Peripheral Nerve Ultrasound - HD

Introduction

Thank you and good morning.

I'll be talking about peripheral nerve ultrasound or first a few comments on what nerves look like normally with ultrasound.

Normal Appearance of Peripheral Nerves on Ultrasound

When you look at the structure of a peripheral nerve, what you'll identify is the individual hypoechoic nerve fascicles, and around those fascicles you'll have the echogenic connective tissue.

When you look at the nerves in short axis, it'll have a honeycomb appearance.

When you're looking at peripheral nerves, it's the short axis image that is most characteristic when identifying nerves.

As you move the transducer proximal and distal, you'll appreciate the tubular configuration of the structure.

What I'm showing here in looking at the carpal tunnel, I'm toggling the transducer and what I'm doing is bringing out anisotropy of the tendons.

Note that anisotropy also involves the nerves, but to a much lesser degree because it's already hypoechoic.

It's just really the echogenic epinephrine that is going to decrease with anisotropy.

By toggling the transducer you can bring out these echogenic layers to show that honeycomb appearance.

As you follow a peripheral nerve more distally, the nerve will arborize.

Obviously these fascicles will diminish in number as they branch off.

Then finally will be down to an individual fascicle.

If you look at the peripheral nerves in long axis, you'll appreciate the hypoechoic fascicles of the nerve and the echogenic connective tissue.

You can see in long axis it looks somewhat similar to a tendon.

They are different, the tendon is more compact and fiber-like, but there's some similarity.

Another issue is when you look at a nerve in long axis, sometimes the nerve will snake around and not be in one plane.

That's why the short axis for me is my starting point when identifying nerve problems.

Focus on Entrapment Neuropathies

This talk is less than 30 minutes.

I'm gonna focus on one area of peripheral nerves and that is entrapment neuropathies.

That's probably one of the most common reasons ultrasound is performed when looking at nerves.

We heard earlier in the last talk about peripheral nerve tumors, so I won't be talking about that.

Pathophysiology of Nerve Compression

What happens to a nerve when it's compressed?

It's been shown with this experimental model that the compression causes ischemia and the first thing you see will be edema.

The edema will correlate with the degree of axonal injury.

This will go on also with demyelination.

Note here that inflammation is really not a part of this.

The term neuritis is somewhat a misnomer, it's really edema and ischemia and then axonal damage with demyelination.

Ultrasound Findings in Entrapment Neuropathies

What are the findings on ultrasound?

Anywhere in the body when you have a nerve that's entrapped, you'll have the nerve enlarged and hypoechoic and edema at and proximal to the entrapment site.

That's why when we're looking at these peripheral nerves, as I move on with this lecture, realize that every nerve has two or three precise anatomic locations where that nerve goes through a constricted area making that prone to entrapment.

When you scan over those potential sites of entrapment, we'll look for an enlargement of the nerve abnormally hypoechoic, and then as the nerve goes into the compressed or entrapped area, it will then either flatten or may still be enlarged.

End Organ Effects on Muscle

One other point before moving on to the specifics, we also have to keep in mind the end organ of the peripheral nerve that we're evaluating.

What is the effect on the muscle?

We know that the earliest sign of denervation is edema and edema will appear as increased echogenicity.

That's because there are increased interfaces with the edema and the muscle tissue.

As the denervation continues, you'll then have fatty degeneration or infiltration.

This will also appear hyperechoic but usually more hyperechoic than the edema itself.

Fat itself is actually hypoechoic on ultrasound, but why is it echogenic?

Remember that the image in ultrasound we're looking at interfaces of tissue with different impedance differences.

What we're looking at is the muscle fibers and the fat interdigitating creating more interfaces, creating this echogenic appearance.

Then finally with more chronic cases you'll have atrophy.

In this case, the muscle will be decreased in size and echogenic.

An important thing here is to compare to the other side.

What I found is that when I put my transducer over an echogenic muscle, the first thing I do is reach for the gain and try to turn it down and make this look like normal muscle.

If you do that, of course you won't be able to pick up the abnormality.

I will go to the asymptomatic side, optimize my gain, and then go to the other side at the same level to look for atrophy and denervation.

Specific Nerves and Entrapment Sites

What I'll be doing is going through each of the nerves and highlighting the critical parts or anatomic locations where we can see entrapment first, beginning at the median nerve, looking at carpal tunnel syndrome.

Median Nerve: Carpal Tunnel Syndrome

As I mentioned, we're looking for enlargement of the nerve and we're looking at the wrist crease, which is at and proximal to the entrapment site.

The number or the area of the nerve will correlate with the severity of carpal tunnel syndrome.

The whole thing about these numbers, however it's how you want to dial your sensitivity and specificity.

In general, these are the numbers that have been quoted in the literature.

Here you can see the normal nerve.

This image is blown up to show the echogenic connective tissue.

Here is an abnormal nerve.

It's more hypoechoic because the echogenic connective tissue is less echogenic.

In long axis we can see the enlarged hypoechoic nerve now being compressed under the retinaculum.

This has been termed the notch sign where basically there's a transition from an enlarged nerve to a smaller diameter nerve due to the compression.

One problem with just having an exact measurement to say what is normal or abnormal is that people have all different sizes of their nerves.

What's been shown to be the most accurate is looking at a comparison of the area proximal to distal.

We measure approximately at the level of the pronator quadratus, we measure at the wrist crease wherever the nerve is the largest, if that area increases by two millimeters squared or more 99% sensitivity and 100% specificity.

This if you do need to measure is quite helpful.

To be honest, I typically do not measure all these nerves when I think they're enlarged.

If I'm looking at a nerve in short axis and I'm scanning distally and as I approach an entrapment area, if that nerve starts to become hypoechoic enlarged and then compresses, I don't care what the number is, that nerve is abnormal.

Also when you push on the nerve that's hypoechoic, they're gonna tell you that the nerve is irritated.

Putting it all together, you could use numbers, but I really make up my mind most of the time just subjectively.

As we leave the carpal tunnel, be aware of the bifid median nerve.

It's a normal variant. The incidence is quoted at 3%, it's probably more like 5%.

Remember that when you have a bifid median nerve, you almost always will have a persistent median artery.

That information can be important if someone is going for an open carpal tunnel release.

If you do see two nerve trunks, the Ventura has shown that you want to combine the area and then you're gonna use four millimeter squared.

Your threshold is a difference between proximal and distal.

The nerve is getting larger by four millimeters squared or more when combining the areas that is abnormal.

Another point I wanna make is that if you see flow blood flow with any nerve that is abnormal and it's been shown that this finding alone is 95% accuracy.

I don't rely on the vascularity, I really am looking for an enlargement of the nerve.

This is another finding that's been described with carpal tunnel syndrome.

Ulnar Nerve Entrapment Locations

We're gonna talk about two entrapment locations for the ulnar nerve.

First ulnar tunnel syndrome.

If you identify the pisiform, you can palpate that on the patient.

You can see that on ultrasound we know between the pisiform and the ulnar artery is the ulnar nerve shown here and here.

This is going to Guyon's canal, a site of potential entrapment.

One potential cause is an accessory abductor digiti minimi.

This is present up to 25% of the population and you have this extra muscle that's really compressing the nerve in this space.

Uncommonly you could have the muscle go between the artery and the nerve.

Most commonly it's over it.

In my experience, I do see this probably 20% of the time.

I have rarely actually seen people have symptoms related to this accessory abductor digiti minimi.

It has been reported in the literature as a potential cause for entrapment.

Then if we look at cubital tunnel syndrome, this is the second most common nerve entrapment of the upper extremity following carpal tunnel.

We're gonna look at the elbow.

Looking at the anatomy here we see the ulnar nerve going behind the medial epicondyle.

This is not the area that's entrapped, it's actually more distal.

The nerve is gonna go underneath Osborne's fascia and the true cubital tunnel is right here, not back here.

The true cubital tunnel is defined by the arcuate ligament and the two heads of the flexor carpi ulnaris.

However, when looking for entrapment, as I mentioned earlier, the nerve will be hypoechoic and enlarged at and proximal to the entrapment site.

Looking behind the epicondyle is where we target the increased size of the nerve.

That's entrapped.

Here's the normal situation behind the medial epicondyle here is the ulnar nerve.

Here is the posterior band of the ulnar collateral ligament.

As we follow this more distally, this is the true cubital tunnel between the two heads of the FCU and under the arcuate ligament, the enlargement that will be proximal.

There's a number here and it changes depending on the years go by and what's been quoted.

Really around 10 millimeters squared could be a rule of thumb, a ratio of 2.8, meaning that if it gets larger by 2.8 size, that is abnormal.

Again, I don't really measure this, I just look at it in short axis.

If it's getting larger and it's symptomatic right at the site, then I'm gonna call it entrapment.

There are different causes for this.

It can be idiopathic overuse.

You could have a joint process that's pushing on the nerve.

You can have an accessory muscle here, the anconeus epitrochlearis.

One point I do want to make and highlight specifically here is that you can have mild hypoechogenicity alone of the ulnar nerve where it goes around the epicondyle.

That is a normal finding.

If you read MRI, we often see edema of that nerve there as well.

The reason is when we bend our elbow, that nerve is sliding behind the epicondyle.

It's more of a physiologic edema, but the point is that it wouldn't be enlarged if it's just a little bit of edema.

A little bit of hypoechogenicity is okay as long as it's not enlarged in this specific location.

Here we're looking behind the medial epicondyle marked enlargement of the ulnar nerve.

We can't even see the echogenic connective tissue around the fascicle.

The MRI turned upside down to simulate the ultrasound.

If we look in long axis, we see it markedly enlarged with a transition to a normal size under the arcuate ligament.

Regardless of the size, it's enlarged, it's hypoechoic, it transitions to normal and it's symptomatic.

That is a compressive neuropathy.

Here's a case of an anconeus epitrochlearis.

If you have extra muscle sitting here, that's this normal variant and can cause secondary compression.

A pitfall here is that you only can really identify this muscle with the elbow completely extended because if you flex the elbow, the triceps is gonna be pulled up into this space and you cannot differentiate the anconeus epitrochlearis from the triceps.

You want to extend the elbow completely.

There should be no muscle between the olecranon and medial epicondyle with the elbow fully extended.

This is the incon, that's normal.

Here's the accessory anconeus epitrochlearis.

As we leave the ulnar nerve, I just wanna mention dynamic imaging 'cause this can cause irritation of the nerve.

Normally when we flex the elbow, the nerve should stay behind the epicondyle.

In the abnormal situation, when we flex the elbow, the nerve will come out anterior.

This occurs up to 20% of the population that's asymptomatic.

Remember when you're scanning here, don't put too much pressure with the transducer 'cause you can inhibit this nerve from snapping back and forth.

This must be differentiated from snapping triceps syndrome.

With snapping triceps syndrome, the nerve dislocates, but the triceps will also show subluxation.

Here's an example of that anterior on the left posterior on the right, here's the epicondyle.

Neutral position, the ulnar nerve is right here.

As we flex, there's one snap and there's the other snap.

That's the triceps coming over the top.

As we extend the triceps is gonna slide back down into normal position right there. And then the ulnar nerve.

This can produce two different snaps.

Radial Nerve Entrapment Sites

Moving onto the radial nerve, there are two sites of potential entrapment.

First supinator syndrome.

If you look anatomically, the radial nerve comes from behind the humerus, goes under the brachioradialis and divides into superficial and deep branches.

The deep branch will go between the two layers of the supinator, which is termed the radial tunnel.

It will then come out of the supinator as the posterior interosseous nerve.

When trying to find this nerve, I find it easiest to start anterior at the level of the distal humerus.

This characteristic clamshell appearance of the humerus and the hyaline cartilage is my bone landmark.

Here's the brachialis and between the brachialis and brachioradialis we'll see the superficial and deep branches of the radial nerve.

You can just track this distally to the potential entrapment site.

What makes this confusing?

There are actually two clinical syndromes associated with the radial nerve at the site.

There's supinator syndrome and radial tunnel syndrome.

Supinator syndrome is the one that we're well aware of because it has motor deficits, abnormal electrodiagnostic studies and that's the one where we typically see nerve enlargement like any other entrapment neuropathy.

The problem with radial tunnel syndrome is they have pain but no motor deficits.

Normal EMG, no nerve enlargement but sometimes you'll see edema and MRI of the muscle.

This is either a very minor form of this, it's somewhat controversial exactly what is causing this problem.

We're gonna focus on supinator syndrome where we actually will see nerve enlargement.

As the deep branch goes into the supinator between these two layers, we see hypoechoic enlargement at and proximal to the entrapment site.

Here is the contralateral normal side here in short axis this is normal.

This is the abnormal nerve that's enlarged proximal to the supinator.

In here it is the deep branch markedly enlarged.

Wartenberg syndrome is we have a problem with the superficial sensory branch of the radial nerve at the level of the wrist.

This branch crosses over the wrist and the first extensor compartment and it's prone to injury maybe doing a phlebotomy or IV placement.

Also you can have entrapment such as this case here.

This person had a radial fracture and surgery with plate fixation and there was scar tissue here.

The superficial branch of the radial nerve is going right into the scar tissue.

Here's the nerve going in to this tissue causing the entrapment.

Suprascapular Nerve: Paralabral Cysts

As we finish the upper extremity, the suprascapular nerve, we're gonna look at paralabral cyst, we call them paralabral cyst, although people also call 'em ganglion cysts or labral cyst.

The reason why we use the term paralabral cyst is because 80 to 90% are associated with labral tears.

We want to convey to the clinician that we just can't simply aspirate this because it's gonna come back unless the labral tear is fixed.

Sometimes we will aspirate these however, as just a temporizing measure until they have definitive surgery.

They can occur behind the scapula at the spinoglenoid glenoid notch or higher up in the suprascapular notch.

Remember that the muscle downstream will be affected.

The more proximal notch we will have supraspinatus and infraspinatus atrophy or more distally beyond the supraspinatus we'll have isolated infraspinatus atrophy.

Here we can see this paralabral cyst.

It's going to the labral tear.

They tend to be multilocular and very viscous like a ganglion, but again they're associated with these labral tears.

Here we can see the increased echogenicity decrease size of the infraspinatus due to the suprascapular nerve entrapment.

There's a pitfall when looking for this paralabral cyst and that is normal suprascapular vein dilation.

If you externally rotate the shoulder and you see this, this is not a ganglion cyst, it's not paralabral cyst.

This is a normal vein and the key is a paralabral cyst will not change in size with internal and external rotation.

Don't bother putting on a doppler waveform or color because the flow is so slow.

It's usually you can't see anything and of course don't put a needle in there because you'll just get blood back.

The idea here is the compressibility is key to tell you that this is a normal finding and not a paralabral cyst.

Lower Extremity Nerves

When wrapping up this talk with the lower extremity, beginning with the common peroneal nerve, we see this nerve paralleling the biceps femoris here.

It goes into this tunnel underneath the peroneus longus muscle and also it's prone to injury due to fibular fracture.

We're gonna talk about entrapment here and here's the nerve and as we move distally, we see it get enlarged as it fits underneath this peroneus longus muscle.

As it tries to fit into this tunnel, it becomes hypoechoic.

When you push on this, this induces symptoms.

Common Peroneal Nerve: Intraneural Ganglion Cysts

Another type of pathology that we see specific to this area is the peroneal intraneural ganglion cyst.

A patient may present with a mass over the fibula area and they may present with foot drop.

In fact, 18% of those with foot drop actually have a ganglion cyst as the cause.

This is something that a lot of clinicians don't realize and once I made this point clear to them, it's really increased the number of cases that we imaged because they used to assume it was always idiopathic.

The idiopathic variety of foot drop is usually someone who crosses their legs, who has weight loss and is thin or has had trauma.

With this ganglion cyst it's actually the opposite.

They actually have a high body mass index.

How does that cause ganglion cyst formation?

If you look at Dr. Spinner's articles from Mayo Clinic, he's described the pathophysiology of these intraneural ganglion cyst.

First of all, we know that the knee joint connects to the tibiofibular joint anatomically in 10 to 20% of the population and people who have a higher body mass index.

As they get older, and they get joint effusion.

With that higher BMI that's gonna increase the pressure in the knee joint, it's gonna force that fluid into the tibiofibular joint.

This fluid can then track along the articular branch of the peroneal nerve creating this ganglion cyst.

They can also occur less commonly from the tibial nerve.

The point here is it's coming from the tibiofibular joint.

By MRI, this can be very challenging because it looks just like one of these veins here.

Unless you're cued in to look here, you can totally miss it.

Here's this articular branch with the ganglion cyst.

Of course ultrasound has the resolution showing this cyst and they're typically multilocular like every ganglion cyst and they call this the signet ring sign where you have the fluid right next to the nerve and the nerve is actually here.

It's gonna track along the nerve as you go proximal.

Here's a companion case.

This is an extended field of view image.

This was the worst case I've ever seen.

We're talking over 15 centimeters, this one up into the sciatic nerve from the tibiofibular joint.

Another teaching point here is when you see a cyst around the fibular head, although we may think, oh tibiofibular synovial cyst.

Keep in mind that if it's tracking along the peroneal nerve, this could be an intraneural ganglion cyst.

This one had atrophy of the anterior compartment.

Shown here is increased echogenicity and edema on the MRI.

Superficial Peroneal Nerve

The superficial peroneal nerve is prone to problems where it pierces the crural fascia about nine centimeters proximal to the fibula.

If I go to the image here, we can see that the nerve comes out between the anterior compartment and the peroneal compartment.

When it pierces the fascia in this area, that's the site where you can have enlargement and entrapment and trauma as well.

Here's a case of a patient who was a ballet dancer and had pain and swelling over the lateral part of the lower leg.

When we had the patient move, we saw a muscle hernia.

Here with the resolution of ultrasound, we can see the superficial branch of the peroneal nerve with the neuroma here where it pierces the fascia and was being compressed by this muscle hernia.

Tibial Nerve: Tarsal Tunnel Syndrome

Moving to the tibial nerve, the area that's prone to entrapment is the tarsal tunnel.

This is an enclosed space between the medial malleolus and the calcaneus.

It contains different tendons here medially as well as the tibial nerve, artery and veins as well.

Causes for tibial nerve entrapment include ganglion cyst, uncommonly varicose veins, maybe tenosynovitis, but there's a whole host of things, probably ganglion cyst is the most common.

What is interesting is, unlike the other entrapment neuropathies I've talked about, nerve enlargement is really not as significant feature here.

My theory here is that when you're looking at nerve entrapment anywhere in the body, the tighter the space of entrapment, the more axonal damage and enlargement of the nerve you'll get.

That's why the cubital tunnel in the elbow, I've seen the most marked enlargement there than anywhere in the body because that space is so small.

If you look at the tarsal tunnel, this is really a very large space.

People may have symptoms of tarsal tunnel syndrome, the nerve may be completely normal, meaning that there's no enlargement, no hypoechogenicity.

Be aware that the findings of the nerve itself may not be that prominent.

In fact, I don't think I've ever seen nerve enlarged, but this is what I typically see is a ganglion cyst like all ganglion cysts in the body.

They tend to be multilocular like this and you can see the nerve being deviated around it.

Morton's Neuroma

Finally, Morton's neuroma looking at the interdigital space, the word neuroma, this isn't truly an entrapment neuropathy, it's not a tumor, it's really edema, fibrosis and necrosis.

Usually the second or third interspace is where this occurs.

Females wearing flexible or high-heeled, narrow-toed shoes are all different reasons why people can get a Morton's neuroma.

What we're looking for is a hypoechoic mass-like area between the metatarsal heads.

A five millimeter threshold has been used.

It's not that you can't see neuromas that are smaller, but when you start finding them smaller, your sensitivity, your specificity goes down.

Because that area is somewhat heterogeneous.

A lot of people will rely on this, I'll call if it's four millimeters, but you just better be sure and see it in two planes.

That's really a discreet hypoechoic mass-like area.

Seeing the nerve in continuity is helpful if you can find it.

What I wanna touch on is the importance of dynamic imaging when looking at Morton's neuroma.

I typically scan from a plantar aspect you can scan from a dorsal aspect, it doesn't matter.

You apply pressure on the other side with your finger to splay the metatarsal heads apart and also to compress the tissue.

We can see it better.

By doing this maneuver you can induce symptoms which helps say yes, this is an interspace that will likely have a Morton's neuroma.

Here it's hypoechoic note here on the MRI, it tends to be more plantar than dorsal and that's an important point I'll make in another moment.

Here in long axis here we can see the neuroma, the nerve going into it.

If you see a hypoechoic area, remember that a bursa, an intermetatarsal bursa, they're typically always associated with these Morton's neuromas.

Some people call this a neuroma-bursa complex.

The hypoechoic here is probably the bursa.

Realize that when we're measuring it, we're probably measuring both.

How can we distinguish first of all a bursa from neuroma?

If you look here at the anatomic drawing here, the bursa is actually between the metatarsal heads, whereas the neuroma's gonna be more plantar.

Like if you look here, this is dorsal, this is plantar, this is the neuroma and this is the more dorsal bursa.

By pushing, what I'm showing here is that the bursa tends to collapse a bit where the neuroma does not.

That's one way of differentiating between the two.

That's one aspect of the dynamic imaging is compressing to say bursa versus neuroma.

The second aspect of dynamic evaluation is a sonographic Mulder's sign.

For me, this is the most important maneuver when looking at Morton's neuromas.

Basically the Mulder's sign is when you squeeze the foot from side to side like this and what will happen is the metatarsal heads will come together and will cause the neuroma to snap or move in a plantar direction.

When performing the Mulder's maneuver, I'm definitely scanning from a plantar aspect 'cause the neuroma's gonna be pushed toward the transducer.

Here is an example of a Morton's neuroma.

These are metatarsal heads plantar here, this is the normal interspace.

You can appreciate the heterogeneous appearance of the normal interspace.

That's why calling a tiny neuroma may be difficult.

By pushing the metatarsal heads together and squeezing the foot, you could see the neuroma moving in a plantar direction.

We can see it better.

We see it's non-compressing, therefore it's not a bursa.

In fact, I really haven't seen a bursa really move in a plantar direction and this is extremely enlarged 'cause the bursa is usually more dorsal.

The last point is with this dynamic maneuver, you can feel the click and the patient will tell you yes, this is creating symptoms.

Summary

To summarize this lecture on peripheral nerve ultrasound, focusing on entrapment neuropathies, I went through each of the nerves in the extremities highlighting the key anatomic locations that are prone to entrapment.

The ultrasound finding everywhere is hypoechoic enlargement at and proximal to the entrapment site.

With the caveat that the tibial nerve often does not show this.

What's nice with ultrasound we can evaluate the entire limb very efficiently, much quicker than MRI.

With higher resolution, we can correlate directly with signs and symptoms, then highlighting the benefit of dynamic imaging, which was important when looking at the ulnar nerve in the elbow.

Also very important in looking at Morton's neuromas.

Thank you very much for your attention.