Hyponatraemia

What it is

Hyponatremia is a low serum sodium, and it is the end of several different paths. Not every low sodium is a true hyponatremia: some are hypertonic and some are pseudohyponatremia, so the serum osmolality is measured before anything else.

Severe hyponatremia is generally defined by a serum sodium level below 120 mmol/L. Severity is based on both the sodium level and neurologic symptoms, and even moderate levels of 125-129 can be life-threatening if onset is rapid.

Causes and risk

Once a true, hypotonic hyponatremia is confirmed, the causes group by volume status:

• Hypovolemic, where total body Na is low: renal salt loss from excessive diuretic use, low aldosterone or ATN; extrarenal salt loss from diarrhea, vomiting, diaphoresis or third spacing
• Euvolemic: SIADH, psychogenic polydipsia, postoperative, hypothyroidism, glucocorticoid deficiency
• Hypervolemic: CHF and heart failure, nephrotic syndrome, liver disease, cirrhosis, renal failure

Hyponatremia is also one of the four laboratory marks of primary adrenal insufficiency, alongside hyperkalemia, hypercalcemia and eosinophilia, and in Addison's disease the sodium is low and the potassium is high.

The etiologies of SIADH are CNS disturbance such as stroke, hemorrhage or trauma; medications such as carbamazepine, SSRIs or NSAIDs; lung disease such as pneumonia; ectopic ADH secretion such as small cell lung cancer; and pain and nausea.