Ultrasound Analysis of Median Nerve in Carpal Tunnel Disease - SD

Introduction to Ultrasound Study of the Median Nerve in Carpal Tunnel Disease

What I will be presenting to you today is the ultrasound study of the median nerve in carpal tunnel disease, as we have structured it at Henry Ford Hospital in Detroit.

I first would like to give credit to Orlin Iff our residents, and Rachel Hulen, our fellow who have done a lot of work looking at normal carpal tunnel and abnormal carpal tunnel.

Joseph Craig, a staff from the Henry Ford Hospital and Dr. Dimar, who is a plastic surgeon driving some of the research we've been doing on carpal tunnel disease, ultrasound of carpal tunnel disease.

I'll be showing the old stuff, the things we've been doing for years and now for more than 14 years in carpal tunnel.

Static anatomy analysis, some of the anatomy and the pathology, but also some of the newer things, newer occupational hazards, occupational injuries that are causing carpal tunnel disease and some of the dynamics that we have noticed on ultrasound in or within the carpal tunnel.

Goal of the Lecture

The goal of my lecture is to stop some here in the room from considering carpal tunnel disease as a static disease and really start thinking of carpal tunnel as a dynamic disease as background.

Background on Carpal Tunnel Syndrome

Carpal tunnel syndrome has previously been described to affect certain occupations, and in particular, we are thinking of typists, transcriptionists musicians, piano players, violinists, but also sonographers and people working with small tools like dentists and dental hygienists, and factory workers.

Improvements in Ultrasound Imaging

Now, within the last 10 years, the equipment has become so sensitive and the imaging of carpal tunnel and of median nerve has dramatically improved over time.

So that accurate identification of the median nerve is now becoming a routine study.

Compression of the median nerve can be reliably identified and localized by ultrasound, and it has been shown in several papers that the cross-sectional diameter of the nerve somewhat correlates with EMG findings, and that there is a cutoff if one measures the surface area of the median nerve of 10 square millimeters or 0.1 square centimeters.

For patients with carpal tunnel disease are typically over that number.

Patients under this number typically have normal median nerves in terms of where to measure what are the anatomical considerations in terms of placement of the transducer.

Transducer Placement and Landmarks

Several papers have shown that the distal palm mark crease is an easy landmark to place a transducer for carpal tunnel measurement in that it's just proximal to the entrance to the carpal tunnel, and that's where most of the swelling is noticed surgically that carpal crease, which is indicated by the arrows, the distal distal palmar wrist crease corresponds to about the radiocarpal joint and is just proximal to the carpal tunnel.

So studies also have shown excellent correlation between ultrasound diagnosis and the increased cross-sectional area with electrographic findings studies.

One of the studies we have published in radiologic clinics of North America, actually, Dr. Dehi Lee, who worked with plastic surgery department at Henry Ford Hospital published that in 1999.

Anatomic Considerations of the Median Nerve

Some other anatomic considerations, the median nerve is actually covered by the flexor retinaculum as shown here, and there are two layers of that flexor retinaculum, a superficial and a deep layer surrounding the flexor pollicis longus.

For your reference, the flexor carpi radialis tendon is actually not part of the carpal tunnel, but some of our measurements are gonna be referred to that tendon at the lateral aspect of the carpal tunnel.

When you take the flexor pollicis away, this is the median nerve.

It's normally coursing deep to the flexor retinaculum for orientation.

This is the flexor pollicis longus at the lateral aspect of the median nerve, and this is the flexor to the middle finger at the medial border of the median nerve, median nerve location relative to bones.

Actually in the middle of the carpal tunnel, the median nerve is located over the capitate bone.

Traditional Teachings and New Findings

Now, traditional teachings in plastic surgery and in the clinical setting have shown that or have always believed that repetitive motion inflection and pronation of the wrist might induce carpal tunnel.

And that's actually the basis for the phalen test.

New findings of studies we have done at Henry Ford have shown that pinching a pinch maneuver in pronation inflection is also implicated in the development of symptoms of carpal tunnel syndrome.

Now, we often visualize the median nerve as a static structure located anterior to the flexor tendons in the carpal tunnel.

However, cadaver dissections that we have done have demonstrated that the median nerve to be a very flexible structure capable of both lateral and sagittal motion within the carpal tunnel here, shown as a cadaver dissection after the transection of the flexor retinaculum showing the location of the median nerve.

Ultrasound Appearance and Motion of the Median Nerve

Now the median nerve on ultrasound has been shown to have a filar hypoechoic or slightly echogenic structure, volar tendon, the flexor tendons, and at the ulnar aspect of the flexor pollicis longus and the flexor carpi radialis.

Now, the flexor carpi radialis is a good ultrasound landmark for referencing the relative motion of the median nerve due to its relative constant position relative to the scaphoid bone.

So just looking at this with ultrasound, this is a cross section to the carp through the carpal tunnel.

This is the radial side of the carpal tunnel.

This is actually the scaphoid bone.

This is the capitate bone, and over that capitate and the very volar soft tissues of the carpal tunnel is the flat structure of the median nerve.

This is the flexor carpi radialis, which is actually sitting outside the carpal tunnel flexor carpi radialis, a good reference point and a constant reference point for the location of the median nerve flex digitorum shown relative to the median nerve flexor carpi radialis.

Movement During Pronation

Now, there is movement of the median nerve during pronation of the hand.

It's not uncommon for the median nerve to demonstrate either ulnar or radial mobility during pronation of the forearm.

We've actually studied that in asymptomatic wrists, showing that there doesn't seem to be a predilection towards what side the nerve is moving.

Sometimes it moves to the radial side, sometimes it moves to the ulnar side of the wrist, but the wrist movement observable by ultrasound showing here in supination the location of the median nerve relative to the capitate bone and relative to the flexor carpi radialis.

However, when you pronate the hand, you will notice that in this patient, the nerve moves towards the radial aspect of the wrist, a significant movement of the median nerve during pronation of the hand.

There is also sagittal motion of the median nerve during pronation, and it also can be a normal finding.

And four out of the 10 volunteers, the median nerve moved towards the volar aspect.

While in about three patients or three volunteers, the nerve moved towards the dorsal aspect.

So again, no real predilection towards what side is moving, but it does move often during the pronation of the hand showing that movement relative to the capitate bone in a normal volunteer median nerve flexor carpi radialis capitate bone, and in this volunteer, there was about six millimeters of a dorsal movement of the median nerve relative to the capitate.

Measurement Technique for Motion

Now, how do we do this movement from supination to pronation?

How is it best accomplished?

We actually start typically as we usually started when we were making static images of the carpal tunnel in supination of the hand, placing the transducer over the distal palmar wrist crease, and we assess the position of the median nerve relative to the flexor carpi radialis and relative to the capitate.

And then we follow that maneuver by pronating the hand placing it first in neutral position, doing the same measurements of the median nerve position relative to the FlexCar radialis and the capitate bone.

Then of course, by subtraction, we came to the movement of the median nerve, both in the coronal and the sagittal plane, the median nerve cross-sectional diameter.

We still measure that with a measuring a surface area, taking this as our cutoff value.

Below the 10, the 0.1 we call normal over 0.1, we call abnormal, showing you that surface area measured by ultrasound.

It used to be when we started out measuring, we only measured the AP diameter and the coronal diameter.

We multiplied those.

And now with the current technology, we always measure surface area.

Instead, the transverse images are also followed by longitudinal images showing the area of maximum swelling, showing here the anatomy of the median nerve in the carpal tunnel, the flexor retinaculum relative to Guyon's canal, and the location of the ulnar nerve.

Again, look at the position of the median nerve relative to capitate and the flexor carpi radialis.

Clinical Experience with Occupational Hazards

So from clinical experience, Dr. Dimar or plastic surgeon had noticed that in certain professions, as in sonography and patient and dental hygienists with carpal tunnel disease and in computer users also radiologists using PACS systems, they often oppose their thumb and their index finger in a sort of a pinch maneuver.

They're performing on their instruments while holding a probe or a mouse or other instrument.

Now that is typically held in a prone neutral or prone flex position of the wrist, and we kind of decided we would try to kind of mimic that maneuver utilizing that pinch maneuver in our ultrasound examination shown here is in the pronated position of the hands, the pinch maneuver of the index to thumb pinching.

Pinch Maneuver in Ultrasound Examination

Now, how do we do practically when we scan?

We have the patient hold for three to five seconds in a fairly tight pinch during the visualization of the median nerve over the carpal tunnel.

Now, how does that look on ultrasound?

You see the patient or the person pinching and you can actually notice how does some movement of the median nerve, but there is no entrapment within the carpal tunnel.

Now, this slide shows you side by side comparison of movement of the median nerve on ultrasound during pinch maneuver in an asymptomatic individual.

And as you see, once the pinching starts, there is some movement of the median nerve, but the median nerve is not entrapped within the carpal tunnel.

So there is some flattening and some change in the position of the median nerve, but the median nerve does not entrap within the carpal tunnel during this simple pronation pinch maneuver.

So there is no sagittal movement with this maneuver.

Now in our subject population, in the normals pinch maneuver revealed no real sagittal motion and nine outta 10 individuals.

There was slight movement of the median nerve.

The one patient who demonstrated some mobility promptly demonstrated return to the pre pinch position after cessation of the pinch maneuver.

Flexion Pinch Maneuver

Now, the next maneuver we routinely now use in practice is flexion pinch.

Now that flexion pinch maneuver two is done with the hand in pronation, and we are holding for about two to three seconds, and then we release and have the wrist come back to neutral position.

When an individual does that, most of the time there is indeed some sagittal movement of the median nerve relative to the carpal tunnel in even in asymptomatic individuals.

Again, you see the movement of the median nerve into the carpal tunnel with the flexion pinch follow the nerve here, flexion pinch the nerve is entrapped or moves ly in the carpal tunnel and moves back out.

So sagittal movement of the nerve is quite common in flex pinching.

Now, the median nerve in this case dives deep into the groove between the flexor pollicis longus and the flexor digitorum tendons.

Upon return of the wrist to neutral the median nerve very spontaneously reduces motion of in the groove between the flexor tendons and the flexor pollicis longus is a normal finding.

Symptomatic Findings in Carpal Tunnel Disease

Now, in several of our symptomatic patients with carpal tunnel disease, we've noticed that the median nerve entraps within the carpal tunnel in between the tendons of the flexor pollicis longus and the flexor digitorum and does not spontaneously reduce to its anatomic position.

This is an example of one of those entrapments.

Notice that with pinching and pronation and with flexion of the wrist, the nerve is entrapped with the carpal tunnel, and this is the median nerve kind of tally located, and there is actually an indentation from the flexor digitorum tendons upon the median nerve and no return to normal causing symptoms of carpal tunnel disease.

So this is a very symptomatic patient.

It's actually a sonographer who developed after years of musculoskeletal sonography carpal tunnel disease.

Summary

In summary, median nerve is a flexible structure and I hope that I could convince you today that you should look at carpal tunnel disease, as in some patients, a dynamic disease of the median nerve sagittal motion during a pinch maneuver while not very common, can be a normal finding, especially when the nerve returns to its normal anatomic position after cessation of the maneuver sagittal motion and the of the tendon in the groove between the flexor digitorum tendons and the flexor pollicis longus during flexion and pinch maneuver is common even in normals, but the nerve should return to its original position when the pinch maneuver and the wrist returns to normal.

Some carpal tunnel syndrome patients demonstrate entrapment of the median nerve in the groove without a return to the anatomic position with cessation of the maneuver.

Now, this may lead to continuous mass effect of the tendons on the median nerve and as possibly an important etiology of carpal tunnel disease in certain professions.

I thank you for your attention.