Since my name was mentioned at the start of the thread I want to explain my story in more detail.
I started having ALS symptoms in 2009 and was diagnosed in August of that year. But the year before, 2008, I had gone to the neurologist for a different problem. I was doing an intensive workout program called P90X which I bought through a TV informercial. Any insomniacs changing the channel in the middle of the night, you've probably seen it advertised. It's a good program but very hard on the body. It includes a yoga workout that has a bunch of positions including one where you lie on your back and swing your legs over your head until they touch the floor. Well I am very inflexible and my legs come no where close to the floor, and when I do this my back is stiff so I end up with all my weight on my neck. I think that was the cause of the problems.
My first symptom was shooting pain through my left shoulder. Shortly afterward, the tip of my left thumb started to go numb. At about this time I found that my left arm was weaker and could not do as many curls as my right arm. So I went to the neurologist, the first time I'd ever been to a neurologist. I had a lot of tests and in the end he diagnosed it as arthritis in my spine causing nerve compression. Specifically there were small bone spurs visible in what are called the foramena, the openings in your backbone where nerves exit. This was thought due to my hard workout and all the stress I was putting on my neck area. I was advised to quit the workout program, which I did. I switched to starting to train to run a marathon.
Now as part of this testing I did have an EMG in my left arm, which was clean. BUT THIS WAS NOT A CASE OF ALS SYMPTOMS WITH A CLEAN EMG. Pardon me for shouting. My symptoms in 2008 were not ALS. First, I had sensory issues in the form of my numb thumb and my shoulder pain (which was exacerbated if I tilted my head back). ALS does not cause numbness and nerve pain. Second, I had a "dirty" MRI which showed nerve compression in my spine that would account for the problems. In short this was a normal garden-variety case of nerve impingement. I changed my workout and got some PT, and it cleared up.
Now it is true, six months later as my marathon training peaked, I started having what turned out to be ALS symptoms. My first symptoms were bulbar, my voice became weak and slightly slurred. I was also really, really tired and had trouble recovering from my training workouts, but I still don't know if that was ALS or just because I suddenly decided to become a marathoner at age 53. But I got tested, had a dirty EMG, and then a couple of months later at UCLA they re-did the EMG and it was worse. So they gave me the diagnosis on the basis of the progression.
But at that time, and even still now 8 months after diagnosis, my left arm where i had the problems in 2008, is the least affected part of my body (well my breathing is still good too, knock on wood). So that is further evidence that what I had in 2008 was not a case of ALS with a clean EMG. It was nerve impingement.
I would assume that any of us with ALS, if you had gone back years before we had symptoms, would have had a clean EMG back then. But most of the time nobody would ever get one since there would be no reason. Once in a while you might have a case like mine where an EMG was given due to some other cause, and so it is not surprising that the EMG is clean since the ALS disease process hasn't gotten going yet.
That is different though from someone who does have symptoms they are afraid might be ALS, whether twitching or atrophy or weakness or whatever, and you get a clean EMG. In that case you can know that your symptoms are not due to ALS (at least, lower motor neuron ALS). Maybe something else is causing it and you can get helped. Or maybe it's just anxiety and your own fears are manifesting because you are worrying about them so much. But it is not ALS, at that time.
Nobody can guarantee that you won't get ALS next year, any more than they can guarantee that you won't be killed in a traffic accident tomorrow. All they can say is that at this time, ALS is not producing your symptoms. My story illustrates exactly this point. ALS was not producing my symptoms in my left arm in 2008. That was the diagnosis and I still think it was correct, even given all that has happened since then.