Here is some basic information about SSPE: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002392/ It takes so long to present because it is an insidious course. Think of a brain tumour, which people can live with for years and never even know they have until the growth becomes so large it begins to affect neurobehaviour. It is only associated with wild-type measles disease and not the vaccine.
SSPE is a late consequence of measles virus infection of the brain. Essentially, a small mutation in the measles virus gives it the capacity to hide in the brain for several years (usually 5 to 7, but it can be much shorter or longer) and then it starts to affect the brain quite quickly - the kids get seizures, can go blind, lose all abilities and eventually die (usually within a year or two, but Micha has been blind and in a wake coma for the past 6 or 7 years).