65 Chapter 3 control group consisted of 54 men and 59 women with a mean age of 54.2 (range 18–83) being an average cohort out of the Dutch population and comparable to our OI cohort regarding age and gender distribution. The Dutch control group had a mean and median FSS of 2.9, SD 1.1. Severe fatigue was defined as FSS score > 5.1 (mean + 2SD) and fatigue was defined as FSS score > 4 (mean + 1SD, n = 113, 15). Results Clinical characteristics We approached 221 OI patients who had visited the expert center to participate in this study and to fill in the questionnaire. The age range of this cohort was 18–80 years. Permission and signed informed consent were received from 151 patients. A group of 52 patients did not complete the questionnaire and was therefore excluded. Therefore, 99 patients (65.1% response rate) were available for analysis. It concerned individuals with type 1 (n = 72), type 3 (n = 13) and type 4 (n = 14). Sixty-one women and 38 men were included. The mean age was 45 (age range 19–80 years). These distributions are comparable to our total OI population 16. Fatigue severity score Participant basic characteristics and total scores The mean and median FSS score of the individuals with OI in our cohort were respectively 4.4 and 4.8, SD 1.4 (95% CI 4.16–4.70). According to the Kolmogorov Smirnov test, the distribution of the FSS mean score was normal (p = 0.105). 42% (n = 42) of the respondents had a mean FSS score of ≥5 whilst 23.1% (n = 23) had a mean FSS score between 4 and 5. The man/woman distribution in the cohort was 40.5% (n = 17)/ 59.5% (n = 25). A single sample t test and the Mann-Whitney U test were conducted to determine if the differences between the FSS score in the OI group versus the American and Dutch controls were statistically significant, concluding that individuals with OI in this cohort have statistically higher fatigue scores than the American control group, t (98) = (15.46), p = (0.000), and the Dutch control group, t (98) = (11.10), p = (0.000). Statements 3 and 4 of the FSS had both higher median scores with a smaller 95% confidence interval of the mean (4.63 CI 4.27–4.99 and 4.66 CI 4.32–4.99) (significance 0.099, 0.067) compared to the other questions. Statements 6 and 8 had also a high median score (4.23, 4.67), but overall more diffuse results as can be seen in the 95% confidence interval (3.86–4.7; 4.22–5.12) (Table 1).
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