Hi. Welcome to the group!
Sorry to hear you are having problems with your legs and walking.
My own MG started with the eyes and head and was diagnosed by a positive blood test. I think the test that most neurologists give when the blood tests (there are more than one) is negative and they suspect MG is the single fiber emg on one of the muscles that is having the problem. From myasthenia.org
"The measurement of jitter by SFEMG is the most sensitive clinical test of neuromuscular transmission and is abnormal in almost all patients with myasthenia gravis. A normal test in a weak muscle excludes the diagnosis of myasthenia gravis, but an abnormal test can occur when other motor unit disorders cause defects in neuromuscular transmission."
Myasthenia Gravis is a failure off the transmission of signals to reach our muscles -- at the junction between nerves and muscles. The path is blocked.
Your symptoms are different than mine were. I got tired after a little walking too (also chewing, my fingers, hands, arms, breathing...), but I recovered with rest in a short time. It limited my walking, but never had problems that persisted more than a few hours, not two weeks like yours from a single walking effort.
The SFEMG test is likely next for you in MG diagnosis.
However another test is if you improve with a medication -- Mestinon (pyridostigmine). It takes effect in about 1/2 hour and lasts for about 3 hours. You might ask your doctor to try a 60mg pill. (it has some digestive side effects so take it with some food). If it immediately makes walking easier, it makes it more likely you have MG. That is the first treatment most of us get. It temporarily improves nerve to muscle transmission.
Good Luck
Russ

Hi, I've just been diagnosed with MG. The antibodies were negative but single fibre EMG was positive. I'm on experimental diagnostic treatment with Mestion and it has confirmed that I have MG. The doctor started with a small dose of 10mg x 3 per day and the dose was increased a bit once a week until it was 60mg x 3 times per day. Then the my symptoms got so better but did not disappeared. We are still figuring out the medicines and dosages but the diagnosis is on board. And that is a relief. I hope you will get diagnosis soon so you can start on treatment.

Slow moving mama,
You described your symptoms very similar to what I am currently experiencing except my symptoms started with bilateral numbness in my feet and legs and progressed to my thighs and butt which is not a symptom of myasthenia. Over the past few months repetitive muscle movement caused me to feel very fatigued and muscle fatigue and it progressively got worse where I had severe difficulty walking. my calf and thigh muscles were getting so weak. If I over do it, which is hardly any activity, then I am also out of commission. I have had trouble taking care of my 18mo old. I also have not had any issues with upper body, face, or breathing difficulties. I trailed mestinon 60 mg three times a day and it helped. I have been able to walk better around the house and continuously down my street without feeling fatigued. I'm still experimenting with the dose but dr suspects MG. Im waiting to do the EMG. I did the first antibody test and it was negative. The med is worth a try.

Hi Slowmovingmama (love the name - describes me as well!), sorry to hear about your situation. It must be so frustrating to have these symptoms but not have a firm diagnosis - I remember those days. Your symptoms sound very similar to mine - I have what my neuro describes as "limb-predominant myasthenia", it's not as common as other forms but definitely exists. I did eventually have issues with breathing, swallowing, chewing, but those came later in my disease process. I'm very similar in that repetitive movements fatigue me extremely quickly, and like you, can take up to a couple of weeks essentially on bed (or recliner) rest to improve.

So, you asked about tests:
-As others mentioned, the mestinon trial is one of the best indicators. For me, mestinon gives some improvement to my major muscles/limbs, but gives the most improvement if I'm bad enough that my chewing/swallowing/breathing is affected. Mestinon is fast-acting, I notice a difference within 15-20 minutes at the most.
-I was negative on my blood tests, but researchers have discovered several other antibodies that cause myasthenia in the past few years - and last I checked, these don't have commercially available tests yet. So, being seronegative doesn't rule it out.
-Single fiber EMG (SFEMG) is considered highly predictive, but must be done properly. (Don't be on any meds like steroids or mestinon, don't have coffee that morning, I think as someone else mentioned it is supposed to be done on the muscle group that causes you the most problems). I had what my neuro calls "equivocal" results because I got 1 abnormal result out of 20, and technically they say you need 2 or more to qualify as MG. But I had coffee that morning (didn't realize it was a no-no), the person administering my test stepped out for 30-45 minutes in the middle of the test to do an interview for a journalist, and they tested my forehead only - facial muscle weakness wasn't an issue for me at that time. So, my neuro dismissed those results as not predictive.
- The most helpful test for me (other than mestinon) was a chest CT/MRI (my neuro had me do both since first one showed a problematic thymus). Because my thymus turned out to be extremely hyperplastic, this plus my response to mestinon and symptoms/exam were sufficient for my neuro to diagnose MG with certainty. Immune suppressing medications, and thymectomy, have also given me some improvement or slowed my symptom progression, which further solidified the neuro's diagnosis.

Since you said the mestinon trial seemed helpful, you may also want to push for your doctor to do the chest CT scan. (They're more likely to approve CT first for unconfirmed MG because it's much cheaper than MRI). My doctor ordered mine before blood tests came back because he was so certain of MG; if I hadn't gotten the hyperplastic thymus results, the MG confirmation might have taken longer (given that I am seronegative). Having a thymectomy slowed my symptom progression significantly for several years.

Happy to chat more if helpful. Good luck!!

Sorry - just realized it was another responder who said that mestinon helped. So, yes, my recommendation would be for you also to do a trial of mestinon! :) (Much cheaper and easier to justify to insurance than a chest CT)

And no, SFEMG isn't 100% predictive (per my neuro) - especially if you are like me and it mainly affects your legs, not your facial muscles, but they only test your facial muscles. :)

Hi, nice to meet every-one, I am new here! The only test that proved that any-thing was wrong with me was a muscle biopsy. They used every test out there, nothing, until the biopsy. Now all the medicine is making me sick, sucks.

Sapira, was your biopsy negative? I've read that is the of the ways to diagnose seronegative MG. Several docs have suggested it to me but wouldn't order it. I see another neuro Wednesday for second visit, a 6 month recall. She said Mestinon is likely working because of non-specific effects. It is working less well and I'm now at 6x60mg per day. I'm now getting hoarse after on 30 minutes of conversation and it takes longer to recover. I've had several episodes of blurry vision too, with extended book reading. So hard being undiagnosed for years now. I don't want to have to have a crisis just to get a diagnosis!
Flutebell

The antibody tests were negative for me. I was positive on the Tensilon Test. Had an EMG muscle test schedule but it was canceled after testing positive for the Tensilon.

Oh and beware of the MUSK AB test.... insurance not ended up not covering it and it costs me $1700.00 out of pocket (Athena Diagnostics)