What Is The Long-term Prognosis Of Lennox-Gastaut Syndrome?

My 5 year old granddaughter has had seizures and infantile spasms since she was 6 weeks old. She was prescribed ACTH 3 years ago which seemed to stop the spasms for about a year and a half. They have returned; and her seizures are much more frequent and of longer duration. We imagine she is now entering the stage of Lennox-Gestaut. We are wondering what her life expectancy may be.