○ SIADH: hypotonic hyponatremia (efective serum osmol<275mOsm/L) within appropriately high urinary concentration (urine osmol>100 mOsm/L) and euvolemia or hypervolemia.
○ Cerebral salt wasting (CSW): similar to SIADH but with extracellular fluid volume depletion due to renal sodium loss (urinary Na > 20 mEq/L).
● minimum W/U: serum [Na+], serum osmolality, urine osmolality, clinical assessment of volume status. If volume status is high or low: urinary [Na+] TSH (to R/O hypothyroidism).
● treatment: based on acuity, severity, symptoms & etiology; as appropriate.
● risk of overly rapid correction: osmotic demyelination (including central pontine myelinolysis).
Generally defined as a serum sodium level of less than 135 mEq/L and is considered severe when the serum sodium level is below 125 mEq/L.
[Na+] <135 mEq/L=mild, <130=moderate, <125=severe hyponatremia.
Syndrome of inappropriate antidiuretic hormone secretion is the most common type of hyponatremia 2).
Due to slow compensatory mechanisms in the brain, a gradual decline in serum sodium is better tolerated than a rapid drop. Symptoms of mild ([Na]<130 mEq/L) or gradual hyponatremia include: anorexia, headache, difficulty concentrating, irritability, dysgeusia and muscle weakness. Severe hyponatremia (<125 mEq/L) or a rapid drop (>0.5 mEq/hr) can cause neuromuscular excitability, cerebral edema, muscle twitching and cramps, nausea/vomiting, confusion, seizures, respiratory arrest and possibly permanent neurologic injury, coma or death.