When I first show this movie on Showtime in high school I was really confused. From the trailer I thought that it would be like Once Bitten. Boy was I wrong. Nick Cage stood out as a performer to be revered and feared for me even then.
I work in healthcare and after thinking back to Nick Cage's symptoms in the movie, I believe that he is suffering from Neurosyphilis!! Neurosyphilis is a form of syphilis that you can develop which attacks the lining of the brain and causes all kinds of mental issues. When an elderly person presents with senility or Alzheimer the first thing the doctor will try to rule out is syphilis.
Cage is banging chicks left and right in the 80s and most likely raw dogging it. He was bound to pick something up and syphilis would definitely by in the top five VD to get in the 80s. This fits with the movie perfectly! You don't watch 8 years of House MD without learning a little something about medicine.