Fibro fog: A type of cognitive dysfunction reported by many people with fibromyalgia. Also sometimes referred to as brain fog, its symptoms include difficulty with concentration, memory deficits, and confusion. The reason for the changes in brain function with fibromyalgia is not clearly understood. People with fibromyalgia experience fatigue and are at risk for sleep deprivation and depression, but research has not supported the idea that these factors are the cause of the so-called fibro fog. Another possible theory is that the nervous system disturbances in fibromyalgia affect the blood vessels of the brain, causing certain areas of the brain to receive less oxygen at times. Also known as fibromyalgia fog.