hypoosmolality

Hypoosmolality, also referred to as a hypoosmolar state, describes a condition in which the osmolality of extracellular fluids is lower than normal. Serum osmolality is normally about 275 to 295 mOsm/kg, and hypoosmolality indicates an excess of free water relative to solutes in the body. In clinical practice, it most often manifests as hypotonic hyponatremia, where low sodium accompanies reduced serum osmolality, though other solutes contribute to the overall osmotic balance.

Causes of hypoosmolality are diverse and are commonly grouped by volume status. Hypovolemic states arise from dehydration or loss of solutes with disproportionate water replacement. Euvolemic states include disorders such as syndrome of inappropriate antidiuretic hormone secretion (SIADH), hypothyroidism, and adrenal insufficiency, where water retention occurs with normal or near-normal fluid volumes. Hypervolemic states occur with heart failure, cirrhosis, and nephrotic syndrome, in which total body water increases and osmolality falls. Excessive water intake from polydipsia, beer potomania, or administration of hypotonic IV fluids can also produce hypoosmolality by diluting body solutes.

Pathophysiology involves impaired water excretion and inappropriate water retention, often mediated by vasopressin (antidiuretic hormone) signaling, leading to water movement into cells and potential cellular swelling, especially in the brain. Clinically, symptoms range from subtle cognitive changes to confusion, seizures, or coma in severe cases, particularly with rapid onset.

Diagnosis starts with measuring serum osmolality. If low, assessment of urine osmolality and urine sodium helps determine whether the kidneys are conserving water and what the likely cause is. Evaluation often includes thyroid and adrenal function, and consideration of medications.

Treatment depends on severity and etiology. Acute, severe symptoms may require cautious hypertonic saline; milder cases or chronic hyponatremia from SIADH or fluid overload are managed with fluid restriction, solute repletion, and treatment of the underlying cause. Correcting osmolality too rapidly risks osmotic demyelination, so therapy is carefully monitored.