Rett: "up to 80% have seizures"
From wikipedia. Citation reference: Jian, Le; Nagarajan, Lakshmi; De Klerk, Nicholas; Ravine, David; Bower, Carol; Anderson, Alison; Williamson, Sarah; Christodoulou, John; Leonard, Helen (2006). "Predictors of seizure onset in Rett syndrome". The Journal of Pediatrics. 149 (4): 542–7. doi:10.1016/j.jpeds.2006.06.015. PMID 17011329.
Also from article, Rett seizures have "wide range of severity" and is one of 4 causes of abrupt death from Rett.