KANSAS CITY, Mo. — One morning at breakfast six years ago, as Dawn Sticklen’s healthy 13-year-old son tried to eat a bowl of cereal, his arms started shaking and got so weak he couldn’t feed himself.“We thought maybe it was because he had been sick for a few days and maybe he was just kinda weak from not eating properly,” said Sticklen, who lives in Joplin.“But he just kept saying it was getting harder and harder to move his arms. So we knew something was wrong. We got him in to see the doctors and they all were like, ‘this doesn’t look right.’ ”Joplin doctors sent the family to Children’s Mer...