The New England Journal of Medicine Downloaded from nejm.org at MIT LIBRARIES on October 8, 2013. For personal use only. No other uses without permission.

From the NEJM Archive. Copyright © 2010 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org at MIT LIBRARIES on October 8, 2013. For personal use only. No other uses without permission.

From the NEJM Archive. Copyright © 2010 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org at MIT LIBRARIES on October 8, 2013. For personal use only. No other uses without permission.

From the NEJM Archive. Copyright © 2010 Massachusetts Medical Society. All rights reserved.