The New England Journal of Medicine Downloaded from nejm.org on January 6, 2013 For personal use only No other uses without permission Copyright © 1994 Massachusetts Medical Society All rights reserved The New England Journal of Medicine Downloaded from nejm.org on January 6, 2013 For personal use only No other uses without permission Copyright © 1994 Massachusetts Medical Society All rights reserved The New England Journal of Medicine Downloaded from nejm.org on January 6, 2013 For personal use only No other uses without permission Copyright © 1994 Massachusetts Medical Society All rights reserved The New England Journal of Medicine Downloaded from nejm.org on January 6, 2013 For personal use only No other uses without permission Copyright © 1994 Massachusetts Medical Society All rights reserved The New England Journal of Medicine Downloaded from nejm.org on January 6, 2013 For personal use only No other uses without permission Copyright © 1994 Massachusetts Medical Society All rights reserved The New England Journal of Medicine Downloaded from nejm.org on January 6, 2013 For personal use only No other uses without permission Copyright © 1994 Massachusetts Medical Society All rights reserved The New England Journal of Medicine Downloaded from nejm.org on January 6, 2013 For personal use only No other uses without permission Copyright © 1994 Massachusetts Medical Society All rights reserved