They worry when you come in with clinical weakness in one limb and present with fasciculations in that *one* limb.
If anyone reads this with twitching only in *one limb* (without weakness) please do not freak out. My BFS started with twitching in one calf for four months. I didn't have any weakness, so they suspected a disc problem at first. Then, one day, I became "like the others." I was thrilled the day my right calf started . . . now it is everywhere.
But to this day, my twitching favors my left leg.