Hypernatraemia

What it is

Hypernatremia is a high serum sodium. The first move is to assess volume status, because euvolemic and hypovolemic hypernatremia are treated differently and the fluid given is not the same.

Causes and risk

Diabetes insipidus is the cause to know. In central DI there is low ADH secretion. In nephrogenic DI the ADH secretion is normal, but the tubules cannot respond to ADH, and that is due to a mutation in the V2 canals.

Central DI is primary and idiopathic, or secondary to brain lesions such as tumors, hypoxic injury or surgery. Nephrogenic DI comes from an ADH receptor mutation, from medications such as lithium and demeclocycline, which will precipitate it, or from electrolyte disturbances such as hypercalcemia and hypokalemia.

Two other causes sit outside the kidney. Conn's syndrome, that is primary hyperaldosteronism, raises the sodium. Urinary alkalinization carries hypernatremia among its complications, alongside metabolic alkalosis, hyperosmolality and fluid retention.

How it presents

Polyuria, nocturia and polydipsia, with hypernatremia and dehydration. The diuresis cannot be stopped, so there will be concentration. In the absence of nocturia, diabetes insipidus is very unlikely, and that single question is worth asking early.

Mild hypernatremia above 150 mEq/L belongs to central DI and nephrogenic DI alike; it is one cell held by the two together, and it does not separate them.