From the Guidelines
Low osmolarity indicates a state of hypo-osmolarity, where the concentration of solutes in the blood is lower than normal, which can lead to cellular swelling and potentially life-threatening complications. In the context of human physiology, normal serum osmolarity ranges from 275-295 mOsm/L, as stated in the ESPEN guideline on clinical nutrition and hydration in geriatrics 1. When osmolarity falls below this range, it's considered hypo-osmolar, which often correlates with hyponatremia (low sodium levels). This condition can lead to water moving into cells, causing them to swell, and in the brain, this cellular swelling can result in cerebral edema, leading to symptoms like headache, confusion, seizures, and in severe cases, coma or death. Low osmolarity commonly occurs due to excessive water intake, inappropriate ADH secretion (SIADH), certain medications like diuretics, or medical conditions such as heart failure, cirrhosis, or kidney disease. Some key points to consider in the management of low osmolarity include:
- The primary indicator of hydration status is plasma or serum osmolality, as stated by the US Panel on Dietary Reference Intakes for Electrolytes and Water 1.
- An action threshold of directly measured serum osmolality >300 mOsm/kg should be used to identify low-intake dehydration in older adults, with a Grade of recommendation B and strong consensus (94% agreement) 1.
- For older adults with measured serum or plasma osmolality >300 mOsm/kg and unable to drink, intravenous fluids shall be considered, with a Grade A recommendation and strong consensus (95%) 1.
- In cases of severe dehydration, intravenous rehydration should be continued until pulse, perfusion, and mental status normalize, and the patient awakens, has no risk factors for aspiration, and has no evidence of ileus 1. Treatment typically involves addressing the underlying cause while carefully correcting the osmolarity, often through fluid restriction or administration of hypertonic solutions in severe cases, and rapid correction must be avoided as it can lead to osmotic demyelination syndrome, a serious neurological complication. It's also important to note that low-osmolarity oral rehydration solutions (ORS) can be given to all age groups, with any cause of diarrhea, and are safe in the presence of hypernatremia as well as hyponatremia (except when edema is present) 1.
From the Research
Definition of Low Osmolarity
Low osmolarity indicates a state of hyponatremia, which is characterized by a low serum sodium concentration, typically less than 135 mmol/L 2, 3, 4, 5, 6.
Causes of Low Osmolarity
The causes of low osmolarity include:
- Dysregulation of arginine vasopressin (AVP) homeostasis 2
- Water retention disorders such as congestive heart failure and cirrhosis 2, 5
- Euvolemic states such as syndrome of inappropriate secretion of antidiuretic hormone 2, 3
- Increased vasopressin activity 6
Treatment of Low Osmolarity
The treatment of low osmolarity includes:
- Vasopressin receptor antagonists (VRAs) such as conivaptan and tolvaptan 2, 3, 5, 6
- Fluid restriction and hypertonic saline 3, 4
- Loop diuretics 4
Effects of Low Osmolarity Treatment
The effects of low osmolarity treatment include: