[1] Johns Hopkins Univ, Sch Med, Dept Med, Baltimore, MD 21231 USA
[2] Johns Hopkins Univ, Sch Med, Dept Neurol, Baltimore, MD 21231 USA
[3] Johns Hopkins Univ, Bloomberg Sch Publ Hlth, Dept Epidemiol, Baltimore, MD USA
[4] Northwestern Univ, Sch Med, Dept Med, Chicago, IL 60611 USA
[5] Univ Calif Los Angeles, Sch Publ Hlth, Dept Epidemiol, Los Angeles, CA 90024 USA