Peripheral Nerve Sonography - HD

Introduction

We're gonna talk about peripheral nerves in the next 25 minutes.

And the good news is there's a lot of overlap with what has been shown earlier today.

We can go quickly and hopefully I'll have time to show a few challenge cases.

I have nothing to disclose except that I love San Diego.

The goals today are to talk about normal and abnormal nerves focusing on nerve entrapment.

And then when we talk a little bit about trauma and also nerve tumors.

Normal Appearance of Nerves

Nerves have a very typical appearance on ultrasound and once you've seen one, you've seen 'em all.

You just have to learn their locations.

Nerves are composed of fascicles, which are hypoechoic, and then intervening connective tissue or perineural tissue, which is hyperechoic.

In the short axis you'll see that it has a honeycomb appearance where you have these hypoechoic fascicles with intervening hyperechoic tissue and surrounding hyperechoic tissue.

In the long axis view, the nerve has a fascicular pattern, so you see kind of like squid ink pasta or a cable with hypoechoic fascicles look in comparison with the underlying tendon, which is relatively more hyperechoic, but also has a fibrillar pattern.

Utility of Ultrasound for Nerves

Ultrasound is very useful in assessing nerves because peripheral nerves are superficial.

We have that great resolution for superficial structures.

We can look at entrapment, which I will talk about in a moment, but also look for masses that involve nerves and for trauma or injury to nerves, which is shown here with this puncture injury.

Nerve Entrapment Appearance

Nerve entrapment also has a predictable appearance on ultrasound, so you can diagnose it in multiple locations.

Once you learn the features at the point of entrapment or compression, there will be a caliber change in the nerve.

We're going from proximal to distal here.

Here is the point of compression distal to that you have a normal appearing nerve, so most of the abnormalities actually proximal to the point of compression.

At that point, the nerve is thickened or enlarged the enlarged cross-sectional area, but also may appear relatively hypoechoic because of the edema that now replaces that hyperechoic connective tissue.

If you look at this example, these are both of the ulnar nerve.

The point of compression is here distal, you have that normal fascicular pattern, but proximal you don't see those hyperechoic bands because the nerve is now predominantly hypoechoic throughout.

One may also look at muscle and look for changes in the echogenicity of the muscle.

Here we're actually looking at a gastrocnemius.

We have the medial and the lateral heads of the gastrocnemius.

But in this side where the patient has symptoms, you can compare the medial head, which has increased in echogenicity in comparison with a lateral head.

That increased echogenicity may be in the acute phase due to a denervation type of edema, but over time may be related to a fatty replacement or fatty atrophy in the involved muscle.

There are several common nerve entrapment syndromes.

I will not be covering all of these, but the point I want to make is once you recognize the normal appearance of the nerve, you may apply that anywhere in the body, but also the normal or expected appearance of an entrapment, you may apply elsewhere.

The only thing to learn now is where to look and what signs you may look for or what the pitfalls are.

Median Nerve Entrapment: Carpal Tunnel Syndrome

The most common of all entrapments is the median nerve entrapment, also known as carpal tunnel syndrome.

Just to go over the anatomy, the carpal tunnel has a bony floor by these carpal bones and a transverse carpal ligament, which is the softer roof of the carpal tunnel.

The median nerve lies inside the tunnel along with the flexor tendons.

In this ultrasound image, the flexor tendons are the hypoechoic bundles down here, and the median nerve is relatively hyperechoic fascicular pattern.

We see the transverse carpal ligament, which is this hypoechoic structure and hypoechoic structure actually overlying the median nerve.

And then we have flexor carpi radialis and the pisiform, just to point out here, this is the ulnar nerve and artery within Guyon's canal.

Patients who present with carpal tunnel syndrome or median nerve entrapment are diagnosed clinically and then can be supplemented with either an EMG or an ultrasound or both.

And they present with symptoms that involve the thumb, the index finger, the middle finger, and the radial aspect of the ring finger.

Sometimes you may read about something called a notch sign, which is just that caliber change in the nerve at the site of compression.

Here we have the enlarged median nerve and then the flattened appearing median nerve.

This is that enlarged median nerve within the tunnel.

There are measurement criteria for median nerve enlargement and entrapment.

Normal cross-sectional area is eight millimeters squared greater than 12 is considered abnormally thickened or enlarged.

And then between eight to 12 is borderline.

Klauser group did a study where they looked at the cross-sectional area of the median nerve at its thickest near the carpal tunnel and then at the level of the pronator quadratus.

And a difference of two millimeters squared or greater is associated with a 99% sensitivity and a hundred percent specificity for entrapment in the tunnel.

Here's a patient who presented with burning in their fingers at night.

And when we look at the median nerve here within the tunnel, it is enlarged measuring 21 millimeters squared in cross-sectional area, certainly well above 12.

Here we are at the level of the pronator quadratus, which is this muscle right here, and we're measuring the median nerve at this level at 13 millimeters squared.

That's definitely greater than two difference.

And this patient does have entrapment of the median nerve, the carpal tunnel.

This is what that long axis view looks like, and you can see that it has this apparent flattening in that area of compression.

One variant to know about is a bifid median nerve because a bifid median nerve, first of all, you want to alert the surgeon about that as well as the presence of a persistent median artery, which having a bifid median nerve and a persistent median artery is associated with a slightly increased risk of having entrapment in the carpal tunnel.

And the criteria for this is to measure the combined cross-sectional area greater than 12 or as Klauser determined in his group a difference of four millimeters squared between the two measurement areas.

A similar case was shown earlier, but this is a patient who presented with carpal tunnel symptoms even after release.

I just wanted to show you a comparison.

This is the asymptomatic normal side with a normal median nerve within the carpal tunnel.

And the symptomatic side with the median nerve has this flattened appearance with an overlying irregular hypoechoic structure.

When you look at it in long axis view, you can see that there is this kind of shadowing irregular structure that's lying over the median nerve and this patient has a recurrence in their symptoms because of this exuberant scar tissue that is formed.

This can be the normal expected postoperative appearance, especially if there's been a release.

But the most important thing is to know that they do have recurrence symptoms and typically the nerve will be tender when you're scanning over that area.

Ulnar Nerve Entrapment: Cubital Tunnel Syndrome

Moving on to the ulnar nerve.

Ulnar nerve is typically most common site of entrapment is in the cubital tunnel.

Here we have a diagram of the ulnar nerve coursing posteriorly posterior to the medial epicondyle.

And as it comes around, it will enter the cubital tunnel, which is formed by the two heads of the flexor carpi ulnaris and an overlying kind of fascia ligament, the arcuate ligament, the border of which is known as Osborne's ligament.

And this is the most common site of entrapment.

Knowing what a nerve entrapment looks like, the nerve proximal to this area will be thickened and hypoechoic, and that is best seen here at the level of the medial epicondyle.

With ulnar nerve entrapment, the symptoms will involve the small finger and the ulnar aspect of the ring finger.

Patients describe symptoms worse at night and may even present with weakness of the hand muscles.

Ten is cross-sectional areas what we use for abnormal.

Here we are looking transversely at the ulnar nerve at the level of the medial epicondyle, which is here, and we have this ulnar nerve, which is measuring 12 millimeters squared in cross-sectional area, abnormally enlarged.

And here again is that appearance we expect to see when there is a nerve entrapment.

The nerve entrapment is not within the tunnel, but just before it enters.

Here's the nerve, which is normal and proximal to that, that nerve is thickened and hypoechoic.

Entrapment may not necessarily be due to known ligamentous structures.

There may be masses or anatomic variants that can cause entrapment.

Here is one example of a variant, the anconeus epitrochlearis muscle, which typically is at the level of the elbow and can cause entrapment symptoms involving the ulnar nerve.

This muscle is seen in up to around 30% of the population and is best seen with the patient's arm in extension.

Here again, we are at the level of the anconeus and the medial epicondyle.

And with this enlarged appearing ulnar nerve and overlying this is this extra structure, it's hypoechoic.

We see it in the short and in the long axis you can see that it's quite broad.

It's like a band.

It has the muscle architecture on ultrasound, and this is the anconeus epitrochlearis or accessory muscle.

This is what happens when you try to image the patient in flexion.

We have extension view and a flexion view, medial epicondyle, ulnar nerve and anconeus muscle.

Over here in flexion, you see that ulnar nerve, but you no longer see the muscle.

One pitfall would be that you would like to scan your patient in extension so that you don't miss this lesion or this muscle.

Ulnar Nerve Entrapment: Guyon's Canal

Anytime somebody presents with a neuropathy, you have to remember that although entrapment syndromes are common, there are other causes.

And as a routine, if someone presents to my office with a neuropathy, median, ulnar, whatever, I will scan the nerve, let's say the ulnar nerve from the wrist all the way up to the arm because sometimes you will find something unexpected and not necessarily entrapment in the cubital tunnel.

The next most common location for an ulnar neuropathy is actually Guyon's canal, which is this image here.

Neuritis, not necessarily an inflammation, but you can have trauma or repeated injury to the ulnar nerve that can cause symptoms, subluxation being one of them or tumors.

This is a patient who actually presented with a forearm lump rule out lipoma.

That lump actually ended up being this, which is an accessory palmaris brevis muscle.

But when I was scanning the patient, he told me that he had an ulnar neuropathy or symptoms of ulnar nerve compression.

And when I looked in Guyon's canal, I actually saw that there was an ulnar nerve, an ulnar artery, and this extra hypoechoic structure, which I then suggested could be an accessory adductor digiti minimi muscle.

And this is what it looked like here at surgery.

This was confirmed, but remember, common things being common.

I scan his entire ulnar nerve all the way up to the cubital tunnel, and his entrapment was actually at the level of the cubital tunnel.

He's been living with his muscle all his life.

And I told his surgeon, even if you remove that muscle, I saw the entrapment at the level of the cubital tunnel.

It's might be something important to check when you're looking for nerve entrapment.

Here's another patient who presented with ulnar neuropathy following an ankle fracture, and how can you get that?

In this case, his ulnar nerve is very thickened, 33 millimeters squared cross-sectional area, focally thickened here, but in this case it was because it was constantly bumping up against his crutches and he was injuring and reinjuring his ulnar nerve.

Not always an entrapment.

Here's another patient who presented with ulnar nerve entrapment and had a transposition.

The diagnosis was done clinically followed by an EMG, so they did the transposition and yet he still continued to have symptoms.

They asked him to scan him and again, doing what I normally do, I scanned from the wrist all the way up the arm and just here where I'm starting in the wrist, we are at the level of Guyon's canal.

We have an ulnar nerve with the adjacent ulnar artery.

But also, as you may have seen earlier in the plenary session, a ganglion cyst, which presents as an anechoic multi lobulated cystic structure, which is in this case causing compression of the ulnar nerve at Guyon's canal.

As shown earlier, the ulnar nerve may also sublux.

Very important to check that when you're seeing a subluxing ulnar nerve, if you're gonna notify the surgeon, you might wanna also check for a snapping triceps.

You don't want 'em to transpose the nerve and still not relieve the symptoms of snapping.

Just taking from earlier, here's that ulnar nerve snapping over the medial epicondyle, but if you look closely, there is a second snapping structure right here, which is the medial triceps.

Again, something important to look for when you're looking at a snapping elbow, but also for an ulnar neuropathy.

Radial Nerve Entrapment

Moving on to the radial nerve.

Patients may present sometimes with rule out radial tunnel or supinator syndrome, which is pathology or entrapment involving the radial nerve.

The other important thing is when patients present with lateral elbow pain and you're looking for a tennis elbow, this may often mimic those symptoms.

Patients who present with lateral elbow pain, I will look for common extensor tendinosis, but also check the radial nerve because you may have one or the other or even both.

With similar symptoms, the radial nerve courses behind the humerus along the spiral groove comes around and moves in front and divides or bifurcates into a deeper motor branch, which depending what you read is called the posterior interosseous nerve or a superficial and a superficial sensory branch, the deeper branch courses and dives between two heads of the supinator muscle.

And right where it dives in there is this arcade of Frohse and that is the more common location for a radial nerve entrapment.

And this is where we're going to look.

Here is a transverse view of the motor branch of the radial nerve or the deep radial nerve and a long axis view.

We already know what it's going to look like right at an entrapment proximal to the level entrapment.

We're gonna see a focal thickening or a decreased echogenicity of that nerve here is what it looks like in transverse.

Peroneal Nerve Entrapment

This was interesting, I got a phone call at the middle of the night from my resident on call and he said, we have a problem because our technologist on call showed up with a foot drop and can't do the transplant doppler studies.

That's a big problem.

The other problem is she has a foot drop.

I had two problems, but anyway, I said that sounds like a peroneal nerve entrapment pretty common.

I wonder if they may even have something compressing the peroneal nerve and knowing the anatomy, the nerve comes down after it bifurcates from the sciatic nerve and comes down laterally and we'll wrap around the neck of the fibula right here and then divide into a superficial and deep branch.

She did get an ultrasound that night that was ordered, and you can see a multi lobulated cystic structure.

He didn't know how to scan the peroneal nerve.

I just see this, but it is in the location where the peroneal nerve lies and we postulated that this person has a ganglion cyst intramuscular in this case, compressing that peroneal nerve.

This study was done 12 years ago and unfortunately the surgeons don't believe that ultrasound could possibly make this diagnosis.

The patient had to wait two weeks.

My technologist had to wait two weeks till she could get her MRI to prove that she had a ganglion cyst that was compressing on her peroneal nerve, which then was subsequently decompressed.

One thing to know about ganglion cysts and nerves is that you may have a not so common, but still pretty often seen an intraneural ganglion cyst.

This most commonly involves the peroneal nerve, but may also be seen involving the tibial nerve and even tracking up into the sciatic nerve.

The mechanism is that when you get these ganglion cysts forming from the nerve joint or from the joint, that fluid may track up small branches that then track up into larger branches of the involved nerve.

Here is a patient who presented with a foot drop.

We were looking at the level of the fibular neck at the peroneal nerve, and we see this cystic structure, which is multi lobulated and extending along, but if you look closely here, this is actually tracking and interdigitating within this peroneal nerve.

We were able to give the diagnosis of an intraneural ganglion cyst that was tracking up into the peroneal nerve, which makes you look like a star and it's nice, and then you get more people sending patients to you.

Morton's Neuroma

Moving on to another form of nerve entrapment that is the Morton's neuroma, and this was shown earlier, but this is a nerve entrapment that involves a digital nerve, a branch of the plantar nerve.

And normally this is seen at the level of the metatarsal head where it is enlarged due to the presence of perineural fibrosis and edema.

Most common location is the third web space.

We're gonna be scanning at this level and many times you may see an associated bursa that is distended with fluid.

Many times our measurements will be actually larger than what is seen surgically, and that's because our measurements will include that thickened and or fluid distended bursa.

These patients may present with electric shocks traveling down their toes, perhaps, with numbness or burning or even a sensation of a pebble under their toes.

Usually this is due to or they get these symptoms when they're wearing tight fitting shoes or when they're wearing high heels.

And Washington DC is a big place for these because it's a walking town.

I do see Morton's neuromas day in and day out.

We're scanning at the level of the metatarsal heads here, and what we're looking is in that intermetatarsal space.

The tissue is typically relatively hypoechoic.

Nothing lives there that you can actually see until you see this hypoechoic nodule.

This is in the second web space in this case, but it's a hypoechoic mass that lives here.

And this is compatible with a Morton's neuroma.

Many times when I'm imaging these, I'm compressing from the dorsal of the foot against the transducer, which is on the plantar aspect of the foot.

And the patient will say, you're actually pressing what hurts.

There's a lot of confirmation when you're doing this.

The other thing you can do is compress, do a transverse compression in the metatarsal heads and see this kind of structure popping, which is that Mulder's click.

And many times you'll actually feel the click.

The other thing you can look for is an entering and an exiting nerve going into the mass or the nodule look out for some fluid again, because you may see a compressible bursa filled with fluid adjacent to this neuroma.

Here's that neuroma in the space.

It's hypoechoic and pretty well defined.

You can see an entering and an exiting nerve.

And again, when I kind of release up on the pressure of the transducer, you can see that there's a little bit of fluid next to that, fluid distending intermetatarsal bursa.

Nerve Tumors and Other Lesions

In the last five minutes, just wanted to show some other things that you can look for when you're looking at the nerves with ultrasound.

One thing is you may find some soft tissue nodules and these are typically either schwannomas or neurofibromas and they have a very similar appearance on ultrasound.

They tend to be hypoechoic and well-defined and can have vascular flow detected with doppler.

They may be painless, but may also present with neuropathy in the distribution of the nerve and patients may even report symptoms when you're scanning over this area.

Schwannomas are typically described as eccentric to the nerve, but many times it's very difficult to tell.

In fact, when you look at this case of a nerve, they look very similar.

Neurofibromas, however may be multiple and you may see them in multiple locations in the setting of neurofibromatosis.

Again, when looking at nerve lesions, you want to look for a kind of an entering and exiting nerve that will help you distinguish it from other lesions.

Soft tissue lesions that present as hypoechoic masses.

One last thing shown earlier is after amputation you may see what is called a stump neuroma or a terminal neuroma.

Some of them are symptomatic, but some of them are not.

And when patients present after amputation with a tender stump, the differential usually right when they come to you is, is there an abscess or some kind of collection there.

But the other thing to look for would be some kind of painful lump, possibly representing that post amputation or stump neuroma.

Again, it has a hypoechoic appearance and you may find that entering nerve, which is very useful.

Case Example: Strengths of Ultrasound

I love this case because this illustrates many of the strengths of ultrasound when assessing abnormalities in the musculoskeletal system.

This patient presented with carpal tunnel entrapment, which was diagnosed clinically and then confirmed with an EMG.

They had a carpal tunnel release from those two things alone, clinical diagnosis and EMG, very common.

However, after the release, they still presented with carpal tunnel symptoms, so they got an MRI, which showed a carpal tunnel release and nothing else.

They did another MRI of the elbow, which is the next most common location for entrapment and that was negative as well.

They sent the patient to me and doing, as I always do, I scan from the wrist and trace the nerve proximally.

I found this as I was tracing up the median nerve.

There's this kind of lobulated elongated structure that was replacing the median nerve.

This is the panoramic view where you can see that there's a mass involving the median nerve and then extending along the course of that nerve.

As it stands, the MRI of the wrist pretty much ended right here and the MRI of the elbow pretty much ended right here.

The nice thing about ultrasound is there's no recipe.

You can actually scan where you wanna go, you can compare with the other side, the patient will tell you you're scanning the area that's bothering me and you're still giving me those symptoms.

The patient went to the OR typically these are done with a patient awake but with a block.

And so after this was peeled off the nerve, the patient said, I still feel symptoms in my thumb.

I wheeled the machine down to the OR and found another neuroma involving a branch of the median nerve extending into the thumb, which he just opted to leave behind because now we found the reason very nice show of what ultrasound can do for you.

Nerve Trauma and Injury

Trauma to the nerve may present with a thickening of the nerve in the setting of a crush injury or traction.

But mainly these people come to me when they have symptoms after an injury just to prove whether there's a transection that needs to be surgically repaired or we can watch and wait due to trauma.

This is a taser injury, somebody was reaching for a taser and got impaled in the hand.

We were scanning at this level and we see this thickening of this branch of a digital nerve in the long axis view.

You can see that is focally thickened here but not transected.

They opted to wait.

Here's another patient who knew Costco could be so dangerous, but at Costco a large cart impaled him on his leg and for weeks and months after he was having this continued pain on the side of his leg and they had peroneal scan, they had looked for hematoma, looked for a muscle injury.

And after finally coming, getting passed down to me, I found this soft tissue lump here.

I said, okay, well I don't know, some kind of scar.

However, in the long view you can see that there's a nerve that leads right up to this lump.

This is a branch of the peroneal or fibular nerve.

This is the superficial branch and this was a little neuroma or some neuroma formed from a transection of that nerve.

Here's another one with a dog bite, where you can see that the nerve is discontinuous and there's a neuroma forming at that stump.

Very useful to diagnose whether you're going to watch or you're going to wait or you're going to operate.

Take Home Points

My take home points are ultrasound is well suited to assess peripheral nerves.

They're superficial, we can see them very well once you've seen one nerve, you know the normal appearance elsewhere and you know the expected appearance when there is an entrapment dynamic may be useful assessing the snapping ulnar nerve and for Morton's neuromas.

But remember, if someone presents with the symptoms of a neuropathy and you don't see an entrapment, you may wanna check whether there are any compressing structures or anatomic variants or any masses involving the nerve or injury to the nerve, even remote or from renegade shopping carts at Costco.

Closing Remarks

I thank you for your attention, especially that we're here in beautiful San Diego, which I just disclosed.

I love.

These are my favorite places, some of my favorite places in San Diego.

We have a Torrey Pines Beach and Torrey Pines hike going on over here.

This is Coronado Beach.

This is La Jolla Shores and this is my favorite bathroom in all of San Diego, which is located in the gas lamp district.

I dare you to find it, but if you do, you'll be blessed with Ryan Gosling watching you as you do your business.

And thank you.