Fibromyalgia: Introduction Fibromyalgia, once called fibrositis, is a condition affecting the muscles or joints that is often difficult to diagnose. It is one of a group of conditions with vague symptoms and may need to be distinguished from chronic fatigue syndrome, lupus, Type 2 diabetes, depression, Lyme disease, multiple sclerosis, or various other conditions. Fibromyalgia and chronic fatigue syndrome share many common characteristics, and may be misdiagnosed as each other, or both misdiagnosed as depression.