The New England Journal of Medicine Downloaded from nejm.org on November 2, 2012. For personal use only. No other uses without permission.

Copyright 1992 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org on November 2, 2012. For personal use only. No other uses without permission.

Copyright 1992 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org on November 2, 2012. For personal use only. No other uses without permission.

Copyright 1992 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org on November 2, 2012. For personal use only. No other uses without permission.

Copyright 1992 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org on November 2, 2012. For personal use only. No other uses without permission.

Copyright 1992 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org on November 2, 2012. For personal use only. No other uses without permission.

Copyright 1992 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org on November 2, 2012. For personal use only. No other uses without permission.

Copyright 1992 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org on November 2, 2012. For personal use only. No other uses without permission.

Copyright 1992 Massachusetts Medical Society. All rights reserved.

The New England Journal of Medicine Downloaded from nejm.org on November 2, 2012. For personal use only. No other uses without permission.

Copyright 1992 Massachusetts Medical Society. All rights reserved.