63 Chapter 3 Introduction Osteogenesis imperfecta (OI) is a rare hereditary disorder with a prevalence of 6–7:100,000 1. OI is primarily characterized by bone fragility. Additional features of OI include blue sclerae, dentinogenesis imperfecta, hearing loss, ligamentous laxity and short stature 2–6. OI is known to be a clinically variable disorder with severity ranging from perinatal lethality to slightly increased fracture frequency with normal life expectancy 3. As such, the clinical classification of OI consists of 5 different types (1–5) 6. In approximately 90% of patients with OI, dominant mutations in the genes COL1A1 and COL1A2 encoding respectively the alpha1 and alpha2 chains of the protein collagen type I, are identified 6. The functional ability of patients with OI, especially ambulation, have been historically attributed to the severity of the skeletal deformities 3, 6 and this has long been the focus of physicians involved in the care of patients with OI. However, many patients visiting our expert center for adults with OI complained about fatigue, which limits their quality of life, and asked whether this could be related to their diagnosis of OI. Previous studies indicate that the quality of life (QoL) of individuals with OI is negatively influenced by reduced function due to fatigue indicating that fatigue is an important factor when considering quality of life in OI patients 7–10. As such, we approached a subgroup of our total group of OI patients to investigate the impact of fatigue on daily functioning compared to control populations. Methods Study design and population A cross-sectional cohort study was undertaken in the national expert center for adult patients with Osteogenesis Imperfecta, Isala Hospital, Zwolle, The Netherlands. All patients who visited the expert center from December 2007 until December 2015 were selected to participate. The main exclusion criteria were unreturned questionnaires. Informed consent was obtained from each participant. The study was registered in the Isala research registry (Nr.190106) and the local Medical Ethical Committee approved the study protocol and granted an exemption because participants are not subject to procedures and are not required to follow rules of behavior. Data collection Many definitions of fatigue exist 11 as well as scales to measure the nature, severity and impact of fatigue in a range of clinical populations 12. To investigate fatigue in patients with OI the Fatigue Severity Scale (FSS) was distributed among all adult patients. The FSS questionnaire is widely used and has been found valid and reliable in different patient groups 13. It is developed to measure
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