According to FemaleFirst, the 16-year-old singer suffers from tachycardia, which means that her heart beats at a faster rate than normal.
In her new autobiography, entitled Miles to Go, Cyrus spoke of her heart condition, saying, "The type of tachycardia I have isn’t dangerous. It won’t hurt me, but it does bother me. There is never a time onstage when I’m not thinking about my heart."