Regression in Rett syndrome: Developmental pathways to its onset.

Rett syndrome (RTT) is an X-linked genetic disorder that occurs predominantly in females. The clinical picture associated with RTT is defined by core and supportive consensus criteria, with a period of behavioural regression being a conditio sine qua non. This review sheds light on atypical neurofunctions and potential behavioural biomarkers before the onset of regression. The main focus lies on (a) motor development, especially on purposeful hand movements and the occurrence of stereotypies; and (b) speech-language and socio-communicative development. We outline potentially specific atypical behavioural patterns in these domains (e.g., vocalisations on inspiratory airstream) and different developmental traits of regression: (i) non-achievement of certain milestones: 'regression', here, might point to the fact that the lack of respective behavioural patterns appeared more and more worrisome with increasing age; and (ii) developmental milestones were achieved and functions deteriorate or even get lost during regression. To conclude, we are not quite there yet, but seem to be on the right track towards defining new and reliable neurofunctional markers for early detection of RTT.