The New England Journal of Medicine Downloaded from nejm.org at BOSTON UNIVERSITY on September 6, 2013. For personal use only. No other uses without permission.

From the NEJM Archive. Copyright © 2009 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org at BOSTON UNIVERSITY on September 6, 2013. For personal use only. No other uses without permission.

From the NEJM Archive. Copyright © 2009 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org at BOSTON UNIVERSITY on September 6, 2013. For personal use only. No other uses without permission.

From the NEJM Archive. Copyright © 2009 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org at BOSTON UNIVERSITY on September 6, 2013. For personal use only. No other uses without permission.

From the NEJM Archive. Copyright © 2009 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org at BOSTON UNIVERSITY on September 6, 2013. For personal use only. No other uses without permission.

From the NEJM Archive. Copyright © 2009 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org at BOSTON UNIVERSITY on September 6, 2013. For personal use only. No other uses without permission.

From the NEJM Archive. Copyright © 2009 Massachusetts Medical Society. All rights reserved.