64 the impact of fatigue on daily functioning 14 and consists of the following nine statements: 1. My motivation is lower when I am fatigued. 2. Exercise brings on my fatigue. 3. I am easily fatigued. 4. Fatigue interferes with my physical functioning. 5. Fatigue causes frequent problems for me. 6. My fatigue prevents sustained physical functioning. 7. Fatigue interferes with carrying out certain duties and responsibilities. 8. Fatigue is among my three most disabling symptoms. 9. Fatigue interferes with my work, family, or social life. The higher the score (on a scale of 1–7), the higher the impact on fatigue in daily living (1 completely disagree, to 7 completely agree.) The questionnaire was sent to the patients in the form of an email containing a link to download a mobile application. If participants were unable to download the application, the questionnaire was sent by email or regular post. To assess how fatigue influences daily living in OI patients we analyzed the distribution of scores for the 9 separate statements. The severity of fatigue was calculated as a mean FSS score of all nine items per patient ranging from 1.0 (no fatigue) to 7.0 (maximum fatigue). Medical records were analyzed from patients who completed the FSS to determine gender, age and the type of OI according to the updated Sillence criteria 3. Means and standard deviation (SD) were given for normally distributed continuous variables. Differences in means comparing OI patients and separate FSS questions were tested using independent t tests and the mean differences were presented as the mean with 95% confidence intervals (95%CI). A two-sided p-value of 0.05 was considered significant. All data were analyzed with SPSS (statistics 24.0.) Control populations To evaluate the impact of fatigue on daily living in OI versus controls, we compared the FSS scores from our cohort with two previous studies that used the FSS. The first study by Krupp et al. 1989 14 investigated fatigue in individuals with MS (multiple sclerosis) and SLE (systemic lupus erythematosus) and in a control group consisting of 20 healthy American individuals selected from volunteers unfamiliar with the study with a mean age of 39.7 years SD 9. The American control group scored a mean of 2.3 SD 0.7. The researchers determined a cut off score > 4 for severe fatigue, influencing daily living 14. The second study concerned the study of Merkies et al. 1999 15 which investigated fatigue in immunemediated polyneuropathies and recruited a Dutch control group (n = 113) from hospital personnel, companions (relatives, friends) of patients visiting their outpatient clinic, and volunteers unfamiliar with their study. These patients declared themselves to be healthy, free from any chronic medical condition, and were not taking medication that could contribute to fatigue. This
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