Peripheral Nerve Ultrasound

Introduction

Hello, my name is John Jacobson.

I'm a musculoskeletal radiologist from the University of Michigan in Ann Arbor, Michigan.

My lecture will be on peripheral nerve ultrasound.

A few disclosures. I'm a consultant for BioClinica and I receive book royalty from Elsevier.

Note that images from this lecture are in the textbook and copyrighted by Elsevier.

Normal Appearance of Peripheral Nerves

What do peripheral nerves look like normally by ultrasound where they're best seen in short axis where they have a characteristic honeycomb appearance, you'll see the individual hypoechoic nerve fascicle separated by the hyper coic connected tissue layers.

This being the median nerve in the carpal tunnel.

Of course, as the nerve travels more distally and arbor rises, you'll have less and less faciles.

When you have a nerve trunk like this, it looks like a honeycomb note here on this cine clip.

I'm toggling the transducer.

I'm doing this on purpose to bring out an atrophy of the adjacent tendons to show how we can differentiate a peripheral nerve from a tendon using anisotropy to our advantage.

Diagnosing Nerve Entrapment

The most common reason we perform ultrasound of peripheral nerves is to diagnose nerve entrapment.

What are the findings of nerve entrapment by ultrasound?

They're the same anywhere in the body and that is when a nerve is traveling into an enclosed space like a fibrosis canal where it's being entrapped, the nerve will become hypo coic and enlarged at in proximal to the entrapment site.

And this is often best appreciated in short axis.

As we scan from proximal distal, you'll see the nerve which is usually uniform, start to get enlarged in hypo coic and then transition back to normal as it enters into the entrap site.

Also, keep in mind when looking for dnar, looking for entrapment neuropathies is to evaluate the end organ or the muscle to look for changes related to the entrapment.

We'll be looking for denervation.

What can happen in this scenario is fat can be infiltrated into the muscle and because the fat is interdigitating with the muscle fibers, there are increased interfaces and therefore the muscle will become normally increased in echogenicity.

With more significant cases, the muscle will decrease in size.

Then we can use the term atrophy.

Here's a case looking at the tibias anterior muscle, the normal short axis with the hypo coic muscle and the echogenic fibro fatty layers.

Here the muscles abnormally hypo coic due to fatty infiltration decreased in size.

Now we can use the term atrophy.

What's important when looking for atrophy is to compare to the other side, set your gain appropriately and then look at the symptomatic side.

'cause many times when we look at this image, we have a tendency to decrease the gain to try the muscle, make the muscle look normal.

Carpal Tunnel Syndrome

We're gonna talk about four nerve entrapment syndromes in this lecture.

First carpal tunnel syndrome, looking at the median nerve in the wrist.

The hallmark of this syndrome would be enlargement of the nerve add en proximal to the entrapment site at the risk crease.

If you look proximally, this is the normal nerve and right at the risk crease under the Retin inoculum we see hypo coic and enlarge.

When moving in short axis from proximal distal, this is the classic finding.

Now if you choose to make measurements you could do circumferential trace underneath the epineurium, and if there's a difference of two millimeters square or more, that's a 99% sensitivity and 100% specificity.

Here's a companion case, again, the normal median nerve and this is abnormal.

The echogenic fiber, connective tissue layers are more hypo coic from edema and effectively look in long axis, the nerve gets larger and larger and more hypo coic as it enters into the carpal tunnel.

As we look at this cine clip, the nerve here going distally is getting larger and more hypo coic as it enters into the carpal tunnel.

Again, normal now becoming hypo coic now becoming enlarged going from proximal to distal As a normal variant.

Some patients may have a bifid median nerve in this case.

Usually there's an interpose persistent median artery in between to diagnose carpal tunnel syndrome.

Here you can simply add the area in four millimeter square is the threshold in this case to diagnose carpal tunnel syndrome.

But the findings are the same, which is hypo coic enlargement of the nerve as it gets close to the entrapment site in in the carpal tunnel.

Cubital Tunnel Syndrome

Let's move on to cubital tunnel syndrome.

In the elbow looking at the ulnar nerve, the anatomy here shows the ulnar nerve going behind the elbow and entering into the true cubital tunnel between the two heads of the flexural narrows underneath the arcuate ligament.

Remember, if we're looking for abnormal hypo coic swelling of a nerve, we're gonna look at and proximal to the entrapment.

We're gonna look behind the median upper condyle for this abnormality.

Here we can see an enlarged ulnar nerve.

As we look distally, it's becoming more and more hypoechoic, more enlarged, and a transition underneath the arc ligament into the cual tunnel.

The number here is 9.5 millimeters square as a threshold, but the numbers have been variable in the literature.

It was seen short axis, the enlargement as it's entering into the true cual tunnel underneath the arcuate ligament As a normal variant.

Some patients may have an synchronous trois Here on this MRI turned upside down to simulate the ultrasound.

We can see this extra illit layer of muscle.

This can cause entrapment and compression of the ulnar nerve.

It's important not to mistake this for a mass.

Note, the fibers here that are characteristic of muscle and the characteristic location.

And course now we're looking at the ulnar nerve.

We also need to think about dynamic evaluation looking for ulnar nerve dislocation 'cause this can produce symptoms as well.

We image while bending the elbow into flex.

Normally the NAR nerve should stay behind the medial epicondyle abnormally.

It can dislocate over the epicondyle.

This can be seen in 20% of asymptomatic volunteers, but repetitive dislocation can indeed cause irritation.

Here we can see the normal ulnar nerve behind the epicondyle with flexion.

It should stay behind the apex of the epicondyle, but in this case, as we flex, we follow the ulnar nerve rather than staying behind.

It moves over the top and snaps right there.

And then as we extend the elbow it reduces be careful not to apply too much pressure 'cause you can inhibit the dislocation of the ulnar nerve.

And as we leave the elbow we need to mention tricep syndrome.

Snapping tricep syndrome is a condition where not only does the ulnar nerve dislocate, but the median head of the triceps shows subluxation over the apex as well.

And you may hear or feel two different snaps.

As I start the video clip, I'm gonna freeze this in neutral position.

As I stop right Here, we can see the apex of the medial peon, the normal position of the ulnar nerve and the triceps, which already looks somewhat large.

Now as we flex, these structures should stay behind the apex as we start the video clip and we flex the elbow, the ulnar nerve snaps and then the tricep snaps and they're in contact with each other.

And as we extend, the triceps will now move back right there and the ulnar nerve second.

You can feel two snaps often through the transducer.

Peroneal Intraneural Ganglion Cyst

Moving on to the peroneal tendon, we're gonna look specifically at the intradural common peroneal cyst.

The peroneal intradural ganglion occurs from fluid extending from the tibial fibular joint.

In fact, it's been shown that in 20% of the patients who have foot drop, this can be the cause for the compression and the foot drop.

The continuity from the joint has been proven with Mr.

Arthrography that if you inject contrast into the knee joint, it can go into the tibial fibular joint.

And then with with weight bearing, it can go into this intra ganglion cyst.

It may also go into the tibial nerve, but less commonly here taken from literature.

Dr. Spinners articles on this will illustrate the pathophysiology of the perineal intraoral ganglion.

The fluid in the tibial fibular joint can connect through this articular branch and work its way up through the common peroneal nerve.

It can go up into the sodic nerve, it can go back on the tibial nerve, it can even come up the tibial nerve.

The key is when you see a cyst around the tibial fibular joint, although you might think about a ovial cyst or a non-specific ganglion, you need to see if it's tracking along the course of the common perineal nerve.

They call this an intradural ganglion cyst.

Here's an example of MRI correlation showing the articular branch with fluid tracking.

Along here is the ganglion cyst next to the common perineal nerve.

They call this the signet ring sign. Here is the cyst.

The nerve is next to it multilocular like most galine cyst, and here it's wrapping around the fibula.

The key is it's gonna track along the common perineal nerve throughout its course.

This case is a very large intraoral ganglion cyst.

It's over 15 centimeters long.

It's going up into the sodic nerve.

Again, multilocular tracking along the common perineal nerve.

Note the edema in the muscle, a sign of denervation by mr, and then the increased echogenicity indicating fatty infiltration.

Morton Neuroma

Then finally we're gonna finish with Morton neuroma involving entrapment of the interdigital nerve in the forefoot.

Between the metatarsal heads, this nerve can be entrapped causing edema, fibrosis and necrosis.

The third inter metatarsal space is more common in the second people are prone to this are those who have pliable shoes, high heeled shoes, and those with narrow toed shoes that will cause compression that with weight-bearing, the metatarsal heads will push on this nerve causing entrapment.

Ultrasound works pretty well when looking for these neuromas five millimeter mass.

You can see these with a hundred percent sensitivity.

You can see them smaller than this, but it becomes more difficult.

Looking for the nerve and continuity in long axis is helpful.

Remember that bursa live in this location so it's important to differentiate bursa from neuroma.

How does one do this? Bursa tend to be more an coic and aromas tend to be more hypoechoic.

Also with compression.

CE tend to compress and norm neuromas tend to not compress.

Here is a neuroma, an MR showing this teardrop a mass coming in a plantar direction.

Here it is by ultrasound.

Note that I tend to scan from a plantar approach 90 degrees to the metatarsals, applying pressure with my other finger, separating out the metatarsal heads, allowing me to see between the metatarsal heads and also trying to induce symptoms by compressing the aroma between my finger and the transducer.

The long axis view is very important 'cause here we can see the emus nerve going into the neuroma.

When you start to see a more an coic area, that's likely the adjacent in our metatarsal bursa.

What has been a very helpful sign to increase accuracy is the MULDER'S sign.

Dr. Mulder described a clinical test where if you press the foot together from the sides and compress the metatarsals together, this can produce a palpable click in symptoms that can indicate more to neuroma.

We use a very similar sign with ultrasound.

We scan from a Pinar aspect in the coronal plane or short axis of the metatarsals from a planter aspect and we'll squeeze the foot from side to side.

And this helps us in three ways in improving our accuracy.

Here's neutral cross section of the metatarsal heads.

This is the normal inter metatarsal space.

This is abnormal as they start the ine clip, we can see the neuroma moving in the plantar direction.

This maneuver helps, helps us in three ways.

First of all, when you squeeze the foot together, the neuromas pushed any plantar direction, so we see it better.

That improves our accuracy.

The second thing, the compression that occurs helps us differentiate the neuroma from a bursa.

The bursa usually compress and don't move to this extent where the neuromas tend not to compress and move more.

And the third reason this helps is the clicking that occurs, reproducing the symptoms is further evidence than indeed this is a morin neuroma.

Take Home Points

The take home points for this lecture on peripheral nerve ultrasound.

First of all, we reviewed entrapment neuropathy.

Basically in the extremities there are specific nerves that have distinct sites of possible entrapment.

And the findings anywhere in the body are the same Hypo coke enlargement at and proximal to the entrapment site.

And if you push the TR with the transducer on the involved nerve, it can reproduce symptoms.

The second point we talked about the intradural ganglion cyst, the classic appearance where it travels along the common perineal nerve.

It's important to differentiate this from a non-specific synovial cyst from the tibial fibular joint.

Then finally, we highlighted the importance of dynamic imaging and peripheral nerve ultrasound, first ulnar nerve in the elbow looking for ulnar nerve dislocation and also snapping tricep syndrome.

Then finally looking at mort in aroma with the molder sign where we can increase our accuracy in the diagnosis of Morton Aroma.

Thank you very much for your attention.