Types of Dehydration
Dehydration is classified into three distinct types based on serum sodium concentration and tonicity: hypertonic (hypernatremic), isotonic (isonatremic), and hypotonic (hyponatremic) dehydration. 1, 2, 3
Classification System
The three types of dehydration differ fundamentally in their pathophysiology and reflect different patterns of water and electrolyte loss:
Hypertonic (Hypernatremic) Dehydration
- Results from pure water loss exceeding sodium loss, causing elevated serum sodium and osmolality 1, 2, 3
- Also called "low-intake dehydration" when caused by insufficient water intake 1
- Particularly common in older adults due to reduced thirst sensation with aging 1
- Characterized by intracellular fluid depletion as water shifts from cells to maintain extracellular volume 4, 5
- Intravascular volume is better preserved initially compared to other types because the hyperosmolar state draws fluid from intracellular compartments 4
- Diagnosed when serum osmolality exceeds 300 mOsm/kg in older adults 1
Isotonic (Isonatremic) Dehydration
- Occurs when water and sodium are lost in proportional amounts, maintaining normal serum sodium concentration 2, 3, 6
- Most common type seen in acute gastroenteritis and food poisoning with vomiting and diarrhea 6
- Both intracellular and extracellular compartments are depleted proportionally 3
- Serum sodium remains within normal range despite volume deficit 3, 6
Hypotonic (Hyponatremic) Dehydration
- Results from sodium loss exceeding water loss, causing low serum sodium concentration 2, 3, 4
- Also termed "volume depletion" when referring to extracellular fluid and sodium deficit 3
- Occurs with excessive sodium losses through vomiting, diarrhea, or renal losses 3, 5
- Most dangerous for intravascular volume as the hypo-osmolar state causes fluid shift into cells, worsening circulatory compromise 4
- Can present with neurologic symptoms due to cellular swelling 5
Critical Distinction: Volume Depletion vs. Dehydration
The term "dehydration" technically refers specifically to total body water deficit (especially intracellular), which is always hypertonic, while "volume depletion" refers to intravascular water and sodium deficit, which can be hypertonic, isotonic, or hypotonic 3. However, in clinical practice, the term dehydration is often used more broadly to encompass all three types 1, 2.
Clinical Implications
Assessment Differences
- Tachycardia, delayed capillary refill, and orthostatic hypotension appear earlier and more prominently in volume depletion (isotonic and hypotonic types) compared to hypertonic dehydration 3, 4
- Hypertonic dehydration may present with better hemodynamic stability initially due to preserved intravascular volume 4
Treatment Varies by Type
- Hypertonic dehydration requires hypotonic fluids to correct both the water deficit and dilute the elevated osmolality 1
- Isotonic and hypotonic dehydration require isotonic fluids (normal saline or lactated Ringer's) to replace both water and electrolytes 7, 3
- Hemodynamically unstable patients require isotonic saline regardless of type until stabilization is achieved 7, 3
Common Pitfall
Do not confuse the classification system: iso- and hypotonic dehydration are caused by illness (vomiting, diarrhea, renal losses) and involve electrolyte disturbances, while hypertonic dehydration is primarily related to inadequate water intake 1. This distinction guides both diagnosis and treatment approach.