Critical care nephrology · KIMS Secunderabad
Hyponatremia — Low Blood Sodium and Why Getting the Treatment Rate Right Is Critical
Hyponatremia — serum sodium below 135 mEq/L — is the most common electrolyte disorder in hospitalised patients, affecting 15 to 30% of inpatients. It is important not only because low sodium itself is dangerous (causing cerebral oedema, confusion, seizures, and death in severe acute cases) but because the treatment — raising the sodium level — carries its own risk if done incorrectly. Correcting sodium too rapidly causes osmotic demyelination syndrome (ODS, formerly central pontine myelinolysis) — irreversible damage to myelin sheaths in the brainstem, causing quadriplegia, dysarthria, dysphagia, and locked-in syndrome. The management of hyponatremia is one of the most carefully calibrated treatments in nephrology — the rate of correction must be neither too slow (allowing brain oedema to persist) nor too fast (causing ODS).
The first and most critical step in hyponatremia management is not treatment — it is classification. The correct treatment depends entirely on the mechanism of sodium loss or dilution: hypovolaemic (total body sodium and water both low — dehydration), euvolaemic (total body sodium normal but water retained — SIADH, hypothyroidism, adrenal insufficiency), or hypervolaemic (both sodium and water retained, but water retention exceeds sodium — heart failure, cirrhosis, nephrotic syndrome, CKD).
Classification by volume status
Hypovolaemic hyponatremia
Volume status: Low total body sodium + low water (more Na than water lost). Causes: Vomiting, diarrhoea, sweating · Diuretic excess (thiazides > loop) · Adrenal insufficiency · Cerebral salt wasting. Urine Na: below 20 (extra-renal loss) · above 40 (renal loss). Treatment: IV 0.9% normal saline — restores volume, ADH falls, free water is excreted, sodium rises.
Euvolaemic hyponatremia
Volume status: Normal total body sodium · Excess water retained. Causes: SIADH (Syndrome of Inappropriate ADH secretion — the most common cause overall · Causes: lung disease, CNS disease, malignancy, drugs — SSRIs, opioids, carbamazepine) · Hypothyroidism · Adrenal insufficiency. Urine Na typically above 40 · Urine osmolality inappropriately high (above 100 mOsm/kg). Treatment: fluid restriction (below 1 litre/day) · tolvaptan (vasopressin V2 receptor antagonist) for SIADH not responding to fluid restriction.
Hypervolaemic hyponatremia
Volume status: Elevated total body sodium AND water (but water excess more than sodium). Causes: Cardiac failure · Liver cirrhosis · Nephrotic syndrome · Advanced CKD. Urine Na: below 20 (avid sodium retention). Treatment: treat the underlying condition (optimise cardiac failure, liver disease, nephrotic syndrome) · fluid restriction · loop diuretics · tolvaptan for refractory cases.
Symptoms — severity correlates with acuity
Mild · 130–135 mEq/L
Often asymptomatic. Nausea, malaise, mild cognitive impairment on formal testing.
Moderate · 125–130 mEq/L
Headache, nausea, vomiting, confusion, gait instability, cognitive impairment.
Severe · Below 120–125 mEq/L (or rapid fall)
Severe confusion, seizures, respiratory arrest, non-cardiogenic pulmonary oedema, cerebral herniation — life-threatening. Acute hyponatremia (rapid fall in hours) is far more dangerous at the same sodium level than chronic hyponatremia (gradual fall over days), because the brain has had less time to adapt.
The correction rate — the most critical treatment decision
Sodium must not be raised faster than 8 to 10 mEq/L in any 24-hour period (or 18 mEq/L in 48 hours) in chronic hyponatremia, to prevent osmotic demyelination syndrome (ODS). In acute symptomatic hyponatremia (seizures, severe confusion), 1 to 2 mEq/L/hour for the first 2 to 3 hours is appropriate to control symptoms — then slow to below 8 mEq/L/24 hours. Sodium is checked every 2 to 4 hours during active correction. If sodium rises too rapidly, hypotonic IV fluids (5% dextrose) or DDAVP are given to re-lower it — a deliberate therapeutic re-lowering to prevent ODS.
Specific treatments at KIMS
Hypovolaemic hyponatremia — IV 0.9% normal saline
Restores intravascular volume, suppresses ADH, allows free water excretion. Sodium rises as dehydration is corrected. Monitor every 4 hours.
SIADH — fluid restriction + tolvaptan
Fluid restriction to below 1 litre per day is the mainstay. Tolvaptan (a selective vasopressin V2 receptor antagonist) blocks ADH action on the collecting duct — free water is excreted as dilute urine and sodium rises without the need for fluid restriction. Tolvaptan must be initiated in a monitored inpatient setting at KIMS — the rate of sodium rise must be tracked closely to prevent ODS from over-rapid correction.
Hypertonic saline (3% NaCl) — for severe symptomatic hyponatremia
Administered as 100ml boluses IV for active seizures or severe neurological symptoms — each 100ml bolus of 3% NaCl raises serum sodium by approximately 1 to 2 mEq/L. Repeat up to 3 times if symptoms persist. Once symptoms are controlled, switch to slower correction strategy.
Treat the underlying condition — always
SIADH from drugs: stop the offending drug. Hypothyroidism: levothyroxine. Adrenal insufficiency: hydrocortisone. Cardiac failure: optimise with SGLT2 inhibitors, diuretics, ACE inhibitors. Cirrhosis: albumin, terlipressin for HRS-related hyponatremia.
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Frequently Asked Questions — Hyponatremia
Low blood sodium (hyponatremia) is almost always caused by excess water in the blood relative to sodium — not by a shortage of dietary salt intake. The most common cause in hospitalised patients is SIADH (syndrome of inappropriate ADH secretion) — excess antidiuretic hormone causes water retention, diluting the sodium. SIADH is caused by: lung disease (pneumonia, tuberculosis — important in India), central nervous system disease (head injury, meningitis, stroke), malignancy (small cell lung cancer), and drugs (SSRIs, opioids, carbamazepine, NSAIDs). Other important causes: heart failure, liver cirrhosis, and nephrotic syndrome (all cause sodium and water retention, but with disproportionate water retention); thiazide diuretics (a very common cause in elderly patients); and hypothyroidism.
Yes — severe acute hyponatremia (sodium below 120 mEq/L developing rapidly, within hours) is life-threatening. The rapidly falling sodium causes cerebral oedema — the brain swells against the rigid skull — causing progressive headache, confusion, seizures, respiratory depression, and brain herniation. Chronic hyponatremia (sodium falling slowly over days to weeks) is less acutely dangerous at the same sodium level because the brain has had time to adapt by extruding osmoles. However, chronic hyponatremia below 130 mEq/L still causes cognitive impairment, gait instability (increasing fall risk in elderly patients — hyponatremia is a major and often unrecognised cause of falls and hip fractures in the elderly), and reduced quality of life.
Osmotic demyelination syndrome (ODS) — also called central pontine myelinolysis — occurs when sodium is corrected too rapidly after chronic hyponatremia. In chronic hyponatremia, brain cells adapt to the low sodium environment by extruding osmotic particles (osmoles) from inside the cells — the brain shrinks slightly but restores its osmotic equilibrium. When sodium is raised rapidly, the now relatively hypo-osmolar brain cells are exposed to a hyper-osmolar extracellular environment — water leaves the cells rapidly, causing the myelin sheaths of brainstem neurons to be damaged as they shrink. The result is ODS — an irreversible demyelinating injury causing locked-in syndrome, quadriplegia, and pseudobulbar palsy. Prevention: never raise sodium more than 8 to 10 mEq/L in 24 hours in chronic hyponatremia.
Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH) is the most common cause of euvolaemic hyponatremia. ADH (antidiuretic hormone, also called vasopressin) is normally released when blood osmolality rises or blood volume falls, causing the kidneys to retain water. In SIADH, ADH is secreted despite normal or low blood osmolality — for reasons unrelated to the normal physiological triggers. The retained free water dilutes the serum sodium. Diagnosis: serum osmolality below 280 mOsm/kg + urine osmolality above 100 mOsm/kg + urine sodium above 40 mEq/L + clinically euvolaemic (not dehydrated or fluid-overloaded) + exclusion of hypothyroidism and adrenal insufficiency.
KIMS Secunderabad — Dr. E. Ravi (Senior Consultant Nephrologist, critical care nephrology), systematic hyponatremia classification (volume status assessment, urine sodium, urine osmolality), tolvaptan for SIADH under monitored inpatient conditions with 2-hourly sodium tracking, hypertonic saline for severe symptomatic hyponatremia, ODS prevention protocol with deliberate re-lowering if correction is too rapid. NABH and NABL accredited. Emergency line: 040-4488-5000.