The current study concerns a longitudinal analysis of registry data collected for the “Radon indication registry for the assessment of pain reduction, increase of quality of life and improvement in body functionality throughout low-dose radon hyperthermia therapy” (registration ID ISRCTN67336967; https://doi.org/10.1186/ISRCTN67336967) in the valley of Gastein in Austria. The registry collects data from individuals visiting the valley of Gastein for the purpose of spa-treatment including radon for a variety of rheumatic diseases. Data are collected by means of standardized questionnaires that are completed by participants directly before commencement of the treatment (baseline), directly after the treatment and 3; 6 and 9 months after the treatment.
For the current study, data provided by participants with AS were included if they completed the questionnaire at each timepoint.
The intervention consisted of an individualized spa-treatment including radon. This so-called low-dose radon balneo/speleo therapy (LDRnBST; radon-therapy) is part of a holistic treatment program for patients with AS and is applied in terms of balneo- and/or speleotherapy. The former includes bathing in water with low activity of radon as applied by the local facilities according to standardized treatment regimens. An intervention including radon-therapy consists of approximately 10 baths with a duration of 20 minutes. Speleotherapy including radon describes the process of relaxation while being exposed to low activity of radon, high humidity and mild hyperthermia (37-41.5°C) in the healing gallery of Gastein (a former gold mine) located in moderate altitude (1280 m) above sea level for an average time of 60 minutes on alternate days.
The intervention had an average duration of 17.5 days (SD 3.5) and took place in the valley of Gastein in the Austrian Alps.
The EuroQol EQ-5D (© EuroQol Research Foundation. EQ-5D™) is a self-reported questionnaire consisting of two parts, a descriptive system comprising 5 dimensions of health (i.e., mobility, self-care, usual activities, pain/discomfort, anxiety/depression) and a visual analogue scale (VAS) capturing participant’s self-rated health status on a 0-100 scale with higher values representing better health. Using the unique score from each of the 5 dimensions of health a utility index score can be calculated (i.e., von Neumann-Morgenstern utility value for current health). (24)
Single values for each of the 5 dimensions reflect the level of problem with each dimension resulting in an individual health state. This health state can be converted into a weighted health state by applying scores from the EQ-5D preference weights extracted from the general population which can take a value from 0 (death) to 1 (full health).
The EQ-5D utility index and EuroQol VAS were used as outcome variables for the current study. In absence of Austrian population weights, German population weights were used to calculate the EQ-5D utility index. (25)
Main independent variable of interest and covariates
The timepoint of survey completion by the participants was used as main independent variable of interest. Covariates were chosen a priori and included age (in years), sex (men/women) and body mass index (BMI; BMI=weight[kg]/height[m]2) due to their already established influence on health and health related QoL. (26-28)
First, descriptive statistics were used to characterize the sample in terms of age, gender and BMI at baseline (i.e., directly before the intervention) and to describe the EQ-5D utility index and VAS-score for each of the timepoints of measurement. Next, two linear regression models were computed to explore the association of timepoint of measurement with a) the EQ-5D utility index and b) the EuroQol VAS-score while adjusting for age, sex and BMI. After each model, margins and their 95% confidence interval (CI) were calculated to produce specific age, gender and BMI standardized estimates for the utility index and VAS score.
P-values ≤0.05 were considered statistically significant. A change of 0.05 in the EQ-5D utility index and of 5.00 in the EuroQol VAS was considered clinically relevant. (29, 30)