Therapeutic Efficacy of the Phlebology-oriented Thermal Cure at the Saint-Eloy Thermal Center in the Treatment of Symptoms of Severe Chronic Venous Insufficiency

Inclusion Criteria: * Adult male or female (≥ 18 years of age). * Patients with severe chronic venous insufficiency of the lower limbs (C4a,b,c - C5 according to the CEAP (Clinical manifestations, Etiology, Anatomic distibution, Pathophysiology) classification (2020 revision) for at least one of the legs) confirmed by venous Doppler ultrasound of the lower limbs. * Patients with an indication for a thermal cure exclusively focused on phlebology. * Patients available to participate in a 3-week consecutive thermal cure at the Saint-Eloy Thermal Center, followed by 6 months of clinical follow-up, in accordance with the protocol schedule. * Patients available to attend the visits scheduled in the protocol and able to complete the data collection documents. * For women of childbearing age: patient using effective contraception. * Patient affiliated with a health insurance plan or beneficiary (Art. L.1121-8-1, French Public Health Code). * Patient who has given their informed consent in writing regarding their participation in the protocol. Exclusion Criteria: * Patients with a contraindication to thermal cure. * Patients who have participated in thermal cure (regardless of type) within the last 9 months. * Patients who have had an ulcer on their lower limbs that has healed within the last 3 months. * Patients without venous trophic disorders of the lower limbs, and in particular patients with isolated class C3 chronic venous insufficiency or less in both lower limbs. * Patients with an open ulcer on a lower limb (class C6). * Patients for whom interventional venous treatment is due to begin within the next 6 months. * Patients scheduled to undergo orthopedic surgery on the lower limbs within the next 6 months. * Patients who are unable to walk. * Patients with a known allergy to sulfur. * Patients with any medically significant findings or significant history that may impact safety, interpretation of results, and/or subject participation in the study at 6 months, in the investigator's opinion. * Patients simultaneously participating in another clinical trial or who have participated in another clinical trial for which the exclusion period has not ended. * Vulnerable persons (Art. L.1121-5 to 8, and L1122-1-2, French Public Health Code).