Dehydration can be mild, moderate or severe. This is determined by the percentage of body weight lost due to fluid loss. Normally, the body loses and regains one to two percent of its weight throughout the day as fluids are continually eliminated and replaced. When dehydration occurs, a higher percentage of body weight is lost. Dehydration can progress from mild to moderate to severe within hours, especially in infants and children. Weight loss of more than 15 to 20 percent is considered a medical emergency and can result in death without emergency intervention.

Dehydration may be classified as follows:

Dehydration may be further defined by the type of fluid/electrolyte imbalance that exists. Knowing which type of imbalance is present in moderate to severe cases of dehydration can ensure that appropriate replacement fluids restore the proper balance of fluids and electrolytes to the body. These types of dehydration include:

• Isotonic dehydration. When both fluid and sodium are lost in equal amounts. It is most common in people taking diuretics (medications that increase urine output), fasting, or people with severe or prolonged diarrhea and vomiting. This accounts for approximately 70 to 80 percent of all pediatric cases of dehydration. This type of dehydration is sometimes referred to as isonatremic dehydration.
  
  
• Hypertonic dehydration. When more fluid than sodium is lost. This is more common in people not getting enough nutrients through their diet (e.g., due to watery diarrhea, excessive vomiting, breastfeeding, eating disorders) or who have diabetes insipidus (water diabetes, which is characterized by frequent urination and thirst). It may also be due to excess diuretic use, infection or fever. It accounts for approximately 10 to 20 percent of all pediatric cases of dehydration with diarrhea.
  
  Hypertonic dehydration is considered the most dangerous of the three types because of the risk of complications. Patients may seem less ill at first and often are not brought in for treatment until the dehydration has become more severe. There are also risks associated with treatment (e.g., rapid rehydration can lead to cerebral edema). This type of dehydration is sometimes referred to as hypernatremic dehydration.
  
  
• Hypotonic dehydration. When more sodium than fluid is lost. This type of dehydration is sometimes referred to as hyponatremic dehydration, and accounts for approximately 10 to 15 percent of all pediatric cases of dehydration with diarrhea. This may occur due to the use of diuretics or kidney disease. Although rare, it can also occur due to drinking too much water, which excessively dilutes the level of sodium in the body and may cause the body’s cells to swell (dilutional hyponatremia). This complication can be life-threatening if swelling causes pressure on the brain (cerebral edema).