Multi-center, randomized, double blind (patient and assessor), sham controlled clinical trial using a parallel 2 group design. Patients will be randomly allocated to receive TENS or sham TENS for 3 weeks. Informed consent of eligible patients will be obtained prior to randomization. Patients will be assigned on a 1:1 basis to the TENS or sham TENS group. Randomization will be centralized using randomization software and an electronic randomization form, generated by the trial coordinating centre (CTU Bern). Patients will be randomized once demographic data and selection criteria are completed by the recruiting physician in an electronic form. Randomization will be stratified according to treatment centre, TENS naivety and clinical severity.