68 FSS results compared to results in two control groups The fatigue scores in our study cohort are significantly higher compared to the Dutch national control group(n = 113) 15 and the American control group (n = 20) 14. Merkies et al. 11 define a mean FSS score ≥ 5.1 as severe fatigue, and a score > 4 and < 5 equates “borderline fatique “ 15. When analysing the FSS results of the OI cohort according to the definitions of Merkies et al. the OI cohort experiences borderline fatigue, influencing daily living, with regard to the mean FSS score. Krupp et al. 14 defined a FSS score of > 4 as moderate to high fatigue level, influencing daily living. When analyzing the FSS results according the definition of Krupp et al. 14 it appears that 42.4% of the respondents (n = 42) had a mean FSS score of five or higher indicating severe fatigue. 23.1% (n = 23) had a score between four and five indicating borderline fatigue. When analysing the results with the definition of Merkies et al., it appears that 38.4% of the respondents (n = 38) had a mean FSS score of five or higher indicating severe fatigue. 27.3% (n = 27) had a score between 4 and 5.1, indicating borderline fatigue. These mean FSS scores are very high compared to the general population, with only 5% of the general population being severely fatigued 15. The presence and severity of fatigue is almost equal across all OI types, which could indicate that OI type and severity of OI is not influencing fatigue. This may demonstrate that although most people with OI type 1 will have reached a higher level of daily functioning than patients with OI type 3 and 4, they still experience comparable impact of fatigue on their daily functioning. The FSS scores in the OI cohort also exceed minimal clinically important difference (MCID) values determined for other patient groups, which are for example 0.4 for SLE and 0.7 for RA (rheumatoid arthritis) 17, 18. Given the above, there appears to be sufficient evidence for the presence of increased occurrence and severity of fatigue in OI patients in the investigated cohort FSS results compared to one similar study involving OI patients A comparable study was recently performed in Norway by Arponen et al. 9. It concerned a cross-sectional study of responses of OI patients matched with healthy controls from Norway to a questionnaire, designed to evaluate levels of experienced fatigue and body pain as well as presence or absence of symptoms related to sleep disturbance or sleep apnoea. Fatigue was evaluated with, among others, the FSS questionnaire which demonstrated a FSS mean score of 5 in patients with OI (n = 56). Interestingly, the Norwegian control group scored a mean FSS score of 4 (n = 56). Arponen et al. concluded that in comparison with age and gender matched controls, adults with OI do not differ in experienced fatigue 9. The Dutch control group 15, has a lower mean FSS score (2.9, n = 113) than the control group in the Norwegian study of Arponen et al (4.0, n = 56, 9). Compared to the American original validation 14
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