilities of ultrasound visualization of lower limb nerves that may be damaged by venous intervention.

General considerations to nerves visualization

Unless other structures in the lower limb, such as tendons and ligaments nerves are not that easy to recognize during ultrasound imaging. At first, there are few anatomical landmarks, like bones, muscles or large vessels that may be used as reference points for superficial nerves lying in subcutaneous soft tissues. Secondly, subcutaneous, sensitive nerves that are often damaged during venous interventions have a very small calibre. They may have just a few fascicles. Using common ultrasound machines with 7.5–10 MHz transducer makes it nearly impossible to distinguish structures of this size and echogenicity from surrounding tissues. The best imaging of peripheral nerves can be achieved by using high-resolution ultrasound probes of 12 and more MHz. To be good at nerve tracking a very precise knowledge of standard anatomy and sonographic anatomy of subcutaneous nerves is also very important.

But not only preoperatively the vision of nerves running next to the veins we aim to treat is important. Also, in postoperative controls, if a patient presents with sensory disturbance after venous procedure ultrasound imaging may be helpful in locating damage and its extent. Probes up to 17 MHz should be used to visualize small nerves. The area of scanning may be limited according to sensory symptoms location and intervention area. The damaged part of the nerve is usually seen even better that in normal condition. Caliber of the injured nerve increases due to its traumatic edema and fibers become less echoic.

Typical points where venous specialists can meet nerves while managing CVD patients

There are some crucial points where invasive procedures may lead to different nerves damage.

1. Groin area. Femoral nerve may be at risk when one is performing high ligation or during tumescent anesthesia for thermal ablation.

2. Inner thigh. Saphenous nerve or its branches are often injured there by pulling stripper or thermally during radiofrequency or laser ablation procedure.

3. Popliteal fossa. The main risk is caused here to tibial and common peroneal nerves by open surgery aimed to interrupt sapheno-popliteal junction.

4. Calf, ankles, foot. Sural nerve is at risk while puncturing or ablating small saphenous vein. The saphenous nerve continuation may be injured by both open and thermal procedures. At ankle joint area many small branches of saphenous nerve (medially), superficial peroneal nerve (laterally and on the foot) and sural nerve (dorsally) may be injured when stab avulsion is performed here

Thus, any vascular specialist should be aware of normal anatomy and sonographic anatomy of peripheral nerves at those particular zones. Very important to know nerves function and main symptoms of their damage ([Table 1]).

Table 1 Lower limb nerves important for venous specialists.

nerve	category (motor/semsory/mixed)	innervation area	symptoms of injury
femoral	mixed	Motor: knee-extensor muscles Sensory: most of anterior and medial aspect of lower limb	Motor: quadriceps extensor weakness, difficulties with extending the knee, feeling of the knee buckling Sensory: numbness, tingling mostly at anterior and medial aspect of lower limb, dull pain at the groin
saphenous	sensory	Anteromedial aspects of knee and calf; medial aspect of foot	Full dissection: loss of cutaneous sensitivity Incomplete dissection or physical trauma: different symptoms, including local numbness, hyperesthesia, pain, paresthesia
tibial	mixed	Motor: gastrocnemius, soleus, plantaris and popliteus muscles Sensory: medial aspect of the calf, plantar aspect of the foot and heel area	Motor: loss of toes and plantar flexion, weakened foot inversion Sensory: lost or weakened sensitivity of the medial calf, and inner plantar aspect of the foot and the heel
common peroneal	mixed	Motor: foot and toes extensors Sensory: lateral calf, anterior and anterolateral aspects of the foot	Motor: foot drop Sensory: loss of sensitivity of lateral calf, anterior and anterolateral aspects of the foot
sural	sensory	Lateral foot and posterolateral aspect of the calf	Loss of sensitivity of lateral foot and posterolateral calf
superficial peroneal	mixed	Motor: short and long peroneal muscles Sensory: lateral lower calf, dorsal foot	Motor: weakened eversion of the foot Sensory: loss of sensitivity of lateral one-third of the lateral calf and of dorsal foot

Groin area: femoral nerve identification

Femoral nerve is one of the largest peripheral nerves. It is formed by ventral branches of lumbosacral plexus receiving neural fibers from LII-LIV.

While scanning sapheno-femoral junction it is quite easy to visualize femoral nerve. It is located laterally to femoral vessels very close to common femoral artery ([Fig. 1]). Continuing distally, femoral nerve is dividing to muscle branches for quadriceps and sartorius muscles and to sensitive saphenous nerve ([Fig. 2]).

Fig. 1 Normal anatomy of femoral vessels and femoral nerve. 1 – femoral nerve; 2 – femoral artery; 3 – femoral vein; 4 – sapheno-femoral junction; arrow – saphenous nerve.

Fig. 2 Skin projections of femoral nerve, its muscles branches (lines) and saphenous nerve (thin line for adductor channel portion, dotted line for subcutaneous portion).

In transverse scan nerve looks as an oval-shape structure with a hyperechoic outer layer (epineurium). Internal structure of the nerve on ultrasound imaging may be described as “salt and pepper” or “honeycombs”. Dotted appearance of small hypo- and hyperechoic parts are nerve fascicles embedded within epineurium ([Fig. 3]).

Fig. 3 Panoramic transverse scan of sapheno-femoral junction area. 1 – femoral artery; 2 – femoral vein; 3 – sapheno-femoral junction. Femoral nerve is marked by dotted circle.

Risk of femoral nerve damage is extremely low as it is located very far from the area of manipulations necessary for both crossectomy and thermal ablation.

Inner thigh: saphenous nerve identification

This is the longest sensory branch of a femoral nerve. It supplies skin of medial aspects of knee joint, calf and inner ankle region. It runs along the lateral side of the superficial femoral artery at upper thigh, lying in the adductor canal. Then, saphenous nerve crosses artery lying on its anterior wall ([Fig. 4]) and heading to its medial side. More distally, at a border of mid- and lower thigh saphenous nerve perforates fascia lata with genus descendens artery between sartorius and gracilis muscles tendons. Here the saphenous nerve becomes subcutaneous and gives an infrapatellar branch ([Fig. 5]). At lower thigh saphenous nerve runs distally close to great saphenous vein (GSV) ([Fig. 6]).

Fig. 4 Saphenous nerve on a mid-thigh locating on anterior wall of common femoral artery. a 1 – femoral vein; 2 – femoral artery. Arrow points to saphenous nerve. b The same picture in B-mode. Saphenous nerve is marked by dotted circle.

Fig. 5 Skin projection of saphenous nerve and its infrapatellar branch (dotted line). Short curved line points at the beginning of the subcutaneous portion of the saphenous nerve.

Fig. 6 Saphenous nerve at lower thigh lying close to great saphenous vein. Nerve is an oval shape hyperechoic structure (arrow).

Saphenous nerve caliber is rather small, i. e. 1–1.5 mm. Its location may significantly vary and no definitive anatomical reference points exist. All this may cause difficulties in imaging of saphenous nerve. The most risky area for saphenous nerve damage starts from the point of its piercing the fascia and locates distally.

Infrapatellar branch supplies skin of lower medial aspect of knee joint area and may be damaged after arthroscopy. Infrapatellar nerve can be theoretically injured by puncturing during tumescent anesthesia for thermal ablation. Such patients may experience pain at thoroughly located area.

Popliteal fossa nerves

In the upper part of popliteal fossa schiatic nerve divides into tibial nerve (medial part) and common peroneal nerve (lateral part) ([Fig. 7], [8]). More distally at a level of popliteal crease tibial nerve can be visualized under fascia close to popliteal vessels. The nerve is located more superficially while popliteal vein is visualized medially to nerve ([Fig. 9]).

Fig. 7 Popliteal fossa nerve’s normal anatomy. 1 – popliteal vein; black star – schiatic nerve; one-headed arrow – tibial nerve; broken arrow – common peroneal nerve; two-headed arrow – medial sural cutaneous nerve (tibial branch) and lateral sural cutaneous nerve (peroneal branch) are forming sural nerve running distally.

Fig. 8 Skin projections of sciatic nerve (thick line) and its branches. Densely dotted line – common peroneal nerve and lateral sural cutaneous nerve. Loosely dotted line – tibial nerve and medial sural cutaneous nerve running to lateral one to form sural nerve (thin line).

Fig. 9 Transverse scan of popliteal fossa. 1 – popliteal vein; one-headed arrow – tibial nerve; broken arrow – common peroneal nerve.

In the outer part of popliteal fossa common peroneal nerve runs close and a bit medially to femoral biceps muscle tendon. The nerve bends fibula’s head being covered by skin and fascia only ([Fig. 10]). It divides here into two branches, superficial and profound peroneal nerves.

Fig. 10 a Longitudinal scan. Common peroneal nerve (marked by arrows) running very close to the skin. b Transverse scan. Common peroneal nerve (marked by arrow) running very close skin. Broken arrow points at biceps tendon.

The manipulation with the highest risk of nerval damage in popliteal fossa is correctly performed crossectomy with dissection of the small saphenous vein down to its junction with the popliteal vein. At this location the tibial nerve can be injured. Thermal ablation is less dangerous, as in this treatment the junction itself is mostly respected. But if the tip of catheter is positioned too close to sapheno-popliteal junction the tibial nerve is also at a risk of damage. It may be also injured during tumescent anesthesia, especially if it is performed with a probe in a longitudinal position. When scanning longitudinally it’s often very difficult to locate the nerve which is not seen with popliteal and small saphenous vein at the same time ([Fig. 11a]), while in transverse scan the tibial nerve can be easily visualized ([Fig. 11b]).

Fig. 11 a Longitudinal scan at popliteal fossa. 1 – popliteal vein; 2 – sapheno-popliteal junction. No nerve is seen. b Transverse scan of the same patient. Tibial nerve is marked by dotted circle.

The same risk exists when one is trying to ablate popliteal fossa perforating vein (Thierry’s perforator). The tibial nerve is lying in a close proximity to perforator ([Fig. 12]) and may be damaged by needle during tumescence.

Fig. 12 Transverse scan of popliteal fossa. 1 – popliteal vein; 2 – popliteal fossa perforator. Arrow points at tibial nerve.

If the nerve is damaged significant enlargement of its caliber is usually observed ([Fig. 13]).

Fig. 13 Post-crossectomy tibial nerve neuropathy. a Transverse scan at a level of popliteal crease. Significantly enlarged hypoechoic tibial nerve is easily visualized (dotted circle). b Longitudinal scan of the same patient with enlarged nerve (arrows). Local edema of nerve is observed. No fascicles are seen.

Real risk exists for common peroneal nerve in the lateral part of popliteal fossa and close to fibula head when stab avulsion is performed. Superficial location of the nerve makes it very easy to damage by hooks for phlebectomy.

Calf: sural nerve identification

Sural nerve is one of the best studied nerves in relation to venous procedures [12]. This is not only due to its frequent involvement as a “collateral loss” in routine invasive venous procedures. Unless many other nerves sural nerve can usually be easily recognized and traced on sonography. It forms by joining of medial sural cutaneous nerve (tibial branch) and lateral sural cutaneous nerve from peroneal nerve ([Fig. 7], [8]). The level of joining is a very individual. In only about 10 % of legs the main trunk of sural nerve forms at upper third of a calf. In about half of legs joining point is located at a mid-calf. One of three sural nerves forms at a distal calf. After completing, sural nerve pierces fascia and runs distally in a very close proximity to small saphenous vein within its compartment ([Fig. 14]). Here it supplies skin of posterior calf.

Fig. 14 a Normal anatomy of sural nerve. 1 – popliteal vein. Two-headed arrow points at medial sural cutaneous nerve and lateral sural cutaneous nerve to form sural nerve. Straight arrow points at sural nerve, broken arrow – to small saphenous vein. b Transverse scan at a mid-calf. 1 – small saphenous vein; arrow points at a sural nerve.

At distal calf sural nerve lies laterally and close to Achilles tendon, then it bends around lateral ankle supplying heel area ([Fig. 15]).

Fig. 15 Transverse scan behind a lateral ankle. 1 – small saphenous vein. Straight arrow points at Achilles tendon, broken arrow – at sural nerve.

Sural nerve injury may occur due to its close relationship to small saphenous vein. Both stripping and thermal ablation are risky for sural nerve at the very intervention. Small branches of sural nerve may be damaged by phlebectomy hooks at a distal calf. Neuropathy may develop indirectly as a consequence of scar adhesive lesion.

Calf: superficial peroneal nerve identification

From its beginning at fibula’s head superficial peroneal nerve descends between short and long peroneal muscles supplying them. Nerve’s small terminal branches provide skin of lateral calf and most of dorsal foot with sensory innervations ([Fig. 16]). Superficial peroneal nerve is easily identified along all it course by sonography ([Fig. 17], [18]). Nerve and its branches may be injured only at distal calf or at a lateral and dorsal foot when phlebectomy is performed. The mid portion that is running deeper between peroneal muscles is not at risk of damage during venous intervention.

Fig. 16 Skin projections of common peroneal (thick dotted line) and superficial peroneal nerve on the lateral aspect of the calf. At upper and mid-calf superficial peroneal nerve runs under fascia between short and long peroneal muscles (line). At lower calf nerve runs subcutaneously (thin dotted line). Location of fibula head is marked by oval and an arrow.

Fig. 17 Transverse a and longitudinal b scans of superficial peroneal nerve at mid-calf (arrows) running between long (1) and short (2) peroneal muscles.

Fig. 18 Transverse scan of superficial peroneal nerve at lower calf after dividing into two branches (arrows). Dashed arrow points at tributary vein, broken arrow points at anterior tibial vessels. Running with them profound peroneal nerve can be seen (curved arrow).

Calf: saphenous nerve identification

From the knee level down to ankle area saphenous nerve runs on anteromedial calf dividing on many tiny branches. They supply skin of medial and partly anterior and posterior calf. The nerve caliber on the calf is getting smaller than on the thigh. That makes imaging very troublesome. Some sweeps of transducer up and down from knee to medial ankle may be needed to locate nerve in transverse section. The caliber of nerve is about 1 mm and only three to five fascicles are often seen ([Fig. 19]). The imaging of small cutaneous branches is impossible due to its very small caliber.

Fig. 19 Saphenous nerve at mid-calf lying close to great saphenous vein (arrow).

Stripping, puncture when making tumescent anesthesia or thermal energy during laser or radiofrequency ablation may damage saphenous nerve and its small branches at calf and inner ankle area. Stab avulsion at medial ankle area is also a risky procedure.

At ankle joint area small branches of saphenous, superficial peroneal and sural nerves may be injured when stab avulsion is performed here. But, it’s impossible to visualize such a small structures.

Conclusions

Modern ultrasound devices allow physicians to visualize nerves effectively, when they know, where they are. Increased number of venous interventions put many patients at risk of nerves injury. This makes venous specialists responsible to prevent this complication as duplex ultrasound is now a mandatory before venous procedure. Nerve imaging at typical risk points should be a part of decision taking prior to intervention. Knowledge of nerves and skills to visualize them prior to intervention are the least to prevent avoidable damage.

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