Hyponatremia is defined as a serum sodium level < 135 mmol/l. A distinction is made between mild hyponatremia (130–135 mmol/l), moderate hyponatremia (< 130 mmol/l), and severe hyponatremia (< 120 mmol/l). In neurosurgery, the sodium level is highly relevant, as low sodium levels promote cerebral swelling and thus increased intracranial pressure.