A 27-year-old woman consulted the neurology clinic because of progressive muscle weakness in legs and arms. She noticed increasing difficulty in walking, with a maximum walking time of 15 minutes when there was a slight upwards slope, climbing stairs, and lifting heavy things. She had more and more frequent falls in which it felt like her legs suddenly could not bear her weight anymore. Once she fell, she wasn’t able to get up from the floor without help. At the time of referral, she also experienced problems rising from a sitting position. She also mentioned problems with repeated movements due to fatigability, for example, when she walked or was cleaning out the dishwasher.

In retrospect, she remembered that she had had a ‘typical’ walk from the age of five and had never been able to run. A subtle tremor of her fingers was already present in kindergarten. Her medical history was otherwise unremarkable. She had two healthy siblings.