Sodium Disorders

A serum sodium concentration is best read as a water-to-solute signal, not as a measure of total body sodium stores. The immediate question is not simply *how low or high is the value?* It is whether the osmotic disturbance is causing cerebral dysfunction now. A person with chronic hyponatraemia at 118 mmol/L may be alert because the brain has adapted. A patient at 128 mmol/L who is vomiting, deeply somnolent, or seizing has cerebral oedema and needs emergency treatment. The converse safety issue follows treatment: a chronically low sodium that rises too quickly can injure the adapted brain. Use three decisions to organize care: 1. Is this an emergency? Neurological or cardiorespiratory symptoms take priority over the absolute sodium value. 2. Is the hyponatraemia truly hypotonic, and what is preventing water excretion? Plasma and urine studies answer this. 3. What correction rate is safe? Duration, risk factors for osmotic demyelination syndrome, urine output, and serial sodium results matter as much as the initial result. Canadian laboratories report sodium in mmol/L. For sodium, mmol/L and mEq/L are numerically equivalent, but use...