Muscle wasting but normal Median nerve

[Linh]

Dear Steve,

I have just recently scanned an elderly patient presenting with skinny thenar eminence compared to other side. The muscles supplied by MN is thin and echogenic (even though the difference is not marked between the 2 sides).
I checked her median nerve at the level of the lunate and radius and it was below 0.9cm2. The ulnar nerve is also < 0.6cm2 at the Gyon’s canal.
She had severe OA so they took her trapezoid out bilaterally.

I also found a complex collection with echogenic material and she said she had Gout from using high blood pressure medication. This collection is just medial to base of the 1st metacarpal. This case really confuses me and the radiologist as to why there is muscle wasting. Could it just be age related? In the video course, you said the most common cause is CTS but is there other cause appropriate to my case?

Best regards,
Linh

[Linh]

I followed up the reports and the complex collection corresponds to chondrocalcinosis seen on X-ray. I guess with the wasting, it might be related to recurrent median nerve irritation from OA?

[Stephen Bird]

Hi Linh,

Interesting case.
In this case I suspect the thenar muscle atrophy is related to the underlying severe OA.
If the trapezoid has been removed there must have been severe STT joint OA changes.
These patients find it difficult to use their thumb in a normal way and I suspect the global atrophy of the thenar muscle group is disuse atrophy related rather than a primary neural problem.
The medial and ulnar nerves sound normal.

Steve

[Linh]

Thanks Steve. Indeed the atrophy was not marked, the muscles looked thin but not markedly echogenic. Disuse due to OA sounds very reasonable.

[Stephen Bird]

Hi Linh,

how are you finding thenar assessment for upper limb nerve chronic dysfunction?

We are finding many cases where it consolidates the diagnosis.

Steve

[Linh]

Hi Steve,

Hope you are well 🙂

I have not encountered a thickened median nerve with denervation changes at the thenar muscle. There was one case when I thought the adductor pollicis appears fatty but it turns out to be just fat when reviewing the previous CT.

On the other hand, I have come across a denervated abductor hallucis with muscle volume loss, fatty infiltration and localised pain at the medial plantar foot. But the medial plantar nerve looks fine.

Very confusing at times with nerve work. But still, it is very challenging and rewarding when we can use US to identify small and intricate network of nerves. There are so many I wish to identify in practice 🙂

Best regards,
Linh

[Stephen Bird]

Yes, it can be tricky, but worth keeping in mind.

Adductor Pollicis is ulnar nerve distribution.

Abductor hallucinations is medial plantar nerve supplied.

So if you see wasting you need to consider a few things:

Is it the only intrinsic muscle in the foot that has atrophy change or are you seeing a more global pattern like you might see with a diabetic neuropathic foot.

If the other intrinsics are normal but that single muscle has atrophy consider an isolated denervation event. Also consider a tendon injury at either end of the muscle or an MTJ tear as possible causes of atrophy.

Steve

[Linh]

Thanks Steve.

It can be confusing at times whether a hyperechoic muscle belly is due to trauma (muscle strain, DOMS), fatty atrophy due to denervation/neuritis or (like you said) due to diabetes.

IN my case, I compared to other side and there is a difference in hypoechogenicity and volume of the abductor hallucis.
Rob taught us to compare the Abductor hallucis to the abductor digiti minimi of the same foot also (Duel screen).

I should have asked if patient is diabetic though. Nice tip!

[Stephen Bird]

Yes, I agree comparison with the other foot intrinsics is a great technique.
You can use FDB , QP or And Dig Min for this purpose.
The comparison will teach you if it is a single muscle problem of or a systemic issue affecting all the intrinsic muscles.

Then to differentiate DOMS from trauma from deinervation the clinical presentation often tells the story.

Steve

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