People with RA according to the revised American Rheumatism Association  or with AS according to the New York  criteria between 18 and 70 years of age and only patients with chronic disease that had been stable for at least 3 months without change in medication were included.
Patients with a change of treatment during 3 months prior to the start of the study; patients with signs of acute inflammatory activity (morning stiffness lasting longer than 1 h or more than three joints actively inflamed) as judged by the consulting rheumatologist (heat treatment may aggravate clinical signs and inflammatory activity in an acute phase of the disease); patients permanently wheelchair-bound or bedridden; patients with the following comorbidities: heart disease, skin disease, malignancy, asthmatic bronchitis, or psychiatric disorders are excluded.
Patients were recruited consecutively from the rheumatology out-patients clinic (JJR) in a general district hospital in Enschede, The Netherlands.
After oral and written information about the study and the possible clinical effects of IR whole-body hyperthermia, patients were invited to participate. Informed consent was obtained from all patients, according to the Declaration of Helsinki.
The patients were treated in the Health Company Infrared Cabin (kindly made available by The Health Company, P.O. Box 321, 2400 AH Alphen a/d Rijn, the Netherlands), which was 130 × 90 × 190 cm in size. The temperature in the cabin can be adjusted from normal ambient room temperature up to 90°C. The patients were seated in the infrared cabin, which has six heating sources; three at the back, two in front besides the entrance, and one under the bench behind the lower legs of the patients. The infrared used has a long wavelength between 5,000 and 1,000,000 nm.
The patients were treated for a period of 4 weeks, twice weekly, with eight IR sessions in the IR cabin (30 min at an ambient temperature of 55°C). According to the manufacturers’ recommendation, before treatment, a preheating time of the IR whole-body hyperthermia equipment of 15 min was used. During the whole study period, the dosages of nonsteroidal anti-inflammatory drugs and disease-modifying antirheumatic drugs were not changed; treatments with physiotherapy, when applied, were not changed, and no corticosteroid injection was given
Clinical measurements were performed 4 weeks before the start, at the start, and at the end of the 4-week period of IR treatments and 4 weeks after the end of the treatment series; all four assessments were at the same time of the day and executed by a trained physiotherapist unaware of the study protocol. The pretreatment period without IR sauna was meant as a control and the 4 weeks after treatment as follow-up.
The patient’s perceptions of pain , stiffness , and fatigue [16–18] were measured on a 100-mm visual analog scale and were considered as primary outcomes.
Secondary outcomes for the RA patients were: Escola Paulista de Medicina Range of Motion (EPM-ROM scale)  and activities and participation scales of the Dutch Arthritis Impact Measurement Scales (DUTCH-AIMS) . For AS patients, Bath Ankylosing Spondylitis Global Score , the Bath Ankylosing Spondylitis Metrology Index (BASMI, a ROM-index) , and the Bath Ankylosing Spondylitis Functional Index  were used. To evaluate possible effects on the disease activity (improvement or exacerbation) for RA patients, the Disease Activity Score using 28 joints (DAS 28)  was calculated, and for AS patients, the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI)  and the erythrocyte sedimentation rate (ESR) after 1 h were assessed. The DAS 28, BASDAI, and ESR were considered as tertiary outcomes. These measurements are fairly routine in clinical research and have been proven to be valid, reliable, and of good sensitivity to change [14–25].
Directly before and after the first IR treatment, the primary outcomes were registered to measure the immediate effect of IR on pain, stiffness, and fatigue. Besides that, during and after the first treatment, well-being (as tertiary outcome) was recorded on a five-point Likert scale (very uncomfortable, uncomfortable, neutral, comfortable, and very comfortable). Well-being was measured at 15 and 30 min after the start of the treatment (patient still in the cabin) and 30 min after the end of treatment (patient out of cabin).
The continuous data were checked for normality. This was done with the descriptive statistics explore command in Social Package for Social Sciences (SPSS) 14.0  by making histograms, scatter graphs, normality plots, normal curves, and carrying out normality tests (Kolmogorov–Smirnov and Shapiro–Wilk). Continuous data were statistically analyzed by means of repeated measure analysis with Bonferroni correction within SPSS 14.0. Results are expressed as mean and SEM (Standard Error of Mean). For nonparametric data (well-being on an ordinal Likert scale), Wilcoxon signed-rank test for related samples was applied. Level of significance (α) was chosen at 0.05.
The ethical committee of the Hospital Medisch Spectrum Twente, Enschede, The Netherlands approved the study design.