Sonja Mensch

11 General introduction and outline of the thesis disorders are for example Down syndrome (Down syndrome foundation., 2016), Prader Willi syndrome (Prader-willi syndrome foundation., 2016) or progressive diseases such as the RETT syndrome (Williamson & Christodoulou., 2006), (RETT syndrome foundation., 2012) and the Tay-Sachs syndrome (Kinderneurologie., 2016). Another cause of brain damage can be malformation of the brain, for example due to corpus callosum agenesis (Paul et al., 2007). While these causes are known, in a great number of cases the exact cause is unknown or never found (van Karnebeek et al., 2005). Children with SMD, as defined in this thesis, are characterised by a severe or profound intellectual disability, comparable to an Intelligence Quotient (IQ) <25. The Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) American Psychiatric Association (APA., 2013). The American Psychiatric Association defines intellectual disabilities as neurodevelopmental disorders that begin in childhood and are characterized by (intellectual) difficulties in cognitive functioning as well as difficulties in conceptual, social, and practical areas of living. The terms ‘mild’, ‘moderate’, ‘severe’, and ‘profound’ are used to describe the severity of the condition. Severe intellectual disabilities manifests themselves as major delays in development; individuals often have the ability to understand speech but otherwise have limited communication skills. Despite being able to learn simple daily routines and to engage in simple self- care, individuals with severe intellectual disabilities need supervision in social settings and often need ‘family care’ in a supervised setting such as a group home. Persons with profound intellectual disability need continuous close supervision and help with self- care activities. They have very limited ability to communicate and often have physical limitations. Individuals with mild to moderate disability are less likely to have associated medical conditions than those with severe or profound intellectual disabilities. Children with SMD also have physical limitations characterized by a low level of motor abilities, comparable to level IV or V of the Gross Motor Function Classification System (GMFCS) for children with cerebral palsy (R. Palisano et al., 2007) (Veugelers et al., 2005). Children and youth in Level IV are able to take a sitting posture (usually supported) but self-mobility is limited; they are more likely to be transported in a manual wheelchair or use powered mobility. Children and youth in Level V have severe limitations in head and trunk control and require extensive assisted technology and physical assistance. Self-mobility is achieved only if the child/youth can learn how to operate a powered wheelchair (Palisano, Rosenbaum, Bartlett, & Livingston., 2007). Furthermore, vitality and quality of their life of these children are threatened by a range of health problems (van Schrojenstein Lantman-de Valk & Walsh., 2008) (Liptak et al., 2001) (Shevell, Dagenais, Hall, & Consortium., 2009), which start at an early age and steadily increase as they grow up. Various studies have demonstrated comorbid conditions in children with SMD such as low bone quality and fractures (Mergler et al.,