A 20-year-old previously healthy man suddenly noticed that he was unable to run. The next day he could not climb the stairs and lost strength in his arms. He was admitted to hospital, and over the next hours he progressively lost muscle power in his arms and legs. Swallowing was progressively impaired, and he noticed minor tingling in both hands and feet. He had had a minor upper respiratory tract infection a week prior to admission.

A 7-year-old girl visited the outpatient clinic because of difficulty walking. She had never managed to run properly, and experienced frequent falls ever since she began walking independently at the age of 18 months. Jumping was not possible, and when stepping up or down, she needed support below her arms. There was no fluctuation of symptoms during the day, but she had suffered from periods that could last several weeks in which using the stairs was completely impossible. She was unable to blow up a balloon and her speech was slow and poorly articulated. There were no complaints about chewing or swallowing. She had a healthy non-identical twin sister and the family history was unremarkable.