Only for experts: What is this Mental Asylum Movie?

Hi. The movie speaks of events happening in a mental asylum facility. Where the doctor is investigating things in the facility as I remember. What made this movie cool was the twist at the end. The doctor keeps speaking with a patient who was locked in his cell alone in the basement most of the time but the doctor was not allowed to go inside his cell until finally at the end he decides to enter the cell to find out that there was no patient at all in the cell and he was actually speaking to his illusions and in fact he was a patient such as everybody else in the facility. Something close to the plot of Shutter Island but I think the film was from the 80s or 90s. Can anybody help with the movie name?