Monday, January 21, 2008

Carpal Tunnel Syndrome- Pitfall

Carpal tunnel syndrome is a common affliction, but other entrapment neuropathies can be masquerade as carpal tunnel syndrome.

Pronator syndrome (also known as pronator teres syndrome) is an entrapment neuropathy of the median nerve that occurs at the level of the elbow. Other entrapment neuropathies of the median nerve are the supracondylar process syndrome, anterior interosseous nerve syndrome, and carpal tunnel syndrome.

Here is a patient with pronator syndrome. There is edema in the pronator teres and FCR (red arrows) and abnormal signal in part of flexor digitorum superficialis (green arrow). There is also edema in the muscles innervated by the anterior interosseous nerve branch of the median nerve, including the flexor pollicis longus and flexor digitorum profundus (white arrows).

In most cases of entrapment neuropathies, the characteristic MRI finding is edema and/or atrophy in those muscles innervated by the nerve in question. In most cases, there is no underlying mass, as most cases of entrapment neuropathies are due to anatomic variants such as anomalous muscles or fascial bands.

Many people expect to find a mass in these cases, but things are usually like the Wonder-Bra: you see the result, but the cause is hidden.


Mustafa said...

Hi Vic,
Congrats on your blog. Hopefully it will be a nice way for all of us to learn more about MSK radiology. As a hand surgeon who treats a lot of entrapment neuropathies, I really enjoyed your latest post. The MRI of pronator entrapment-induced muscle changes was especially cool.
Isn't it important to point out, however, that for most entrapment neuropathies, the characteristic MRI is completely normal? Are there any new findings to suggest that we are missing diagnoses where these muscle changes can be seen on MRI, or were the cases described particularly advanced?

I'd love to hear your thoughts on this.

Take care, Mustafa Haque

Vic David MD said...

Hi Mustafa,

It's great to get the perspective of a hand surgeon, thanks for your post.

You are absolutely correct, in most cases of entrapment neuropathies the MRI is completely normal. When denervation is complete and sudden the MRI will always be positive within a few days of the event, but this is obvious clinically and the MRI is superfluous. As you know, in some areas of the body, the clinical picture of nerve entrapment can be confusing (e.g. suprascapular nerve entrapment in shoulder) and MRI is more useful in this setting.

The prolongation of T2 relaxation time seen in the setting of acute and subacute denervation is thought to reflect alterations in blood flow. One will also see enhancement of the muscle after IV gadolinium. To date, MR spectroscopy and diffusion imaging have not had an impact on this issue.