One thing I have learned in my own efforts to research different possibilities based on my symptoms is that there are hundreds (if not thousands) of possibilities out there. The body is tremendously complex, but there are a limited number of ways we experience symptoms (fever, cough, twitching, soreness, weakness, etc...). Zenarcher once used a great analogy of a dead telephone to explain the difference between ALS and MG (In MG the phone is unplugged. In ALS the phone lines are gone). In both cases the phone is dead, but for different reasons.
I also like imagining a symptom like twitching to be like static or snow on a TV set (Though with digital broadcasting and cable I wonder if my grandkids will grow up in a world without snow). We could all have static on our TV's, and even call around the night of the big game to compare, ("Do you have snow?" "You mean like blizzard snow, and no picture, or occasional snow across the picture?"). The cause for each of us could be different: bad cable, crappy antenna, kid has the microwave on, station in our town is having issues. The symptoms would remain the same.
The diagnosed process attempts to sort through the symptoms and clinical findings . Doctors start with the usual suspects, because, based on the very definition of probability, that's what its most likely to be.
It becomes challenging when doctors have to wade into deeper waters of rarer disorders, or those requiring knowledge of multiple specialties. First of all, even in a large practice the Dr. may never have seen the disease first hand, let alone diagnosed multiple cases of it. Second, because they are very uncommon or not completely understood, black and white diagnostic tools might not exist. That is why many of us end up at academic medical centers where they see a higher volume of unusual cases, and where there is ready access to multi-disciplinary teams and the latest diagnostic techniques.
Finally, illnesses do not always play ball. Just because something is not a "presenting" symptom or common finding doesn't mean that it never occurs with the illness, or that presenting symptoms/findings of one illnesses don't occur in rare examples of other illnesses. I like to take the positive spin on this. I saw an abstract the other day about a couple of cases of MG where the patients involved also had symptoms of tongue atrophy and fasciculation. So if the doctors are telling you there are holes in the diagnostic puzzle that point away from ALS, I would take that as a win (especially if the specialist has ALS/Neuromuscular disease experience).
I'm sure if a bunch of us were grouped together there might be a fascinating paper in all this. If not, we could all at least all go out to a nice place to eat or a show afterwards.
Robert