Anacrusis, I feel the need to do a little fact checking here.
MG affects any of the 640 skeletal muscles. It is considered a "head and down" disease because it often affects the head/neck/bulbar muscles first (usually). Guillain-Barre is considered a legs and up syndrome, for example.
MG can be asymmetrical or symmetrical. The right side of my face, for example, tends to droop first or more. When my MG gets worse, my entire face droops. Yeah, even my nose. Not attractive pictures. But it's good to document by taking photos when this does happen to you.
When any muscles get weaker, they can feel like they're numb. Like my legs after walking in a store, which I can't do. Think of muscle strength on a spectrum from strong to paralyzed. Can someone with paralyzed legs feel them? No. When your muscles don't get enough acetylcholine and they can't move well or at all, they can feel numb. Does that make sense? So while "sensation" is not a primary symptom of MG, it is a secondary one. So is pain. When muscles get too weak, like when marathon runners go past the "safety zone," their muscles can cramp up and be painful too; just like in MG.
Since a lot of people with autoimmune diseases can also have Antiphospholipid Antibody Syndrome and be prone to a clot and, therefore, a TIA or stroke, if you have one-sided weakness that persists in spite of Mestinon or other treatment, get to an ER right away. It's always best to be checked out if you don't think it's MG making your muscles weaker.
The Congenital Myasthenia Syndromes are a collection of a variety of genetic syndromes - caused by genetic mutations - and have basically 3 subtypes (generalizing to make a point). In one, people don't have any acetylcholine. In others, like sodium channel ones, they have too much. In still others, like DOK 7, they don't have enough.
So Mestinon will work for those who don't have enough acetylcholine. 3,4 DAP is another drug that is sometimes used.
Mike, I don't recall if they actually did the blood work to determine if you had a CMS or not. The antibody tests you had done show you have myasthenia gravis of autoimmune origin. If they didn't find a genetic mutation, then it is not a CMS that you have. They usually have to do specialized blood tests (at Mayo Rochester or UC Davis) plus a specialized muscle biopsy (which is dangerous due to where it's done and how much tissue is taken). You could say that you have genetic myasthenia gravis, meaning that a particular gene in your family is dominant that predisposes you and others to have MG. But it is NOT CMS. It's not the same thing.
And while these diseases can be genetic, it doesn't mean that if you have a particular HLA typing of a certain disease that you will get that disease. And you can not have an HLA for a disease and can still get a disease due to things like toxins mutating your DNA/RNA!
Whomever the "expert" was that told your sister, Mike, that she can't have MG with only a modulating antibody and a negative SFEMG is completely WRONG. Oh, no, how dare I question a doctor?
There are many subsets of MG. I have modulating only AB's, borderline SFEMG, highly positive Tensilon test, beyond positive response to Mestinon, diagnosis by both a neuro-ophthalmologist and MG expert, breathing tests indicative of a neuromuscular disease, have had an MG crisis and even more exacerbations. So what your sister was told is a load of bull, IMO.
They're all covering their legal behinds.
I hope that helps to clarify a few things. No, I'm not a doctor. Wouldn't it be great if all doctors actually did their jobs well? Doctoring is not about absolute facts and diseases don't follow algorithms.