CHAPTER 1 16 of having a mitochondrial disorder, and the need and challenges in addressing the child’s perspective and in using adequate evaluation assessments in person-centred health care for these children. The chapter concludes with an outline of this thesis following the path from the needs of the child from their own perspective to assessing performance of meaningful activities in a reliable, valid and responsive way. Children with mitochondrial disorder Mitochondrial disorders are a heterogeneous group of disorders with variable genetic cause and variable phenotype. The prevalence of mitochondrial diseases is estimated at 5 to 23 per 100.0007-9, depending on the method used. The prevalence of childhood mitochondrial disease contributes to about one-tenth of the cases9. In addition, 23 in 100,000 people carry pathogenic mutations and either have disease or are at risk of disease in the future9. Although the estimated prevalence does not seem that high, mitochondrial disorders are one of the most common inherited errors of metabolism7. Mitochondrial disorders can be caused by mutations of genes encoded by either nuclear DNA (nDNA) or mitochondrial DNA (mtDNA)10. Since 2010, genetic diagnostics have improved and mutations are easier to detect. Mutations have been identified in more than 230 different genes10, 11, and the number of identified genes associated with mitochondrial disorder is continuously increasing12. This heterogeneity is also reflected in the clinical phenotypes associated with mitochondrial disorder, with a large range of symptoms and impairments13-15. Figure 1 Multi-system involvement of mitochondrial disorders16
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