Sherlock steps forward and takes hold of Ted's arm, both to steady him and to stop him from following through with the very understandable urge to wake Watson up. He had felt it himself earlier, but knew that it wouldn't go well, and that it was more important to learn how long she was seizing. Still, it's hard for him as well to watch his partner and best friend twitching like that, and his stricken expression shows it.

The doctor looks over and taps a finger to her lips to warn them to be quiet.