Hyponatremia (Na+ < 135 mEq/L) is the most common electrolyte disorder, classified by timing, volume status, and tonicity. In acute hyponatremia (< 48 hours), water shifts rapidly into brain cells (osmotic gradient), causing cerebral edema, increased ICP, and potentially fatal herniation — this requires urgent treatment. In chronic hyponatremia (> 48 hours or unknown duration), the brain adapts by extruding organic osmolytes (glutamate, taurine, myo-inositol) over 24-48 hours to restore cell volume — the brain is 'osmo-adapted.' This adaptation is protective against edema but creates vulnerability to overly rapid correction: if serum sodium is raised too quickly, water is pulled out of brain cells faster than osmolytes can be re-accumulated, causing oligodendrocyte apoptosis and osmotic demyelination syndrome (ODS/central pontine myelinolysis). Causes by volume status: hypovolemic (diuretics, GI losses, adrenal insufficiency), euvolemic (SIADH — most common, hypothyroidism, adrenal insufficiency, psychogenic polydipsia), hypervolemic (CHF, cirrhosis, nephrotic syndrome). Tonicity assessment distinguishes true hypotonic hyponatremia from pseudohyponatremia (hyperlipidemia, hyperproteinemia) and translocational (hyperglycemia — corrected Na+ rises 1.6-2.4 mEq/L per 100 mg/dL glucose above 100).