RTT clinical severity was assessed using either the total clinical severity score (CSS), a validated clinical rating specifically designed for RTT, or the "compressed" CSS both based on 13 individual ordinal categories measuring clinical features common in RTT [24], and phenotypical severity was also measured by the use of the Rett Syndrome Behaviour Questionnaire (RSBQ) [25].
Cass, "The Rett Syndrome Behaviour Questionnaire (RSBQ): refining the behavioural phenotype of Rett syndrome," Journal of Child Psychology and Psychiatry and Allied Disciplines, vol.