What is aortic dissection, the medical emergency linked to Graham's death?

Sen. Lindsey Graham’s (R-S.C.) sudden death on Saturday at the age of 71 has been attributed to an aortic dissection, raising questions over the rare but often fatal condition.  Francisco Diaz, D.C.’s chief medical examiner, said in a statement Sunday that his office’s preliminary findings indicate that “aortic dissection due to arteriosclerosis cardiovascular disease” was the cause of Graham’s death. Aortic dissection...