1. Department of Neurology; Mayo Clinic; 200 First Street SW Rochester Minnesota 55905 USA
2. Department of Neurology; University of Michigan; Ann Arbor Michigan USA
3. Department of Neurology; Queen's University; Kingston Ontario Canada
4. Department of Neurology; University of Maryland; Baltimore Maryland USA
5. Division of Biomedical Statistics and Informatics; Mayo Clinic; Rochester Minnesota USA
6. Division of Epidemiology; Mayo Clinic; Rochester Minnesota USA