BOSTON -- Katherine Guevara, an experienced marathoner who didn't follow doctors' recommendations, said if there had been a bowl of pretzels at the finish line, she would have eaten them to recoup the salt she lost by drinking two cups of water at each station along the 26-mile route.
Guevara, who finished in 3 hours and 38 minutes, was the first case of hyponatremia in the Boston Marathon medical tent yesterday -- a condition that killed charity runner Cynthia Lucero of Waltham last year.
Three other cases followed Guevara, and one of them was hospitalized.
Hyponatremia is caused by overloading the body with fluids, which dilute sodium levels in the blood.
"She's a real case," said Dr. Arthur Siegel. "A woman who could've been a Cynthia Lucero this year. It proves how important the testing is."
Siegel, a physician at McLean Hospital in Belmont, has been studying blood samples from cases like Lucero's to determine what other factors cause hyponatremia, or water intoxication.
Most runners aren't aware of the dangers of hyponatremia, but Guevara, 26, of Virginia knew she needed salt because she felt dizzy and disorientated. Her blood test showed she was right, and she was immediately given salty, hot broth to drink by Siegel.
"She's a classic case," he said. "She's a full-blown case."
Hyponatremia is common in petite women, and charity runners who are on the course for five or six hours and drink continuously during the race.
The condition is also common among ultra-marathoners, who run as many as 50 to 100 miles, and tri-athletes, who compete for long periods of time. These athletes are encouraged to take sports drinks to replace electrolytes.
But Guevara, who is an ultra-marathoner, said she doesn't like Gatorade, and loaded up on three liters of water before the race and drank throughout. She still entered the tent in a treatable condition.
Lucero's death -- only the second in Marathon history and the first from hyponatremia -- has changed the way medical officials treat runners.
Each registered Marathoner and charity runner received information about how to properly hydrate themselves.
They were told to weigh themselves on practice runs before Marathon day -- if they gained weight they were drinking too much; if they lost weight, they needed more fluids.
Guevara did not follow these directions and actually had gained 2 pounds on the course, coming into the tent at 118 pounds.
Runners can suffer from hyponatremia hours after running the Marathon, said Siegel.
If blood sodium levels indicate runners are hyponatremic, they should not drink any fluids until they urinate, he said, so the condition won't worsen.
Once urination resumes, runners should drink no more than 6 to 8 ounces of fluids per hour for the next six hours. If they are nauseous or vomit or faint, they should immediately visit an emergency room.
Runners can confuse the condition with dehydration, which has similar symptoms, and often continue drinking, worsening their conditions.
In severe cases, hyponatremia causes seizures, coma and death as a result of brain swelling. Lucero drank 16 ounces of fluid five minutes before she collapsed, Siegel said. Lucero collapsed at mile 22 and later died at Brigham & Women's Hospital.
Siegel believes an antidiuretic hormone that prevents the kidney from secreting fluid is also at play when runners develop hyponatremia.
In addition to testing blood sodium levels before administering IV fluids, Siegel was conducting a pilot study yesterday using a test called an osmolality. The test measures how concentrated or diluted a runner's body fluids are and if there is more of the suspected antidiuretic hormone present.
If the test is shown to work -- connecting hyponatremics with elevated levels of the hormone, called arginine vasopressin -- it could become a new standard at marathons across the country.
The hormone is made in the brain and is in charge of regulating water excretion by the kidney.
Siegel was in charge yesterday of determining who had dangerous sodium levels and delivering test results to physicians who tailor the amount of fluids given to runners based on the results.
Terry Shirey, director of scientific affairs at Nova Biomedical of Waltham, said for the last four years the company has provided equipment at the medical tent to test blood sodium levels.
"The idea is to get a sodium result on the runner's blood as fast as we can after they've crossed the finish line," Shirey said.
Joan Casey, medical coordinator for the Boston Athletic Association, said a couple of years ago -- before Lucero's death -- Guevara might have been treated for dehydration with more fluids.