154 Summary Osteogenesis imperfecta (OI) is a rare congenital connective tissue disease, also known as “brittle bone disease”, because it leads to fragile bones and a high incidence of fractures. OI is a clinically and genetically heterogeneous disorder and consists of 5 types (OI types 1-5). In most cases, brittle bones are caused by decreased and/or abnormal production of collagen type 1 production due to a dominant pathogenic variant in either the COL1A1 or COL1A2 gene. The phenotype is influenced by the gene involved, the specific position of the variant and the variant type. Collagen type 1 is mainly found in bones, but also in several other structures of the human body such as teeth, ligaments and tendons, and to a lesser extent in sclera, blood vessels and internal organs. The deficiency of the collagen type 1 protein, or the synthesis of an abnormal collagen type 1, not only affects bone fragility, but can also cause other symptoms that can manifest throughout the body. These symptoms can have a negative impact on quality of life. Although other symptoms of OI, such as easy bruising and fatigue, are often mentioned, some of them have hardly been studied. The aim of this thesis was to better understand the quality of life and bleeding tendency in individuals with OI. To improve patient care and counselling and increase therapeutic capabilities, it is important to look at OI more broadly than just a disease with bone fragility. The following research questions were addressed: 1. What is the quality of life in people with OI compared with control populations? 2. What is the impact of fatigue on daily functioning in people with OI compared to control populations? 3. What is the prevalence of bleeding tendency in OI compared with a control population? 4. What are the clinical manifestations of bleeding tendency in OI? Is diagnostic testing for bleeding disorders indicated? 5. Which bleeding events are most clinically relevant in OI? What can be learned from therapeutic considerations in other mild bleeding disorders?
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