The New England Journal of Medicine Downloaded from nejm.org on March 17, 2015. For personal use only. No other uses without permission. Copyright 1990 Massachusetts Medical Society. All rights reserved. The New England Journal of Medicine Downloaded from nejm.org on March 17, 2015. For personal use only. No other uses without permission. Copyright 1990 Massachusetts Medical Society. All rights reserved. The New England Journal of Medicine Downloaded from nejm.org on March 17, 2015. For personal use only. No other uses without permission. Copyright 1990 Massachusetts Medical Society. All rights reserved. The New England Journal of Medicine Downloaded from nejm.org on March 17, 2015. For personal use only. No other uses without permission. Copyright 1990 Massachusetts Medical Society. All rights reserved. The New England Journal of Medicine Downloaded from nejm.org on March 17, 2015. For personal use only. No other uses without permission. Copyright 1990 Massachusetts Medical Society. All rights reserved. The New England Journal of Medicine Downloaded from nejm.org on March 17, 2015. For personal use only. No other uses without permission. Copyright 1990 Massachusetts Medical Society. All rights reserved. The New England Journal of Medicine Downloaded from nejm.org on March 17, 2015. For personal use only. No other uses without permission. Copyright 1990 Massachusetts Medical Society. All rights reserved.