1. 1 Neurology Service, Department of Veterans Affairs Medical Centre, Washington, DC 20422, USA
2. 2 Department of Neurology, Georgetown University Medical School, Washington, DC 20007, USA
3. 3 VA Multiple Sclerosis Centres of Excellence, Baltimore, MD 21201, USA
4. 4 School of Pharmacy, University of Maryland, Baltimore, MD 21201, USA
5. 5 Veterans Health Administration, Epidemiology Program, Washington, DC 20420, USA
6. 6 Rehabilitation Care Service, Veterans Affairs Medical Centre, Seattle, WA 98108, USA
7. 7 Department of Physical Medicine and Rehabilitation, University of Washington School of Medicine and Department of Epidemiology, University of Washington School of Public Health, Seattle, WA 98195, USA