Mother-of-two Ashley Summers, of Indiana, died after drinking two liters of water in just 20 minutes.
The 35-year-old, who was feeling dehydrated and wanted to quench her thirst, collapsed and never regained consciousness.
Hyponatremia occurs when levels of sodium in blood drop to abnormally low levels.
Sodium is vital for regulating the amount of water in the body and controlling blood pressure, nerves and muscles.
When the concentration of the electrolyte is off-balance, extra water causes the body's cells and makes them swell.
This is most dangerous if it occurs in the brain, with the organ unable to swell due to the skull, according to the National Kidney Foundation.
As a result, it can lead to seizures or a coma, which can be fatal.