October 2023: The Syndrome of Inappropriate Antidiuresis
The issue
Hyponatremia — a low blood‑sodium level — is the most common electrolyte problem, affecting about 5 % of all adults and 35 % of people in the hospital. In older in‑patients, 25 – 40 % of hyponatremia is caused by the syndrome of inappropriate antidiuresis (SIAD), in which excess antidiuretic hormone makes the kidneys retain water even though the body is already well hydrated.
What do I need to know?
Because water dilutes the blood, sodium falls and brain cells swell. Rapid or severe SIAD can bring headache, weakness, seizures, or coma; slower, chronic cases may “just” cause confusion, balance problems, or brittle bones, and these subtle signs are often mis‑attributed to aging. Common triggers include certain cancers, lung or brain disorders, many medications (notably antidepressants), pain, nausea, and even surgery. First‑line treatment is usually fluid restriction; if that fails, salt or protein supplements, oral urea, or the vasopressin‑blocker tolvaptan can raise sodium. Fewer than one patient in twenty needs emergency care, but when severe symptoms strike, physicians give rapid doses of 3 % saline to reverse brain swelling.
Potential risk of SIAD
Mild, symptom‑free hyponatremia (sodium 130 – 134 mmol/L).
Recommended Actions
Repeat sodium and weight in one week.
Review all medicines (particularly antidepressants, diuretics, pain drugs).
Begin gentle fluid restriction (e.g., ≤ 1.5 L/day) and eat normal salt and protein.
Imminent risk of SIAD
Moderate hyponatremia (125 – 129 mmol/L) or new lethargy, gait unsteadiness, nausea.
Recommended Actions
Check urine sodium/osmolality to confirm SIAD and exclude thyroid or adrenal problems.
Tighten fluids to ≤ 1 L/day; consider adding salt tablets or oral urea.
Stop or change any offending drug after talking with the prescriber.
Confirmed SIAD with danger
Sodium < 125 mmol/L or seizures, severe confusion, vomiting, or respiratory distress.
Recommended Actions
Seek emergency care; doctors will give 100 ml boluses of 3 % saline and admit for close monitoring.
Once stabilized, a specialist may prescribe tolvaptan (often 7.5 mg daily) or long‑term urea while fluid restriction continues.
Care team should prevent over‑rapid correction by checking sodium every 4–6 hours and using desmopressin if needed.
What can I do?
Ask the clinician, “Am I at potential, imminent, or confirmed risk, and what number are we aiming for?” Keep a daily log of fluid intake, body weight, and any symptoms such as headaches, unsteadiness, or confusion. If on fluid restriction, measure beverages with a marked pitcher and favor foods higher in protein and salt (unless another condition forbids it). Family members can help track medicines, watch for new neurologic signs, and ensure lab appointments are kept so sodium rises safely without overshooting.
