Course Diarrhea and Dehydration

Dehydration is usually classified into 3 types based on the amount of sodium in the blood: isotonic, hypotonic (hyponatremia), and hypertonic (hypernatremia). In clinical practice, the first 2 can be grouped into a single isohypotonic category since they share similar physiologic characteristics, clinical presentations, and treatments. In this case, net water and electrolyte loss is either hypertonic (resulting in hypotonic dehydration) or isotonic (resulting in isotonic dehydration) compared to normal plasma osmolarity. As a result of these losses, extracellular fluid volume (EFV) is significantly reduced, with no or little decrease in intracellular fluid volume (IFV). Reduced EFV is responsible for most of the clinical signs of dehydration, which are therefore very evident.

Hypertonic dehydration occurs when net fluid losses are hypotonic in comparison to normal plasma osmolarity. In this case, the osmolar balance between the intracellular and extracellular compartment leads to the shift of water from the intracellular to the extracellular space. Because EFV is thus compensated and less affected, clinical signs of dehydration are less obvious. The loss of intracellular fluid results in intracellular dehydration evidenced by specific clinical features.