I've had the worst ALS fear of anyone I've never known, hands-down, so I know everything you've been going through.
I've had muscle twitches all over, sometimes more localized, while other times completely random. The twitches include buzzing in my left hand, and in the arches of both feeet, too.
I've seen a neurologist and had three EMG exams, and it's basically been ruining my life for the last four months.
I first convinced myself of atrophy in my left leg, as it appeared thinner than the right. I then convinced myself of left palm atrophy as the thenar tendons are much more pronounced than in my right hand. I have most lately convinced myself of atrophy on the outside of my left shoulder, as it seems a little less thick than on my right arm.
I explain all of this because what does it amount to? All things on our bodies are different, and specifically, nothing on one side of the body is going to match exactly with that on the other side of the body. I seriously doubt you have actual atrophy, and I also seriously doubt that you have profound weakness. However, you'll have answers soon enough, after your appointment tomorrow.
Good luck! And, keep us posted.