There are rules covering insanity for AD&D but most of them deal with a mind-shattering magical effect or monstrous encounter as the cause. There wasn't really an established table for gradual insanity. This is a fairly simple system for dealing with gradual insanity in Dungeons & Dragons. Under this system the onset is gradual as your sanity erodes over time. As this occurs you become more susceptible to episodes and these episodes become more severe.