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.

Disclosures

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.

Nerve Entrapment

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

Findings of 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.
So 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.
So 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.
So 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.

Specific Nerve Entrapment Syndromes

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

Carpal Tunnel Syndrome

First carpal tunnel syndrome,
looking at the median nerve in the wrist.
So the hallmark of this syndrome would be enlargement
of the nerve add en proximal
to the entrapment site at the risk crease.
So 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.
So 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.
So remember, if we're looking for abnormal hypo coic
swelling of a nerve, we're gonna look at
and proximal to the entrapment.
So we're gonna look behind the median upper condyle
for this abnormality.
So 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.
So 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.
So 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.
So as I start the video clip,
I'm gonna freeze this in neutral position.
So 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.
So 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.
So 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.
So 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.
So 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.
So 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.
So 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.
So 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 uh shoes, high heeled shoes,
and those with narrow toed uh shoes
that will cause compression that with weight-bearing,
the metatarsal heads will push on this nerve
causing entrapment.
So 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.
So how does one do this? Well, 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.
So 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.
So what has been a very helpful sign
to increase accuracy is the MULDER'S sign.
Now 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.
Well, 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.
So 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.
So 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.
So 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

So 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.

Conclusion

Thank you very much for your attention.