ULTRASOUND OF NORMAL PERIPHERAL NERVE

BY: DR. BHISHM SEVENDRABaroda Medical College, Gujarat

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Ultrasound Anatomy and Nerve scanning Technique

The femoral artery is palpated behind the midpoint of the inguinal ligament and a linear high frequency (8–12 MHz) probe should be placed perpendicular to the course of the femoral nerve.

Medial to lateral sliding movements of the transducer, aid visualization of the pulsatile femoral artery.

Structures that can be visualized are the fascia iliaca, femoral artery, and immediately below and lateral , femoral nerve in a wedge-shaped space .

The femoral nerve is typically hyperechoic and lies in a sulcus in iliopsoas inferiorly.

The femoral nerve may show anisotropic (directionally dependent) behaviour and angling the probe slightly cranially or caudally may help in visualization of the nerve.

Ultrasound of Femoral Nerve

Femoral triangle

Other structures visualized include the distal divisions of the common femoral artery into superficial and profunda femoris arteries and the compressible femoral vein, located infero-medially relative to the pulsating femoral artery.

Despite the relatively superficial position of the femoral nerve, visualization may sometimes be difficult. This may be because of anatomical variations, such as proximal division of the nerve, or because of similar echogenicity to surrounding tissues.

The patient is positioned supine with the operator on the right-hand side and vice versa for a left-handed operator.

The ultrasound machine should be placed on the opposite side so that the operator's line of sight, and the screen is in a straight line.

Ultrasound anatomy of femoral nerve in the upper thigh. FL, fascia lata; FI, fascia iliaca; FN, femoral nerve; FA, femoral artery; FV, femoral vein; ILPSM, iliopsoas muscle; PM, pectineus muscle. The yellow line indicates needle path.

Ultrasound of Popliteal Sciatic Nerve

Accessing the nerve proximally is made challenging by its deep lie.

  However, the nerve becomes superficial as it approaches the popliteal fossa.

Anatomy

The sciatic nerve originates from the lumbar and sacral plexi (L4 – S3) and travels deep in the posterior thigh.

  Proximal to the popliteal fossa, the sciatic nerve lies more superficial, between the long head of the biceps femoris laterally and the semimembranous muscles medially.

  In this location, the nerve generally bifurcates into the common peroneal and tibial nerves. These nerves continue into the popliteal fossa where they lie superficial and lateral to the popliteal vessels.

 

From the popliteal fossa, the tibial nerve continues down the back of the leg to wrap around the posterior medial malleolus. It provides innervation to the majority of the posterior lower extremity, knee joint and plantar surface of the foot. The common peroneal nerve passes posterior to the head of the fibular and then winds anterior, where it divides into deep and superficial branches.

Scanning Technique

ideally, the patient is on prone position .   scanning can be performed with the

patient on their side or even supine .  A high-frequency (6-18 MHz), linear

array probe is used with appropriate depth adjustment, usually between 2 to 6 centimeters depth setting.  

The probe is placed transversely across the popliteal fossa at the popliteal crease.

  The femur is first identified by its’

characteristic hyperechoic stripe and dense posterior shadowing.

  Next, the pulsatile popliteal artery and corresponding vein are located superficial and medial to the femur. 

At this level, the sciatic nerve has generally bifurcated into the tibial and common peroneal nerves.  

The tibial nerve is more easily visualized than the common peroneal nerve and is found superficial and lateral to the artery. 

The common peroneal nerve is found lateral to the tibial nerve at a similar depth.

  If visualization of the nerves is difficult, the probe is fanned or rocked to find the ideal angle of incidence for nerve visualization.

If visualization is still challenging, the patient should plantar flex the ankle to elevate the peroneal nerve and dorsiflex the ankle to elevate the tibial nerve if possible.  This is commonly referred to as the “seesaw” sign.

 Once either or both of the nerves are identified, they are followed proximally until they form the sciatic nerve.

Anatomy The sciatic nerve (from sacral plexus L4-S3) divides above the poplitel fossa into the posterior tibial nerve and common peroneal nerve. 

The posterior tibial nerve travels in the posterior leg with the posterior tibial artery, in the fascial plane between the superficial and deep muscle groups.

It is a mixed sensory and motor nerve. 

Further distally, it passes posterior to the medial malleolus and posterior to the posterior tibial artery at the ankle. 

Ultrasound of posterior tibial nerve

Scanning Technique

A high-frequency (10 – 15 MHz) linear array probe is applied in a transverse plane posterior to the medial malleolus .

The posterior tibial artery is identified by its pulsations and lack of compressibility.

 Anteromedial to the artery lie the flexor digitorum tendons and the tibialis posterior tendon.

Flexing of the toes will result in movement of the hyperechoic tendons on the image.

The oval tibial nerve is found posterolateral to the posterior tibial artery. 

Ultrasound of Median Nerve

The median nerve is a mixed sensory and motor nerve that is formed from elements of the medial and lateral cords of the brachial plexus.

It travels in a neurovascular bundle in the upper arm and remains tightly associated with the brachial artery as it passes through the cubital fossa in the elbow.

Anatomy

In the forearm, the median nerve is found between the muscle bodies of the flexor digitorum superficialis and flexor digitorum profundus before passing through the carpal tunnel in the wrist.

Its motor branches supply the deep volar muscles in the forearm and thenar eminence of the hand. Its sensory distribution is limited to the radial aspect of the palm.

Distal to the elbow, the median nerve gives off the anterior interosseous nerve, which supplies the deep volar muscles of the forearm.

Median Nerve – Elbow

In the antecubital fossa, the median nerve lies medial to the brachial artery.

With the arm extended and externally rotated, a high-frequency (8-18 MHz) probe is placed transversely over the brachial artery pulse.

Scanning Technique

Once the brachial artery is identified, depth is adjusted to visualize just beyond the artery.  The brachial vein location varies and a tourniquet will aid in identifying it.

Medial to the artery, the median nerve is exhibits its classic honeycomb appearance

Median Nerve – Forearm

At the level of the mid-forearm, the median nerve is found embedded in the muscles of the volar compartment.

Its classic honeycomb appearance should stand out against the surrounding hypoechoic muscle.

Scanning Technique

If the nerve is difficult to identify, it may be traced from the carpal tunnel.

The median nerve will persist when followed, unlike the tendons on the carpal tunnel, which will transition into muscle bodies.

Median Nerve – Wrist

scanning is performed using a high-frequency (8-18 MHz) linear array probe.

Scanning starts on the volar wrist, transversely at the first carpal crease.

Several round, fibrinous structures are identified superficial to the carpal bones representing the tendons and median nerve in the carpal tunnel.

 Scanning Technique

Fanning or rocking the probe and will change the appearance of these structures from hypo-echoic to hyper-echoic. This effect is term anisotropy and is more apparent in tendons than in nerves.

The median nerve is identified by its less dramatic anisotropy and lack of movement with flexion and extension of the digits.

Alternatively, the structures can be traced into the forearm, where the median nerve persists and the tendons transition into muscle.

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Ultrasound of Radial Nerve

Anatomy

The radial nerve is a mixed sensory and motor nerve that arises from the posterior cord of the brachial plexus.

It travels in the spiral groove of the humerus in the upper arm and descends anterior to the lateral epicondyle in the elbow.

At the elbow it divides into the superficial branch, which provides cutaneous innervation to part of the hand, and the deep branch, which innervates the dorsal compartment of the forearm.

The superficial branch travels with the radial artery in the mid-forearm, but divides away from the artery in the distal forearm as it separates into smaller cutaneous branches.

Scanning Technique

The radial nerve is found sonographically by tracing the radial artery from distal to proximal.

At the level of the mid-forearm, the cutaneous branches will reform the radial nerve lateral to the artery. The nerve may appear triangular or oval at this level.

 Proximal to the elbow,   probe is placed over the triceps muscle in transverse orientation localizing the brachial artery.  Lateral to it, the radial nerve can be found crossing over the humeral shaft in mid-upper arm.

Ultrasound of Ulnar Nerve Anatomy

The ulnar nerve originates from the C8-T1 nerve roots, which form parts of the medial cord of the brachial plexus. 

It is a mixed sensory and motor nerve and provides sensory innervation to the medial aspect of the hand. 

It descends on the posteromedial aspect of the humerus and enters the flexor compartment of the forearm by passing between the olecranon and medial epicondyle of the humerus (“cubital tunnel”) and through the two heads of the flexor carpi ulnaris. 

The nerve then travels near the ulnar bone and medial to the ulnar artery in the distal forearm. 

About 5 to 10 cm proximal to the wrist dorsal and palmar cutaneous branches split and provides innervation to the medial aspect of the hand. 

Artery and nerve are found deep to the tendon of the flexor carpi ulnaris and enter the palm superficial to the flexor retinaculum, via the ulnar canal.

Scanning Techniques

Ulnar Nerve at the Wrist

In the wrist, the hyperechoic appearing ulnar nerve lies medially, adjacent to the artery.

 The nerve is located between the ulnar artery and the flexor carpi ulnaris at the level of the proximal volar skin crease.

A high-frequency (10-18 MHz), linear array probe is used with appropriate depth adjustment.

image of the ulnar nerve at the level of the wrist.

Ulnar Nerve at Forearm

To locate the ulnar nerve in the forearm, one can trace the artery proximally until the nerve becomes visible in the mid-forearm located medial to the vessel. 

The ulnar artery, which is in close proximity to the nerve, facilitates sonographic detection.

Ulnar Nerve at Elbow

The ulnar nerve lies in the cubital tunnel on the medial aspect of the elbow.

The ulnar nerve can be easily located in the groove between the olecranon and the medial epicondyle.

  The probe orientation should be transverse and needle orientation in-plane.

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