Both of my index fingernails start curving down as soon as they reach the end of my finger. Any explanations?

It has always been just the index fingers most of my adult life...have noticed in past 6 mos or so that my middle fingers are starting to do the same, and the curvature of the index fingers is getting worse. Am I just getting old, (am 45) or is this the sign of something else? 3 Doctors have not been able to give me any clues. But one suggested I see a cardiologist. ?? The index fingernails also tend to curve towards my thumb rather than grow straight off the end of my finger.Both of my index fingernails start curving down as soon as they reach the end of my finger. Any explanations?
Have you had your cholesterol checked?? My uncle had this happen to his nails and his cholesterol levels were sky high.