Classification of Disorders
Depending on how the osmolality of the extracellular fluid changes compared to normal, three main types of hyperhydration are distinguished:
- Hypoosmotic — accompanied by a decrease in the concentration of osmotically active substances.
- Hyperosmotic — characterized by abnormally high osmolality.
- Isosmotic — occurs with normal plasma osmotic pressure.
Hypoosmotic Hyperhydration
In this form, excess extracellular fluid is combined with low osmolality. Fluid accumulates in both the extracellular and intracellular compartments.
Main Causes:
- Water intoxication: occurs with repeated enteral administration of electrolyte-free water. Due to sodium ion deficiency, urinary osmotic pressure drops below 50 mOsm/kg, and water begins to be reabsorbed even in the absence of antidiuretic hormone (ADH).
- Increased ADH production (e.g., in SIADH / syndrome of inappropriate antidiuretic hormone secretion).
- Severe heart failure with inadequate hormone secretion.
- Renal failure with a sharp drop in excretory function.
Clinical Manifestations: Hypervolemia and hemodilution (dilution of the blood) are observed within the vascular bed. The kidneys respond with polyuria due to high filtration pressure. Due to the osmotic gradient, water rushes into the cells. This causes erythrocyte hemolysis and cytolysis — the destruction of tissues with the release of intracellular macromolecules and enzymes into the plasma. Vomiting and diarrhea may occur from the gastrointestinal tract as a consequence of intoxication.
Hypoosmotic syndrome is particularly dangerous for the brain: neuronal swelling leads to increased intracranial pressure, lethargy, apathy, altered consciousness, and seizures. Note: peripheral edema is usually absent because water shifts intracellularly.
Hyperosmotic Hyperhydration
In this case, the osmolality of the extracellular fluid is abnormally elevated and exceeds intracellular osmolality.
Pathogenesis and Etiology: High external osmotic pressure creates a gradient. Water is transported from cells to the interstitium, causing cellular dehydration. Intracellular osmotic pressure also rises.
Such a state is caused by:
- Iatrogenic factors: administration of excessive salt solutions without monitoring their plasma levels.
- Ingestion of seawater: salts stimulate hypothalamic receptors, causing intense thirst. Repeated drinking of salt water only increases the dehydration of cells (including brain neurons).
- Endocrine disorders: hyperaldosteronism, leading to excessive renal sodium reabsorption.
- Renal pathologies: tubulopathies and enzymopathies with reduced salt excretion.
Consequences: Symptoms are dictated by the increase in plasma volume. Hypervolemia develops, circulating blood volume increases, cardiac output rises (which may later drop due to heart failure), and blood pressure and central venous pressure increase. Life-threatening conditions are possible: cerebral edema, pulmonary edema, severe hypoxia, and neuropsychiatric disorders.
Isosmotic Hyperhydration
Characterized by an increase in extracellular fluid volume while maintaining normal osmolality.
Most often, the pathology occurs with massive infusions of isotonic solutions (e.g., Natrii chloridum, Kalii chloridum, sodium bicarbonate) or against the background of pronounced circulatory failure leading to fluid retention.
Important feature: even with the development of pronounced hyperhydration with generalized edema, central nervous system functions are generally not impaired. This is the main clinical distinction from hypo- and hyperosmotic states.