Study of NMDA Antagonists and Neuropathic Pain

Inclusion Criteria: * \> 18 years old * patient suffering chronic neuropathic pain * All chronic pain is retained except central or diabetic pain * Answering patient at ketamine in pain treatment by investigator, and having already received ketamine * Patient who completed before ketamine the following evaluation : DN4, QCD, QDSA, PGIC questionnaires, HAD scale, Leed's sleep evaluation and SF36. * Patient who completed at the end of treatment the following evaluation : numerical scale and PGIC * Sufficient cooperation and understanding to comply to the requirements of study * Acceptance to give a written concert * Affiliation at system of French social security * Inscription or acceptation of inscription at national register of voluntaries participant at research Exclusion Criteria: * Against-indication at memantine or dextromethorphan administration : hypersensitivity at active substance or excipients, hypertension, antecedent cerebrovascular accident, severe cardiac insufficiency * Patient with medical or surgical antecedents * Patient with progressive disease at balance of inclusion * Patient treated by an IMAO * Woman in childbearing age not using effective contraceptive method, pregnant or lactating woman * Patient who participated in another clinical trial, located in exclusion period or received benefits \> 4500 euros during 12 months before the beginning of trial * Patient with cooperation and understanding insufficiency to comply to the requirements of protocol * Patient with social protection * No affiliation at system of French social security