A 33-year-old man was referred by his nephrologist who treats him with potassium supplementation for a hypokalaemic periodic paralysis, which had been diagnosed by the finding of heterozygosity for the missense mutation c.1853 G->A; p.Arg528His in the voltage-gated calcium channel (CAGNA1S) gene. He now visited our department in search of an explanation and treatment for an apparently fixed (permanent) weakness in his upper legs, which hampered climbing stairs.

The attacks of flaccid weakness had begun when he was 13 years old. The frequency had been increasing starting a few years ago. He now experiences mild attacks four to five times a week, with a heavy feeling in the affected limb during hours or a whole day.