From the Guidelines
Low osmolality indicates a decreased concentration of solutes in the blood or other body fluids compared to normal levels, with values below 275 mOsm/kg considered low, as stated in the most recent guideline 1.
Definition and Causes
Low osmolality can result from several causes including excessive water intake, syndrome of inappropriate antidiuretic hormone secretion (SIADH), heart failure, liver cirrhosis, or certain medications like diuretics.
Clinical Significance
This condition is clinically significant because it often correlates with hyponatremia (low sodium levels), which can lead to neurological symptoms when severe.
Treatment
Treatment depends on the underlying cause but may include fluid restriction, salt supplementation, or medications like tolvaptan that antagonize ADH effects, as recommended in 1.
Key Considerations
Rapid correction should be avoided as it can lead to osmotic demyelination syndrome, a serious neurological complication. The body normally maintains osmolality within a tight range through mechanisms involving antidiuretic hormone (ADH) and thirst regulation. When osmolality decreases, the kidneys should excrete more dilute urine to restore balance.
Management in Specific Conditions
In patients with congenital nephrogenic diabetes insipidus, the typical emergency is hypernatraemic dehydration, which should be treated with water, and salt-containing solutions should be avoided, as stated in 1.
- The initial rate of fluid administration should be based on physiological demand.
- Routine maintenance rate is usually a good starting volume.
- Application at this rate will result in a slow decrease in plasma osmolality, unless withdrawing diuretics and COX inhibitors.
From the FDA Drug Label
Removal of excess free body water increases serum osmolality and serum sodium concentrations.
Low osmolality indicates a state of hyponatremia or excess free body water.
- Key points:
- Low osmolality is associated with diluted serum sodium concentrations.
- Removal of excess free body water can help increase serum osmolality and serum sodium concentrations.
- Monitoring of serum sodium levels is necessary to prevent hypernatremia during treatment with tolvaptan 2.
From the Research
Low Osmolality Indication
- Low osmolality is often associated with hyponatremia, a condition where the sodium levels in the blood are lower than normal 3, 4.
- In patients with heart failure, low discharge serum osmolality was independently predictive of worse postdischarge mortality and readmission 5.
- Low osmolality can be an indicator of various clinical outcomes, including increased risk of mortality and readmission in patients with heart failure 5.
- It can also be related to the syndrome of inappropriate antidiuretic hormone secretion (SIADH), which can lead to hyponatremia and other complications 3, 4.
Clinical Implications
- Low osmolality can be a significant predictor of adverse outcomes in patients with heart failure, and its measurement can be useful in risk stratification and management 5.
- The assessment of osmolality can help clinicians identify patients at risk for hyponatremia and other fluid-electrolyte disorders, allowing for timely intervention and treatment 3, 4.
- The use of arginine vasopressin receptor antagonists, such as tolvaptan, can be an effective treatment option for patients with euvolemic and hypervolemic hyponatremia 6, 4.
Diagnostic Approach
- The diagnostic approach to low osmolality should involve the evaluation of serum sodium levels, urine osmolality, and other relevant clinical parameters 3, 7.
- The calculation of osmolal gap can be a useful tool in identifying the presence of unmeasured osmotically active particles and guiding further diagnostic and therapeutic efforts 7.