In the scene where Nygma wakes to find himself stuck with needles to supposedly help with his recuperation, Myrtle refers to herself as "The Riddlette", to which Ngyma remarks that he thinks "rehashing someone else's idea isn't very original". That's a self-referential joke from the writers, referring to the similarity between the sequences involving Nygma recovering in Myrtle's room and the plot of the movie
Misery (1990) (based on the Stephen King book of the same name).