EFFICACY of USUAL MANAGEMENT of CHRONIC IDIOPATHIC ANO-PERINEAL PAIN by USING LOCAL ANESTHETIC INFILTRATION

The goal of this clinical trial is to evaluate the efficacy of a posterior perineal block performed under neurostimulation at 1 month after local anesthetic infiltration in the treatment of chronic idiopathic ano-perineal pain. The main questions aims to answer are : 1. The efficacy of a posterior perineal block performed under neurostimulation on pain 1 month after local anesthetic infiltration in the treatment of chronic idiopathic ano-perineal pain. 2. The rate of maintenance at 3 months of a positive response to the first infiltration. 3. The pain-free rate at 1 month after the first infiltration. 4. Evolution of quality of life and pain impact between inclusion and one month after the first infiltration. 5. Changes in anxiety and depressive disorders associated with pain between inclusion and one month after the first infiltration. 6. The evolution of symptoms related to central sensitization (both somatic and emotional) between inclusion and one month after the first infiltration. Participants will : * be managed with a posterior perineal block of lidocaine or saline solution under neurostimulation. * complete a questionnaire on pain, anxiety and depression at inclusion and one month after injection of the research product (lidocaine or saline solution). Researchers will compare between the two treatment arms (lidocaine versus saline solution) to see the rate of patients with a reduction of at least 3 points on a visual analog pain scale between inclusion and 1 month after the first injection.