The disease is termed 'multiple' because it usually affects many parts of the nervous system, and it is characterized by frequent relapses followed by periods of partial and sometimes complete recovery. It is thus multiple both anatomically and chronologically. The spinal cord is perhaps the most frequently affected; its involvement may cause periods of partial to complete paralysis of the legs, and, at times, of the trunk and arms. With or without this weakness or paralysis, there may be fatigue, difficulty reasoning and thinking, lack of coordination, staggering and tremor, or there may be poor coordination plus tremor of the extremities, sometimes of the body and head. Numbness, tingling, and various sensory changes may also occur. Eye symptoms are also common, with nystagmus, periods of double or blurred vision, and even temporary to permanent blindness in one or both eyes. In advanced cases there may be slurred speech and sometimes difficulty with bladder and bowel control.