Complex sentence: GPI was originally considered to be a type of madness due to a dissolute character, when it was first identified in the eighteenth century, until the cause-effect connection with syphilis was discovered in the late 1880s.
Split into simpler sentences: GPI was originally considered to be a type of madness due to a dissolute character, when first identified in the 18th Century. Then the cause-effect connection with syphilis was discovered in the late 1880s.