- Definitions
- Pathophysiology
- Systems Affected
- Genetics
- Incidence/Prevalence
- Geographic Distribution
- Signalment
- Signs
- Causes
- Risk Factors
BASICS
- Osmolality represents the number of dissociated solute particles per kilogram of solvent and is expressed as mOsm/kg
- Osmolarity represents the number of dissociated solute particles per liter of solvent and is expressed as mOsm/L
- Osmolality is a thermodynamically more precise term than osmolarity, because osmolality is based on weight, which is temperature independent, whereas osmolarity is dependent on volume and is temperature dependent
- Osmotic pressure governs the movement of water across membranes because of differences in solute content
- Hyperosmolality and hyperosmolarity are defined in horses as serum concentrations of osmoles above the reference interval. Normal intervals are >270–300 mOsm/kg and >270–300 mOsm/L, respectively
- Osmolal or osmolar gap is the difference between the measured and the calculated estimate of osmolality (normal gap range –5 to 15 mOsm/L)
- 1 osmole is the gram molecular weight of a nondissociable substance and contains Avogadro's number of particles. Effective osmoles are particles in solution that cannot freely move across cell membranes and therefore create an osmotic effect
- Tonicity is the effective osmolality of a solution due to the concentration of solutes that can cause a fluid shift across semipermeable membranes
- A solvent is a liquid holding another substance in solution; for osmolality, this is water
- A solute is the dissolved substance in the solution
- A solution, in this discussion, contains solutes within a solvent
- In serum/plasma, Na+ is the principal cation balanced by many anions (Cl–, HCO3–, protein, sulfate, phosphate). Na+ is the most osmotically active particle, with some contribution from Cl– and HCO3–, and lesser contributions from glucose and urea. Animals that are hypernatremic are hyperosmolal, and hyponatremia is associated with hypo-osmolality
- Water loss increases the concentrations of solutes in serum/plasma, thereby increasing blood osmolality
- Dissolved particles that cannot move between adjacent compartments exert osmotic pressure and cause water movement to equilibrate solute concentrations between compartments
- Hypertonicity causes water to shift from the ICF to the ECF, resulting in cell shrinkage. Not all cases of hyperosmolality produce hypertonicity
- Serum osmolality is a measure of ECF osmolality. The ECF compartment comprises one-third of total body weight, with the remaining two-thirds in the ICF compartment
- The osmolalities of ECF and ICF are equal, but the ionic compositions differ. ECF solutes are primarily Na+ and Cl–, with small concentrations of HCO3–, K+, phosphate, Ca2+, and Mg2+, whereas ICF is high in K+ and phosphate, with lower concentrations of Na+, Cl–, and Ca2+
- Blood volume, hydration status, and ADH are involved in controlling the ECF volume. Hypovolemia stimulates carotid and aortic baroreceptors to respond to changes in blood pressure, causing ADH secretion
- Hyperosmolality stimulates ADH secretion from the neurohypophysis. The kidney's ability to produce urine of various concentrations maintains body regulation of osmolality. The hypothalamic thirst center is stimulated and causes an increase in water consumption to counteract serum hyperosmolality
- Rapid increases in serum osmolality cause water movement along its concentration gradient, from ICF to ECF spaces, resulting in neuronal dehydration, cell shrinkage, and cell death
- Nervous—rapid fluid shifts can lead to brain swelling
- Cardiovascular—hypotension and depressed ventricular contractility (due to loss of circulatory fluid volume)
- Renal/urologic—decreases urine output
- Multiple systems can be affected by hypoxia secondary to hypovolemia
- Excessive thirst may be the first sign of hyperosmolality
- Signs are primarily neurologic and behavioral
- Severity of signs relates to how quickly hyperosmolality occurs rather than the absolute magnitude of change
- Signs are most likely to develop with serum osmolality >350 mOsm/kg
- A history of water deprivation
- Improper reconstitution of milk replacer in orphan foals
- Owner might report signs such as anorexia, lethargy, incoordination, increased thirst
- Increased solutes—hypernatremia, hyperglycemia, severe azotemia, ethylene glycol toxicosis, propylene glycol toxicosis, salt poisoning, hypovolemic or septic shock, administration of mannitol, radiographic contrast solution, parenteral nutrition solutions, and lactate in patients with lactic acidosis
- Decreased ECF volume—dehydration or hypovolemia (e.g. GI loss, renal loss, cutaneous loss, third-space loss, low water consumption) and polyuria without adequate compensatory polydipsia
- Predisposing medical conditions—renal failure, diabetes insipidus, diabetes mellitus, pituitary pars intermedia dysfunction, equine metabolic syndrome, heat stroke
- Therapeutic hyperosmolar solutions—hypertonic saline (hypovolemic shock), NaHCO3 (renal tubular acidosis, diarrhea), sodium iodide (antimicrobial), and mannitol (cerebral edema)
- Hyperthermia
- Limited access to water
DIAGNOSIS
TREATMENT
MEDICATIONS
FOLLOW-UP
MISCELLANEOUS