Subacute sclerosing panencephalitis (SSPE) is a progressive neurological disorder of children and young adults that affects the central nervous system (CNS). It is a slow, but persistent, viral infection caused by a defective measles virus. SSPE has been reported from all parts of the world, but is considered rare in developed countries, with fewer than ten cases per year reported in the United States. The incidence of SSPE declined by at least 90 percent in countries that practiced widespread immunization with measles vaccine. The incidence of SSPE is still high in developing countries, such as India and Eastern Europe. There is a higher incidence among males than females (male/female: 3/1). Most youngsters with SSPE have a history of measles infection at an early age, usually younger than two years, followed by a latent period of six to eight years before neurological symptoms begin. Despite the long interval between the measles infection and the onset of SSPE, researchers think that the infection of the brain occurs soon after the primary bout with measles and progresses slowly. Why it persists and progresses is still not clear. The initial symptoms of SSPE are subtle and include mild mental deterioration (such as memory loss) and changes in behavior (such as irritability), followed by disturbances in motor function, including uncontrollable involuntary jerking movements of the head, trunk, or limbs called myoclonic jerks. Seizures may also occur. Some people may become blind. In advanced stages of the disease, individuals may lose the ability to walk, as their muscles stiffen or spasm. There is progressive deterioration to a comatose state, and then to a persistent vegetative state. Death is usually the result of fever, heart failure, or the brain's inability to control the autonomic nervous system.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.