Hyponatremia
Cynthia was in excellent health. 
She was extremely diligent and dedicated in her training. 

She had successfully completed a marathon 2 years prior to running in Boston. 

By all accounts she looked great all throughout the 19 miles of the 2002 Boston Marathon. 

Yet, she became unsteady and collapsed between miles 19 and 20.  She was placed in an ambulance and seized on her way to the hospital.

She never regained consciousness.
Since Cynthia's passing there have been several reports intended to warn athletes about the dangers of hyponatremia so that runners can be better informed.
Runner's World - Drink to your health
Boston Globe - August, 2002
The Chicago Sun Times - Dangers of marathons -October, 2002
MetroWest Daily - November, 2002

Coolrunning.com - Tips for runners
MetroWest Daily - Official's Warning - April 16, 2003
MetroWest Daily - A close call - April 22, 2003
Boston Globe - Medical Coverage at Marathon - April 18, 2006

Unfortunately, just a few months after Cynthia's tragedy, another runner apparently  succumbed to the same syndrome that claimed her life while participating in the Marine Corps Marathon.
The Washington Times - October 30, 2002
The Washington Times - October 24, 2003
Washington Post - October 24, 2003
More recently, a 22 year old runner was hospitalized and subsequently died of hyponatremia after completing the London Marthon in April 2007.

The exact cause of hyponatremia is a topic of some debate in the medical community.
MetroWest Daily News - November 5, 2003
Medscape report - July 30, 2003
British Journal of Medicine - July 19, 2003
Sound Medicine Expert Interview - January 9, 2005
New England Journal of Medicine - April 14, 2005. (You may need to download Adobe Reader to view this file)

Current research attempting to better characterize the mechanisms responsible for hyponatremia in these cases can help develop recommendations for improved monitoring and treatment of runners and ultimately prevent the potentially tragic consequences of this condition
American Journal of Medicine - 2007
Sports Medicine - 2007

It is our hope that the seriousness of this rare syndrome can be shared with all runners and that efforts are made to prevent tragedies such as these from happening to healthy, active people in the prime of their life.