<qa>  What is the cause of FMS?Fibromyalgia syndrome FMS is a long-term disorder that is characterized by widespread body pain and tender points in joints, muscles, tendons, and other soft tissues. Other symptoms associated with the disorder include fatigue and depression. The cause of FMS is unknown, and conventional treatments are often unsuccessful in adequately relieving pain. For people with severe, unrelenting pain, which is referred to as refractory FMS, opioid maintenance therapy may be an option, but it comes with the dangerous potential of addiction and is therefore avoided if possible. Vagus nerve stimulation VNS, in which a nerve located in the neck receives electrical input, may be an option for providing pain relief. The purpose of this study is to determine the safety, tolerability, and effectiveness of VNS as a treatment for people with refractory FMS. 