Dysnatremias-what causes them and how should they be treated?

Patients commonly present with or develop disorders of sodium [Na⁺] metabolism in intensive care. There is a strong correlation between [Na⁺] in plasma and extracellular fluid (ECF) volume. A low [Na⁺] (<135 mmol/L), known as hyponatremia, represents a relative excess of free water in ECF compared with [Na⁺]. This may be acute or chronic and may be associated with an array of neurological symptoms or may be asymptomatic. Hyponatremia may be associated with normal or increased body water (isovolemic or hypervolemic) or low body water (hypovolemic). The syndrome of inappropriate antidiuretic hormone secretion is caused by various cancers and brain injuries and results in renal water retention. Acute symptomatic hyponatremia should be treated immediately with hypertonic saline (3%) to increase [Na⁺] by 5 mmol/L and control symptoms. The goal is to reduce the risk of brain injury while avoiding the risk of excessively rapid correction, with associated demyelination. The maximum increase in [Na⁺] should be <10 mmol/L in the first 24 hours (and <8 mmol/L subsequently). Hypernatremia is associated with profound dehydration or isovolemic or hypervolemic [Na⁺] overload, usually iatrogenic. Dehydration is treated by repleting the free water deficit. In acute symptomatic hypernatremia, the goal is to stabilize the balance of [Na⁺] and ECF volume to avoid neurologic injury associated with the condition or over rapid correction. We recommend that the clinician correct the [Na⁺] at a rate of 0.5-1 mmol/L per hour, depending on whether the disorder is chronic or acute. The maximum rate of correction is 8-12 mmol/L per day. Hypervolemic hypernatremia is treated with hypotonic fluids and loop and thiazide diuretics.