How do doctors determine SFN is NOT ALS or MS?

Hi, @susan0514 and @domiha ~
I can really identify with the anxiety of being without a diagnosis ~ here where I live (Iowa) I waited for many months for the opportunity to make an appointment with a neurologist. As it happened, the consultations I had were with his P.A. (Physician's Assistant) but he performed the EMG at a separate appointment. The referring doctor, my pain specialist, was looking for a possible diagnosis of MS or Parkinson's. I was told that the only definitive test for MS was the brain MRI but unless I missed it, I have not seen that information mentioned in this thread. As it turned out, my diagnosis, based on the EMG, brain MRI and labs was an extension of the axonal-demyelinating sensor-motor peripheral neuropathy that I have been dealing with for several years, now exhibiting more extensively. According to the diagnosis I received, there is an irregularity (my word of interpretation) in the brain which explains my mobility and gait problems which are ongoing in spite of physical and occupational therapies.
I'm not sure if this information is helpful at all but I certainly wish you (and all who find themselves in the midst of unanswered concerns) the very best outcomes of your searches and tests!

@domiha - Mike - When I first started dealing with the rather quicker progression of PN around 2016, the doc I saw at that time ordered the MRI of the brain. Prior to the test, I did not know why he ordered it, he just said let's get this done. He called me a few days later to let me know it was normal. My guess is the MRI was part of the screening process to rule out ALS or other brain abnormalities. Another MRI was done in late 2018, no change. I always tried to concentrate on each part of any good news I got. Great, it isn't this, it isn't that. After going through a ton of tests at various teaching hospitals from 2016 to 2019, I finally flew to Mayo in MN in early 2020. They too came up with no cause. It took me a while to realize that not having a cause for my PN is fine. I wish you the best on any future testing keeping in mind, sometimes there is no conclusive answer. Ed

It is very helpful, thank you for your kind words!

I think doing the biopsy on the thigh also (and in my case, the knee) answers the question of length-dependent small fiber neuropathy vs non-length dependent. I believe a podiatrist cannot work above the ankle. It was nice of him to do what he could though. My biopsies showed that I have length-dependent SFN, meaning it was only showing at the ankle two years ago, but will progress up from there.

I saw the NP at the office of my PCP a few weeks ago after I thought I had possibly experienced a brief TIA, but that's a whole other story. Basically, I was talking and suddenly for a couple of minutes some of my words were not coming out right. I felt no other symptoms. Afterwards, all was back to normal, but I did go to the doctor to just get checked. The NP sent me for an MRI of my brain, which showed nothing of concern, so that was a bit of relief.
Yes, the podiatrist was kind to do the skin biopsy and send me for the EMG, but any follow up on results would need to be through a neurologist. As I've said, I'm having a hard time getting to a neurologist, but I do have an appointment with a neurophysiologist next week, and from what I read they are more into testing. I also read that "Radiculopathy is often mistaken for neuropathy because both conditions cause similar symptoms, such as pain, weakness, numbness, and tingling." Since these symptoms of tingling/burning, muscle weakness in legs, etc, began following my laminectomy, and since the EMG showed signs of some continuing nerve involvement in my L4/L5/S1 area, perhaps this is actually radiculopathy? The skin biopsy results showed "beginning signs of SFN."
If and when I ever get in to see a neurologist, perhaps I will be able to get some answers.... that will perhaps lead to more questions.
The spine surgeon who did my laminectomy suggested at one point that a fusion of L4/L5/S1 "might" resolve the numbness and tingling in my feet and the weakness in my lower legs. I have NO DISC left between L5/S1. Maybe most of these symptoms I'm dealing with are not primarily from PN.... but the nerve involvement and compression where the nerves exit the spinal column in my lumbar area.
All of you are dealing with your own symptoms and situations, and I appreciate your comments and thoughts and input more than you can know! The "not knowing" is driving me nuts. I, too, understand that neurologists are often difficult to get appointments with, as there are so many patients needing help. But, as I wait ... as patiently as possible.... could my situation be worsening? And could this wait mean that some of these symptoms will be irreversible? In the end, it is what it is.
To ALL who have replied to my questions in this thread, I thank you!!! I wish each and every one of you the very best in your own journeys!!
Mike