18/01/2022 at 11:09 am #29450Lena TruongParticipant
Happy New Year, hope you had a good break. I just have a question for you, is it possible to diagnose Baxter nerve entrapment in the foot with ultrasound? If possible, can you give us a few tips and tricks?
18/01/2022 at 11:34 am #29452Stephen BirdKeymaster
Yes it is possible.
The patient usually presents with heel pain similar to plantar fasciitis and they have a normal plantar fascia on ultrasound. They are often a runner or have had repetitive heel trauma from another reason.
There is a paper which shows that you can see the Baxters nerve (It is now called the first branch of the lateral plantar nerve) arising from the lateral plantar nerve, however in a practical sense this is no help as the area where the nerve becomes entrapped is deep to the plantar fascia where we can not see it.
So my examination is like a supra scapular nerve or axillary nerve where I don’t really look at the nerve itself for an abnormality, but rather look for the denervation changes in the muscle it feeds.
In the case of the first branch of the lateral plantar nerve it only supplies one muscle with motor neural feed and that muscle is abductor digits minimise on the plantar aspect of the foot at the lateral aspect.
The And Dig Min is the muscle you see just lateral to the central muscles of FDB and QP. You know you have the correct muscle when you can see the petronius longus tendon passing under the cuboid bone and the muscle on top of petronius longus is And Dig Min.
If you are seeing the base of the 5th metatarsal or the shaft of the 5th metatarsal you are too distal and you will be looking at a different muscle ( flexor digit minimise brevis)
So line up the cuboid and the petronius longus and you can’t be looking at anything else other than the correct muscle.
Once you have found the And Dig Min look at how black the muscle is. If it is nice and black its normal. If it has denervation atrophy it will be small and echogenic.
Remember to make a diagnosis of Baxters neuropathy you need to find isolated wasting of ONLY the And Dig Min muscle. I compare it with flexor digitorum brevis and quadratus plantae in the middle of the plantar foot to look for normal appearing FDB and PQ with isolated wasting of And Dig Min.
If they are all wasted symmetrically then you likely have an old diabetic neuropathic foot rather than an isolated Baxters problem.
Another god thing to understand is that the lateral band of the plantar fascia runs along the surface of abductor digits minimi.
Have a crack and let me know how you go,
04/02/2022 at 2:18 pm #30716Lena TruongParticipant
Thank you for the reply! I definitely will give it a try and let you know how I go 🙂
06/03/2023 at 9:21 am #47721Sandra St CyrParticipant
I would like to add that there can be a variant to the nerve distribution and it is possible to have wasting of all of the muscles mentioned (plantar foot). Not common, but possible.
I agree that it is typically ABductor Dig min, but according to a paper that my msk Rad showed me, that is not always the case.
We did have such a scenario, unilateral plantar numbness and wasting of the muscle bellies of QP, FDB and ABDig min, and the patient was not diabetic – having said that, I have not followed up that case to see if it was Baxter’s neuropathy
06/03/2023 at 12:32 pm #47730Stephen BirdKeymaster
It is a good point you raise Sandy,
Yes indeed there are so many variations in anatomy.
I have the privilege of conducting four cadaver workshops each year and you start to wonder if there is actually a person who is built according to classic anatomical guidelines.
There are so many variations.
In this case I would use the logic that a Baxters neuropathy that effects all of the plantar intrinsic muscles should be unilateral in nature and also other muscles like the extensor digitorum brevis on the dorsum of the foot and the calf muscles should be spared from the process.
Also the lack of a diabetes history would strengthen the case that it is an isolated neural failure.
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