The aim of this clinical trial is to assess the efficacy of the balneotherapy program (therapeutic orientation: Phlebology) in terms of chronic venous disease improvement and related quality of life, in patients presented with advanced chronic venous insufficiency (i.e., with C4-C5 of severity classification).

The multicenter randomized controlled trial (RCT) "Thermes & Veines" that aimed at evaluating balneotherapy in patients with advanced chronic venous insufficiency is considered as the reference study. The French National Academy of Medicine encourages the re-use of data of published RCT when available. In this context, the current study is designed as a single-arm prospective study with indirect comparison using propension score. The Control group consists of the 197 patients which were allocated to the Control group of the "Thermes & Veines".

All patients enrolled in the current study benefit of 18-days of spa treatment with Mineral Water of Royat, and examination with vascular practitioner at enrollment and 6 months after the beginning of spa treatment.

Chronic Vascular Disease (CVD) related quality of life and symptoms, as well as skin trophic changes from baseline are assessed at 6 months. Variations from baseline are compared in Control and Spa treatment groups. The indirect comparison methodology is based on the reuse of individual data from a "control" group and adjustment after calculating the propensity score.

Inclusion Criteria:

• Primary or post-thrombotic CVD with confirmed diagnosis by by duplex ultrasound examination (last available exam);
• With CVI classified as C4a, C4b, or C5)
• Available for balneotherapy program within the 2 next months
• With health insurance affiliation.

Exclusion Criteria:

• Active venous thrombosis, recent or ongoing erysipelas, peripheral arterial pathology (ankle brachial index < 0.70).
• Walking difficulty
• Neurologic diseases of the lower limbs
• Presenting or likely to present a contraindication to thermal treatments, with planned surgery;
• Having already benefited from a thermal treatment whatever the indication during the last 6 months
• Chronic infectious disease, cancer, heart, kidney or liver failure;
• Pregnant, breastfeeding women or women planning a pregnancy within the year;
• Persons deprived of their liberty or subject to psychiatric care or subject to a measure of legal protection or unable to express their consent;
• Patient likely to not respect the protocol or not to be able to attend the visits, particularly given the study follow-up;
• Living at more than 30 minutes from the thermal site or who cannot be accommodated less than 30 minutes from the thermal site;
• Already included in a clinical trial or in the exclusion period of a clinical trial.

Patients meeting the eligibility criteria and living, or with possible accomodation, at less than 30 minutes from Royat (Auvergne, France)