Ketamine and Neuropathic Pain

The primary outcome is to compare the analgesic efficacy of intravenous ketamine treatment to that of a placebo in patients with refractory neuropathic pain. The secondary outcomes are: 1. - To compare the additive analgesic efficacy of prior administration of magnesium sulfate to that of placebo and ketamine only, on the effectiveness of intravenous ketamine treatment, 2. - To study the evolution time of pain and analgesia after the intravenous administration of ketamine and placebo, 3. - To study the correlation of the analgesic response to administered products respectively ketamine, ketamine and magnesium sulfate, placebo.